Infrared photodetectors operating at near room temperature

Recent progress in the field of infrared photodetectors operating in the 3-16 μm spectral range at near room-temperatures (200-300 K) is reported. The fundamental limits to performance of the devices are imposed by the statistical nature of thermal generation-recombination processes and related noise. The figure of merit of a material for photodetectors is the ratio of absorption coefficient to the volume generation rate, α/G. Comparing various materials one can find that narrow bandgap semiconductors exhibit much higher α/G than extrinsic materials. While HgCdTe remains the most important material for near room temperature infrared photodetectors, HgMnTe, HgZnTe, InAsSb and Tl-based III-V compounds are possible candidates to replace it. The performance of near room temperature infrared (NRTIR) photodetectors can be improved by reduction of the total rate of generation and recombination in the active region of the device. Various methods to achieve this are discussed including selection of semiconductor, non-equilibrium mode of operation, and elimination of parasitic thermal generation on surfaces and contacts. Further improvement can be obtained by reduction of the physical volume of the detector without reduction of the apparent “optical” area of the device by the use of optical concentrators and optically resonant cavities. Similar methods are applicable to cryogenically cooled detectors operating at very long wavelengths (16-30 μm). Various practical NRTIR photodetectors are described, such as photoconductive, photoelectromagnetic and photovoltaic detectors. Photovoltaic devices are the most promising type of NRTIR, since they require no bias and exhibit no low frequency noise. However, conventional single junction photovoltaic detectors suffer from very low quantum efficiency and low resistance. These limitations can be removed with novel solutions based on multijunction heterostructures which are capable of achieving high performance in practice