The performance of GaAs power MESFET's using backside copper metallization has been evaluated. 10 nm Ta metal was used as the diffusion barrier between GaAs and Cu for copper film metallization in this study. Microstructural characterization shows that the Cu/Ta films with GaAs remained stable up to 400 degreesC, indicating that Ta is a good diffusion barrier for Cu in GaAs MESFET's. A copper metallized 6 mm power MESFET was thermal stressed to test the device stability. After annealing at 200 degreesC for 3 h, the devices showed very little degradation in power performance, and the thermal resistance of the device was 65 degreesC mm/W with 1.4 W/mm DC input power. Results in this study demonstrate that the feasibility of using Cu/Ta films for the backside metallization of GaAs power devices with stable electrical and thermal characteristics. (C) 2002 Elsevier Science Ltd. All rights reserved.