Classification Of Solids on the Basis Of Band Theory:

Based on the energy band structure, the arrangement of electrons and forbidden bands, solid materials are classified into the following three categories -

1. Conductors
2. Insulators, and
3. Semiconductors

Let us discuss these materials briefly and move on to semiconducting materials,

Conductors:

Materials which conduct electric current when a potential difference is applied across them are known as conductors. In case of a conductor, the valence band is completely filled, while the conduction band is half filled, as shown in Fig below.

Therefore, when a small potential difference is applied to a solid material, it provides sufficient energy to the electron in the valence band to shift to the conduction band Thus, the shifting of electrons from the valence band to the unfilled conduction band results in the flow of current in the material. Examples for some good conductors arc copper, lithium, etc.

Insulators:

Solid materials which do not conduct electric current under normal conditions are known as insulators. In insulators, the valence band is completely filled and it has no electron in the conduction band. Further, the forbidden energy gap will be very high when compared with a conductor. The energy band diagram of an insulator (for example, ebonite) is shown in Fig. below.

Figure: Energy bands in insulators

Therefore, the energy required to shift an electron from the valence band to the conduction band to make electrical conduction is very high. Hence, it is not possible to provide enough energy by an ordinary electric field. However, one can achieve electrical conduction in an insulator with very high voltage known as breakdown voltage.

Semiconductors:

Semiconductors (for example, silicon or germanium) are materials whose electrical conductivity lies between that of conductors and insulators. The conductivity of semiconductors is of the order of 104 to 10-4 mho m-1. The magnitude of the forbidden energy gap of a semiconductor lies in between the forbidden energy gap of insulators and conductors, as shown in Fig below.