High-temperature dependence, up to 150 °C, of the photoresponsivity of ZnS, ZnSTe, and ZnSSe photodiodes was investigated in this study. It was found that, in general, the responsivity at higher temperatures will shift to longer wavelengths because of band-gap narrowing. A remarkable observation is that the near-band-edge responsivities of these diodes increase at higher temperature. We believe that this observation is attributed to the change of the density-of-state distribution due to lattice expansion at high temperatures, and a simplified model is used to illustrate this hypothesis. © 2000 American Institute of Physics.