In　this　article,　the　synergistic　effect　of　negative　gate　bias　and　radiation　on　SiC　metal-oxide-semiconductor　fieldeffect　transistors　(MOSFETs)　is　investigated.　Under　negative　gate　bias　(-5,　-10,　and　-15　V),　a　slight　decrease　in　the　threshold　voltage　(Vth)　of　non-irradiated　SiC　MOSFETs　is　observed.　The　variation　of　trapped-oxide　charge　(Not)　and　SiC/SiO2　interface　trap　charge　(　Nit)　can　be　obtained　through　the　subthreshold　swing　and　mid-gap　voltage　method.　Although　the　Vth　of　non-irradiated　SiC　MOSFETs　remains　unchanged,　significant　increases　in　Nit　and　Not　are　observed　due　to　the　opposite　effect　of　Not　and　Nit　on　Vth.　This　phenomenon　is　attributed　to　holes　drifting　to　the　interface　and　tunneling　into　the　oxide　layer　under　the　negative　gate　bias.　It　is　found　through　the　Cg-Vgs　characteristic　curves　that　holes　tunneling　into　the　oxide　layer　are　mostly　from　the　channel　region.　For　irradiated　SiC　MOSFETs,　a　large　number　of　radiation-induced　holes　exist　in　the　oxide　layer　above　the　channel　region　and　JFET　region,　resulting　in　a　decrease　in　Vth,　which　is　mainly　caused　by　an　increase　in　Not.