INTRODUCTION TO SEMICONDUCTOR
Diode :- Materials such as germanium, silicon, carbon, etc. are not good conductors like copper nor insulators like glass. In other words, the resistivity of these substances lies in between conductors and insulators. Such materials are known as semiconductors. Semiconductors have some useful characteristics and are being extensively used in electronics devices. For instance, a transistor—a semiconductor device is very fast replacing to huge vacuum tubes in almost all applications. Transistors are only one of the semiconductors devices family; and thousands of other semiconductor devices are becoming increasingly popular. In this article, we shall focus our attention on the different parameters of semiconductors.
What is Semiconductor
It is not easy to explain a semiconductor if we want to take into account all its physical properties. However, normally, a semiconductor is elaborate on the basis of current conductivity as under: A semiconductor is a material which has resistivity (10−4 to 0.5 Ωm) in between insulators and conductors e.g. germanium, selenium, carbon, silicon, etc. The reader may wonder, when a semiconductor is not a good conductor nor an insulator, then why not to classify it as a resistance material? The answer shall be readily here if we study the following table :
Comparing the resistivities of the above substance, it is apparent that the resistivity of germanium (semiconductor) is large as compared to copper (conductor) but it is a little bit low when compared with glass (insulator). This shows that the semiconductor resistivity lies in mid of conductor resistivity and insulators resistivity. However, it will be wrong if we take the semiconductor as a resistance substance. For example, nichrome, which is one of the highest resistance matter, has resistivity very lower as compare to germanium. This shows that electrically germanium can’t be cotegraized as a conductor neither insulator nor a resistance material. This gave such materials like germanium the name of semiconductors. It is interesting to note that it is not only the resistivity alone which decides whether material is semiconductor or not. For example, it is just possible to make an alloy whose resistivity falls within the range of semiconductors materials but the alloy cannot become in contrary of semiconductor. In fact, semiconductors have a huge number of peculiar chargecterstics that distinguish them from conductors, insulators, and resistance subistance.
Properties of Semiconductors
(i) The resistivity of a semiconductor material is less than an insulator material but more than a conductor materail.
(ii) Semiconductors materail has a negative temperature co-efficient of resistance i.e. the resistance
of a semiconductor materails decreases with the increase in temperature and vice-versa. For example, germanium is actually an insulator materail at low temperatures but it becomes a good conductor at more temperatures.
(iii) When a suitable metallic impurity (e.g. arsenic, gallium etc.) is combine with a semiconductor material, its electricity conducting properties change appreciably. This charectersitc is most important and is discussed later in detail.
Bonds in Semiconductors
The atoms of all element are bound together by the bonding action of valence electrons. This
bonding is because of the fact that it is the tendency of every atom to complete its last orbit by acquiring 8 electrons in it. However, in most of the materials, the last orbit is not complete i.e. the last orbit does not have 8 electrons. This thing makes the atom active to enter into bargain with other atoms to complete its 8 electrons in the last orbit. To do so, the atom may share, lose or gain valence electrons with other atoms. In semiconductors materails, bonds are made by sharing of valence electrons. Such bonds are known as co-valent bonds. In the creation of a co-valent bond, each atom shares equal number of valence electrons and the contributed electrons are shared by the atoms engaged in the formation of the bond.
Fig. 5.1 shows the co-valent bonds between germanium atoms. A germanium atom has four electrons in valence orbit. It is the tendency of germanium atom to have in last orbit 8 electrons To do so, each germanium atom itself positions between four other germanium atoms as shown in Fig.5.1 (i). Each neighbouring atom contribute one valence electron with the central atom. In this business of sharing electrons, the central atom completes its last orbit by having Eight electrons revolving around the nucleus. In this way, the central atom sets up co-valent bonds. Fig. 5.1 (ii) shows the bonding diagram. The following points may be keep in mind regarding the co-valent bonds :
(i) Co-valent bonds are created by sharing of valence electrons.
(ii) In the formation of co-valent bond, every valence electron of an atom creates direct bond with the valence electron of an adjacent atom. In other words, valence electrons are associated with specified atoms. For this thing, valence electrons in a semiconductor materials are not free.
A material in which the atoms or molecules are arranged in an orderly pattern is called a crystal. All semi-conductors materails have crystalline structure. For example, referring to Fig. 5.1, it is clear that each atom is surrounded by neighbouring atoms in a repetitive manner. Therefore, a little piece of germanium is generally known as germanium crystal.
Commonly Used Semiconductors
There are many semiconductors meterials available, but very little of them have a practical application in electronics world. The two most frequently used subistance are germanium (Ge) and silicon (Si). It is due to the energy needed to break their co-valent bonds (i.e. energy required to release an electron from their valence bands) is very little; being nearl 0.7 eV for germanium and nearly 1.1 eV for silicon. Therefore, we shall discuss these two semiconductors materials in detail.
Germanium has become the model materials among the semiconductors world; the main reason being that it can be purified relatively very well and crystallised very easily. Germanium is an earth element and in 1886 was discovered. It is recovered from the ash of some coals or from the flue dust of zinc smelters. Normally, recovered germanium is in the shape of germanium dioxide powder which is then converted to pure germanium.
The atomic number of germanium element is 32. Therefore, it has 32 electrons and 32 protons. 2
electrons are in the 1st orbit, 8 electrons in the 2nd, 8 electrons in the 3rd and 4 electrons in the outer or valence orbit [See Fig. 5.2 (i)]. It is clear that germanium atom has 4 valence electrons i.e., it is a tetravalent element. Fig. 5.2 (ii) shows how the many germanium atoms are held through co-valent bonds. As the atoms are arranged in an orderly form, therefore, germanium has a crystalline structure.
Silicon is an element which is most of the common rocks. In reality, sand is silicon dioxide. The silicon compounds are chemically decreases to silicon which is 100% pure for use as a semiconductor materai.
The atomic number of silicon is fourteen. Therefore, it has fourteen protons and fourteen electrons. 2 electrons are in the 1st orbit, 8 electrons in the 2nd orbit and 4 electrons in the 3rd orbit [See Fig. 5.3 (i)]. It is clear that silicon atom has 4 valence electrons i.e. it is a tetravalent element. Fig. 5.3
(ii) shows how many silicon atoms are bound through co-valent bonds. such as germanium, silicon
atoms are also arranged in an orderly patteren. Therefore, silicon has crystalline structure materail.
Energy Band Description of Semiconductors
It has already been explaind that a semiconductor material is a material whose resistivity lies mid of the conductors and insulators. The resistivity is of the 10−4 to 0.5 ohm meter. However, a semiconductor materail can be explain much more comprehensively on the basis of energy bands/levels as under :
A semiconductor is a material that has an almost complete filled valence band and nearly zero conduction band with a very little energy gap (j 1 eV) separating the two. Figs. 5.4 and 5.5 show the energy band diagrams of silicon and germanium. It may be observed that the forbidden energy gap is very short; being 0.7 eV for germanium and 1.1 eV for silicon. Therefore, relatively little energy is required by their valence electrons to move over to the conduction band. Even at normal room temperature, few of the valence electrons may acquire sufficient energy to move into the conduction band and thus become free electrons. However, at this room temperature, the number of free electrons available is very *small. Therefore, at normal room temperature, a piece of germanium or silicon is not a good conductor material nor an insulator material. For this point, such materials are known as semiconductors. The energy band description is very extremely helpful in understanding or learning the current flowing through a semiconductor material. Therefore, we shall mostly use this concept in our further discussion.
Effect of Temperature on Semiconductors.
The electrical conductivity of a semiconductor material changes appreciably with changing of temperature. This is a very important point.
(i) At absolute zero.
At absolute zero temperature, all the electrons are very tightly bound by the semiconductor material atoms. The inner orbit electrons of semiconductor will bound whereas the valence electrons are engaged in covalent bonding. At this temperature, the covalent bonds are very tight and there are zero free electrons. Therefore, the semiconductor material behaves as a perfect insulator [SeeFig. 5.6 (i)]. In terms of energy band description, the valence band is completely filled and there is a huge energy gap between the valence and conduction band. Therefore, zero valence electron can be moved to the conduction band to become a free electron. It is because of the non-availability of free electrons that a semiconductor material behaves as an insulator material.
(ii) Above absolute zero.
When the temperature is increased, some of the covalent bonds in the semiconductor material break because of the thermal energy provided. The breaking of bonds sets those electrons free which are engaged in the creating of these bonds. The result is that some free electrons exist in the semiconductor material. These free electrons can constitute a small electric current if the potential difference is
applied across the semiconductor material [See Fig. 5.7 (i)]. This describes that the resistance of semiconductor material reduces with the increase in temperature i.e. it has a negative temperature coefficient of resistance. It may be added that at normal room temperature, the current through a semiconductor material is too small to be of any practical current value.
Fig. 5.7 (ii) describes the energy band diagram. As the temperature is increased, few of the valence electrons acquire sufficient energy to move into the conduction band and thus become free electrons. Under the influence of the electric field, these free electrons will constitute an electric current. It may be pointed that every time a valence electron moves into the conduction band, a hole is a generator in the valence band. As we shall read in the next article (whatswho) , holes also play major to flowing current. In fact, hole current is the most significant concept in semiconductors materials.
watch video on effect of temperature on semiconductor
What is Hole Current ?
At Normal room temperature, few of the covalent bonds in pure semiconductor material break, setting up free electrons. Under the effect of the electric field, these free electrons constitute an electric current. At the same time, another current – the hole current – also flows in the semiconductor. When a covalent bond is broken because of the thermal energy, the removal of one electron leaves an area i.e. a missing or leaved electron in the covalent bond. This missing electron is known as a hole that behaves as a positive charge. For one electron set free, one hole is created. Therefore, thermal energy creates hole-electron pairs; there being as many holes as the free electrons. The current conduction by holes can be explained as follows :
The hole shows a missing electron. Suppose the valence electron at L (See Fig. 5.8) has become free electron due to thermal energy. This creates a hole in the co-valent bond at L. The hole is a strong centre of attraction **for the electron. A valence electron (say at M) from nearby co-valent bond comes to fill in the hole at L. This results in the creation of hole at M. Another valence electron (say at N) in turn may leave its bond to fill the hole at M, thus creating a hole at N. Thus the hole having a positive charge has moved from L to N i.e. towards the negative terminal of supply. This constitutes hole current. It may be noted that hole current is due to the movement of ***valence electrons from one covalent bond to another bond. The reader may wonder why to call it a hole current when the conduction is again by electrons (of course valence electrons !). The answer is that the basic reason for current flow is the presence of holes in the covalent bonds. Therefore, it is more appropriate to consider the current as the movement of holes.
