The influence of surface chemical treatments and of deposition of a SiO2 surface passivation layer on carrier distributions and mobility in AlxGa1-xN/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 SiO2 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 SiO2 layer by wet etching. These observations are quantitatively consistent with a shift in Fermi level at the AlxGa1-xN surface of approximately 1 eV upon deposition of SiO2, indicating that the AlxGa1-xN/SiO2 interface has a different, and possibly much lower, density of electronic states compared to the AlxGa1-xN free surface. © 2001 American Institute of Physics.