Carrier transport mechanisms in narrow-gap photodiodes

Carrier transport mechanisms in photodiodes from narrow-gap HgCdTe and PbSnTe semiconductors for long wavelength infra-red (LWIR) and medium wavelength infra-red (MWIR) applications are discussed in connection with their use in hybrid multielement arrays for T approximate to 77-150 K temperature range. It is shown that the bulk diffusion and thermal generation-recombination (GR) currents in depletion region determine carrier transport mechanisms in these photodiodes at zero bias. The transport mechanisms are changed for reverse-biased voltages at which diodes are operating with silicon read-out devices in hybrid arrays. In this case the dark current is determined not only by the GR recombination (Schockley-Read-Hall (SRH) recombination) via the centres in the gap,but preferentially by two non-thermal mechanisms: a) interband tunnelling via the traps in the gap, and b) direct band-to-band tunnelling. The last one is important only at rather high electron concentrations n greater than or equal to 10(16) cm(-3) or n greater than or equal to 2.10(18) cm(-3) in HgCdTe and PbSnTe photodiodes, respectively and at large reverse-bias (V greater than or equal to 0.2 V) voltages. The parameters of the traps (density of states, energy level position, capture cross section) have been estimated from the fit of calculated transport characteristics to the measured dependences on bias and temperature in the photodiodes of both both investigated compounds. One level of traps in the gap for (SRH) recombination and trap-assisted tunnelling (TAT) mechanisms were used in fitting procedure. The best fit was obtained for the model in which the level of the traps was near the middle or slightly shifted from the middle of the gap towards the conduction band edge.