Energy band description. The hole current can be beautifully explained in terms of energy bands. Suppose due to thermal energy, an electron leaves the valence band to enter into the conduction band as shown in Fig. 5.9.This leaves a vacancy at L. Now the valence electron at M comes to fill the hole at L. The result is that hole disappears at L and appears at M. Next, the valence electron at N moves into the hole at M. Consequently, hole is created at N. It is clear that valence electrons move along the path PNML whereas holes move in the opposite direction i.e. along the path LMNP.
What is Intrinsic Semiconductor ?
A semiconductor in an extremely pure form is known as an intrinsic semiconductor. In an intrinsic semiconductor, even at room temperature, hole-electron pairs are created. When electric field is applied across an intrinsic semiconductor, the current conduction takes place by two processes, namely ; by free electrons and holes as shown in Fig. 5.10. The free electrons are produced due to the breaking up of some covalent bonds by thermal energy. At the same time, holes are created in the covalent bonds. Under the influence of electric field, conduction through the semiconductor is by both free electrons and holes. Therefore, the total current inside the semiconductor is the sum of currents due to free electrons and holes.
It may be noted that current in the external wires is fully electronic i.e. by electrons. What about the holes ? Referring to Fig. 5.10, holes being positively charged move towards the negative terminal of supply. As the holes reach the negative terminal B, electrons enter the semiconductor crystal near the terminal and combine with holes, thus cancelling them. At the same time, the loosely held electrons near the positive terminal A are attracted away from their atoms into the positive terminal. This creates new holes near the positive terminal which again drift towards the negative terminal.
Whats is Extrinsic Semiconductor ?
The intrinsic semiconductor has little current conduction capability at room temperature. To be
useful in electronic devices, the pure semiconductor must be altered so as to significantly increase its conducting properties. This is achieved by adding a small amount of suitable impurity to a semiconductor. It is then called impurity or extrinsic semiconductor. The process of adding impurities to a semiconductor is known as doping. The amount and type of such impurities have to be closely controlled during the preparation of extrinsic semiconductor. Generally, for 108 atoms of semiconductor, one impurity atom is added. The purpose of adding impurity is to increase either the number of free electrons or holes in the semiconductor crystal. As we shall see, if a pentavalent impurity (having 5 valence electrons) is added to the semiconductor, a large number of free electrons are produced in the semiconductor. On the other hand, addition of trivalent impurity (having 3 valence electrons) creates a large number of holes in the semiconductor crystal. Depending upon the type of impurity added, extrinsic semiconductors are classified into:
(i) n-type semiconductor
(ii) p-type semiconductor
what is n-type Semiconductor?
When a small amount of pentavalent impurity is added to a pure semiconductor, it is known as
n-type semiconductor. The addition of pentavalent impurity provides a large number of free electrons in the semiconductor crystal.
Typical examples of pentavalent impurities are arsenic (At. No. 33) and antimony (At. No. 51). Such impurities which produce n-type semiconductor are known as donor impurities because they donate or provide free electrons to the semiconductor crystal. To explain the formation of n-type semiconductor, consider a pure germanium crystal. We know that germanium atom has four valence electrons. When a small amount of pentavalent impurity like arsenic is added to germanium crystal, a large number of free electrons become available in the crystal. The reason is simple. Arsenic is pentavalent i.e. its atom has five valence electrons. An arsenic atom fits in the germanium crystal in such a way that its four valence electrons form covalent bonds with four germanium atoms. The fifth valence electron of arsenic atom finds no place in co-valent bonds and is thus free as shown in Fig. 5.11. Therefore, for each arsenic atom added, one free electron will be available in the germanium crystal. Though each arsenic atom provides one free electron, yet an extremely small amount of arsenic impurity provides enough atoms to supply millions of free electrons. Fig. 5.12 shows the energy band description of n-type semi-conductor. The addition of pentavalent impurity has produced a number of conduction band electrons i.e., free electrons. The four valence electrons of pentavalent atom form covalent bonds with four neighbouring germanium atoms. The fifth left over valence electron of the pentavalent atom cannot be accommodated in the valence band and travels to the conduction band.
The following points may be noted carefully :
(i) Many new free electrons are produced by the addition of pentavalent impurity.
(ii) Thermal energy of room temperature still generates a few hole-electron pairs. However, the
number of free electrons provided by the pentavalent impurity far exceeds the number of holes. It is due to this predominance of electrons over holes that it is called n-type semiconductor (n stands for negative). n-type conductivity. The current conduction in an n-type semiconductor is predominantly by free electrons i.e. negative charges and is called n-type or electron type conductivity. To understand n-type conductivity, refer to Fig. 5.13. When p.d. is applied across the n-type semiconductor, the free electrons (donated by impurity) in the crystal will be directed towards the positive terminal, constituting electric current. As the current flow through the crystal is by free electrons which are carriers of negative charge, therefore, this type of conductivity is called negative or n-type conductivity. It may be noted that conduction is just as in ordinary metals like copper. Fig. 5.11
What is p-type Semiconductor ?
When a small amount of trivalent impurity is added to a pure semiconductor, it is called p-type
semiconductor. The addition of trivalent impurity provides a large number of holes in the semiconductor. Typical examples of trivalent impurities are gallium (At. No. 31) and indium (At. No. 49). Such impurities which produce p-type semiconductor are known as acceptor impurities because the holes created can accept the electrons. To explain the formation of p-type semiconductor, consider a pure germanium crystal. When a small amount of trivalent impurity like gallium is added to germanium crystal, there exists a large number of holes in the crystal. The reason is simple. Gallium is trivalent i.e. its atom has three valence electrons. Each atom of
gallium fits into the germanium crystal but now only three co-valent bonds can be formed. It is because three valence electrons of gallium atom can form only three single co-valent bonds with three germanium atoms as shown in Fig. 5.14. In the fourth co-valent bond, only germanium atom contributes one valence electron while gallium has no valence electron to contribute as all its three valence electrons are already engaged in the co-valent bonds with neighbouring germanium atoms. In other words, fourth bond is incomplete; being short of one electron. This missing electron is called a hole. Therefore, for each gallium atom added, one hole is created. A small amount of gallium provides millions of holes. Fig. 5.15 shows the energy band description of the p-type semiconductor. The addition of trivalent impurity has produced a large number of holes. However, there are a few conduction band electrons due to thermal energy associated with room temperature. But the holes far outnumber the conduction band electrons. It is due to the predominance of holes over free electrons that it is called p-type semiconductor ( p stands for positive).
p-type conductivity. The current conduction in p-type semiconductor is predominantly by holes
i.e. positive charges and is called p-type or hole-type conductivity. To understand p-type conductivity, refer to Fig. 5.16. When p.d. is applied to the p-type semiconductor, the holes (donated by the impurity) are shifted from one co-valent bond to another. As the holes are positively charged, therefore, they are directed towards the negative terminal, constituting what is known as hole current. It may be noted that in p-type conductivity, the valence electrons move from one co-valent bond to another unlike the n-type where current conduction is by free electrons.
Charge on n-type and p-type Semiconductors
As discussed before, in n-type semiconductor, current conduction is due to excess of electrons whereas in a p-type semiconductor, conduction is by holes. The reader may think that n-type material has a net negative charge and p-type a net positive charge. But this conclusion is wrong. It is true that n-type semiconductor has excess of electrons but these extra electrons were supplied by the atoms of donor impurity and each atom of donor impurity is electrically neutral. When the impurity atom is added, the term “excess electrons” refers to an excess with regard to the number of electrons needed to fill the co-valent bonds in the semiconductor crystal. The extra electrons are free electrons and increase the conductivity of the semiconductor. The situation with regard to p-type semiconductor is also similar. It follows, therefore, that n-type as well as p-type semiconductor is electrically neutral.
Majority and Minority Carriers
It has already been discussed that due to the effect of impurity, n-type material has a large number of free electrons whereas p-type material has a large number of holes. However, it may be recalled that even at room temperature, some of the co-valent bonds break, thus releasing an equal number of free electrons and holes. An n-type material has its share of electron-hole pairs (released due to breaking of bonds at room temperature) but in addition has a much larger quantity of free electrons due to the effect of impurity. These impurity-caused free electrons are not associated with holes. Consequently, an n-type material has a large number of free electrons and a small number of holes as shown in Fig. 5.17 (i). The free electrons in this case are considered majority carriers — since the majority portion of current in n-type material is by the flow of free electrons — and the holes are the minority carriers.
Similarly, in a p-type material, holes outnumber the free electrons as shown in Fig. 5.17 (ii).
Therefore, holes are the majority carriers and free electrons are the minority carriers.\
What is PN junction ?
When a p-type semiconductor is suitably joined to n-type semiconductors, the contact surface is called PN junction. Most semiconductor devices contain one or more PN junctions. The PN junction is of great importance because it is in effect, the control element for semiconductor devices. Thorough knowledge of the formation and properties of PN junction can enable the reader to understand the semiconductor devices.
Formation of PN junction
In actual practice, the characteristic properties of PN junction will not be apparent if a p-type block is just brought in contact with the n-type block. In fact, PN junction is fabricated by special techniques. One common method of making PN junction is called alloying. In this method, a small block of indium (trivalent impurity) is placed on an n-type germanium slab as shown in Fig. 5.18 (i). The system is then heated to a temperature of about 500ºC. The indium and some of the germanium melt to form a small puddle of the molten germanium-indium mixture as shown in Fig. 5.18 (ii). The temperature is then lowered and puddle begins to solidify. Under proper conditions, the atoms of indium impurity will be suitably adjusted in the germanium slab to form a single crystal. The addition of indium overcomes the excess of electrons in the n-type germanium to such an extent that it creates a p-type region. As the process goes on, the remaining molten mixture becomes increasingly rich in indium. When all germanium has been redeposited, the remaining material appears as indium button which is frozen on to the outer surface of the crystallized portion as shown in Fig. 5.18 (iii). This button serves as a suitable base for soldering on leads.
