Photodiodes are themost sensitive photodetectors, being widely used for thermal imaging in high-performance cameras for both commercial and governmental purposes. In the thermal IR region, MWIR photodiodes (3- to 5-μm spectral range) are the most widely used andmostmature, being incorporated into arrays of hundreds of thousands to millions of elements and being widely used in applications from tactical thermal imaging to space-based astronomy. They have wavelengths sufficiently long to detect thermal infrared but short enough not to be very susceptible to tunneling (which can limit sensitivity).
Probably the most widely used high-performance MWIR photodiode material is InSb. However, InSb is a binary compound with a fixed spectral range (which increases undesirably with operating temperature) and has currents limited by defects, rather than fundamental mechanisms. Therefore it reaches its best performance at low temperatures (typically ~80-100 K). InSb is used mainly for its maturity and low fabrication cost; however, its limitations make it unsuitable to realize the low system size, weight, and power, and low life-cycle cost benefits attainable from substantially higher operating temperature.
Fortunately, two other material systems have the potential to attain high MWIR performance at high operating temperatures. Mercury cadmium telluride (HgCdTe), discovered over 50 years ago in Britain, has consistently shown the highest performance of any MWIR photodiode material at or above liquid nitrogen temperatures (80 K), and its adjustable energy gap allows tailoring of the spectral response precisely to the intended operating temperature. Moreover, it has matured to the point where its performance is limited not by defects but by fundamental Auger mechanisms that arise from the band structure itself.
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