Abstract
The non-equilibrium operation of narrow energy-gap semiconductor devices has been studied in the context of infrared detection. The fundamental mechanisms which determine medium and long wavelength infrared detector sensitivities have been reviewed, and it is proposed that the dominant factors which have necessitated the cooling of such devices in order to minimise thermal noise can be suppressed. This is contrary to previous assumptions, and is a consequence of the operation of detectors in a non-equilibrium mode where the free carrier densities are held substantially below their normal values. The prospect is offered, therefore, of the removal of the cooling requirement with less than a factor of two loss in performance. The properties of non-equilibrium minority carrier current phenomena in wide-gap semiconductors have been reviewed. The first application of these phenomena to narrow-gap devices is presented here and, specifically, the effects of minority carrier accumulation, exclusion and extraction in cadmium mercury telluride infrared detectors are demonstrated. Accumulation has been studied in swept-out extrinsic devices, where it has been shown that the responsivity can be increased by a factor of five, without loss of frequency response. Consequently, the SPRITE photoconductors have been re-designed to optimise the effects of accumulation, giving a 50% Improvement in spatial resolution. Minority carrier exclusion has been examined in intrinsic devices, where non-equilibrium carrier reductions by a factor of forty have been demonstrated at room temperature, with electric fields sufficiently low to prevent carrier heating. Increases in responsivity of up to a factor of twenty and improvements in detectivity by a factor of three have been observed, confirming that the dominant Auger generation mechanism can be suppressed. Observation of the maximum performance improvement was prevented, however, by excess surface generation and 1/f noise. Photovoltaic devices for the suppression of Auger generation through the use of extraction have been described, and the phenomena has been demonstrated at room temperature. Bipolar transistor operation has also been demonstrated for the first time in CMT. This was observed both under low temperature, extrinsic conditions and near ambient temperature where performance is maintained by extraction.