The common belief that IR photodetectors must be cooled to achieve a high sensitivity is substantiated by the huge and noisy thermal generation in their small threshold energy optical transitions. Cryogenic cooling of detectors has always been the burden of sensitive infrared systems, particularly those operating in the middle-wavelength (MWIR) and long-wavelength (LWIR) range of the IR spectrum. In contrast, infrared detectors with limited cooling have obvious advantages, including the elimination of power-consuming cryogenics; a reduction in size, weight, and cost; and greater reliability. Despite numerous research initiatives and the attractions of ambient temperature operation and low-cost potential—room-temperature IR detector technology enjoyed only limited success in competition with cooled photon detectors. Recent considerations of the fundamental detector mechanisms suggest, however, that near-perfect detection can be achieved without the need for cryogenic cooling. The limitations to perfect detection without cooling are of technological rather than fundamental nature.

The importance of sensitive and fast detection of long wavelength IR radiation without cooling was recognized in Poland in early 60’s and became the main subject of Polish scientists. They proposed numerous concepts and practical solutions related to uncooled detection. In early 80's Vigo System S.A. was founded to commercialize uncooled IR detectors.

In this paper recent progress in fast and sensitive detection of the middle and long wavelength infrared radiation using uncooled or minimally cooled photodetectors is discussed. Photoconductive and photoelectromagnetic detectors are still in production but they are gradually replaced with photovoltaic devices which offer inherent advantages of no need for electric or magnetic bias. Sophisticated device architectures based on Hg_1-xCd_xTe multilayer heterostructures have been proposed and implemented. The heterostructures have been grown by low temperature metalorganic vapor phase deposition (MOCVD). Thermal generation of charge carriers is minimized by careful optimization of the device architecture and reduction of the volume of active regions by monolithic integration of the devices with suitable microoptics. Solutions to other specific problems of high-temperature detection, such as poor collection efficiency due to a short diffusion length, the Johnson-Nyquist noise of parasitic impedances, and interfacing of very low resistance devices to electronics have been found. The dynamic resistance of the devices is increased by the use of multiple photovoltaic cells connected in series.

The present research and fabrication program includes devices which are optimized for any wavelength within a wide 1-16 µm spectral range and 200-300 K operation temperatures. The gap between performance of practical uncooled devices and fundamental limits set by quantum noise of the signal or background radiation is steadily narrowing. Picosecond range response time has been obtained in high frequency optimized devices.

The uncooled or minimally cooled infrared devices have found numerous civilian and military applications in areas such as thermography, process control, sensitive heterodyne detection, fast pyrometry, Fourier and laser spectrophotometry, imaging interferometry, laser technology and metrology, long-wavelength optical communication, ultransensitive (ppb and ppt range) gas analyzers, imaging spectrophotometers, thermal wave nondestructive material testing, and many others.

Key words: HgCdTe, heterostructures, MOCVD, infrared photodetectors, uncooled operation

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1. Room temperature infrared photodetectors