The effects of regions made semi‐insulating by ion implantation on the base current of a GaAs/AlGaAs heterojunction bipolar transistor (HBT) have been investigated. The implanted region, which overlaps both ends of the emitter and base fingers in the device structures studied, is intended to reduce parasitic capacitances. It is shown that the current between the base and emitter fingers at these implant isolation edges can be divided into two distinct components: a leakage current which is linear with bias and a recombination current which has an ideality of 2.0. Both of these parasitic current components reduce the HBT current gain, particularly at low current densities. A measurement technique was developed to separate these two currents using dc current‐voltage characteristics of the device.