Analysis of the effect of surface passivant charges on HgCdTe photoconductive detectors

The effect of fixed surface charge density (Q(1)) on the performance of HgCdTe (MCT) photoconductive detectors is presented in this study. The figure of merit such as the specific detectivity (D*) of Hg0.78Cd0.22Te photoconductive detectors operating at 77 K is calculated with 300 K background and 2 pi field of view (FOV). The equations leading to the above calculations are developed for the case of a one-dimensional model. Our calculations show that: (i) D* shows maxima with varying Q(1); (ii) improvement in D* due to accumulation is observed for values of Q(1) up to 5 x 10(10) cm(-2) for materials with low as well as high bulk minority carrier lifetimes (tau(b)); and (iii) D* is limited by shunt resistance due to passivants having Q(1) > 5 x 10(10) cm(-2) for low-tau(b) materials. Furthermore, D* is found to degrade by an order of magnitude from its peak value with increase in Q(1) from 5 x 10(10) cm(-2) to 1 x 10(13) cm(-2) in a material having tau(b) = 10 ns, whereas no such degradation in D* is seen for the same range of Q(1) in a material having tau(b) greater than or equal to 1 mu s. In addition, our calculations predict that the accumulation layer improves D* by a factor of 1.5 for surfaces having an initial surface recombination velocity as high as 10(5) cm s(-1).