In this letter, we investigate a metalization process for reducing the contact resistance on undoped GaN layers. The Si metal source was diffused successfully into the GaN films by using SiOx/Si/GaN/Al2O3 structures. By using a high-temperature annealing process, we diffused and activated the Si atoms into the GaN film. This caused a heavy doped n-type GaN layer to be formed near the GaN surface. Under high temperatures, such as a diffusion process at 1000°C, the as-deposited Ni/Al/Ti contact had good ohmic properties and a low specific contact resistivity (ρc) of 1.6×10-3 Ω cm2. Rapid thermal annealing the contact at 800°C for 30 s caused the ρc to decrease rapidly to 5.6×10-7 Ω cm2. The Ni/Al/Ti contact characteristics on the GaN films diffused at various temperatures are also discussed.