Al, An, Ti/Al and Ti/Au contacts were prepared on n-GaN and annealed up to 900 degrees C. The structure, phase and morphology were studied by cross-sectional transmission and scanning electron microscopy as well as by X-ray diffraction (XRD), the electrical behaviour by current-voltage measurements. It was obtained that annealing resulted in interdiffusion, lateral diffusion along the surface, alloying and bowling up of the metal layers. The current-voltage characteristics of as-deposited Al and Ti/Al contacts were linear, while the Au and Ti/Au contacts exhibited rectifying behaviour. Except the Ti/Au contact which became linear, the contacts degraded during heat treatment at 900 degrees C. The surface of Au and Ti/Au contacts annealed at 900 degrees C have shown fractal-like structures revealed by scanning electron microscopy. Transmission electron microscopy and XRD investigations of the Ti/Au contact revealed that Au diffused into the n-GaN layer at 900 degrees C. X-ray diffraction examinations showed, that new Ti2N, Au2Ga and Ga3Ti2 interface phases formed in Ti/Au contact at 900 degrees C, new Ti2N phase formed in Ti/Al contact at 700 and 900 degrees C, as well as new AlN interface phase developed in Ti/Al contact at 900 degrees C. (c) 2006 Elsevier B.V. All rights reserved.