A semiconductor is a material with an electrical conductivity that is intermediate between that of an insulator and a conductor. A semiconductor behaves as an insulator at very low temperatures, and has an appreciable electrical conductivity at room temperature although much lower conductivity than a conductor. Commonly used semiconducting materials are silicon, germanium, gallium arsenide and indium phosphide.
The distinction between a semiconductor and an insulator is slightly more arbitrary. A semiconductor has a band gap which is small enough such that its conduction band is appreciably thermally populated with electrons at room temperature, whilst an insulator has a band gap which is too wide for there to be appreciable thermal electrons in its conduction band at room temperature.