Abstract: The disclosure relates to a method for producing graded band gap mercury cadmium telluride, preferaby in narrow band gap mercury cadmium telluride, to reduce tunneling and the like by causing the surface region of the mercury cadmium telluride to lose mercury or by replacing the mercury with another group IIB element, such as cadmium or zinc. Cadmium or zinc films are deposited on the surface of a mercury cadmium telluride substrate and the substrate is then subjected to a low temperature anneal to replace the mercury at the substrate surface on a graded basis. In the case of a P-type mercury cadmium telluride substrate, the film can be that of a group I metal such as gold, silver or copper. Low temperature annealling is again used. The deposited film can be selectively disposed on the substrate to provide localized regions with graded band gas in the mercury cadmium telluride substrate.