The influence of surface chemical treatments and of deposition of a SiO 2 surface passivation layer on carrier distributions and mobility in Al x Ga 1Ϫx N/GaN heterostructure field-effect-transistor epitaxial layer structures is investigated. Surface chemical treatments are found to exert little influence on carrier distribution and mobility. Deposition of a SiO 2 surface passivation layer is found to induce an increase in electron concentration in the transistor channel and a decrease in mobility. These changes are largely reversed upon removal of the SiO 2 layer by wet etching. These observations are quantitatively consistent with a shift in Fermi level at the Al x Ga 1Ϫx N surface of approximately 1 eV upon deposition of SiO 2 , indicating that the Al x Ga 1Ϫx N/SiO 2 interface has a different, and possibly much lower, density of electronic states compared to the Al x Ga 1Ϫx N free surface.