In this paper, we present a technique to fabricate normally off GaN-based high-electron mobility transistor (HEMT) by sputtering and post-annealing p-NiOx capping layer. The p-NiOx layer is produced by sputtering at room temperature and post-annealing at 500 degrees C for 30min in pure O-2 environment to achieve high hole concentration. The Vth shifts from -3 V in the conventional transistor to 0.33 V, and on/off current ratio became 10(7). The forward and reverse gate breakdown increase from 3.5 V and -78V to 10V and -198V, respectively. The reverse gate leakage current is 10(-9) A/mm, and the off-state drain-leakage current is 10(-8) A/mm. The V-th hysteresis is extremely small at about 33 mV. We also investigate the mechanism that increases hole concentration of p-NiOx after annealing in oxygen environment resulted from the change of Ni2+ to Ni3+ and the surge of (111)-orientation. (C) 2017 Elsevier B.V. All rights reserved.