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Luminance, conduction current and threshold voltage of ZnS:Mn ac thin film electroluminescent display devices were measured as functions of device temperature (10 K-300 K) and risetime of the excitation voltage pulse (2 μs-50 μs). Results provided insight into the electron ejection mechanism at the insulator-phosphor interfaces. It was found that the distribution of interface state electrons at the beginning of the excitation voltage pulse varied substantially with device temperature. Pure tunneling is thought to be the dominant electron ejection mechanism at the beginning of the voltage pulse while phonon-assisted tunneling is responsible for altering the interface electron distribution during the interval between the pulses. A delay of several microseconds was observed in the build up of the transferred charge. It is attributed to the relatively small population of electrons available at the insulator-phosphor interface.