Properties of metal/n-GaN Schottky and PCVD-SiO(2)n-GaN interfaces, including the effect of thermal annealing, have been investigated by I-V-T, C-V and C-t measurements. I-V-T characteristics of the Schottky diodes indicate that reported large discrepancies in the Schottky barrier height (SBH) between I-V and C-V measurements, scattered Richardson constants, and I-V shoulders are well explained by superposed cut-rent arising from "surface patches" with low SBHs. A low-temperature annealing in N-2 is highly effective to improve the uniformity of the SBH. For the as-deposited SiO2/n-GaN sample, the interface Fermi level locates at about 0.2 eV from the conduction band edge under thermal equilibrium condition, and the value increased to 0.5 eV after annealing in H-2 at 500 degrees C. A relatively small band bending was also confirmed from C-t measurement. The minimum interface state density of 2 x 10(11) cm(-2) eV(-1) for the as-deposited interface reduced to one-third after the annealing.