infrared detectorsDevices sensitive to infrared radiation in the region from 0.8 μm to about 400 μm. Present-day detectors are nearly all operated at cryogenic temperatures: 77 K using liquid nitrogen or 4 K or lower by using liquid helium. They are made from very pure semiconductor material, such as silicon or germanium, either as 1 mm cubes or smaller or as square arrays of tiny pixels (as with CCDs). For either form, electrical connections to the semiconductor allow monitoring of the electrical signal that is generated when the device is struck by infrared photons. Different conversion mechanisms from incident photon to electrical signal are involved, depending upon the detector (see bolometer; photoconductive detector; photovoltaic detector).
‘Doping’ the detector material with a particular impurity determines the wavelength region in which the detector will have its best response; impurities used include indium, gallium, copper, arsenic, antimony, beryllium, and boron. For example, a gallium-doped germanium detector (denoted, using chemical symbols, as Ge:Ga) has a useful wavelength region of 40–180 μm while an arsenic-doped silicon detector (denoted Si:As) has a useful range of 8–14 μm. A detector denoted InSb is made of indium antimonide, which is a compound rather than a crystalline mixture. Detectors of Ge:Ga can have their response limit extended to 205 μm by slightly squashing or stressing the material of the detector. Infrared detectors are susceptible to hits from cosmic ray particles, and work is progressing on detectors that are less vulnerable to this phenomenon.