Method of manufacturing semiconductor device

A method of manufacturing a semiconductor device comprising:

• forming an electron transit layer above a substrate;
• forming an electron supply layer above the electron transit layer;
• forming a protective film above the electron transit layer;
• forming a zinc oxide film above the protective film;
• forming a sacrifice layer above the zinc oxide film;
• forming a first opening and a second opening in the sacrifice layer and the zinc oxide film;
• forming a third opening connecting to the first opening and a fourth opening connecting to the second opening in the protective film, the electron supply layer, and the electron transit layer;
• forming, by acid treatment using a weakly acidic solution, a first gap in a first portion exposed to the first opening of the zinc oxide film, and a second gap in a second portion exposed to the second opening of the zinc oxide film;
• forming, after the acid treatment, a source region containing a first conductive impurity on a bottom surface of the third opening and a drain region containing the first conductive impurity on a bottom surface of the fourth opening;
• and
• removing the zinc oxide film after forming the source region and the drain region.