Based on band theory explain the conductivity of solids.

Explanation

In solids, molecular orbitals of the atoms form a band which is called valence band and the next empty band in which electrons can move is called conduction band.
In conductors, the conduction bandis close to valence band and therefore the electrons can easily go into the conduction band, under an applied electric field causing conductivity.
In insulators, gap between filled valence band and hte next higher conduction band is large electrons cannot jump to it and such a substance has very small conductivity. They are called insulators.
In case of semiconductors, the gap between the valence band and conduction band is small. Therefore, some electrons may jump to conduction band and show some conductivity. Conductivity of semiconductors increases with temperature.