Interface properties of metal/GaN, AlGaN Schottky structures formed on oxide-etched and intentionally oxidized surfaces are studied by I-V-T and C-V-T measurements. The characteristics support the previously proposed "surface patch" model, where the patches with low Schottky barrier heights (SBHs) cause a leakage current. The true SBHs were obtained from high-temperature I-V curves with S-values of 0.28 and 0.20 for n- and p-GaN samples, respectively. Thermally oxidized GaN surfaces represented a reduction of the effective SBH for Ni/n-GaN sample. I-V-T characteristics showed that the leakage current due to surface patches is comparable with that of the oxide-etched sample. The current drift was less than 0.1% within the range of 1-10(4) s. On the other hand, anodized GaN surface led to increase of the SBH up to 0.3 eV, owing to formation of a relatively thick oxide layer. The oxide-etched Au/n-Al0.2Ga0.8N sample on HEMT wafer represented a considerably reduced effective SBH at RT, because of a tunneling leakage current. However, the true SBH was deduced to be 1.4 eV from the I-V-T characteristics. The annealing in N-2 flow was again effective to improve the interface properties. (C) 2003 Elsevier Science B.V. All rights reserved.