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Devices that respond to incident ultraviolet, visible, or infrared electromagnetic radiation by giving rise to an output signal, usually electrical. Based upon the manner of their interaction with radiation, they fall into three categories. Photon detectors are those in which incident photons change the number of free carriers (electrons or holes) in a semiconductor (internal photoeffect) or cause the emission of free electrons from the surface of a metal or semiconductor (external photoeffect, photoemission). Thermal detectors respond to the temperature rise of the detecting material due to the absorption of radiation, by changing some property of the material such as its electrical resistance. Detectors based upon wave-interaction effects exploit the wavelike nature of electromagnetic radiation, for example by mixing the electric-field vectors of two coherent sources of radiation to generate sum and difference optical frequencies.
The most widely used photon effects are photoconductivity, the photovoltaic effect, and the photoemissive effect. Photoconductivity, an internal photon effect, is the decrease in electrical resistance of a semiconductor caused by the increased numbers of free carriers produced by the absorbed radiation. See Photoconductive cell
The photovoltaic effect, also an internal photoeffect, occurs at a pn junction in a semiconductor or at a metal-semiconductor interface (Schottky barrier). Absorbed radiation produces free hole-electron pairs which are separated by the potential barrier at the pn junction or Schottky barrier, thereby giving rise to a photovoltage. This is the principle employed in a solar cell. See Solar cell
The photoemissive effect, also known as the external photoeffect, is the emission of an electron from the surface of a metal or semiconductor (cathode) into a vacuum or gas due to the absorption of a photon by the cathode. The photocurrent is collected by a positively biased anode. Internal amplification of the photoexcited electron current can be achieved by means of secondary electron emission at internal structures (dynodes). Such a vacuum tube is known as a photomultiplier. Internal amplification by means of an avalanche effect in a gas is employed in a Geiger tube. See Photoelectric devices
Semiconductors are key to the development of most photon detectors. These materials are characterized by a forbidden energy gap which determines the minimum energy that a photon must have to produce a free hole-electron pair in an intrinsic photoeffect. Since the energy of a photon is inversely proportional to its wavelength, the minimum energy requirement establishes a long-wavelength limit of an intrinsic photoeffect. It is also possible to produce free electrons or free holes by photoexcitation at donor or acceptor sites in the semiconductor; this is known as an extrinsic photoeffect. Here the long-wavelength limit of the photoeffect is determined by the minimum energy (ionization energy) required to photoexcite a free electron from a donor site or a free hole from an acceptor site. See Semiconductor
The choice of materials also plays a role in thermal detectors. The most widely used thermal detector is a bolometer, that is, a temperature-sensitive resistor in the form of a thin metallic or semiconductor film (although superconducting films are also used). Incident electromagnetic radiation absorbed by the film causes its temperature to rise, thereby changing its electrical resistance. The change in resistance is measured by passing a current through the film and measuring the change in voltage. Materials with a high temperature coefficient of resistance are desired for bolometers, a criterion which usually favors semiconductors over metals. See Bolometer