Chemical and electrical properties of the surfaces of GaN and GaN/AlGaN heterostructures were systematically investigated by x-ray photoelectron spectroscopy (XPS), capacitance-voltage, and current-voltage measurements. From in situ XPS study, relatively smaller band bending of 0.6 eV was seen at the GaN (2 X 2) surface grown by radio frequency-assisted molecular beam epitaxy on the metalorganic vapor phase epitaxy GaN template. After exposing the sample surface to air, strong band bending took place at the surface. The surface treatment in NH4OH solution and N-2 plasma was found to reduce the surface Fermi level pinning. Surface passivation process of GaN utilizing SiNx film by electron-cyclotron-resonance assisted plasma chemical vapor deposition (ECR-CVD) achieved low interface state density, 2 X 10(11)cm(-2) eV(-1). No pronounced stress remained at the SiNx/GaN interface, which was confirmed by Raman spectroscopy. The present NH4OH/ECR-N-2 plasma treatment was also found to be effective in realizing well-ordered and nearly oxide-free surface of a GaN/AlGaN heterostructure. The subsequent passivation process using the ECR-CVD SiNx, film enhanced the drain current in the gateless GaN/AlGaN high electron mobility transistor. A surface passivation process utilizing an ultrathin Al-oxide layer reduced leakage current and improved gate controllability of two-dimensional electron gas in the Schottky gate contact fabricated on the GaN/AlGaN heterostructures.