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Chapter 6:
HgCdTe FPA Technologies
In this chapter we will consider the performance of HgCdTe staring FPAs operating in all of the IR spectral bands, under a wide variety of extreme conditions, such as high operating temperatures, reduced backgrounds, and large reverse bias, as indicated by data currently published in the open literature. As previously discussed, this will penalize architectures in which the finished photodiode contains metal vacancies due to the short lifetimes and high dark current generation rates associated with this defect. S-R lifetimes associated with neutral S-R centers are long across the entire HgCdTe composition range, as judged by measurements of depletion current versus temperature for the various cutoff wavelengths from SWIR to LWIR, with indicated values of 200 ms to 50 ms. For tactical background fluxes in the MWIR and LWIR bands, this eliminates concerns with regard to dark current from depletion regions within the detector unit cell, and also implies that the predominant diffusion current will be due to Auger generation for present-day doping concentrations. Auger-limited dark currents vary directly as the majority carrier concentration in the absorber volume of the unit cell. Thus, from a diffusion-limited NETD point of view, the highest operating temperatures will be achieved with low-doped P-type absorbers. For these diffusion-limited circumstances, a flat-band biased nBn architecture offers no dark current advantage over the photodiode. However, it should be pointed out that as HgCdTe transitions into the SWIR region of the spectrum, depletion current does become relevant, even at room temperature, due to the rapid decrease in ni, and there is merit to using the nBn architecture to achieve diffusion-limited operation in this spectral band.
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