The results of electrical and luminescence measurements on a new, low voltage, dc, thin film electroluminescent device structure are presented. The devices incorporate a two-phase Si-rich-SiO2/SiO2 electron injector layer which provides control of current, and an active luminescent ZnS:Mn layer in which light is generated by hot electron impact excitation of the Mn2+ activator in high electric field. Separation of the processes of current control and light generation into different layers permits the effects of space charge and the average field distributions to be determined. The electroluminescence intensity is simply proportional to the average power dissipated in the ZnS:Mn layer when the average field is in the range 0.6-1.2 MV cm-1, and when field distortion due to electron trapping in the SiO2 layer is small. When the field locally in the ZnS:Mn layer exceeds ∼1.7 MV cm-1, lattice ionization competes with impact excitation of Mn2+ and the quantum efficiency falls. A simplified model assumes that in a quasi-steady-state condition the majority of the device current is carried by electrons accumulated in the satellite (L, X) valleys in the conduction band of ZnS. The results are compared with previous studies, and their general significance as regards the limiting efficiency of high field electroluminescent devices using ZnS is discussed.