Module 6 : PHYSICS OF SEMICONDUCTOR DEVICES
Lecture 30 : Conductors, Insulators and Semiconductors
For an insulator there is a wide gap ( $ \sim 6 - 7$ eV) between the lower occupied band,known as the valence band , and the higher unoccupied band, called the conduction band . No electron can exist in this forbidden gap To promote electrons from lower levels to higher levels would require a great amount of energy. It is incorrect to say that electrons in an insulator are not free to move around. In fact, they do. However, as there are as many electrons as there are states, the electrons only trade places resulting in no net movement of charges.
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  Semiconductors, like insulator have band gaps. However, the gap between the top of the valence band and the bottom of the conduction band is much narrower than in an insulator. For comparison, the gap in case of Silicon is 1.1 eV while that for diamond, which is an insulator is about 6 eV.
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