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A photovoltaic detector based on an N+-InAs0.55Sb0.15P0.30/n0-InAs0.89Sb0.$ d11/P+-InAs0.55Sb0.15P0.30 double heterostructure (DH) suitable for operation in the mid-infrared (MIR) spectral region (2 to 5 μm) at room temperature has been studied. A physics based closed form model of the device has been developed to investigate the relative importance of the different mechanisms which determine dark current and photoresponse. The results obtained on the basis of the model have been compared and contrasted with those obtained from experimental measurements on DH detectors fabricated previously in our laboratory using liquid phase epitaxy (LPE). The model helps to explain the various physical mechanisms that shape the characteristics of the device under room temperature operation. It can also be used to optimize the performance of the photodetector in respect of dark current, responsivity and detectivity. A comparison of theoretical predictions and experimental results revealed that Shockley-Read-Hall (SRH) recombination is more important than Auger recombination in determining the room temperature detector performance when the concentration of nonradiative recombination centers in our material exceeds 1017 cm-3. Furthermore, compositional grading in the cladding regions of the double heterostructure has been found to be responsible for the reduction of the detectivity of the device in the shorter wavelength region.