Properties of PN Junction
At the instant of pn-junction formation, the free electrons near the junction in the n region begin to
diffuse across the junction into the p region where they combine with holes near the junction. The
result is that n region loses free electrons as they diffuse into the junction. This creates a layer of
positive charges (pentavalent ions) near the junction. As the electrons move across the junction, the p region loses holes as the electrons and holes combine. The result is that there is a layer of negative charges (trivalent ions) near the junction. These two layers of positive and negative charges form the depletion region (or depletion layer). The term depletion is due to the fact that near the junction, the region is depleted (i.e. emptied) of charge carriers (free electrons and holes) due to diffusion across the junction. It may be noted that the depletion layer is formed very quickly and is very thin compared to the n region and the p region. For clarity, the width of the depletion layer is shown exaggerated.
Once pn junction is formed and depletion layer created, the diffusion of free electrons stops. In
In other words, the depletion region acts as a barrier to the further movement of free electrons across the junction. The positive and negative charges set up an electric field. This is shown by a black arrow in Fig. 5.19 (i). The electric field is a barrier to the free electrons in the n-region. There exists a potential difference across the depletion layer and is called barrier potential (V0). The barrier potential of an on junction depends upon several factors including the type of semiconductor material, the amount of doping and temperature. The typical barrier potential is approximate:
For silicon, V0= 0.7 V ; For germanium, V0= 0.3 V Fig. 5.20 shows the potential (V0) distribution curve.
Applying D.C. Voltage Across pn Junction or Biasing a pn Junction
In electronics, the term bias refers to the use of d.c. voltage to establish certain operating conditions for an electronic device. In relation to a pn junction, there are following two bias conditions :
- Forward biasing 2. Reverse biasing
When external d.c. the voltage applied to the junction is in such a direction that it cancels the potential barrier, thus permitting current flow, it is called forward biasing. To apply forward bias, connect the positive terminal of the battery to p-type and negative terminal to n-type as shown in Fig. 5.21. The applied forward potential establishes an electric field that acts against the field due to the potential barrier. Therefore, the resultant field is weakened and the barrier height is reduced at the junction as shown in Fig. 5.21. As potential barrier voltage is very small (0.1 to 0.3 V), therefore, a small forward voltage is sufficient to completely eliminate the barrier. Once the potential barrier is eliminated by the forward voltage, junction resistance becomes almost zero and a low resistance path is established for the entire circuit. Therefore, current flows in the circuit. This is called forward current. With a forward bias to pn junction, the following points are worth noting :
(i) The potential barrier is reduced and at some forward voltage (0.1 to 0.3 V), it is eliminated
(ii) The junction offers low resistance (called forward resistance, Rf) to current flow.
(iii) Current flows in the circuit due to the establishment of a low resistance path. The magnitude of current depends upon the applied forward voltage.
When the external d.c. the voltage applied to the junction is in such a direction that potential barrier is increased, it is called reverse biasing. To apply reverse bias, connect the negative terminal of the battery to p-type and positive terminal to n-type as shown in Fig. 5.22. It is clear that applied reverse voltage establishes an electric field which acts in the same direction as the field due to potential barrier. Therefore, the resultant field at the junction is strengthened and the barrier height is increased as shown in Fig. 5.22. The increased potential barrier prevents the flow of charge carriers across the junction. Thus, a high resistance path is established for the entire circuit and hence the current does not flow. With a reverse bias to PN junction, the following points are worth noting :
(i) The potential barrier is increased.
(ii) The junction offers very high resistance (called reverse resistance, Rr) to current flow.
(iii) No current flows in the circuit due to the establishment of a high resistance path.
Conclusion. From the above discussion, it follows that with a reverse bias to the junction, a high resistance path is established and hence no current flow occurs. On the other hand, with a forward bias to the junction, a low resistance path is set up and hence current flows in the circuit.
Current Flow in a Forward Biased pn Junction
We shall now see how current flows across the pn junction when it is forward biased. Fig. 5.23 shows a forward-biased pn junction. Under the influence of forwarding voltage, the free electrons in n-type move *towards the junction, leaving behind positively charged atoms. However, more electrons arrive from the negative battery terminal and enter the n-region to take up their places. As the free electrons reach the junction, they become **valence electrons. As valence electrons, they move through the holes in the p-region. The valence electrons move towards the left in the p-region which is equivalent to the holes moving to the right. When the valence electrons reach the left end of the crystal, they flow into the positive terminal of the battery
The mechanism of current flow in a forward biased pn junction can be summed up as under :
(i) The free electrons from the negative terminal continue to pour into the n-region while the
free electrons in the n-region move towards the junction.
(ii) The electrons travel through the n-region as free-electrons i.e. current in n-region is by free
(iii) When these electrons reach the junction, they combine with holes and become valence electrons.
(iv) The electrons travel through p-region as valence electrons i.e. current in the p-region is by holes.
(v) When these valence electrons reach the left end of crystal, they flow into the positive terminal of the battery. From the above discussion, it is concluded that in n-type region, current is carried by free electrons whereas in p-type region, it is carried by holes. However, in the external connecting wires, the current is carried by free electrons
Volt-Ampere Characteristics of pn Junction
Volt-ampere or V-I characteristic of a pn junction (also called a crystal or semiconductor diode) is the curve between the voltage across the junction and the circuit current. Usually, voltage is taken along the x-axis and current along the y-axis. Fig. 5.24 shows the *circuit arrangement for determining the V-I characteristics of a pn junction. The characteristics can be studied under three heads, namely; zero external voltage, forward bias, and reverse bias.
(i) Zero external voltage. When the external voltage is zero, i.e. circuit is open at K, the potential barrier at the junction does not permit current flow. Therefore, the circuit current is zero as
indicated by point O in Fig. 5.25.
(ii) Forward bias. With a forward bias to the pn junction i.e. p-type connected to the positive terminal and n-type connected to the negative terminal, the potential barrier is reduced. At some forward voltage (0.7 V for Si and 0.3 V for Ge), the potential barrier is altogether eliminated and current starts flowing in the circuit. From now onwards, the current increases with the increase in forward voltage. Thus, a rising curve OB is obtained with forward bias as shown in Fig. 5.25. From the forward characteristic, it is seen that at first (region OA), the current increases very slowly and the curve is non-linear. It is because the externally applied voltage is used up in overcoming the potential barrier. However, once the external voltage exceeds the potential barrier voltage, the pn junction behaves like an ordinary conductor. Therefore, the current rises very sharply with an increase in external voltage (region AB on the curve). The curve is almost linear.
(iii) Reverse bias. With reverse bias to the pn junction i.e.p-type connected to negative terminal and n-type connected to the positive terminal, the potential barrier at the junction is increased. Therefore, the junction resistance becomes very high and practically no current flows through the circuit. However, in practice, a very small current (of the order of µA) flows in the circuit with reverse bias as shown in the reverse characteristic. This is called reverse *saturation current (Is) and is due to the minority carriers. It may be recalled that there are a few free electrons in p-type material and a few holes in n-type material. These undesirable free electrons in p-type and holes in n-type are called minority carriers. As shown in Fig. 5.26, to these minority carriers, the applied reverse bias appears as forward bias. Therefore, a **small current flows in the reverse direction. If reverse voltage is increased continuously, the kinetic energy of electrons (minority carriers) may become high enough to knock out electrons from the semiconductor atoms. At this stage breakdown of the junction occurs, characterized by a sudden rise of reverse current and a sudden fall of the resistance of barrier region. This may destroy the junction permanently.
Note. The forward current through a pn junction is due to the majority carriers produced by the impurity. However, reverse current is due to the minority carriers produced due to the breaking of some covalent bonds at room temperature.
Two important terms often used with pn junction (i.e. crystal diode) are breakdown voltage and knee voltage. We shall now explain these two terms in detail.
(i) Breakdown voltage.
It is the minimum reverse voltage at which pn junction breaks down with a sudden rise in reverse current. Under normal reverse voltage, a very little reverse current flows through a pn junction. However, if the reverse voltage attains a high value, the junction may break down with sudden rise in reverse current. For understanding this point, refer to Fig. 5.27. Even at room temperature, some hole-electron pairs (minority carriers) are produced in the depletion layer as shown in Fig. 5.27 (I). With reverse bias, the electrons move towards the positive terminal of supply. At large reverse voltage, these electrons acquire high enough velocities to dislodge valence electrons from semiconductor atoms as shown in Fig. 5.27 (ii). The newly liberated electrons in turn free other valence electrons. In this way, we get an avalanche of free electrons. Therefore, the pn junction conducts a very large reverse current. Once the breakdown voltage is reached, the high reverse current may damage the junction. Therefore, care should be taken that reverse voltage across a pn junction is always less than the breakdown voltage.
(ii) Knee voltage
It is the forward voltage at which the current through the junction starts to increase rapidly. When a diode is forward biased, it conducts current very slowly until we overcome the potential barrier. For the silicon pn junction, the potential barrier is 0.7 V whereas it is 0.3 V for germanium junction. It is clear from Fig. 5.28 that knee voltage for silicon When a p-type semiconductor is suitably joined to n-type semiconductors, the contact surface is called PN junction. Once the applied forward voltage exceeds the knee voltage, the current starts increasing rapidly. It may be added here that in order to get useful current through a pn junction, the applied voltage must be more than the knee voltage.
Note. The potential barrier voltage is also known as the turn-on voltage. This is obtained by taking the straight-line portion of the forward characteristic and extending it back to the horizontal axis.
Limitations in the Operating Conditions of PN Junction
Every pn junction has limiting values of maximum forward current, peak inverse voltage, and maximum power rating. The pn junction will give satisfactory performance if it is operated within these limiting values. However, if these values are exceeded, the pn junction may be destroyed due to excessive heat.
