The effect of trap tunneling on the detector performance of long cutoff wavelength (λco) Hg1-xCdxTe photodiodes was investigated with the use of a parametric model. The development of this model follows closely the formulation by Sah for treating the case of excess currents in gold-doped narrow silicon junctions. The trap tunneling limited 's for long-wavelength Hg1-xCdxTe photodiodes with different p- and n-side doping concentrations and at different temperatures were calculated using this model as a function of p-side trap density, trap location, and junction impurity concentration gradient. The calculated results are in agreement with those measured from actual photodiodes. In particular, the somewhat unexpected temperature dependence of the measured product at low-temperatures can be satisfactorily accounted for. The present tunneling model also adequately explains the observed soft reverse breakdown characteristics for these devices and their behavior as a function of temperature.