The effects of N-2 and Ar plasma exposure on GaAs/AlGaAs heterojunction bipolar transistors (HBT) were investigated with all inductively coupled plasma (ICP) system. The plasma damage was characterized by evaluating device dc current gain and base collector reverse breakdown voltage as functions of ICP source power and rf chuck power. Recombination centers, surface damage, and deep level defects created by the N-2 and Ar discharges are the dominant mechanisms for device degradations. Exposure time and chamber pressure are also critical to device de characteristics. We also found that HBT devices degrade more seriously in N-2 plasma than in Ar plasma.