The effect of a Au overlayer on the thermal stability of Pt transparent ohmic contacts on p-type GaN was investigated. The contact resistivity for the Pt contact (100 Angstrom) rapidly increased from 3.7 x 10(-4) to 8.8 Omega cm(2) after annealing for 16 h at 550degreesC. However, no degradation in the contact resistivity was found by incorporating the Au overlayer (50 Angstrom) on the Pt contact (50 Angstrom). The degradation of the Pt contact originated from the out-diffusion of N atoms producing N vacancies below the contact, resulting in a decrease in the net concentration of holes. For the Pt/Au contact, the Au layer acts as a sink for Ga atoms, causing the production of Ga vacancies below the contact. This suppresses the decrease of net concentration of holes, leading to a thermally stable ohmic contact.