Question

describe how the energy gap of a semiconductor is different from that of insulator and conductor .

Answer 1

Insulator: materials through which electricity cannot pass are called insulators. Plastic, glass, wood etc are the examples of insulators. The valence band of those material remains full of electrons. The conduction band of those material remains empty. The forbidden energy gap between the conduction band and the valence band is widest. The difference is more than 10ev. Crossing the forbidden energy gap from valence band to conduction band requires large amount of energy.

Conductors:  
valence band and the conduction band of conductors overlap each other. There is no forbidden energy gap here so Eg=0. At absolute zero temperature large number of electrons remain in the conduction band. The resistance of conductor is very low, a large number charge carriers are available here. So, the electricity can pass easily through the conductors.

Now, semiconductors

Semiconductors are those materials whose electrical conductivity is between conductors and insulators. The forbidden energy gap of a semiconductor is nearly same as insulator. The energy gap is narrower. The value of Eg =1.1eV for silicon crystal and Eg =0.7eV for germanium at ok. It can easily overcome due to thermal agitation or light. A semiconductor has partially full valence band and partially full conduction band at the room temperature. The conduction band remains fully empty and the valence band remains full of electrons at absolute zero temperature. So, silicon and germanium are insulators at absolute zero temperature. On the other hand with the increasing of temperature the electrical conductivity of semiconductors increase.