Damage introduction into GaAs/AlGaAs high electron mobility transistor (HEMT) structures during either pattern transfer or gate mesa etching steps has been characterized. For O2reactive ion etching of the polydimethylglutarimide (PMGI) planarizing layer in a trilevel resist mask, the threshold dc bias for observable damage introduction in the AlGaAs donor layer is ∼200 V. This threshold bias for damage is a function of the PMGI overetch time and for extended times (>10 min), a decrease in saturated drain‐source current (IDSS) of the HEMTs can be detected for oxygen ions accelerated through a bias of ∼150 V. The use of combined electron cyclotron resonance (ECR)/radio frequency (rf) O2discharges enhances the PMGI etch rate without creating additional damage to the device, and 0.25‐μm gate widths have been demonstrated. Gate mesa formation by etching the GaAs cap with CCl2F2/O2or CH4/H2/Ar discharges is shown to produce damage in the underlying AlGaAs at dc negative biases of 125–150 V. In addition, substantial hydrogen passivation of the Si dopants occurs for the latter mixture. Recovery of the initial carrier concentration in the damaged HEMT occurs around 400 °C, provided the maximum dc biases were kept to ≤400 V during the dry etch step. Finally, changes in the surface chemistry of the exposed AlGaAs after various post‐RIE processing steps have been monitored by x‐ray photoelectron spectroscopy. Complete removal of AlF3was obtained only after careful H2O and NH4OH: H2O rinsing, while chlorides are removed by H2O alone.