The formation of high‐resistivity (>107&OHgr;/&laplac;) regions in GaAs‐AlGaAs heterojunction bipolar transistor (HBT) structures by oxygen and hydrogen ion implantation has been investigated as a function of ion dose and subsequent annealing temperature (400–700 °C). Isolation leakage currents as low as 8 &mgr;A mm−1at 6 V can be achieved between 100‐&mgr;m‐wide ohmic contacts separated by a 16 &mgr;m spacing. The isolation of these 1.8‐&mgr;m‐thick heterojunctions requires up to six different energy oxygen implants (40–400 keV) and three different energy proton implants (100–200 keV) with doses in the mid 1012cm−2range for O+and 5×1014cm−2for H+ions. Similar results can be achieved by substituting a MeV energy oxygen implant for the proton implants. The optimum post‐implant annealing temperature depends on the ion dose but is in the range 500–600 °C. The evolution of the sheet resistance of the implanted GaAs‐AlGaAs material with annealing is consistent with a reduction in tunneling probabilities of trapped carriers between deep level states for temperatures up to ∼600 °C, followed by significant annealing of these deep levels. Small geometry (2×9 &mgr;m2) HBTs exhibiting current gain of 44 and cutoff frequencyfTas high as 45 GHz are demonstrated using implant isolation.