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The buildup of persistent photoconductivity, presently a controversial phenomenon, is observed by measuring densities and mobilities of photoinduced excess electrons in thin ^-GaAs layers between successive illuminations. Evidence from this novel type of analysis supports a model assuming charge separation by macroscopic potential barriers. We explain quantitatively how the photon dose logarithmically increases the number, but not necessarily the density, of persisting carriers and ascribe mobility enhancements to screening of ionized impurities.Many semiconductors exhibit persistent photocurrents (PP): A photoinduced conductivity increment persists after the illumination, often with immeasurably long time constants/"'^ Two conflicting interpretations presently exist. The first assumes macroscopic potential barriers, such as junctions or surface barriers, which separate spatially the photogenerated electron/hole pairs to suppress their recombination.^"^ The second interpretation postulates microscopic barriers against recombination due to impurity atoms with large lattice relaxations.^"'^ Interest in this unusual phenomenon has been recently rekindled. Studies of two-dimensional electron gases at semiconductor interfaces rely on PP to enhance electron densities.^ Interpretations of atomistic properties of deep impurities in semiconductors have been based on PP observations.^*"^ Enhancement of carrier mobility by spatial separation of liberated carriers from their dopant ions ("modulation doping")^ niay be related to PP. Quantitative information is needed to describe lateral charge transport in very thin semiconductor layers near surfaces. This would be of interest for quantum effects from reduced dimensionality^^ as well as for applications, such as charge-coupled devices^^ or memories.^ This paper describes PP in well-defined layers of GaAs at low photon excitation levels. We are able to observe and explain for the first time the transient buildup of PP by using the Hall effect with high resolution to measure both density n and mobility /i of the excess electrons in thin wtype layers on high-resistivity substrates. This technique, which yields more information than simple conductance measurements, is shown here to furnish quantitative details about geometric structure and electronic transport. The method thus promises to become a novel technique for analysis of semiconductors interfaces. We obtain clear evidence for the model, assuming macroscopic barriers.

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