INTRODUCTION TO SEMICONDUCTOR
Semiconductor:- 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.
We know that current is the movement of free electrons from one atom to the other when there is a potential difference. In conductors no forbidden gap is present between the conduction band and valence band. In many cases both the bands overlap each other. The valence electrons are loosely bound to the nucleus in conductors. Usually metals or conductors have low ionization energy and so they tend to lose electrons very easily. When an electric current is applied the delocalized electrons are free to move within the structure. This is the case that happens in normal temperature.
When the temperature increases the vibrations of the metal ions in the lattice structure increases. The atoms starts to vibrate with higher amplitude. These vibrations in turn causes frequent collisions between the free electrons and the other electrons. Each collision drain out some energy of the free electrons and causing them unable to move. Thus it restricts the movement of the delocalized electrons. When the collision happens the drift velocity of the electrons decreases. This means that the resistivity of the metal increases and thus current flow in the metal is decreased. The resistivity increases means that the conductivity of the material decreases.
For metals or conductors, it is said that they have a positive temperature co – efficient. The value α is positive. For most of the metals, the resistivity increases linearly with increase in temperature for a range of 500K. Examples for positive temperature co – efficient include, silver, copper, gold etc.
Solid-state materials are commonly grouped into three classes: insulators, semiconductors, and conductors. (At low temperatures some conductors, semiconductors, and insulators may become superconductors.) The figure shows the conductivities σ (and the corresponding resistivities ρ = 1/σ) that are associated with some important materials in each of the three classes. Insulators, such as fused quartz and glass, have very low conductivities, on the order of 10^18 to 10^10 siemens per centimetre; and conductors, such as aluminum, have high conductivities, typically from 104 to 106 siemens per centimetre. The conductivities of semiconductors are between these extremes and are generally sensitive to temperature, illumination, magnetic fields, and minute amounts of impurity atoms. For example, the addition of about 10 atoms of boron (known as a dopant) per million atoms of silicon can increase its electrical conductivity a thousandfold (partially accounting for the wide variability shown in the preceding figure).
The study of semiconductor materials began in the early 19th century. The elemental semiconductors are those composed of single species of atoms, such as silicon (Si), germanium (Ge), and tin (Sn) in column IV and selenium (Se) and tellurium (Te) in column VI of the periodic table. There are, however, numerous compound semiconductors, which are composed of two or more elements. Gallium arsenide (GaAs), for example, is a binary III-V compound, which is a combination of gallium (Ga) from column III and arsenic (As) from column V. Ternary compounds can be formed by elements from three different columns—for instance, mercury indium telluride (HgIn2Te4), a II-III-VI compound. They also can be formed by elements from two columns, such as aluminum gallium arsenide (AlxGa1 − xAs), which is a ternary III-V compound, where both Al and Ga are from column III and the subscript x is related to the composition of the two elements from 100 percent Al (x = 1) to 100 percent Ga (x = 0). Pure silicon is the most important material for integrated circuit applications, and III-V binary and ternary compounds are most significant for light emission.
Prior to the invention of the bipolar transistor in 1947, semiconductors were used only as two-terminal devices, such as rectifiers and photodiodes. During the early 1950s germanium was the major semiconductor material. However, it proved unsuitable for many applications, because devices made of the material exhibited high leakage currents at only moderately elevated temperatures. Since the early 1960s silicon has become by far the most widely used semiconductor, virtually supplanting germanium as a material for device fabrication. The main reasons for this are twofold: (1) silicon devices exhibit much lower leakage currents, and (2) silicon dioxide (SiO2), which is a high-quality insulator, is easy to incorporate as part of a silicon-based device. Thus, silicon technology has become very advanced and pervasive, with silicon devices constituting more than 95 percent of all semiconductor products sold worldwide.
Many of the compound semiconductors have some specific electrical and optical properties that are superior to their counterparts in silicon. These semiconductors, especially gallium arsenide, are used mainly for optoelectronic and certain radio frequency (RF) applications.
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Reference: Principles Of Electronics By V K Mehta And Rohit Mehta