Cu-metallized interconnects for GaN high-electron-mobility transistors (HEMTs) on Si substrate using a Pt/Cu diffusion barrier layer are investigated. Auger electron spectroscopy (AES) depth profiles indicate that the GaN/Au/Ti/Pt/Ti/Cu thin metal structure is thermally stable up to 350 °C. The Cu-metallized devices using the proposed metal scheme exhibited DC characteristics comparable to those of conventional Aumetallized GaN devices even after annealing at 350 °C for 30 min. No degradation in current with time was observed when the device was tested under 28 V high-voltage stress for 24 h at room temperature. These results indicate that the Cu-metallized airbridges with the Ti/Pt/Ti/Cu diffusion barrier layer can be used for GaN HEMT fabrication.