photoconductivity

The increase in electrical conductivity caused by the excitation of additional free charge carriers by light of sufficiently high energy in semiconductors and insulators. Effectively a radiation-controlled electrical resistance, a photoconductor can be used for a variety of light- and particle-detection applications, as well as a light-controlled switch. Other major applications in which photoconductivity plays a central role are television cameras (vidicons), normal silver halide emulsion photography, and the very large field of electrophotographic reproduction. See also Optical detectors; Optical modulators; Particle detector; Photography.

Although all insulators and semiconductors may be said to be photoconductive, that is, they show some increase in electrical conductivity when illuminated by light of sufficiently high energy to create free carriers, only a few materials show a large enough change, that is, show a large enough photosensitivity, to be practically useful in applications of photoconductors.

Since the electrical conductivity σ of a material is given by the product of the carrier density, its charge, and its mobility, an increase in the conductivity can be formally due to either an increase in carrier density or an increase in mobility. Although cases are found in which both types of effects are observable, photoconductivity in single-crystal materials is due primarily to an increase in earner density. In polycrystalline materials, on the other hand, where transport may be limited by potential barriers between the crystalline grains, an increase in mobility due to photoexcitation effects on these intergrain barriers may dominate the photoconductivity.

The variation of photoconductivity with photon energy is called the spectral response of the photoconductor. Spectral response curves typically show a fairly well-defined maximum at a photon energy close to that of the bandgap of the material, that is, the minimum energy required to excite an electron from a bond in the material into a higher-lying conduction band where it is free to contribute to the conductivity. This energy ranges from 3.7 eV, in the ultraviolet, for zinc sulfide (ZnS) to 0.2 eV, in the infrared, for cooled lead selenide (PbSe).

Another major characteristic of a photoconductor of practical concern is the rate at which the conductivity changes with changes in photoexcitation intensity. If a steady photoexcitation is turned off at some time, for example, the length of time required for the current to decrease to 1/e of its initial value is called the decay time of photoconductivity, t_d. The magnitude of the decay time is determined by the lifetime π and by the density of carriers trapped in imperfections as a result of the previous photoexcitation, which must now also be released in order to return to the thermal equilibrium situation. See also Photoconductive cell.

The noun has one meaning:

Meaning #1: change in the electrical conductivity of a substance as a result of absorbing electromagnetic radiation
  Synonym: photoconduction

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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. When light is absorbed by a material like a semiconductor, the number of free electrons and electron 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 band gap, or by exciting the impurities within the band gap. 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; and selenium, as employed in early television and xerography.^{[citation needed]}

See also, photoconductor and photodiode.

Application

solar cell
photoconductive antenna

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