The high selectivity between the etch rates of GaAs and AlxGa1−xAs or AlAs obtained in plasmas containing both chlorine and fluorine is widely used to obtain accurate and uniform etching of molecular beam epitaxy grown high electron mobility transistor and (less commonly) metal‐semiconductor field‐effect transistor (MESFET) layers. The influence of gas residence time on the selectivity of GaAs to Al0.3Ga0.7As in CCl2F2reactive ion etching (RIE) at 50 V dc bias is investigated, finding that maximum selectivity (>4000 to 1) and maximum GaAs etch rate (1.75 μm/min) are achieved with a residence time for CCl2F2of between two and four seconds. No evidence of any significant damage to MESFET layers when overetching a 50 Å Al0.3Ga0.7As etch stop was observed at these RIE conditions by either Hall measurements or byCVandIVcharacterization of Schottky diodes. MESFETs with 0.2 μm gates recessed in CCl2F2show good dc and microwave performance.