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Narrow gate margin has been the critical limiting factor for the p-gate normally-off GaN HEMTs, imposing significant challenges in both gate-drive design and gate reliability. In this work, by developing dopant-free p-type polarization doping technique in composition-graded InGaN layer, high-quality Schottky contact between the gate metal and cap layer was demonstrated, achieving excellent gate current blocking performance (10-6 mA/mm) after the turning-on of the gate heterojunction structure. Resultantly, normally-off GaN HEMTs with enhanced gate breakdown voltage up to 15.2 V was realized, being especially beneficial for the simplification of gate drive design and the safe operation of gate terminal.