The etching characteristics of GaAs and Al0.22Ga0.78As in citric acid/hydrogen peroxide/ammonium hydroxide etching solution were studied. The selectivity of GaAs to Al0.22Ga0.78As was as high as 200 at 20 degrees C (510 at 0 degrees C) by optimizing the pH and citric acid/hydrogen peroxide ratio. To our knowledge, this is the best result. The etch rate of GaAs is 1000 Angstrom/min and permits a good control of the etched depth. The effects of the etching solution temperature and the doping level of GaAs on the selectivity were determined. The solution stability study over the time proves that the process is very reliable. This wet etching was applied to the gate recess etching of a heterojunction field effect transistor (HFET). Observations and measurements by electron beam microscopy are presented for short gate lengths. They show a very good surface morphology and that the gate recess width is independent of the gate length. It only depends on the etching time. The breakdown voltage value can be adjusted by the gate recess width. The possibilities of this wet etching are consequently demonstrated to be particularly appropriate to the realization of millimeter power HFETs.