The changes in atomic composition and surface states at the surface of AlGaN caused by photowashing treatment were studied by synchrotron radiation photoemission spectroscopy (SRPES). The effect of surface treatment on the electrical properties of AlGaN/GaN HFETs was examined by both current-voltage (I-V) and transconductance dispersion measurements. From these, the origin of changes in electrical properties caused by the photowashing treatment was proposed.