In terms of band theory, what is the difference between a conductor and a semi-conductor?
Semiconductor: In instrinsic semiconductors, the energy gap, Eg, is relatively small (≈ = 1 eV). At absolute zero, conduction band is empty and valence band is full. Therefore, the solid behaves as an insulator at very low temperatures. However, at room temperature, some electrons from the top of valence band acquires enough thermal energy to jump into the conduction band. They leave behind a number of holes. Both electrons and holes movement contribute to conduction.
In extrinsic semi-conductor of n-type, the donor energy level is close to the bottom of conduction band.
In p-type the acceptor energy level is close to the top of valence band.