Ti/Al/Ni/Cu ohmic contact for AlGaN/GaN structure has been fabricated. The Ni layer played an important role in achieving low specific contact resistance (rc), smooth morphology, and excellent edge acuity. With a 50-Å Ni layer, a rc of 1.35 × 10-6 Ω-cm 2 and a root-mean-square roughness of 7.65 nm have been realized. The characterization results indicated that no evidence of Cu diffusion into the semiconductor layers. The formation of Al-Cu and Ti-Cu alloys might have confined the Cu within the ohmic metal. In the absence of gold, the surface roughening caused by Au-Al alloy in conventional Ti/Al/Ni/Au structure was also prevented.