Charge transfer in ZnS-type electroluminescence

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Abstract
In this paper, a simple model for the charge transfer in thin-film, ac-driven electroluminescent structures of doped ZnS type is proposed. We first review the minimal assumptions needed to account for the carrier emission under high-field conditions and its subsequent feedback on the field strength; it opens the way to a quantitative description of the field variation with time and the conduction current under arbitrary low-frequency drive conditions and without using any adjustable parameter. The charge-voltage relationships are also examined in detail. Conduction in the phosphor layer is assumed to originate from deep levels (traps) in the phosphor forbidden band gap, located at the insulator-phosphor interface. These levels act for charge storage, too. When a discrete trap level is considered, field clamping in the active layer is obtained; when a smooth interface-state energy density is assumed, deviations from the field clamping are possible and simply related to the interface parameters. Most of the work is analytical and the model is shown to exhibit the main qualitative features of nonmemory devices.