To improve etching controllability and the yield without reducing selectivity performance by setting the pH of a wet etching liquid of citric acid- hydrogen peroxide-ammonia-water system to a specific range in GaAs/AlGaAs hetero structure and mixing an ascorbic acid to the etching liquid.

An HEMT element is in three-stage recess offset gate structure and a recess process is performed for three times until a gate is formed. More specifically, by utilizing a resist mask, first- and second-stage recess etching can be made. As the etching liquid, citric acid-hydrogen peroxide-ammonia-water system is used. Then, pH is adjusted to a range of 6-8. Then, a gate mask 15 is formed by multiplayer structure and electron beam lithography, and insulation film dry etching using an insulation film 12 and a flattening film 13, and the three-stage gate recess machining is made. More specifically, an n-GaAs layer is eliminated by selective etching and an opening is formed by side etching.