Energy bands in solids consists of three parts

1. Valence                          →           Completely filled band
2. Conduction band           →           Upper unfilled band
3. Forbidden gap                →           Separation between valence band and conduction band.

Metals, Insulators and Semiconductors:

Metal:- For metals either conduction and valence band overlap or the conduction band is partially filled. In either case large number of electrons are present for conduction and these electrons can easily move under the action of even a small applied electric field.

Insulators:- Insulators the valence band is almost completely filled while conduction band is empty. The energy gap for diamonds is 6eV.

Semiconductors:- Semiconductors are materials that have a small energy gap f the order of 1 eV. At 0K (absolute zero), the semiconductors behave like insulators.

Type of semiconductors:-

Intrinsic Semiconductors (pure)

Extrinsic Semiconductors (impure)