photoconductivity
American Heritage Dictionary:
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pho·to·con·duc·tiv·i·ty
(fō'tō-kŏn'dŭk-tĭv'ĭ-tē) 
n., pl., -ties.
Electrical conductivity affected by exposure to light.
photoconduction pho'to·con·duc'tion n.
photoconductive pho'to·con·duc'tive adj.
n., pl., -ties.
Electrical conductivity affected by exposure to light.
photoconduction pho'to·con·duc'tion n.
photoconductive pho'to·con·duc'tive adj.
McGraw-Hill Science & Technology Encyclopedia:
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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, td. 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.
Oxford Dictionary of Biochemistry:
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photoconductive
describing an electrical device that is capable of changing its conductance on exposure to light or other electromagnetic radiation.
—photoconductivity n.
| photochromism, photochemistry, photochemical reaction | |
| photodamage, photodetector, photodimerization |
Wikipedia on Answers.com:
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Photoconductivity
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This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2010) |
Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation.[1]
When light is absorbed by a material such as a semiconductor, the number of free electrons and electron holes changes and raises its electrical conductivity. To cause excitation, the light that strikes the semiconductor must have enough energy to raise electrons across the band gap, or to excite 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 of the material varies the current flowing through the circuit.
Classic examples of photoconductive materials include the conductive polymer polyvinylcarbazole,[2] used extensively in photocopying (xerography); lead sulfide, used in infrared detection applications, such as the U.S. Sidewinder and Russian Atoll heat-seeking missiles; and selenium, employed in early television and xerography.
Applications
Main article: Photoresistor
When a photoconductive material is connected as part of a circuit, it functions as a resistor whose resistance depends on the light intensity. In this context the material is called a photoresistor (also called light-dependent resistor or photoconductor). The most common application of photoresistors is as photodetectors, i.e. devices that measure light intensity. Photoresistors are not the only type of photodetector--other types include CCDs, photodiodes, phototransistors, and others--but they are among the most common photodetectors. Some photodector applications in which photoresistors are often used include camera light meters, street lights, clock radios, and infrared detectors.
For more details, see photoresistor.
See also
References
- ^ DeWerd, L. A.; P. R. Moran (1978). "Solid-state electrophotography with Al2O3". Medical Physics 5 (1): 23–26. Bibcode 1978MedPh...5...23D. doi:10.1118/1.594505. PMID 634229.
- ^ Law, Kock Yee (1993). "Organic photoconductive materials: recent trends and developments". Chemical Reviews, American Chemical Society 93: 449–486. doi:10.1021/cr00017a020. http://pubs.acs.org/doi/abs/10.1021/cr00017a020.
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American Heritage Dictionary
The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.
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McGraw-Hill Science & Technology Encyclopedia
McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.
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Oxford Dictionary of Biochemistry
Oxford University Press. Oxford Dictionary of Biochemistry and Molecular Biology © 1997, 2000, 2006 All rights reserved.
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