Indium antimonide (InSb) is a crystalline compound made from the elements indium (In) and antimony (Sb). It is a narrow-gap semiconductor material from the III-V group used in infrared detectors, including thermal imaging cameras, FLIR systems, infrared homing missile guidance systems, and in infrared astronomy. Indium antimonide detectors are sensitive to infrared wavelengths between 1 and 5 μm. Indium antimonide was a very common detector in the old, single-detector mechanically scanned thermal imaging systems. Another application is as a terahertz radiation source as it is a strong photo-Dember emitter. History The intermetallic compound was first reported by Liu and Peretti in 1951, who gave its homogeneity range, structure type, and lattice constant. Polycrystalline ingots of InSb were prepared by Heinrich Welker in 1952, although they were not very pure by today's semiconductor standards. Welker was interested in systematically studying the semiconducting properties of the III-V compounds. He noted how InSb appeared to have a small direct band gap and a very high electron mobility. InSb crystals have been grown by slow cooling from liquid melt at least since 1954. Physical properties InSb has the appearance of dark-grey silvery metal pieces or powder with vitreous lustre. When subjected to temperatures over 500 °C, it melts and decomposes, liberating antimony and antimony oxide vapors. The crystal structure is zincblende with a 0.648 nm lattice constant. Electronic properties InSb is a narrow direct band gap semiconductor with an energy band gap of 0.17 eV at 300 K and 0.23 eV at 80 K. Undoped InSb possesses the largest ambient-temperature electron mobility (78000 cm2/V⋅s), electron drift velocity, and ballistic length (up to 0.7 μm at 300 K) of any known semiconductor, except for carbon nanotubes. Indium antimonide photodiode detectors are photovoltaic, generating electric current when subjected to infrared radiation. InSb's internal quantum efficiency is effectively 100% but is a function of the thickness particularly for near bandedge photons. Like all narrow bandgap materials InSb detectors require periodic recalibrations, increasing the complexity of the imaging system. This added complexity is worthwhile where extreme sensitivity is required, e.g. in long-range military thermal imaging systems. InSb detectors also require cooling, as they have to operate at cryogenic temperatures (typically 80 K). Large arrays (up to 2048×2048 pixels) are available. HgCdTe and PtSi are materials with similar use. A layer of indium antimonide sandwiched between layers of aluminium indium antimonide can act as a quantum well. In such