GaAs-based metal semiconductor field effect transistors (MESFETs), heterojunction bipolar transistors (HBTs), and high electron mobility transistors (HEMTs) have been exposed to ECR SiH4/NH3 discharges for deposition of SiNx passivating layers. The effect of source power, rf chuck power, pressure, and plasma composition have been investigated. Effects due to both ion damage and hydrogenation of dopants are observed. For both HEMTs and MESFETs there are no conditions where substantial increases in channel sheet resistivity are not observed, due primarily to (Si-H)degrees complex formation. In HBTs the carbon-doped base layer is the most susceptible layer to hydrogenation. Ion damage in all three devices is minimized at low rf chuck power, moderate ECR source power, and high deposition rates.