Definition
Semiconductor
A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator. Its conductivity can be controlled and modified by external factors such as temperature, light, electric fields, and impurity doping.
The defining characteristic is a small band gap (typically –) between the valence band and the conduction band. At absolute zero, a pure semiconductor behaves like an insulator. At room temperature, thermal energy excites some electrons across the gap, creating mobile charge carriers.