DETECTIVITY LIMITS FOR DIFFUSED JUNCTION PbSnTe DETECTORS

Abstract

The resistance–area product (R0A) of diffused junction PbSnTe photo-voltaic detectors under conditions of zero bias voltage is calculated for linearly graded and one-sided abrupt junctions. Equations are developed to demonstrate the overall dependence of R0A on grading constant when minority carrier diffusion current, depletion layer current, and tunneling current are taken into account for the linearly graded junction. Similarly, for the one-sided abrupt junction the dependence of R0A on the carrier concentration of the lightly doped side is shown for the diffusion, depletion, and tunneling mechanisms. The calculations are carried out for two devices of practical interest: Pb0·982Sn0·018Te diffused with Sb with a 5 μm cutoff at an operating temperature of 170°K, and Pb0·79Sn0·21Te diffused with Cd with an 11 μm cutoff at 77°K. The junctions formed by Sb diffusion obey the linearly graded model, whereas the Cd junctions formed in unannealed substrates are one-sided abrupt. Upon comparing measured R0A products with calculated values, we have established approximate values for the lifetimes within the depletion layers for each of these devices. The consequences of these results for the thermal noise-limited detectivity (D*) of these detectors are shown by plotting D* vs R0A. Within this framework, it can be argued that the inherently short lifetimes of PbSnTe play the dominant role in placing the upper limits on achievable D*.