(i) Maximum forward current. It is the highest instantaneous forward current that a on junction can conduct without damage to the junction. The manufacturer’s datasheet usually specifies this rating. If the forward current in a pn junction is more than this rating, the junction will be destroyed due to overheating.
(ii) Peak inverse voltage (PIV). It is the maximum reverse voltage that can be applied to the pn junction without damage to the junction. If the reverse voltage across the junction exceeds its PIV, the junction may be destroyed due to excessive heat. The peak inverse voltage is of particular importance in rectifier service. A pn junction i.e. a crystal diode is used as a rectifier to change alternating current into direct current. In such applications, care should be taken that reverse voltage across the diode during negative half-cycle of a.c. does not exceed the PIV of diode.
(iii) Maximum power rating. It is the maximum power that can be dissipated at the junction without damaging it. The power dissipated at the junction is equal to the product of junction current and the voltage across the junction. This is a very important consideration and is invariably specified by the manufacturer in the datasheet.
What is Semiconductor Diode
It has already been discussed in the previous article that a pn junction conducts current easily when forward biased and practically no current flows when it is reverse biased. This unilateral conduction characteristic of pn junction (i.e. semiconductor diode) is similar to that of a vacuum diode. Therefore, like a vacuum diode, a semiconductor diode can also accomplish the job of rectification i.e. change alternating current to direct current. However, semiconductor diodes have become more *popular as they are smaller in size, cheaper and robust and usually operate with greater efficiency. In this chapter, we shall focus our attention on the circuit performance and applications of semiconductor diodes.
A pn junction is known as a semiconductor diode or *crystal diode. The outstanding property of a crystal diode to conduct current in one direction only permits it to be used as a rectifier. A crystal diode is usually represented by the schematic symbol shown in Fig. 6.1. The arrow in the symbol indicates the direction of easier conventional current flow.
A semiconductor diode has two terminals. When it is connected in a circuit, one thing to decide is whether the diode is forward or reverse biased. There is an easy rule to ascertain it. If the external circuit is trying to push the conventional current in the direction of arrow, the diode is forward biased. On the other hand, if the conventional current is trying to flow opposite to arrowhead, the diode is reverse biased.
Putting in simple words :
(i) If arrowhead of diode symbol is positive w.r.t. bar of the symbol, the diode is forward biased.
(ii) If the arrowhead of diode symbol is negative w.r.t. bar, the diode is reverse biased.
Identification of crystal diode terminals. While using a crystal diode, it is often necessary to know which end is arrowhead and which end is bar. For this purpose, the following methods are available :
(i) Some manufacturers actually paint the symbol on the body of the diode e.g. BY127, BY114
crystal diodes manufactured by BEL [See Fig. 6.2 (I)].
(ii) Sometimes, red and blue marks are used on the body of the crystal diode. Red mark denotes
arrow whereas blue mark indicates bar e.g. OA80 crystal diode [See Fig. 6.2 (ii)].
Crystal Diode as a Rectifier
Fig. 6.3 illustrates the rectifying action of a crystal diode. The a.c. the input voltage to be rectified, the diode and load RL are connected in series. The d.c. the output is obtained across the load as explained in the following discussion. During the positive half-cycle of a.c. the input voltage, the arrowhead becomes positive w.r.t. bar. Therefore, the diode is forward biased and conducts current in the circuit. The result is that a positive half-cycle of input voltage appears across RL as shown. However, during the negative half-cycle of input a.c. voltage, the diode becomes reverse biased because now the arrowhead is negative w.r.t. bar. Therefore, the diode does not conduct and no voltage appears across load RL. The result is that output consists of positive half-cycles of input a.c. voltage while the negative half-cycles are suppressed. In this way, crystal diode has been able to do rectification i.e. change a.c. into d.c. It may be seen that output across RL is pulsating d.c. It is interesting to see that the behavior of the diode is like a switch. When the diode is forward biased, it behaves like a closed switch and connects the a.c. supply to the load RL. However, when the diode is reverse biased, it behaves like an open switch and disconnects the a.c. supply from the load RL. This switching action of diode permits only the positive half-cycles of input a.c. voltage to appear across RL
Example 6.1. In each diode circuit of Fig. 6.4, find whether the diodes are forward or reverse biased.
(i) Refer to Fig. 6.4 (i). The conventional current coming out of battery flows in the branch circuits. In diode D1, the conventional current flows in the direction of arrowhead and hence this diode is forward biased. However, in diode D2, the conventional current flows opposite to arrowhead and hence this diode is reverse biased.
(ii) Refer to Fig. 6.4 (ii). During the positive half-cycle of input a.c. voltage, the conventional current flows in the direction of arrowhead and hence diode is forward biased. However, during the negative half-cycle of input a.c. voltage, the diode is reverse biased.
(iii) Refer to Fig. 6.4 (iii). During the positive half-cycle of input a.c. voltage, conventional current flows in the direction of the arrowhead in D1 but it flows opposite to arrowhead in D2. Therefore, during positive half-cycle, diode D1 is forward biased and diode D2 reverse biased. However, during the negative half-cycle of input a.c. voltage, diode D2 is forward biased and D1 is reverse biased.
(iv) Refer to Fig. 6.4 (iv). During the positive half-cycle of input a.c. voltage, both the diodes are
reverse biased. However, during the negative half-cycle of input a.c. voltage, both the diodes are
Resistance of Crystal Diode
It has already been discussed that a forward biased diode conducts easily whereas a reverse biased
diode practically conducts no current. It means that forward resistance of a diode is quite small as
compared with its reverse resistance.
The resistance offered by the diode to forward bias is known as forward resistance. This resistance is not the same for the flow of direct current as for the changing current. Accordingly; this resistance is of two types, namely; d.c. forward resistance and a.c. forward resistance.
(i) d.c. forward resistance. It is the opposition offered by the diode to the direct current. It is measured by the ratio of d.c. the voltage across the diode to the resulting d.c. current through it. Thus, referring to the forward characteristic in Fig. 6.5, it is clear that when a forward voltage is OA, the forward current is OB.
(ii) a.c. forward resistance. It is the opposition offered by the diode to the changing forward current. It is measured by the ratio of change in voltage across diode to the resulting change in current through it i.e.
The a.c. forward resistance is more significant as the diodes are generally used with alternating voltages. The a.c. forward resistance can be determined from the forward characteristic as shown in
Fig. 6.6. If P is the operating point at any instant, then the forward voltage is ob and forward current is one. To find the a.c. forward resistance, vary the forward voltage on both sides of the operating point equally as shown in Fig. 6.6 where ab = bc. It is clear from this figure that :
For forward voltage oa, circuit current is od.
For forward voltage oc, circuit current is of
It may be mentioned here that forward resistance of a crystal diode is very small, ranging from 1
to 25 Ω.
The resistance offered by the diode to the reverse bias is known as reverse resistance. It can be d.c. reverse resistance or a.c. reverse resistance depending upon whether the reverse bias is a direct or changing voltage. Ideally, the reverse resistance of a diode is infinite. However, in practice, the reverse resistance is not infinite because, for any value of reverse bias, there does exist a small leakage current. It may be emphasized here that reverse resistance is very large compared to the forward resistance. In germanium diodes, the ratio of reverse to forward resistance is 40000: 1 while for silicon this ratio is 1000000: 1
Equivalent Circuit of Crystal Diode
It is generally profitable to replace a device or system by its equivalent circuit. An equivalent circuit
of a device (e.g. crystal diode, transistor etc.) is a combination of electric elements, which when connected in a circuit, acts exactly as does the device when connected in the same circuit. Once the
device is replaced by its equivalent circuit, the resulting network can be solved by traditional circuit
analysis techniques. We shall now find the equivalent circuit of a crystal diode.
Approximate Equivalent circuit
When the forward voltage VF is applied across a diode, it will not conduct till the potential barrier V0 at the junction is overcome. When the forward voltage exceeds the potential barrier voltage, the diode starts conducting as shown in Fig. 6.7 (i). The forward current If flowing through the diode causes a voltage drop in its internal resistance rf. Therefore, the forward voltage VF applied across the actual diode has to overcome :
(a) potential barrier V0
(b) internal drop If rf
For a silicon diode, V0= 0.7 V whereas for a germanium diode, V0= 0.3 V. Therefore, an approximate equivalent circuit for a crystal diode is a switch in series with a battery V0 and internal resistance rf as shown in Fig. 6.7 (ii). This approximate equivalent circuit of a diode is very helpful in studying the performance of the diode in a circuit.
Simplified Equivalent circuit.
For most applications, the internal resistance rf of the crystal diode can be ignored in comparison to other elements in the equivalent circuit. The equivalent circuit then reduces to the one shown in Fig. 6.8 (ii). This simplified equivalent circuit of the crystal diode is frequently used in diode-circuit analysis.
Ideal diode model.
An ideal diode is one that behaves as a perfect conductor when forward biased and as a perfect insulator when reversing biased. Obviously, in such a hypothetical situation, forward resistance rf= 0 and potential barrier V0 is considered negligible. It may be mentioned here that although ideal diode is never found in practice, yet diode circuit analysis is made on this basis. Therefore, while discussing diode circuits, the diode will be assumed ideal unless and until stated otherwise.
Crystal Diode Equivalent Circuits
It is desirable to sum up the various models of crystal diode equivalent circuit in the tabular form
Example 6.2. An a.c. voltage of peak value 20 V is connected in series with a silicon diode and load resistance of 500 Ω. If the forward resistance of diode is 10 Ω,
find :(i) peak current through diode (ii) peak output voltage
What will be these values if the diode is assumed to be ideal?
Peak input voltage = 20 V
Forward resistance, rf= 10 Ω
Load resistance, RL= 500 Ω
Potential barrier voltage, V0= 0.7 V
The diode will conduct during the positive half-cycles of a.c. input voltage only. The equivalent
circuit is shown in Fig. 6.9 (ii).
Comments. It is clear from the above example that output voltage is nearly the same whether the
actual diode is used or the diode is considered ideal. This is due to the fact that the input voltage is quite large as compared with V0 and voltage drop in rf. Therefore, nearly the whole input forward voltage appears across the load. For this reason, diode circuit analysis is generally made on the ideal diode basis.
