Effects of sweepout on HgCdTe detectors under high bias conditions

A model is developed for the sweepout of minority carriers under high bias conditions in a HgCdTe semiconductor. The equations for the signal and generation-recombination (g-r) noise are presented under high bias conditions. Results show that, during sweepout, g-r noise is affected differently than is signal. A peak in the signal to noise ratio (SNR) is obtained for an optimum bias across the detector. Solution of the ambipolar transport equation shows that the sweepout of excess carriers causes the peak in SNR. The signal is shown to have the usual 6 db/octave rolloff in the high bias case as it does in the low bias case. The g-r noise at high bias is shown to have a sinc (fT_a) rolloff where Ta is twice the effective sweep-out lifetime, as predicted previously⁽¹⁾. The results, modified for a long diffusion length (i.e., L_D ≈ 13 μm) alters the equations and thereby eliminates the higher frequency nulls in the sinc (fT_a) rolloff for large geometry detectors, whereas the nulls are evident in small geometry detectors. Experimental data on 12-μm HgCdTe detectors is presented and correlated with the theory. Frequency spectra of the g-r noise and signal response from a modulated CO₂ laser, a PbSe laser, and a GaAs diode are presented.