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Related to photoconductive cell: photovoltaic cell, photoemissive cell
photoconductive cell(foh-toh-kŏn-duk -tiv) An electronic device that consists of a layer of semiconductor sandwiched between two electrical contacts. When light, infrared, or ultraviolet radiation falls on the sandwich its electrical conductivity increases markedly as a result of photoconductivity: the incident photons are absorbed in the material where they produce free electrical charge carriers that change the conductivity. A current therefore flows in the external circuit of the device. The current is proportional to the amount of radiation falling on the cell.
(also photoresistive cell, photoresistor), a semiconductor device whose electrical conductivity or resistance changes upon exposure to optical radiation (see). An electric current flows through a photo-conductive cell that is connected to a circuit containing a DC source. When the cell is illuminated, the current increases as a result of the generation of a photoelectric current that is proportional to the level of the actuating signal and is independent of the polarity of the applied voltage. The generation of a photoelectric current or the production of a change in the voltage across a photoconductive cell by such a current is used to detect radiation (seeRADIATION DETECTOR, OPTICAL DETECTOR, and OPTRON).
Photoconductive cells are fabricated from Se, Te, Ge (either pure or doped with Au, Cu, or Zn), Si, PbS, PbSe, PbTe, InSb, InAs, CdS, CdSe, or HgCdTe. A characteristic feature of such semiconductor materials is the small width of the forbidden band; for example, the width of the forbidden band is 0.18 electron volt for InSb. The semiconductor is deposited as a thin film on a glass or quartz substrate or is cut as a thin wafer from a single crystal. The film or wafer is provided with two contacts, or electrodes. The substrate, photosensitive film or wafer, and electrodes are placed in a protective housing.
The most important parameters of a photoconductive cell are the luminous sensitivity, threshold sensitivity, and time constant. The luminous sensitivity, which ranges from 103 to 108 volts/watt, is defined as the voltage change per unit intensity of the incident radiation at the rated supply voltage. The threshold sensitivity is the magnitude of the minimum signal detected by a cell per unit operating-frequency bandwidth and may be as low as 10 –12 watt per hertz½. The time constant, which characterizes the inertia of a cell, ranges from 10–3 to 10–8 sec. Some photoconductive cells are cooled in order to increase the threshold sensitivity and extend the range of radiation wavelengths that can be detected by the photosensitive layer. Thus, the cooling of a PbS photoconductive cell to 78°K increases the threshold sensitivity by an order of magnitude and extends the maximum wavelength of detectable radiation from 3.3 micrometers (µm) to 5 µm; the cooling of a Zn-doped Ge photoconductive cell to 4°K increases the limit of the cell’s sensitivity to 40 μm.
REFERENCESMarkov, M. N. Priemniki infrakrasnogo izlucheniia. Moscow, 1968.
Aksenenko, M. D., and E. A. Krasovskii. Fotorezistory. Moscow, 1973.
I. F. USOLTSEV
photoconductive cell[¦fōd·ō·kən′dək·tiv ′sel]
A device for detecting electromagnetic radiation (photons) by variation of the electrical conductivity of a substance (a photoconductor) upon absorption of the radiation by this substance. During operation the cell is connected in series with an electrical source and current-sensitive meter, or in series with an electrical source and resistor. Current in the cell, as indicated by the meter, is a measure of the photon intensity, as is the voltage drop across the series resistor. Photoconductive cells are made from a variety of semiconducting materials in the single-crystal or polycrystalline form. See Photoelectric devices