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.
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.
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.
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 :
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.
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 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.
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.
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.
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.
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