Abstract: Optical absorption transition effects are a fundamental physical process in semiconductor photo－electronic detectors. This paper reviews the research on this effect in narrow gap semiconductors for infrared photo－electronic detectors. The theoretical and experimental aspects of inter－band optical transitions are discussed for narrow gap semiconductors. Applications of the expressions for the intrinsic absorption coefficient with respect to the characterization of HgCdTe thin films and the cut－off wavelength of the detectors, as well as the negative luminescence discovered in recent years in mid－infrared HgCdTe photodiodes, are described.