Effects of photowashing treatment on electrical properties of GaAs metal-semiconductor field-effect transistors (MESFETs) were investigated using x-ray photoemission spectroscopy. The binding energy of the Ga-As bond shifted toward lower binding energies and the ratio of Ga/As was increased, namely the formation of the Ga-rich surface. This suggests that acceptor-type defects Ga-As(-) were produced by the photowashing treatment and the level for Fermi energy pinning at the surface moved to acceptor states. The Fermi energy pinning caused by Ga-As(-) results in an increase of the depletion layer width at the ungated region of the MESFET via the increase of band bending from the surface. Therefore the drain current density at a positive gate bias and the leakage current at cate-to-drain were simultaneously reduced.