This article shows that the presence of low‐temperature‐grown GaAs (LT‐GaAs) in LT‐GaAs/AlAs/GaAs:Si heterostructures increases the Al/Ga interdiffusion at the heterostructure interfaces. The interdiffusion enhancement is attributed to the presence of Ga vacancies (VGa) in the As‐rich LT‐GaAs, which diffuses from a supersaturation ofVGafrozen‐in during sample growth. Chemical mapping, which distinguishes between the AlAs and GaAs lattices at an atomic scale, is used to measure the Al concentration gradient in adjacent GaAs:Si layers. A correlation is observed between the Al/Ga interdiffusion and the gate breakdown voltage in metal‐insulator field‐effect transistor structures containing LT‐GaAs. ©1996 American Institute of Physics.