Homogeneous reactions, spatially nonuniform

If the whole semiconductor/electrolyte interface is illuminated uniformly, both conjugate reactions proceed at the same rate over the same areas on the interface. The stationary potential of an illuminated semiconductor is thus a mixed potential. If the surface of a semiconductor, homogeneous in its composition and properties, is illuminated nonuniformly, in the illuminated and nonillumi-nated areas conditions will not be identical for electrochemical reactions. Here the conjugate reactions appear to be spatially separated, so that we can speak about local anodes and cathodes. This situation is deliberately created, for example, for selective light-sensitive etching of semiconductors (see Section V.2). 

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