The electroluminescence emission spectrum of forward-biased MIS diodes on high-purity single crystal CdS has been studied over the temperature range 50-290K.At room temperature the emission consisted of two intense bands in the green at 5135±25Å and 5300±15Å

while at lower temperatures considerably more structure was observed.Thus at 50K free and bound ex-citon emission was particularly prominent. Evidence is presented to suggest that the band at 5135±25A at room temperature is associated with free ex-citon recombination following scattering from free electrons in the conduction band.The band at 5300±15A is attributed to free exciton recombination with the emission of two longitudinal optical phonons.The photoluminescence emission of as-grown CdS crystals has been compared with that of crystals rendered semiconducting by heat treatment in molten cadmium.This treatment suppressed the free-to-bound(HES) and bound-to-bound edge emission.As well as the I1 bound exciton line

which was also preferentially excited ion electroluminescence.