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.

A small impurity is added to germanium to get p-type semiconductor. This impurity is a Trivalent substance

To obtain a p-type germanium semiconductor, it must be doped with As