Voltage　and　current　degrade　the　AlGaN/GaN　high　electron　mobility　transistors　(HEMTs)　under　on-state　stress.　To　determine　which　one　dominates　the　degradation,　two　on-state　stresses　which　have　equal　power　are　exerted　on　AlGaN/GaN　HEMTs:　high　voltage　and　low　current　on　sample　A,　low　voltage　and　high　current　on　sample　B.　In　the　former　stress,　drain-source　voltage　(V-DS)　is　28　V,　drain-source　current　(I-DS)　is　75　mA/mm.　In　the　latter　stress,　V-DS　is　14　V　and　I-DS　is　150　mA/mm.　The　package　temperatures　of　samples　A　and　B　are　kept　at　150　degrees　C.　The　samples　are　measured　every　24　hours,　with　an　extra　measurement　at　the　8th　hour　in　the　first　24　hours　(note　that　the　time　refers　to　the　stressing　time).　There　is　an　interval　of　4　hours　between　the　stressing　and　the　measurement.　The　device　parameters　include　drain-source　current-voltage　(I-DS-V-DS)　characteristics,　large-signal　parasitic　source　resistance　(R-S),　large-signal　parasitic　drain　resistance　(R-D),　and　transfer　characteristics　between　I-DS　and　gate-source　voltage　(V-GS).　The　emission　microscope　(EMMI)　is　used　to　study　the　leakage　current　after　experiment.　The　I-DS-V-DS　characteristics　of　sample　B　are　kept　constant　after　being　stressed,　while　that　of　device　A　shifts　downward　after　being　stressed.　R-S　of　sample　A,　R-S　of　sample　B,　and　R-D　of　sample　B　increase　slightly,　R-D　of　sample　A　increases　more　obviously　with　most　change　happening　in　the　first　8　hours.　I-DS-V-GS　characteristics　of　sample　B　kept　constant,　I-DS-V-GS　characteristics　of　sample　A　shift　downward.　The　changes　of　threshold　voltage　(V-GS(th))　is　obtained　from　the　transfer　characteristics,　and　it　is　similar　to　the　changes　of　transfer　characteristics.　The　V-GS(th)　magnitude　(absolute　value)　of　sample　A　decreases　obviously　while　that　of　sample　B　decreases　slightly.　The　measurements　show　that　the　device　under　low　voltage　and　high　current　stress　degrades　little　and　the　device　under　high　voltage　and　low　current　stress　degrades　more　obviously.　The　EMMI　images　show　that　the　leakage　of　sample　A　is　greater　than　that　of　sample　B.　The　analyses　of　the　parameter　change,　experiment　setting　and　EMMI　image　indicate　that　the　voltage,　rather　than　the　current,　dominates　the　degradation　for　AlGaN/GaN　HEMTs.　The　influences　of　hot　electron　effect,　gate　electron　injection,　and　self-heating　are　recoverable,　and　they　vanish　in　the　interval　between　the　stressing　and　the　measurements.　The　permanent　degradation　of　device　parameter　is　caused　by　the　inverse　piezoelectric　effect　induced　by　high　electrical　field　between　the　gate　and　the　drain.　Besides,　it　is　found　that　sudden　failure　without　precursor　is　easy　to　happen　to　the　device　under　low　voltage　and　high　current　stress.　The　microscope　image　of　damaged　area　shows　that　the　failure　is　due　to　hot　spot　induced　by　high　current　density.