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The presence of mobile charges on semiconductor surfaces has been postulated by several authors in order to account for certain surface phenomena. For example, Brown 1 in his early measurements of inversion-layer channels offered this hypothesis to explain channel "conditioning," i.e., bias-and ambient-dependent conductance variations. Statz and co-workers 2 postulated motion of charges at the surface which were thought to change the conductivity of an underlying channel.Atalla and co-workers 3 discussed a model in which an electric field separates ions of opposite charges on the oxidized surface of a silicon p-n junction when reverse bias is applied across the junction to produce an electric field with a large tangential component parallel to the surface. This "Atalla model" of ionic separation was used to explain the spreading of channels as observed by photocurrents and other electrical characteristics. The theory includes the idea that the ionic charge produces electric fields through the oxide that may approach the breakdown strength.The present work was undertaken after reaching the conclusion, not discussed by Atalla, Bray, and Lindner, 3 that the high electric fields through the oxide should produce "work functions" or contact differences in potential comparable to the applied reverse bias across the junction. A direct measurement of these potentials would confirm the surface-charge hypothesis and the Atalla model, as well as furnish a new tool for studying surface effects.

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