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The I–V characteristics of HgCdTe midwavelength infrared (MWIR) double-layer heterojunction photodiodes at temperatures below 120 K are dominated by thermal generation–recombination in the depletion region. The diode R0A’s, determined from the slope of the I–V curves, are in excess of 1 × 108 Ω cm2 at 77 K when measured in the dark but are much lower when measured with ambient backgrounds. In order to better understand this phenomenon, the I–V characteristics of numerous MWIR HgCdTe photodiodes were measured as functions of background flux using a blackbody source and several narrowband spectral filters. Based on the measured R0A at 100 K in the dark, where thermal G–R currents dominate, the G–R lifetimes are approximately equal to the Auger lifetime for the minority carriers in the lightly doped n-type base layer. At backgrounds in excess of 1 × 1012 photons/cm2 s, the R0A decreases inversely with background flux. The magnitude of the effect at wavelengths much shorter than the cutoff agrees well with estimates based on the depletion modulation theory. At wavelengths very close to the cutoff, the effect is largest and agrees well with a theory based on optical generation of carriers in the depletion region. Thus, the background dependence of R0A is due to the combined effects of depletion modulation and optical generation in the depletion region.