Photoconductivity

Photoconductivity is an optical and electrical phenomenon in which a material becomes more conductive due to the absorption of electro-magnetic radiation such as visible light, ultraviolet light, or gamma radiation. To be photoconductive a semiconductor must be in thermal equilibrium, which contains free electrons and holes. When light is absorbed by the semiconductor, the configuration of electrons and holes changes and raises the electrical conductivity of the semiconductor. To cause excitation the light that strikes the semiconductor must have enough energy to raise electrons across the forbidden bandgap or by exciting the impurities within the bandgap. When a bias voltage and a load resistor are used in series with the semiconductor, a voltage drop across the load resistors can be measured when the change in electrical conductivity varies the current flowing through the circuit. A classic example of photoconduction is in the polymer polyvinylcarbazole which is used extensively in photocoping.See photoconductor.