The formation of p+ and high resistivity layers in GaAs-AIGaAs heterostructures is useful in a number of applications, including the fabrication of heterojunction bipolar transistors (HBTs). Control of the implanted dopant profile during annealing is paramount for rapidly diffusing acceptor species such as Be and Mg, and we show SIMS data of the dopant profile in implanted heterojunctions as a function of annealing temperature in the range 700–900°C for 1–5 sec. Annealing of implanted Be at ≥800°C even at a dose of 2×10l5cm−2 results in complete activation whereas Mg shows only 30% activation under these conditions. There is no significant interface disordering visible by TEM or RBS for either case. Multiple energy O implants (up to and including 1 MeV) were used to render the entire heterostructure resistive; for doses of ∼1013cm−2, the O bombarded layers showed resistivities of 108 Ω/□ after 525°C annealing.