Example 6.3. Find the current through the diode in the circuit shown in Fig. 6.10 (i). Assume
the diode to be ideal.
Example 6.4. Calculate the current through 48 Ω resistor in the circuit shown in Fig. 6.11 (i).
Assume the diodes to be of silicon and forward resistance of each diode is 1 Ω.
Solution. Diodes D1 and D3 are forward biased while diodes D2 and D4 are reverse biased. We can, therefore, consider the branches containing diodes D2 and D4 as “open”. Replacing diodes D1 and D3 by their equivalent circuits and making the branches containing diodes D2 and D4 open, we get
the circuit shown in Fig. 6.11 (ii). Note that for a silicon diode, the barrier voltage is 0.7 V.
Example 6.5. Determine the current I in the circuit shown in Fig. 6.12 (i). Assume the diodes to
be of silicon and forward resistance of diodes to be zero.
Solution. The conditions of the problem suggest that diode D1 is forward biased and diode D2 is
reverse biased. We can, therefore, consider the branch containing diode D2 as open as shown in
Fig. 6.12 (ii). Further, diode D1 can be replaced by its simplified equivalent circuit.
Example 6.6. Find the voltage VA in the circuit shown in Fig. 6.13 (i). Use simplified model
Solution. It appears that when the applied voltage is switched on, both the diodes will turn “on”. But that is not so. When voltage is applied, germanium diode (V0= 0.3 V) will turn on first and a level of 0.3V is maintained across the parallel circuit. The silicon diode never gets the opportunity to have 0.7 V across it and, therefore, remains in an open-circuit state as shown in Fig. 6.13 (ii).
VA = 20 − 0.3 = 19.7 V
Example 6.7. Find VQ and ID in the network shown in Fig. 6.14 (i). Use a simplified model.
Solution. Replace the diodes by their simplified models. The resulting circuit will be as shown in
Fig. 6.14 (ii). By symmetry, current in each branch is ID so that current in-branch CD is 2ID. Applying Kirchhoff’s voltage law to the closed-circuit ABCDA, we have,
Example 6.8. Determine current through each diode in the circuit shown in Fig. 6.15 (i). Use a simplified model. Assume diodes to be similar.
Solution. The applied voltage forward biases each diode so that they conduct current in the same
direction. Fig. 6.15 (ii) shows the equivalent circuit using a simplified model. Referring to Fig. 6.15 (ii),
Comments. Note the use of placing the diodes in parallel. If the current rating of each diode is 20
20 mA and a single diode is used in this circuit, a current of 28.6 mA would flow through the diode,
thus damaging the device. By placing them in parallel, the current is limited to a safe value of 14.3 mA for the same terminal voltage.
Example 6.9. Determine the currents I1, I2, and I3 for the network shown in Fig. 6.16(i). Use a simplified model for the diodes.
Solution. An inspection of the circuit shown in Fig. 6.16 (i) shows that both diodes D1 and D2 are forward biased. Using simplified model for the diodes, the circuit shown in Fig. 6.16 (i) becomes
the one shown in Fig. 6.16 (ii). The voltage across R2 (= 3.3 k Ω) is 0.7V.
Example 6.10. Determine if the diode (ideal) in Fig. 6.17 (i) is forward biased or reverse biased.
Solution. Let us assume that diode in Fig. 6.17 (i) is OFF i.e. it is reverse biased. The circuit
then becomes as shown in Fig. 6.17 (ii). Referring to Fig. 6.17 (ii), we have,
Example 6.11. Determine the state of the diode for the circuit shown in Fig. 6.18 (i) and find ID and VD. Assume simplified model for the diode.
Solution. Let us assume that the diode is ON. Therefore, we can replace the diode with a 0.7V
battery as shown in Fig. 6.18 (ii). Referring to Fig. 6.18 (ii), we have,
Since the diode current is negative, the diode must be OFF and the true value of diode current is ID = 0 mA. Our initial assumption was wrong. In order to analyze the circuit properly, we should replace the diode in Fig. 6.18 (i) with an open circuit as shown in Fig. 6.19. The voltage VD across the diode is
We know that 0.7V is required to turn ON the diode. Since VD
is only 0.4V, the answer confirms
that the diode is OFF
While discussing the diode circuits, the reader will generally come across the following terms :
It is the current flowing through a forward-biased diode. Every diode has a maximum value of forwarding current which it can safely carry. If this value is exceeded, the diode
may be destroyed due to excessive heat. For this reason, the manufacturers’ datasheet specifies the maximum forward current that a diode can handle safely.
Peak inverse voltage
It is the maximum reverse voltage that a diode can withstand without destroying the junction. If the reverse voltage across a diode exceeds this value, the reverse current increases sharply and breaks down the junction due to excessive heat. Peak inverse voltage is extremely important when diode is used as a rectifier. In rectifier service, it has to be ensured that reverse voltage across the diode does not exceed its PIV during the negative half-cycle of input a.c. voltage. As a matter of fact, PIV consideration is generally the deciding factor in diode rectifier circuits. The peak inverse voltage may be between 10V and 10 kV depending upon the type of diode.
Reverse current or leakage current
It is the current that flows through a reverse-biased diode. This current is due to minority carriers. Under normal operating voltages, the reverse current is quite small. Its value is extremely small (< 1μ A) for silicon diodes but it is appreciable (j 100 μA) for germanium diodes. It may be noted that the reverse current is usually very small as compared with the forward current. For example, the forward current for a typical diode might range up to 100 mA while the reverse current might be only a few μA—a ratio of many thousands between forward and reverse currents.
Whats is Voltage Stabilisation
A rectifier with an appropriate filter serves as a good source of d.c. output. However, the major
disadvantage of such a power supply is that the output voltage changes with the variations in the input voltage or load. Thus, if the input voltage increases, the d.c. output voltage of the rectifier also increases. Similarly, if the load current increases, the output voltage falls due to the voltage drop in the rectifying element, filter chokes, transformer winding etc. In many electronic applications, it is desired that the output voltage should remain constant regardless of the variations in the input voltage or load. In order to ensure this, a voltage stabilising device, called voltage stabiliser is used. Several stabilising circuits have been designed but only zener diode as a voltage stabiliser will be discussed.
Whats is Zener Diode
It has already been discussed that when the reverse bias on a crystal diode is increased, a critical
voltage, called breakdown voltage is reached where the reverse current increases sharply to a high
value. The breakdown region is the knee of the reverse characteristic as shown in Fig. 6.52. The
satisfactory explanation of this breakdown of the junction was first given by the American scientist C. Zener. Therefore, the breakdown voltage is sometimes called zener voltage and the sudden increase in current is known as zener current.
The breakdown or zener voltage depends upon the amount of doping. If the diode is heavily
doped, depletion layer will be thin and consequently the breakdown of the junction will occur at a lower reverse voltage. On the other hand, a lightly doped diode has a higher breakdown voltage.
When an ordinary crystal diode is properly doped so that it has a sharp breakdown voltage, it is called a zener diode. A properly doped crystal diode which has a sharp breakdown voltage is known as a Zener diode. Fig. 6.53 shows the symbol of a zener diode. It may be seen that it is just like an ordinary diode except that the bar is turned into z-shape. The following points may be noted about the zener diode:
(i) A zener diode is like an ordinary diode except that it is properly doped so as to have a sharp breakdown voltage.
(ii) A zener diode is always reverse connected i.e. it is always reverse biased.
(iii) A zener diode has sharp breakdown voltage, called zener voltage VZ.
(iv) When forward biased, its characteristics are just those of ordinary diode.
(v) The zener diode is not immediately burnt just because it has entered the *breakdown region. As long as the external circuit connected to the diode limits the diode current to less than burn out value, the diode will not burn out.
Equivalent Circuit of Zener Diode
The analysis of circuits using zener diodes can be made quite easily by replacing the zener diode by its equivalent circuit.
(i) “On” state. When reverse voltage across a zener diode is equal to or more than break down voltage VZ, the current increases very sharply. In this region, the curve is almost vertical. It means that voltage across zener diode is constant at VZ even though the current through it changes. Therefore, in the breakdown region, an **ideal zener diode can be represented by a battery of voltage VZ as shown in Fig. 6.54 (ii). Under such conditions, the zener diode is said to be in the “ON” state.
(ii) “OFF” state. When the reverse voltage across the zener diode is less than VZ but greater
than 0 V, the zener diode is in the “OFF” state. Under such conditions, the zener diode can be
represented by an open-circuit as shown in Fig. 6.55 (ii).
Zener Diode as Voltage Stabiliser
A zener diode can be used as a voltage regulator to provide a constant voltage from a source whose voltage may vary over sufficient range. The circuit arrangement is shown in Fig. 6.56 (i). The Zener diode of zener voltage VZ is reverse connected across the load RL across which constant output is desired. The series resistance R absorbs the output voltage fluctuations so as to maintain constant voltage across the load. It may be noted that the zener will maintain a constant voltage VZ(= E0) across the load so long as the input voltage does not fall below VZ
When the circuit is properly designed, the load voltage E0 remains essentially constant (equal to VZ) even though the input voltage Ei and load resistance RL may vary over a wide range.
(i) Suppose the input voltage increases. Since the zener is in the breakdown region, the Zener diode is equivalent to a battery VZ as shown in Fig. 6.56 (ii). It is clear that output voltage remains constant at VZ (= E0). The excess voltage is dropped across the series resistance R. This will cause an increase in the value of total current I. The zener will conduct the increase of current in I while the load current remains constant. Hence, output voltage E0 remains constant irrespective of the changes in the input voltage Ei
(ii) Now suppose that input voltage is constant but the load resistance RL decreases. This will cause an increase in load current. The extra current cannot come from the source because drop in R
(and hence source current I) will not change as the zener is within its regulating range. The additional load current will come from a decrease in zener current IZ. Consequently, the output voltage stays at constant value.
