The?forbidden gap?between the valence band and conduction band is very large in insulators. The energy?gap?of insulator is approximately equal to 15 electron volts (eV). The electrons in valence band cannot move because they are locked up between the atoms.

The main difference between conductors, semiconductors and insulators is because of Width of forbidden energy gap

In an insulator, the forbidden energy gap between the valance band and conduction band is of the order of 5ev

An?intrinsic semiconductor?is an undoped semiconductor. This means that holes in the valence band are vacancies created by electrons that have been thermally excited to the conduction band, as opposed to doped?semiconductors?where holes or electrons are supplied by a ?foreign? atom acting as an impurity.