Some papers have been published on luminescence of free exciton under high density excitation in CdS. But most of the research about exciton-exciton interaction are concentrated on photoluminescence below 77K. So far no report is found on free exciton emission in forward biased CdS MIS diodes with high pulse current density.Undoped CdS crystals were grown by sublimation in our labora tory.Dice with dimensions of 5×2×1mm

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were annealed in molten cadmium at 650℃ to reduce their resistivity to the range of 0.1-5Ω·cm. The MIS structure was prepared by making an ohmic contact on the substrate and evaporating an insulating layer and Au contact on the opposite surface. PL and EL were excited by 3371Å line of a pulsed nitrogen laser or by the pulse with high current density.