Classify metals, semiconductors and insulators based on the band theory of solids.

Explanation

In the case of conductors, CB and VB overlap with each other. Free electrons are the charge carriers. There are a number of electrons available to conduct electricity at room temperature. In the the case of insulators, there is a wide gap between CB and VB. Forbidden gap energy is greater than 5 eV. There are no electrons in the CB to conduct electricity.
In the case of semiconductors. Forbidden gap energy is less than 5 eV. There are equal number of holes and electrons in a pure semiconductor. However these charge are too low so as to conduct electricity at room temperature.