We have developed a novel selective citric acid-based etchant used for GaAs over AlGaAs, which consists of citric acid,NH4OH,andH2O2. The etching rate ratio of GaAs toAl0.15Ga0.85Aswas as high as 80 by optimizing thepH and citric acid/H2O2ratio. The etch stop mechanism was investigated using x-ray photoelectron spectroscopy. The etching is stopped at the AlGaAs surface due toAl2O3formation, whose density is higher than that on a nonselectively etched surface. This selective citric acid-based etchant was applied to the fabrication of GaAs/AlGaAs heterostructure field effect transistors (HFETs). The threshold voltages of the HFETs exhibit excellent uniformity (the standard deviation of 32 mV across the 3 in. wafer), demonstrating the applicability of this selective etchant to the gate recess process.