In　this　article,　the　failure　behavior　and　mechanism　of　p-GaN　gate　AlGaN/GaN　high-electron-mobility　transistors　(HEMTs)　in　the　third　quadrant　under　repetitive　surge　current　stress　are　investigated.　Repetitive　stress　tests　with　different　surge　current　peak　values　I-peak　are　carried　out.　At　high　I-peak　,　as　the　stress　cycle　increases,　the　evolution　of　the　peak　value　of　surge　voltage　induced　by　surge　current　shows　an　obvious　turning　phenomenon　and　a　significant　increasing　trend.　When　the　surge　voltage　reaches　a　certain　value,　the　gate-to-source　breakdown　occurs,　and　then,　the　device　is　burned　out.　We　propose　that　the　degradation　of　the　third-quadrant　conduction　characteristics　results　in　the　change　of　surge　voltage,　and　excessive　electric　field　intensity　induced　by　high　gate-to-drain　voltage　V(GD　)causes　the　gate　Schottky　junction　breakdown.　It　is　confirmed　by　further　experiments,　electrical　performance　characterization,　and　simulation.　Inconsistent　degradation　of　the　two-dimensional　electron　gas　(2DEG)　channel　in　various　regions　causes　the　aforementioned　turning　phenomenon.　As　the　stress　cycle　increases,　the　channel　degradation　in　the　gate-to-source/drain　access　region　occupies　a　dominant　position.　In　this　situation,　the　V-GD　increases　continuously,　which　will　enhance　the　tunneling　effect　at　the　Schottky　junction,　until　breakdown　occurs.　Besides,　the　device　shows　better　surge　current　reliability　at　higher　gate-to-source　voltage.　These　results　provide　important　insights　into　the　improvement　of　GaN　HEMTs　reliability.