Currents in narrow-gap photodiodes

Dark carrier transport mechanisms in Hg1-xCdxTe (x = 0.19-0.265) photodiodes in the temperature range 70-150 K and PbTe1-ySy/Pb1-xSnxTe (x = 0.2, y = 0.03) heterojunctions at T = 80 K are discussed. Two major current mechanisms were included into balance equations for the p-n junction: trap-assisted tunnelling (TAT) and Shockley-Reed-Hall generation-recombination processes for a defect trap level in the gap. Other current mechanisms (band-to-band tunnelling, bulk diffusion, etc) were taken into account as additive contributions. For TAT Anderson's matrix element of the impurity ionization was used and the tunnelling rate characteristics were calculated in the k-p approximation with constant barrier field. Using donor and acceptor concentrations in n- and p-type regions of the diode, trap level concentration, trap level energy and the in-junction trap level lifetimes as fitting parameters, a relatively good agreement with the experimental data for HgCdTe and PbSnTe diodes with large zero-resistance-area products R(0)A (e.g. for the 10 mu m spectral region R(0)A > 10 Omega cm(2)) was obtained, which allows one to deduce the parameters of mercury-cadmium telluride and lead-tin telluride photodiodes from the parameters of the materials used for the diode fabrication. For the diodes with poor R(0)A characteristics (R(0)A < 1 Omega cm(2) for the 10 mu m spectral region) the agreement with the experimental data is not so good, which is apparently caused by the presence of several types of traps with different energy positions inside the gap and with different properties.