The　phenomenon　of　self-changing　on　the　device　parameters　and　characteristics　after　a　step　voltage　stress　was　applied　to　the　gate　is　studied　in　AlGaN/GaN　high　electron　mobility　transistors.　The　device　was　measured　every　5　min　after　the　stress　was　removed.　The　large-signal　parasitic　source　(drain)　resistance,　transfer　characteristics,　threshold　voltage,　drain-source　current,　gate-source　(drain)　reverse　current-voltage　characteristics　changed　spontaneously　after　the　removal　of　the　stress.　The　time　constant　of　the　self-changing　was　about　25-27　min.　The　gate-source　(drain)　capacitance-voltage　characteristics　were　constant　during　this　process.　Electrons　were　trapped　by　the　surface　states　and　traps　in　the　AlGaN　barrier　layer　when　the　device　was　under　stress.　The　traps　in　the　AlGaN　barrier　layer　then　released　electrons　in　less　than　10　s.　The　surface　states　released　electrons　continuously　during　the　entire　measurement　stage,　leading　to　the　self-changing　of　mearsurement　result.　©　2015　Chinese　Institute　of　Electronics.