Voltage drop across R = Ei− E0
Current through R, I = Iz+IL
Solving Zener Diode Circuits
The analysis of zener diode circuits is quite similar to that applied to the analysis of semiconductor
diodes. The first step is to determine the state of zener diode i.e., whether the zener is in the “on”
state or “off” state. Next, the zener is replaced by its appropriate model. Finally, the unknown
quantities are determined from the resulting circuit.
- Ei and RL fixed. This is the simplest case and is shown in Fig. 6.57 (i). Here the applied voltage Ei as well as load RL is fixed. The first step is to find the state of zener diode. This can be determined by removing the zener from the circuit and calculating the voltage V across the resulting open-circuit as shown in Fig. 6.57 (ii)
If V ≥ VZ , the zener diode is in the “on” state and its equivalent model can be substituted as shown in Fig. 6.58 (i). If V < VZ , the diode is in the “off” state as shown in Fig. 6.58 (ii).
(i) On state. Referring to circuit shown in Fig. 6.58 (i),
(ii) Off state. Referring to the circuit shown in Fig. 6.58 (ii),
- Fixed Ei and Variable RL. This case is shown in Fig. 6.59. Here the applied voltage (Ei) is fixed while load resistance RL (and hence load current IL) changes. Note that there is a definite range of RL values (and hence IL values) which will ensure the zener diode to be in “on” state. Let us calculate that range of values.
(i) RLmin and ILmax. Once the zener is in the “on” state, load voltage E0 (= VZ) is constant. As a result, when load resistance is minimum (i.e., RLmin), load current will be maximum (IL=E0/RL). In order to find the minimum load resistance that will turn the zener on, we simply calculate the value of RL that will result in E0= VZi.e.,
This is the minimum value of load resistance that will ensure that zener is in the “on” state. Any value of load resistance less than this value will result in a voltage E0 across the load less than VZ and the zener will be in the “off” state.
(ii) ILmin and RLmax. It is easy to see that when load resistance is maximum, load current is minimum.
Now, Zener current, IZ= I − IL
When the zener is in the “on” state, I remains **fixed. This means that when IL is maximum, IZ will be minimum. On the other hand, when IL is minimum, IZ is maximum. If the maximum current that a zener can carry safely is IZM, then,
If the load resistance exceeds this limiting value, the current through zener will exceed IZM and the device may burn out.
Fixed RL and Variable Ei. This case is shown in Fig. 6.60. Here the load resistance RL is fixed while the applied voltage (Ei) changes. Note that there is a definite range of Ei values that will ensure that zener diode is in the “on” state. Let us calculate that range of values.
(i) Ei (min). To determine the minimum applied voltage that will turn the Zener on, simply calculate the value of Ei that will result in load voltage E0 = VZ i.e.,
(ii) Ei (max)
Now, current through R, I = IZ+IL
Since IL(= E0/RL= VZ/RL) is fixed, the value of I will be maximum when zener current is maximum i.e.,
Imax = IZM + IL
Now Ei= I R + E0
Since E0(= VZ) is constant, the input voltage will be maximum when I is maximum.
∴ Ei (max)= Imax R + VZ
Example 6.25. For the circuit shown in Fig. 6.61 (i), find :
(i) the output voltage (ii) the voltage drop across series resistance
(iii) the current through zener diode.
Solution. If you remove the zener diode in Fig. 6.61 (i), the voltage V across the open-circuit is
given by :
Example 6.26. For the circuit shown in Fig. 6.62 (i), find the maximum and minimum values of
zener diode current.
Solution. The first step is to determine the state of the zener diode. It is easy to see that for the
given range of voltages (80 − 120 V), the voltage across the zener is greater than VZ(= 50 V). Hence the zener diode will be in the “on” state for this range of applied voltages. Consequently, it can be replaced by a battery of 50 V as shown in Fig. 6.62 (ii).
Maximum zener current. The zener will conduct *maximum current when the input voltage is
maximum i.e. 120 V. Under such conditions :
Minimum Zener current. The zener will conduct minimum current when the input voltage is
minimum i.e. 80 V. Under such conditions, we have,
Example 6.27. A 7.2 V zener is used in the circuit shown in Fig. 6.63 and the load current is to
vary from 12 to 100 mA. Find the value of series resistance R to maintain a voltage of 7.2 V across
the load. The input voltage is constant at 12V and the minimum zener current is 10 mA.
The voltage across R is to remain constant at 12 − 7.2 = 4.8 V as the load current changes from
12 to 100 mA. The minimum zener current will occur when the load current is maximum.
If R = 43.5 Ω is inserted in the circuit, the output voltage will remain constant over the regulating range. As the load current IL decreases, the zener current IZ will increase to such a value that IZ+IL=110 mA. Note that if load resistance is open-circuited, then IL= 0 and zener current becomes 110 mA.
Example 6.28. The zener diode shown in Fig. 6.64 has VZ= 18 V. The voltage across the load stays at 18 V as long as IZ is maintained between 200 mA and 2 A. Find the value of series resistance R so that E0 remains 18 V while input voltage Ei is free to vary between 22 V to 28V.
Solution. The zener current will be minimum (i.e. 200 mA) when the input voltage is minimum
(i.e. 22 V). The load current stays at constant value IL = VZ/ RL = 18 V/18 Ω = 1 A = 1000 mA.
Example 6.29. A 10-V zener diode is used to regulate the voltage across a variable load resistor
[See fig. 6.65]. The input voltage varies between 13 V and 16 V and the load current varies between 10mA and 85 mA. The minimum zener current is 15 mA. Calculate the value of series resistance R.
Solution. The zener will conduct minimum current (i.e. 15 mA) when input voltage is minimum
(i.e. 13 V).
Example 6.30. The circuit of Fig. 6.66 uses two zener diodes, each rated at 15 V, 200 mA. If the
circuit is connected to a 45-volt unregulated supply, determine :
(i) The regulated output voltage (ii) The value of series resistance R
Solution. When the desired regulated output voltage is higher than the rated voltage of the
zener, two or more zeners are connected in series as shown in Fig. 6.66. However, in such circuits,
care must be taken to select those zeners that have the same current rating.
Example 6.31. What value of series resistance is required when three 10-watt, 10-volt, 1000 mA zener diodes are connected in series to obtain a 30-volt regulated output from a 45 volt d.c.
power source ?
Solution. Fig. 6.67 shows the desired circuit. The worst case is at no load because then zeners
carry the maximum current.
Example 6.32. Over what range of input voltage will the zener circuit shown in Fig. 6.68
maintain 30 V across 2000 Ω load, assuming that series resistance R = 200 Ω and zener current
rating is 25 mA ?
Solution. The minimum input voltage required will be when IZ= 0. Under this condition,
Therefore, the input voltage range over which the circuit will maintain 30 V across the load is
33 V to 38 V.
Example 6.33. In the circuit shown in Fig. 6.69, the voltage across the load is to be maintained at 12 V as load current varies from 0 to 200 mA. Design the regulator. Also find the maximum wattage rating of zener diode.
Solution. By designing the regulator here means to find the values of VZ and R. Since the load voltage is to be maintained at 12 V, we will use a zener diode of zener voltage 12 V i.e.,
VZ = 12 V
The voltage across R is to remain constant at 16 − 12 = 4 V as the load current changes from 0 to
200 mA. The minimum zener current will occur when the load current is maximum.
Maximum power rating of zener is PZM = VZ IZM = (12 V) (200 mA) = 2.4 W
Example. 6.34. Fig. 6.70 shows the basic zener diode circuits. What will be the circuit behaviour
if the zener is (i) working properly (ii) shorted (iii) open-circuited?
Solution. Zener diodes cannot be tested individually with a multimeter. It is because multimeters
usually do not have enough input voltage to put the zener into breakdown region.
(i) If the zener diode is working properly, the voltage V0 across the load (= 5 kΩ) will be nearly
6V [See Fig. 6.70 (i)].
(ii) If the zener diode is short [See Fig. 6.70 (ii)], you will measure V0 as 0V. The same problem
could also be caused by a shorted load resistor (= 5kΩ) or an opened source resistor (= 1 kΩ). The
only way to tell which device has failed is to remove the resistors and check them with an ohmmeter. If the resistors are good, then zener diode is bad.
(iii) If the zener diode is open-circuited, the voltage V0 across the load (= 5 kΩ) will be 10V.
Example 6.35. Fig. 6.71 shows regulated power supply using a zener diode. What will be the
circuit behaviour if (i) filter capacitor shorts (ii) filter capacitor opens?
Solution. The common faults in a zener voltage regulator are shorted filter capacitor or opened
(i) When filter capacitor shorts. When the filter capacitor shorts, the primary fuse will blow. The reason for this is illustrated in Fig. 6.71. When the filter capacitor shorts, it shorts out the load
resistance RL. This has the same effect as wiring the two sides of the bridge together (See Fig. 6.71).
If you trace from the high side of the bridge to the low side, you will see that the only resistance across the secondary of the transformer is the forward resistance of the two ON diodes. This effectively shorts out the transformer secondary. The result is that excessive current flows in the secondary and hence in the primary. Consequently, the primary fuse will blow.
(ii) When filter capacitor opens. When the filter capacitor opens, it will cause the ripple in the
power supply output to increase drastically. At the same time, the d.c. output voltage will show a
significant drop. Since an open filter capacitor is the only fault that will cause both of these symptoms, no further testing is necessary. If both symptoms appear, replace the filter capacitor.
Crystal Diodes versus Vacuum Diodes
Semiconductor diodes (or crystal diodes) have a number of advantages and disadvantages as compared to their electron-tube counterparts (i.e., vacuum diodes).
(i) They are smaller, more rugged and have a longer life.
(ii) They are simpler and inherently cheaper.
(iii) They require no filament power. As a result, they produce less heat than the equivalent
(i) They are extremely heat sensitive. Even a slight rise in temperature increases the current
appreciably. Should the temperature *exceed the rated value of the diode, the increased flow of
current may produce enough heat to ruin the pn junction. On the other hand, vacuum diodes function normally over a wide range of temperature changes. It may be noted that silicon is better than germanium as a semiconductor material. Whereas germanium diode should not be operated at temperatures higher than 80ºC, silicon diodes may operate safely at temperatures upto about 200ºC.
(ii) They can handle small currents and low inverse voltages as compared to vacuum diodes.
