photoconductivity, the increase in the electricalconductivity of certain materials when they are exposed tolight ofsufficient energy. Photoconductivity serves as a tool to understand the internal processes in these materials, and it is also widely used to detect the presence of light and measure its intensity in light-sensitive devices.

Certain crystallinesemiconductors, such assilicon,germanium, lead sulfide, and cadmium sulfide, and the related semimetalselenium, are strongly photoconductive. Normally, semiconductors are relatively poor electricalconductors because they have only a small number ofelectrons that are free to move under a voltage. Most of the electrons are bound to their atomic lattice in the set of energy states called thevalence band. But if external energy is provided, some electrons are raised to theconduction band, where they can move and carry current. Photoconductivity ensues when the material is bombarded with photons of sufficient energy to raise electrons across theband gap, a forbidden region between the valence and conduction bands. In cadmium sulfide this energy is 2.42electron volts (eV), corresponding to a photon of wavelength 512 nanometres (1 nm = 10⁻⁹ metre), which is visible green light. In lead sulfide the gap energy is 0.41 eV, making this material sensitive toinfrared light.

Because the current ceases when the light is removed, photoconductive materials form the basis of light-controlled electrical switches. These materials are also used to detect infrared radiation in military applications such as guiding missiles to heat-producing targets. Photoconductivity has broad commercial application in the process ofphotocopying, orxerography, which originally used selenium but now relies on photoconductivepolymers.See alsophotoelectric effect.

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