The effect of H-2 and D-2 plasma exposure on the dc and rf performance of implanted GaAs metal semiconductor field effect transistors (MESFETs) was examined as a function of the plasma parameters. This process simulates a worst-case scenario during the initial stages of dielectric passivation deposition onto the MESFET. The fact that the active channel is so close to the surface makes the MESFET extremely sensitive to effects such as hydrogen passivation, creation of midgap trap states or formation of donor-like surface states through loss of As. A wide variety of both dc (gate current, drain-source current, channel sheet resistances, threshold voltage, reverse breakdown voltage, transconductance and gate ideality factor) and rf (f(T), f(MAX), gate-source resistance, drain-source resistance, drain and source resistance, intrinsic transconductance, gate-source, gate-drain, and drain-source capacitance and gate-source resistance) parameters were measured as a function of changes in ion energy and flux in the plasma.