This work focused on the studies of the high-temperature dependence of the responsivity of ZnS, ZnSTe and ZnSSe photodiodes. 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 temperature and a simplified model is used to illustrate this hypothesis.