The study of diffusion and drift of Si in AlxGa1−xAs by means of capacitance‐voltage measurements reveals that low substrate temperatures during growth by molecular beam epitaxy are required to achieve &dgr;‐function‐like doping profiles. The diffusion coefficient of Si in Al0.3Ga0.7As is determined. We further show theoretically that the random Poisson distribution (usually assumed for dopant distributions in semiconductors) should be modified at high dopant concentrations due to repulsive interactions of impurities.