A consequence of small band gap ( ) in semiconductors is that it is possible to generate additional carriers by illuminating a sample of semiconductor by a light of frequency greater than . This leads to an increased conductivity in the sample and the phenomenon is known as intrinsics photoconductivity . The effect is not very pronounced at high temperatures except when the illumination is by an intense beam of light. At low temperatures, illumination results in excitation of localized carriers to conduction or valence band.

Even when an incident photon does not have sufficient energy to produce an electron- hole paie, it can still produce an excitation at the impurity centres by creating a free electron - bound hole pair (for excitation at donor level) or a free hole - bound electron (for acceptor level). If is the impurity ionization energy, the radiation frequency for extrinsic photoconductivity should be at least .