The control of p-type dopants is very important in producing high performance minority carrier devices such as heterojunction bipolar transistors (HBT) and lasers. In this study, an electrical characterization technique is described which is very sensitive to the p-type dopant profile in a heterojunction. Both the placement of the dopant, i.e. the as-grown profile, and thermal diffusion effects have been investigated. The factors which control the initial placement and subsequent diffusion of the dopant species have been determined and used to produce device-quality GaAs/Al0.30Ga0.70As p+/n heterojunctions.