Energy bands : In a solid , the energy of electrons lie within certain range. The energy levels of allowed energy are in the form of bands, these bands are separated by regions of forbidden energy called band gaps.

Distinguish features : 

(a) In conductors : valence band and conduction band overlap each other.

In semiconductors : Valence band and conduction band are separated by a small energy gap.

In insulators : They are separated by a large energy gap.

(b) In conductors : Large number of free electrons are available in conduction band.

In semiconductor : A very small number of electrons are available for electrical conduction. 

In insulators : Conduction band is almost empty i.e., no electron is available for conduction.

Effect of temperature :

(i) In conductors : At high temperature , the collisions of electrons become more frequent with the atoms / molecules at lattice site in the metals as a result the conductivity decreases (or resistivity increases).

(ii) In semiconductors : As the temperature of the semiconducting material increases, more electrons hole pairs becomes available in the conduction band and valence band , and hence the conductivity increases or the resistivity decreases. 

(ii) In insulators : The energy band between conduction band and valence band is very large , so it is unsurpassable for small temperature rise. So , there is no change in their behaviour.