Microstructural changes in Ni/Au contacts to p-type GaN as a function of annealing temperature were investigated using X-ray diffraction, field emission scanning electron microscopy, and cross-sectional transmission electron microscopy combined with energy dispersive spectroscopy. The results obtained were used to interpret the electrical properties of Ni/Au ohmic contacts to p-type GaN. The contact resistivity decreased from 1.4 X 10(-2) to 6.1 X 10(-4) Omega cm(2) after annealing at 600 degreesC. The reduction in contact resistivity resulted from the dissolution of Ga atoms into the Au-Ni solid solution produced during annealing. Au atoms diffused to the: GaN substrate through grain boundaries and reacted with Ni atoms at the grain boundaries of Ni, producing an Au-Ni solid solution. This resulted in the evolution of microstructure with island-shaped Ni grains surrounded by the Au-Ni solid solution. At this stage, Ga atoms outdiffused from the GaN substrate and dissolved into the Au-Ni solid solution, leading to the generation of Ga vacancies below the contact. Thus, the net concentration of holes below the contact increased, and so the contact resistivity was reduced. When the sample was annealed at 800 degreesC, the Au layer completely reacted with the Ni layer, uniformly producing an Au-Ni solid solution containing Au-Ga and Ni-Ga compounds. Simultaneously, N atoms reacted with Ni, and produced cubic Ni4N in the vicinity of the metal/GaN interface. Consequently, N vacancies, acting as donors in GaN, were generated below the contact, leading to an increase of contact resistivity to 4.0 X 10(-2) Omega cm(2).