Silicon migration during MBE growth of (Al, Ga)As and (Al, Ga)As/GaAs or AlAs/GaAs superlattices has been studied by electrochemical C-V and secondary ion mass spectrometry (SIMS) concentration-depth profiling. It is found to be concentration dependent, with no preferential migration towards or away from the growth front. At high concentrations, superlattice disordering during growth is observed using photovoltage and transmission electron microscopy (TEM) techniques. On the basis of C-V, SIMS and TEM data we propose that silicon migration occurs as the result of a concentration-dependent diffusion process. This is substantiated by measurements of the two-dimensional electron-gas mobility in selectively doped heterojunctions as a function of growth temperature and silicon concentration. © 1986 Springer-Verlag.