(iii) They cannot stand an overload even for a short period. Any slight overload, even a transient pulse, may permanently damage the crystal diode. On the other hand, vacuum diodes can stand an overload for a short period and when the overload is removed, the tube will generally recover
What is LED ?
A light-emitting diode (LED) is a diode that gives off visible light when forward biased. Light-emitting diodes are not made from silicon or germanium but are made by using elements like gallium, phosphorus, and arsenic. By varying the quantities of these elements, it is possible to produce light of different wavelengths with colors that include red, green, yellow, and blue. For example, when a LED is manufactured using gallium arsenide, it will produce a red light. If the LED is made with gallium phosphide, it will produce a green light.
When light-emitting diode (LED) is forward biased as shown in Fig. 7.2 (i), the electrons
from the n-type material cross the pn junction and recombine with holes in the p-type material. Recall that these free electrons are in the conduction band and at a higher energy level than the holes in the valence band. When recombination takes place, the recombining electrons release energy in the form of heat and light. In germanium and silicon diodes, almost the entire energy is given up in the form of heat and emitted light is insignificant. However, in materials like gallium arsenide, the number of photons of light energy is sufficient to produce quite intense visible light.
Fig. 7.2 (ii) shows the schematic symbol for a LED. The arrows are shown as pointing away from the diode, indicating that light is being emitted by the device when forward biased. Although LEDs are available in several colours (red, green, yellow and orange are the most common), the schematic symbol is the same for all LEDs.
There is nothing in the symbol to indicate the colour of a particular LED. Fig. 7.3 shows the graph between radiated light and the forward current of the Light-Emitting Diode. It is clear from the graph that the intensity of radiated light is directly proportional to the forward current of Light-Emitting Diode.
What is LED Voltage and Current?
The forward voltage ratings of most LEDs is from 1V to 3V and forward current ratings range from 20 mA to 100 mA. In order that current through the LED does not exceed the safe value, a resistor RS is connected in series with it as shown in Fig. 7.4. The input voltage is VS and the voltage across LED is VD.
Example 7.1. What value of series resistor is required to limit the current through a LED to
20 mA with a forward voltage drop of 1.6 V when connected to a 10V supply ?
Note that resistor RS is also called current-limiting resistor.
Example 7.2. What is current through the LED in the circuit shown in Fig. 7.5 ? Assume that voltage drop across the LED is 2 V.
Advantages of LED
The light-emitting diode is a solid-state light source. LEDs have replaced incandescent lamps in many applications because they have the following advantages :
(i) Low voltage
(ii) Longer life (more than 20 years)
(iii) Fast on-off switching Protecting Light-Emitting Diode against reverse bias. The LEDs have low reverse voltage ratings. For example, a typical Light-Emitting Diode may have a maximum reverse voltage rating of 3V. This means that if a reverse voltage greater than 3 V is applied to the LED, the LED may be destroyed. Therefore, one must be careful not to use LEDs with a high level of reverse bias. One way to protect a LED is to connect a rectifier diode in parallel with LED as shown in Fig. 7.6. If reverse voltage greater than the reverse voltage rating of LED is accidentally applied, the rectifier diode will be turned on. This protects the LED from damage.
A LED that emits one colour when forward biased and another colour when reverse biased is called a multicolour LED. One commonly used schematic symbol for these LEDs is shown in Fig. 7.7. Multicolour LEDs
actually contain two pn junctions that are connected in reverse-parallel i.e. they are in parallel with
anode of one being connected to the cathode of the other. If positive potential is applied to the top terminal as shown in Fig. 7.7 (i), the pn junction on the left will light. Note that the device current passes through the left pn junction. If the polarity of the voltage source is reversed as shown in Fig. 7.7 (ii), the pn junction on the right will light. Note that the direction of device current has reversed and is now passing through the right pn junction.
Multicolour LEDs are typically red when biased in one direction and green when biased in the
other. If a multicolour LED is switched fast enough between two polarities, the LED will produce a
third colour. A red/green LED will produce a yellow light when rapidly switched back and forth
between biasing polarities.
Applications of LEDs
The LED is a low-power device. The power rating of a LED is of the order of milliwatts. This means that it is useful as an indicator but not good for illumination. Probably the two most common applications for visible LEDs are (i) as a power indicator (ii) seven segment display.
(i) As a power indicator. A LED can be used to indicate whether the power is on or not. Fig. 7.8 shows the simple use of the LED as a power indicator. When the switch S is closed, power is applied to the load. At the same time current also flows through the LED which lights, indicating power is on. The resistor RS in series with the LED ensures that current rating of the LED is not exceeded.
(ii) Seven-segment display. LEDs are often grouped to form seven-segment display.
Fig. 7.9 (i) shows the front of a seven segment display. It contains seven LEDs (A, B, C, D, E, F and
G) shaped in a figure of *8. Each LED is called a **segment. If a particular LED is forward biased,
that LED or segment will light and produces a bar of light. By forward biasing various combinations
of seven LEDs, it is possible to display any number from 0 to 9. For example, if LEDs A, B, C, D and G
are lit (by forward biasing them), the display will show the number 3. Similarly, if LEDs C, D, E, F, A
and G are lit, the display will show the number 6. To get the number 0, all segments except G are lit.
Fig. 7.9 (ii) shows the schematic diagram of seven-segment display. External series resistors are
included to limit currents to safe levels. Note that the anodes of all seven LEDs are connected to a
common positive voltage source of +5 V. This arrangement is known as *common-anode type. In
order to light a particular LED, say A, we ground the point A in Fig. 7.9 (ii). It forward biases the LED
A which will be lit.
What is Photo diode ?
A photo diode is a reverse-biased silicon or germanium pn junction in which reverse current increases when the junction is exposed to light. The reverse current in a photo-diode is directly proportional to the intensity of light falling on its pn junction. This means that greater the intensity of light falling on the pn junction of photo-diode, the greater will be the reverse current.
When a rectifier diode is reverse biased, it has a very small reverse leakage current. The same is true for a photodiode. The reverse current is produced by thermally generated electron hole pairs which are swept across the junction by the electric field created by the reverse voltage. In a rectifier diode, the reverse current increases with temperature due to an increase in the number of electron-hole pairs. A photodiode differs from a rectifier diode in that when its pn junction is exposed to light, the reverse current increases with the increase in light intensity and vice-versa. This is explained as follows.
When light (photons) falls on the **pn junction, the energy is imparted by the photons to the atoms in the junction. This will create more free electrons (and more holes). These additional free electrons will increase the reverse current. As the intensity of light incident on the on junction increases, the reverse current also increases. In other words, as the incident light intensity increases, the resistance of the device (photodiode) ***decreases. Photo-diode package. Fig. 7.10 (i) shows a typical photo-diode package. It consists of a on junction mounted on an insulated substrate and sealed inside a metal case. A glass window is mounted on top of the case to allow light to enter and strike the pn junction. The two leads extending from the case are labelled anode and cathode. The cathode is typically identified by a tab extending from the side of the case.
Fig. 7.10 (ii) shows the schematic symbol of a photodiode. The inward arrows represent the
Photo diode Operation
Fig. 7.11 shows the basic circuit. The circuit has reverse biased photodiode, resistor R and d.c. supply. The operation of the photodiode is as under :
(i) When no light is incident on the pn junction of photo-diode, the reverse current Ir is extremely small. This is called dark current. The resistance of photo-diode with no incident light is called dark resistance (RR). Dark resistance of photo-diode, RR = Dark current VR
(ii) When light is incident on the pn junction of the photodiode, there is a transfer of energy from the incident light (photons) to the atoms in the junction. This will create more free electrons (and more holes). These additional free electrons will increase the reverse current.
(iii) As the intensity of light increases, the reverse current IR goes on increasing till it becomes maximum. This is called saturation current.
Characteristics of Photo-diode
There are two important characteristics of photodiode.
(i) Reverse current-Illumination curve.
Fig. 7.12 shows the graph between reverse current (IR) and illumination (E) of a photodiode. The reverse current is shown on the vertical axis and is measured in µA. The illumination is indicated on the horizontal axis and is measured in mW/cm2. Note that graph is a straight line passing through the origin.
∴ IR = m E
where m = slope of the straight line The quantity m is called the sensitivity of the photodiode.
(ii) Reverse voltage-Reverse current curve. Fig. 7.13 shows the graph between reverse current (IR) and reverse voltage (VR) for various illumination levels. It is clear that for a given reverse-biased voltage VR, the reverse current IR increases as the illumination (E) on the pn junction of photodiode is increased.
Applications of Photo-diodes
There are a large number of applications of photodiodes. However, we shall give two applications of photodiodes by way of illustration.
(i) Alarm circuit using photodiode. Fig. 7.14
shows the use of photodiode in an alarm system. Light
from a light source is allowed to fall on a photodiode fitted in the doorway. The reverse current IR will continue to flow so long as the light beam is not broken. If a person passes through the door, light beam is broken and the reverse current drops to the dark current level. As a result, an alarm is sounded.
(ii) A counter circuit using photo-diode. A photodiode may be used to count items on a conveyor belt. Fig. 7.15 shows a photo-diode circuit used in a system that counts objects as they pass by on a conveyor. In this circuit, a source of light sends a concentrated beam of light across a conveyor to a photo-diode. As the object passes, the light beam is broken, IR drops to the dark current level and the count is increased by one.
Example 7.3. From the reverse current-Illumination curve for a photo-diode shown in Fig. 7.16, determine the dark resistance. Assume a reverse-biased voltage of 10 V.
Example 7.4. A photo-diode is exposed to light with an illumination of 2.5 mW/cm2. If the sensitivity of the photo-diode for the given conditions is 37.4 µA/mW/cm2, find the reverse current through the device.
Reverse current = Sensitivity × Illumination or IR = m × E = 37.4 × 2.5 = 93.5 µA
What is Optoisolator ?
