The dependence of the electrical conductivity, measured either in darkness or after exposure to different durations of light, on temperature and exposure time have been studied for Ge0.20Te0.75Bi0.05 thin films of different thicknesses. The conductivity showed an initial gradual decrease with time, ending with saturation. The activation energy was found to increase by increasing the period of light soaking. The results were explained on the basis of light saturation of the vacant states which bend the band edges at the semiconductor-metal interface.