Photoconductivity

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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, infrared 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. Two classic examples of photoconductive materials are the polymer polyvinylcarbazole, which is used extensively in photocopying (xerography); and lead sulfide, used in infrared detection applications, such as the U.S. Sidewinder and Russian Atoll heat-seeking missiles.