Surface passivation is a key issue in compound semiconductor device technology. The high density of surface states on unpassivated surfaces can lead to excessive non-radiative recombination at the surface, affecting optical devices, or provide leakage and low-field breakdown in electronic devices. Our previous studies on low energy, low-dose hydrogen ion treatment carried out at room temperature showed long-term improvement in the optical properties of near surface quantum wells. We have accordingly applied this process to GaAs-based pseudomorphic HEMTs (PHEMT) in order to improve their power performance. Although our process is designed so that the hydrogen reactions are confined to the surface of the substrate, a critical factor in the success of this treatment is the extent of in-diffusion of the hydrogen, and the possibility of dopant passivation. PHEMT structures were hydrogenated at various conditions and both Hall mobility and carrier density were monitored. For a low hydrogen ion dose (3 × 1016 cm−2) at 80 eV energy, some degradation of Hall mobility and carrier density was noted after the treatment, but full recovery of both parameters was achieved after a 400°C thermal anneal. Much higher hydrogen doses resulted in severe degradation of mobility and carrier density, which were only partially recovered after thermal anneal. Measurements on actual PHEMT devices showed an approximately 15% decrease in the transconductance, and in addition, a 60% decrease in the gate-to-drain leakage current after irradiation with 80 eV hydrogen ions at a dose of 3 × 1016 cm−2. The decrease of the leakage current indicates that passivation is taking place. The decrease of the transconductance suggests that hydrogen may be diffusing into the regions of the dopants. Optimization of the hydrogenation parameters should allow leakage reduction without sacrifice of transconductance.