A 2-D numerical device simulator has been modified to account for the electrical characteristics of mercury cadmium telluride (MCT). The authors compare the simulated current-voltage (I-V) results with measurements. For diodes not dominated by tunneling or other surface effects, the simulated curves match the measurement within the experimental limitations. The modified code can be used to investigate the changes in the I-V characteristics of irradiated diodes. By adding the appropriate models. The authors show the results of an investigation of the band-to-band tunneling in these diodes, which theory predicts is a dominant current-producing mechanism after irradiation. The I-V curves closely match those of irradiated diodes if radiation-induced trapped surface charge is taken into account. >