The use of a reactive ion etch process with high selectivity for etching GaAs layers and stopping on underlying AlGaAs layers is reported. A key feature is the high degree of control that can be maintained over the GaAs‐to‐AlGaAs selectivity by changing the SF6/SiCl4ratio while keeping other etch parameters such as pressure, dc bias, or power constant. Values of the GaAs‐to‐AlGaAs selectivity can be varied from 1 to 500. Diluent gases such as helium can be added to reduce the etch rate, which is important to avoid damage to sensitive device structures and to the overhang profile of resist materials used for liftoff metallization. The application of this etch process for high electron mobility transistor fabrication is discussed.