The effects of photowashing treatment on the electrical properties of an AlGaN/GaN heterostructure field-effect transistor were studied by observing changes in atomic composition and band bending at the surface of AlGaN through synchrotron radiation photoemission spectroscopy. The surface treatment produced group-III oxides on the surface of AlGaN, leaving N vacancies behind. Both the gate leakage current (I-GD) and drain current (I-DS) simultaneously decreased after the treatment. The decrease of I-GD was due to a delay in movement of the electrons, namely, trapping and detrapping. The trapped electrons reduced the effective channel thickness, and led to the reduction of IDS