Detectivity limits for diffused junction PbSnTe detectors

Abstract

The resistance-area product ( R 0 A) 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 R 0 A 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 R 0 A 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: Pb 0.982Sn 0.018Te diffused with Sb with a 5 μm cutoff at an operating temperature of 170°K, and Pb 0.79Sn 0.21Te 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 R 0 A 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 R 0 A. 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 ∗.