An optoisolator (also called optocoupler) is a device that uses light to couple a signal from its input (a photoemitter e.g., a LED) to its output (a photodetector e.g., a photo-diode). Fig. 7.17 shows a LED-photo diode optoisolator. The LED is on the left and the photo-diode is on the right. The arrangement shown in Fig. 7.17 is referred to as optocoupling because the output from the LED circuit is coupled via light to the photo-diode circuit. When the LED is energised, current flows through the LED. The light from the LED hits the photo diode and sets up a reverse current through resistor R2. The voltage across the photo-diode is given by :
Vout = VSS – I R2
The output voltage depends on how large the reverse current is. If we vary the LED supply, the amount of light changes and this causes the photo diode current to change. As a result, Vout changes. The key advantage of an optoisolator is the electrical isolation between the input and output circuits; the only contact between the input and output circuits is the stream of light.
What is Tunnel Diode
A tunnel diode is a pn junction that exhibits negative resistance between two values of forward
voltage (i.e., between peak-point voltage and valley-point voltage). A conventional diode exhibits *positive resistance when it is forward biased or reverse biased. However, if a semiconductor junction diode is heavily doped with impurities, it exhibits negative resistance (i.e. current decreases as the voltage is increased) in certain regions in the forward direction. Such a diode is called tunnel diode.
Theory of Tunnel Diode.
The tunnel diode is basically a pn junction with heavy doping of p-type and n-type semiconductor materials. In fact, a tunnel diode is doped approximately 1000 times as heavily as a conventional diode. This heavy doping results in a large number of majority carriers. Because of the large number of carriers, most are not used during the initial recombination that produces the depletion layer. As a result, the depletion layer is very narrow. In comparison with conventional diode, the depletion layer of a tunnel diode is 100 times narrower. The operation of a tunnel diode depends upon the tunneling effect and hence the name.
The heavy doping provides a large number of majority carriers. Because of the large number of carriers, there is much drift activity in p and n sections. This causes many valence electrons to have their energy levels raised closer to the conduction region. Therefore, it takes only a very small applied forward voltage to cause conduction. The movement of valence electrons from the valence energy band to the conduction band with little or no applied forward voltage is called tunneling. Valence electrons seem to tunnel through the forbidden energy band. As the forward voltage is first increased, the diode current rises rapidly due to tunneling effect. Soon the tunneling effect is reduced and current flow starts to decrease as the forward voltage across the diode is increased. The tunnel diode is said to have entered the negative resistance region. As the voltage is further increased, the tunneling effect plays less and less part until a valley-point is reached. From now onwards, the tunnel diode behaves as ordinary diode i.e., diode current increases with the increase in forward voltage.
V-I Characteristic of Tunnel Diode.
Fig. 7.18 (i) shows the V-I characteristic of a typical tunnel diode.
(i) As the forward voltage across the tunnel diode is increased from zero, electrons from the region “tunnel” through the potential barrier to the p-region. As the forward voltage increases, the diode current also increases until the peak-point P is reached. The diode current has now reached peak current IP (= 2.2 mA) at about peak-point voltage VP (= 0.07 V). Until now the diode has exhibited positive resistance.
(ii) As the voltage is increased beyond VP, the tunneling action starts decreasing and the diode
current decreases as the forward voltage is increased until valley-point V is reached at valley-point
voltage VV (= 0.7V). In the region between peak-point and valley-point (i.e., between points P and V), the diode exhibits negative resistance i.e., as the forward bias is increased, the current decreases. This suggests that tunnel diode, when operated in the negative resistance region, can be used as an oscillator or a switch.
(iii) When forward bias is increased beyond valley-point voltage VV (= 0.7 V), the tunnel diode behaves as a normal diode. In other words, from point V onwards, the diode current increases with
the increase in forward voltage i.e., the diode exhibits positive resistance once again. Fig. 7.18. (ii)
shows the symbol of tunnel diode. It may be noted that a tunnel diode has a high reverse current but operation under this condition is not generally used.
Tunnel Diode Oscillator
A tunnel diode is always operated in the negative resistance region. When operated in this region, it works very well in an oscillator. Fig. 7.19 (i) shows a parallel resonant circuit. Note that RP is the parallel equivalent of the series winding resistance of the coil. When the tank circuit is set into oscillations by applying voltage as shown in Fig. 7.19. (ii), damped oscillations are produced. It is
because energy is lost in the resistance RP of the tank circuit.
If a tunnel diode is placed in series with the tank circuit and biased at the centre of the negative resistance portion of its characteristic as shown in Fig. 7.20, undamped oscillations are produced at
the output. It is because the negative-resistance characteristic of the tunnel diode counteracts the
positive-resistance characteristic of the tank circuit. The circuit shown in Fig. 7.20 is called tunnel diode oscillator or negative resistance oscillator. The negative resistance oscillator has one major drawback. While the circuit works very well at extreme high frequencies (upper mega hertz range), it cannot be used efficiently at low frequencies. Low-frequency oscillators generally use transistors.
What is Varactor Diode ?
A junction diode which acts as a variable capacitor under changing reverse bias is known as a
varactor diode. When a pn junction is formed, depletion layer is created in the junction area. Since there are no charge carriers within the depletion zone, the zone acts as an insulator. The p-type material with holes (considered positive) as majority carriers and n-type material with electrons (−ve charge) as majority carriers act as charged plates. Thus the diode may be considered as a capacitor with n-region and pregion forming oppositely charged plates and with depletion zone between them acting as a dielectric. This is illustrated in Fig. 7.21 (i). A varactor diode is specially constructed to have high capacitance under reverse bias. Fig. 7.21 (ii) shows the symbol of varactor diode. The values of capacitance of varactor diodes are in the picofarad (10−12 F) range
where CT = Total capacitance of th junction
ε = Permittivity of the semiconductor material
A = Cross-sectional area of the junction
Wd= Width of the depletion layer
When reverse voltage across a varactor diode is increased, the width Wd of the depletion layer increases. Therefore, the total junction capacitance CT of the junction decreases. On the other hand, if the reverse voltage across the diode is lowered, the width Wd of the depletion layer decreases. Consequently, the total junction capacitance CT increases.
Fig. 7.22 shows the curve between reverse bias voltage VR across varactor diode and total junction capacitance CT. Note that CT can be changed simply by changing the voltage VR. For this reason, a varactor diode is sometimes called voltage-controlled capacitor.
Application of Varactor Diode
We have discussed that we can increase or decrease the junction capacitance of varactor diode simply by changing the reverse bias on the diode. This makes a varactor diode ideal for use in circuits that require voltage-controlled tuning. Fig.7.23 shows the use of varactor diode in a tuned circuit. Note that the capacitance of the varactor is in parallel with the inductor. The varactor and the inductor form a parallel LC circuit. For normal operation, a varactor diode is always operated under reverse bias. In fact, this condition is met in the circuit shown in Fig. 7.23. The resistance RW in the circuit is the winding resistance of the inductor. This winding resistance is in series with the potentiometer R1. Thus R1 and RW
form a voltage divider that is used to determine the amount of reverse bias across the varactor diode D1 and therefore its capacitance. By adjusting the setting of R1, we can vary the diode capacitance. This, in turn, varies the resonant frequency of the LC circuit. The resonant frequency fr of the LC circuit is given by;
If the amount of varactor reverse bias is decreased, the value of C of the varactor increases. The
increase in C will cause the resonant frequency of the circuit to decrease. Thus, a decrease in reverse bias causes a decrease in resonant frequency and vice-versa.
Example 7.5. The LC tank circuit shown in Fig. 7.23 has a 1 mH inductor. The varactor has capacitance of 100 pF when reverse bias is 5V d.c. Determine the resonant frequency of the circuit
for this reverse bias.
What is Shockley Diode ?
Named after its inventor, a Shockley diode is a PNPN device having two terminals as shown in Fig.
7.24 (i). This *device acts as a switch and consists of four alternate P-type and N-type layers in a single crystal. The various layers are labelled as P1, N1, P2 and N2 for identification. Since a P-region adjacent to an N-region may be considered a junction diode, the Shockley diode is equivalent to three junction diodes connected in series as shown in Fig. 7.24 (ii). The symbol of Shockley diode is shown in Fig. 7.24 (iii).
Working of Scockley Diode
(i) When Shockley diode is forward biased (i.e., anode is positive w.r.t. cathode), diodes D1 and D3
would be forward-biased while diode D2 would be reverse-biased. Since diode D2 offers very high resistance (being reverse biased) and the three diodes are in series, the Shockley diode presents a very high resistance. As the *forward voltage increases, the reverse bias across D2 is also increased.
At some forward voltage (called breakover voltage VBO), reverse breakdown of D2 occurs. Since this breakdown results in reduced resistance, the Shockley diode presents a very low resistance. From now onwards, the Shockley diode behaves as a conventional forward-biased diode; the forward current being determined by the applied voltage and external load resistance. This behaviour of Shockley diode is indicated on its V-I characteristic in Fig. 7.25.
(ii) When Shockley diode is reverse biased (i.e., anode is negative w.r.t. cathode), diodes D1 and D3 would be reverse-biased while diode D2 would be forward-biased. If reverse voltage is increased
sufficiently, the reverse voltage breakdown (point A in Fig. 7.25) of Shockley diode is reached. At
this point, diodes D1 and D3 would go into reverse-voltage breakdown, the reverse current flowing through them would rise rapidly and the heat produced by this current flow could ruin the entire device. For this reason, Shockley diode should never be operated with a reverse voltage sufficient to reach the reverse-voltage breakdown point.
Conclusion of shockley diode
. The above discussion reveals that Shockley diode behaves like a switch. So long as the forward voltage is less than breakover voltage, Shockley diode offers very high resistance (i.e., switch is open) and practically conducts no current. At voltages above the break-over value, Shockley diode presents a very low resistance (i.e. switch is closed) and Shockley diode conducts heavily. It may be noted that Shockley diode is also known as PNPN diode or four layer diode or reverse blocking diode thyristor. Note. Once Shockley diode is turned ON (i.e., it starts conducting), the only way to turn it OFF is to reduce the applied voltage to such a value so that current flowing through Shockley diode drops below its holding current (IH) value. Diode D2 then comes out of its reverse-breakdown state and its high-resistance value is restored. This, in turn, causes the entire Shockley diode to revert to its high resistance (switch open) state.
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