A dead‐time‐free selective dry etching of GaAs/AlGaAs using mixed plasma of BCl3and CHF3has been developed. The selectivity of the etching for GaAs/AlGaAs is strongly dependent on the flow ratioR, whereR=(flow of CHF3)/(flow of CHF3+ flow of BCl3). It was also found that the angle of the sidewall can be varied from overhanging to tapered by controllingR. No undercutting was observed. Complete vertical recess structures with sub‐quarter micrometer width were obtained atR=40% where a selectivity of 30 was attained. The roughness of the etched surface was also strongly dependent onR. A smooth etched surface was obtained whereRwas larger than 40% even for bulk GaAs. Complete square‐shaped vertical recessed structures with sub‐quarter micrometer width were obtained in a GaAs/AlGaAs epitaxial wafer. This etching process is very applicable for the fabrication of GaAs/AlGaAs heterojunction devices such as the GaAs modulation doped field‐effect transistor and the heterojunction bipolar transistor.