The avalanche breakdown voltage (Vb) has been measured in a range of bulk AlxGa1−xAs alloys and Al0.3Ga0.7As/GaAs multilayer structures. The bulk alloys show a linear dependence ofVbonxup to at leastx=0.6. Multilayers with thin (≤100 A˚) dimensions follow this trend, withVbbeing determined by the average Al fraction of the multilayer ‘‘pseudoalloy’’. For thicker (≥500 A˚) layersVbtends to a mean of the bulk values of the layers. The transition from pseudoalloy to bulklike behavior is interpreted in terms of the momentum and energy relaxation lengths, estimated for bulk GaAs using a Monte Carlo solution of the semiclassical transport equation. ©1995 American Institute of Physics.