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The metal-thin-film insulator-semiconductor (MIS) structure is currently receiving much attention in solar-cell studies. Both theoretical and practical investigations indicate that this structure offers a means of overcoming the principal deficiency of Schottky barrier solar cells, namely low open-circuit photovoltage, while maintaining the attractive features that have led the metal-semiconductor junction to be considered as a possible alternative to the p-n junction for large-area, terrestrial, solar-cell applications. The thin insulating layer allows control over not only the magnitude of the dark current flowing through the diode, but also the dominant type (majority or minority carrier) of this current. Desirably low values of dark current have been postulated for majority carrier devices incorporating suitable charge-trapping centres, located either within the insulator or at the semiconductor-insulator interface, and for minority carrier devices employing suitable insulator thicknesses, metal work functions, and semiconductor resistivities. Theories based on these models are reviewed in this paper and their relevance to explaining photovoltage enhancement in practical Si and GaAs MIS cells is examined. The factors affecting other salient solar-cell properties (photocurrent, fill factor, conversion efficiency) are also considered and suggestions as to the parameters limiting present device performance are given.