Ohmic contacts with a low contact resistance and low surface roughness are essential to ensure the optimal device performance of AlGaN/GaN high electron mobility transistors. A tantalum metal layer and Si3N4 or SiO2 thin films deposited by plasma enhanced chemical vapor deposition are here used as an effective diffusion barrier and protection layer, respectively, in the standard Ti/Al/metal/Au ohmic metallization scheme to obtain high quality ohmic contacts. The Si3N4 or SiO2 thickness significantly affects the surface roughness. The optimal Ti/Al/Ta/Au ohmic contact is prepared with a 40-nmthick Ta barrier layer and a 50-nm thick SiO2 rapid thermal annealing protection layer. Compared with conventional ohmic contact stacks, the optimal contact exhibits a low contact resistance of approximately 0.05 Omega . mm and a nano-scale surface roughness, with a root-mean-square deviation of approximately 9 nm. Lastly, the ohmic contact was used to optimize AlGaN/GaN high electron mobility transistor with a 0.5-mu m gate length, exhibiting a maximum drain current density of 760 mA/mm, a peak transconductance of 420 mS/mm, a unity-gain cut-off frequency of 24.6 GHz, and a maximum frequency of oscillation of 45.4 GHz. The X-band power performance has an output power density of 7.4 W/mm, an output power of 32 dBm, and a power added efficiency of 38% at an input power of 21 dBm. (c) 2013 Elsevier B.V. All rights reserved.