Effects of device processing on chemical and electronic properties of AlGaN surfaces were investigated. The X-ray photoelectron spectroscopy analysis showed serious deterioration such as stoichiometry disorder and nitrogen deficiency (N deficiency) at the AlGaN surfaces processed by high-temperature annealing, H-2-plasma cleaning, dry etching in CH4/H-2/Ar plasma and deposition Of SiO2. This resulted in high density of surface states at the processed AlGaN surface. Furthermore, the N deficiency introduced a localized deep donor level related to N vacancy (V-N) at AlGaN surfaces. Such electronic states governed by a V-N-related deep donor and surface state continuum can cause strong Fermi level pinning at the AlGaN surface, reduction of the barrier height and excess leakage currents at the AlGaN Schottky interface and serious drain current collapse in AlGaN/GaN heterostructure field effect transistors. The SiNx- or Al2O3-based passivation scheme with a combination of a remote N-2-plasma treatment was found to be effective in suppressing formation Of VN-related surface defects at AlGaN surfaces. (C) 2004 Elsevier B.V. All rights reserved.