Band Therory

Conductors

In a conductor there are no band gaps and electrons can move easily using a continuous, partially filled conduction band.

Insulators

An insulator has a large gap between the lower energy levels (the valence band) and the upper conduction band.

The valence band is full as no electrons can move up the conduction band which is empty as a result.

The material can't conduct as only the electrons in a conduction band can move easily.

Semiconductors

In a semiconductor, the gap between the valence band and conduction band is smaller and at room temperature there is sufficient energy available to move some electrons from the valence band into the conduction band allowing some conduction to take place.

An increase in temperature increases the conductivity of a semiconductor.

NOTE:-    The difference between insulators and semiconductors is due to a small amount of impurity added to a semiconductor which affects the energy bands. This process is called doping.