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　(Ipeak)　are　carried　out.　At　high　Ipeak　,　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　(VGD　)　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　VGD　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.　　©　1963-2012　IEEE.