Classification of Materials into Metals, Semiconductors and Insulators on the Basis of Band Theory

CLASSIFICATION OF MATERIALS INTO METALS, SEMICONDUCTORS AND INSULATORS ON THE BASIS OF BAND THEORY

Based on band theory, and on the basis of the presence of forbidden band gap the materials are classified into three catogories, viz.

(i) Metals (or) Conductors,

(ii) Semi-conductors and

(iii) Insulators.

(i) Metals (or) Conductors

In conductors, there is no forbidden band gap. Here the valence and conduction band overlap each other as shown in Fig. 1.37. In metals the availability of free electrons will be very high due to the overlapping of conduction band and valence band. Hence, even when a small external field is applied to it, the electrons from valence band freely enters into the conduction band and produces current. Examples: Copper, Aluminium, Iron etc.,a

(ii) Semi Conductors

In semiconductors, the forbidden band gap is very small, in the order 0.5 eV to 1.5 eV and hence there will be a very small gap between the valence and conduction band as shown in Fig. 1.38

Examples: Germanium, Silicon etc.,

Generally, in semiconductors the availability of free electrons in conduction band will be less compared to metals, due to the presence of forbidden band gap between the valence band and conduction band. Therefore, when external field of energy, equal to (or) greater than forbidden band gap energy (E) is applied to a semiconductor, immediately the conduction will take place.

(iii) Insulators

In Insulators, the forbidden gap is very wide, in the range of 3 eV to 5.47 eV and hence there will be a very large gap between the valence band and conduction band as shown in Fig. 1.39. Since the forbidden band gap energy is very very high in the case of insulators, very large amount of energy (external field) is required for conduction to occur.

Examples: Diamond, Dielectrics etc.,