Low-damage, low-temperature, and easy-to-implement hydrogen-plasma-based termination is attractive for fabricating implantation- and etching-free GaN power p-n diodes. This work investigates in detail the hydrogenation process and unveils the critical role of thermal annealing. A subsequent thermal annealing is key to thermally driving down hydrogen to fully hydrogenate p-GaN to form the termination. The devices showed a specific on-resistance of 0.4 mΩ cm2 and a breakdown voltage (BV) of ∼1.4 kV. They also exhibited improved BV compared with mesa-etched devices. High temperature performance was also investigated. These results can serve as important references for future developments of GaN power electronics.