The effects of oxygen plasma conditions on the performance of InGaAs/InP heterostructure bipolar transistors (HBTs) have been studied by comparing the HBT's characteristics, such as current gain and breakdown voltage, before and after treatment. The base-emitter junction characteristics of InGaAs/InP HBTs were unaffected by oxygen plasma treatments, for exposure periods of up to 10 min and rf powers <200 W. Higher rf powers degrade the current gain. In contrast, the base-collector junction was degraded even for short periods and low rf powers; the base-collector leakage current increased and the breakdown voltage decreased. Further reduction of the current gain was observed when dielectric films were deposited by plasma-enhanced chemical vapor deposition on oxygen treated devices. The dielectric film does not reduce the gain of devices that were not treated by oxygen plasma.