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Insulator = Forbidden energy band for an insulator (e.g. diamond) is quite large bev. Since the electron current cannot acquire sufficient applied energy, conduction is impossible and hence diamond is an insulator. Semiconductor = Width of forbidden energy region is medium, practical semiconductor materials are Silicon & germanium. Eg = Es depends somewhat on temperature, it has been determined experimentally that Es decreases with temperature. Metal = A solid which contains overlapping valence & conduction bands.
