Insulator: The valence band of those materials ** full of electrons. The conduction band of those materials ** empty. The forbidden energy gap between the conduction band and the valence band is widest. The difference is more than 4 eV. Crossing the forbidden energy gap from valence band to conduction band require large amount of energy. Mica, glass, eboniote etc are the examples of insulators.
Semiconductors: A semiconductor ** partially full valence band and partially full conduction band at the room temperature The energy gap is narrower. The conduction band ** full empty of a semiconductor where the valence band *** full of electrons at absolute zero temperature. The value of Eg =1.1eV for silicon crystal and Eg=0.7eV for germanium. It can easily overcome due to thermal agitation or light. 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.
