The transient behavior of the electrical and the optical properties under pulse excitation has been studied and analyzed in detail on the electroluminescent devices made of Au-ZnS: NdF3- Ta2O5-Ta composite films. The ZnS-Ta2O5interface is capable of accommodating a large number of trapped electrons, up to 1013cm-2, with the majority of the interfacial states located at a fixed energy below the conduction band of ZnS. Carrier injection is achieved by electrons tunneling through the Au-ZnS Schottky barrier at a field of 2.6×106V/cm. The threshold field of impact excitation for 1 percent NdF3in ZnS was found to be 1.5×106V/cm; while the coefficient of impact excitation at 2.6×106V/cm was estimated to be 1750 cm-1. In addition, the emission time constants of several rare earth fluorides were also studied.