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This paper correlates measurements of minority carrier lifetime using photoconductivity transient or photoconductive decay (PCD) with measurements and analysis of current–voltage (I–V) on P + n photodiodes of the same layer. This analysis is done at the primary temperature of operation near 77 K. Measurements of minority carrier lifetime in n-type molecular beam epitaxy Hg1−x Cd x Te (MCT) on variable substrates of GaAs and CdZnTe are presented. The minority carrier lifetime from I–V analysis of P + n long-wave infrared (LWIR) photodiodes on CdZnTe (CZT) and GaAs substrates is similar to PCD analysis. Deviations occur for MCT/GaAs in the low-temperature extrinsic region associated with Shockley–Read–Hall (SRH) at 78 K in contrast to MCT/CZT at similar temperatures. The x-values in the formula Hg1−x Cd x Te for samples grown on CdZnTe and GaAs are 0.22 and 0.235, respectively. I–V analysis of variable-area LWIR photodiodes shows a calculated minority carrier diffusion coefficient of 2.3 cm2/s on lattice-matched CZT and 3 cm2/s on non-lattice-matched GaAs with diffusion lengths of approximately 24 μm and 20 μm, respectively.