A record-low Ohmic contact resistance is obtained on p++-GaN/p-GaN/GaN/AlGaN/GaN epitaxial structure designed for GaN CMOS circuit. Ni/Ag/NiO (1/120/6 nm) after 500 °C/360 s annealing in air atmosphere demonstrates an ultralow contact resistance (1.67 Ω mm) and the lowest specific contact resistivity (6.48 × 10–7 Ω cm2) reported so far. The employment of a NiO capping layer on Ni/Ag metal stack has improved the annealing thermal stability and electrical properties.