The dc and rf performance of AlGaAs/GaAs high electron mobility transistors were measured after exposure to low pressure (5 mTorr) inductively coupled plasmas of H-2 or D-2 for various source (100-400 W) and chuck powers (10-100 W) and durations (1-4 min). The plasma exposure decreases drain-source current, increases gate leakage and increases the gate ideality factor, even at very low plasma powers and for short process durations. A number of physical and chemical effects are found to be responsible for the observed behavior, including preferential loss of As from the surface, creation of deep trap states by energetic ion bombardment and passivation of Si donors by formation of Si-H or Si-D neutral complexes.