We have developed a novel selective citric acid-based etchant used for GaAs over AlGaAs, which consists of citric acid NH4OH, and H2O2. The etching rate ratio of GaAs to Al0.15Ga0.85 was as high as 80 by optimizing the pH and citric acid/H2O2 ratio. The etch stop mechanism was investigated using x-ray photoelectron spectroscopy. The etching is stopped at the AlGaAs surface due to Al2O3 formation, 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.