Semiconductors and Its Classification | Electrical Engineering

Semiconductors are materials which have electrical conductivities lying between those of good conductors and insulators. The resistivity of semiconductors varies from 10^-5 to 10⁴ ohm-m as compared to the values ranging from 10^-8 to 10^-6 ohm-m for conductors and from 10⁷ to 10⁸ ohm-m for insulators.

There are elemental semiconductors such as Ge and Si which belong to IV group of the periodic table and have resistivity of about 0.6 × 10³ to 1.5 × 10³ ohm-m respectively. Besides these, there are certain compound semiconductors such as Gallium arsenide, Indium phosphide, Cadmium sulphide etc. which are formed from the combinations of the elements of groups III or V or groups II and VI.

Another important characteristic of the semiconductors is that they have small band gap. The band gap of semiconductors varies from 0.2 to 2.5 eV which is quite small as compared to that of insulators. This property determines the wavelength of radiation which can be emitted or absorbed by the semiconductors and hence helps to construct devices such as light emitting diodes and lasers.

All the semiconductors have negative temperature coefficient of resistance. The importance of semiconductors is further increased due to the fact that the conductivity and the effective band gaps of these materials can be modified by the introduction of impurities which strongly affect their electronic and optical properties.

Depending upon the nature of impurities added, the semiconductors are classified as follows:

(i) Pure or intrinsic semiconductors.

(ii) Impure or extrinsic semiconductors.

i. Intrinsic Semiconductor:

A pure semiconductor is called intrinsic semiconductor. Here no free electrons are available since all the covalent bonds are complete. At absolute zero, there is a vacant conduction band separated by an energy gap Eg from a filled valence band. Thus at 0° K, the electric conduction is not possible. A pure semiconductor, therefore, behaves as an insulator. It exhibits a peculiar behaviour even at room temperature or with rise in temperature.

The resistance of a semiconductor decreases with increase in temperature. When an electric field is applied to an intrinsic semiconductor at a temperature greater than 0°K, conduction electrons move to the anode and, the holes (when an electron is liberated into the conduction band, a positively charged hole is created in valence band) move to cathode. Hence semiconductor current consists of movement of electrons and holes in opposite direction.

Since the electrons and holes are created in pairs, the conduction band electron concentration ‘n’ is equal to the concentration of holes ‘p’ in the valence band. Each of this intrinsic carrier concentration is commonly referred to as n_i. Thus for an intrinsic material;

N = p = n_i or np = n_i²

At a given temperature there is certain concentration of electron-hole pair n_i and at the same time there is recombination of electron hole pairs. Recombination occurs when an electron in the conduction band makes a transition to an empty state in the valence band.

ii. Extrinsic Semiconductor:

In a pure semiconductor, which behaves like an insulator under ordinary conditions, if small amount of certain metallic impurity is added it attains current conducting properties. The impure semiconductor is then called impurity semiconductor or extrinsic semiconductor. The process of adding impurity (extremely in small amounts, about 1 part in 10⁸) to a semiconductor to make it extrinsic (impurity) semiconductor is called Doping.

Generally following doping agents are used:

(i) Pentavalent atom having fine valence electrons (arsenic, antimony, phosphorus) called donor atoms.

(ii) Trivalent atoms having three valence electrons (gallium, aluminium, boron) called acceptor atoms.

We know that the impurity states of the impurity atom lie within the forbidden gap, specifically a few hundredths of an electron volt below the conduction band as shown in Fig. 7.4(a). If these are occupied and if the temperature is raised, these states may be ionized and may donate electrons to the conduction band. Now just note why the conduction becomes possible in impure semiconductors at low temperatures and results into a drastic conductivity value at the comparable temperature.

The associated impurity states are also located in the forbidden gap, in this case just above the valence band edge as shown in Fig. 7.4(b). If these are initially empty (a hole is certainly as empty state), and if the temperature is raised, these may become occupied by electrons from the valence band; these excited electrons leave behind holes in the valence band.

One may also say that at finite temperature, the holes acquire thermal energy and move downward into the valence band, just as electrons in the donor state moving up into the conduction band. This interpretation is essentially equivalent to the earlier one; that is, the electrons moving up into the empty hole states. Holes in the valence band are carriers of positive charge and thus contribute to the conductivity.

It is usual for semiconductors to contain both the donor type and the acceptor type impurities simultaneously. When this happens, the hydrogenic impurity levels for both types of impurity appear in the band gap. However, the bound donor electrons and the bound acceptor holes do not represent the ground state of the crystal.

In fact, the ground state of the donor electrons would be in the empty acceptor states, thus tending to fill the later. Since the acceptor levels are below the donor levels, a lower state of energy of the crystal will be reached if electrons from the donors fill the acceptor levels. It is called the neutralization or the compensation of the carriers with each other.

Now, if there are an equal number of acceptors and donors present in the crystal, then this process would lead to all the acceptor levels being occupied and all the donor levels being empty. Under this condition the Fermi level is halfway between the two impurity levels which, if m_e = m_h, is exactly as E_g/2 and the crystal then behaves identically as if it is an intrinsic semiconductor.

Such specimen are said to be compensated. This result is technologically important in the preparation of intrinsic semiconductors, i.e., we may prepare samples with comparable number of donors and acceptors rather than with just zero concentration of these.

The former preparation is much easier and at the same time gives the desired sampled. Generally, it is difficult to add equal number of donors and accepters and the sample is, therefore, essentially an extrinsic one, i.e. either n-type or p-type—depending upon whether donors exceed to acceptors or acceptors exceed to donors.

With the addition of suitable impurities to semiconductor, two types of semiconductors obtained are:

(i) n-type semiconductor, and

(ii) p-type semiconductor.

(i) n-Type Semiconductor:

The presence of even a minute quantity of impurity pentavalent, can produce n-type semiconductor. If the impurity atom has one valence electron more than the semiconductor atom which it has substituted, this extra electron will be loosely bound to the atom.

For example, an atom of germanium possesses four valence electrons; when it is replaced in the crystal lattice of the substance by an impurity atom of antimony (Sb) which has five valence electrons, the fifth valence electron (free electron) produced extrinsic n-type conductivity even at room temperature.

Such an impurity induced into a semiconductor is called donor impurity (or donor). The conducting properties of germanium will depend upon the amount of antimony (i.e., impurity) added. This means that controlled conductivity can be obtained by the proper addition of impurity. Fig. 7.5 shows extrinsic semiconductor (n-type) at T = 0°K showing the loosely bound excess electron contributed by the donor atom.

(ii) p-Type Semiconductor:

p-type extrinsic semiconductor can be produced if the impurity atom (trivalent) has one valence electron less than the semiconductor atom that it has replaced in the crystal lattice.

This impurity atom cannot fill all the interatomic bonds and the free bond can accept an electron from the neighbouring bond; leaving behind a vacancy or hole. Such an impurity is called an acceptor impurity (or acceptor). Fig. 7.6 shows structure of p-type semiconductor at 0°K.