The stabilities of the electrical properties and microstructures of Pt, PtAu, NiAu, and TaTi ohmic contacts after contact formation were studied. The Pt and PtAu contacts annealed in an O2 and N2 mixed gas ambient had the specific contact resistance (ρc) of high-10-3 Ω cm2 and strong adhesion to the GaN substrates. The NiAu contact annealed in the partial O2 ambient had poor adhesion to the GaN, although the ρc value of low-10-3 Ω cm2 was obtained. The TaTi contact had the lowest ρc values of less than 10-4 Ω cm2. After contact formation, the Pt and NiAu ohmic contacts prepared by annealing in the partial O2 ambient showed the excellent electrical and microstructural stabilities during room temperature storage and current injection. However, the contact resistance of the TaTi contact prepared by annealing at 800 °C increased during room temperature storage, and the mechanical failure of the contact was also observed after injecting current as low as 4 kA/cm2. From the present experiments, it was concluded that the Pt and PtAu contacts were the most reliable ohmic contact materials for p-GaN among four contact metals. © 2001 American Institute of Physics.