High　electron　mobility　transistors　(HEMTs)　based　on　GaN　have　a　promising　prospect　in　the　fields　of　high　frequency　and　high　power　due　to　their　advantages　of　high　output　power　density,　high　operating　frequency　and　high　operating　temperature.　At　present,　great　progress　has　been　made　in　material　growth　and　fabrication　processes　of　HEMTs.　However,　the　trapping　effect　produced　by　the　defect　limits　the　development　of　HEMTs.　In　this　paper,　the　surface　states,　interface　defects　and　bulk　defects　of　HEMT　devices　and　the　causes　of　these　defects　were　discussed.　The　phenomenon　caused　by　trap　effect　such　as　the　current　collapse,　gate　lag,　drain　lag　and　kink　effect　were　also　described　in　detail.　The　main　approaches　to　improve　the　defects　were　summarized　from　structure　design　and　process　design.　And　the　latest　progresses　focusing　on　the　four　aspects　of　the　capping　layer,　surface　treatment,　passivation　layer　and　field　plate　structure　were　reviewed.　Further　optimization　in　the　defect　of　the　GaN　based　HEMT　devices　was　explored　at　the　end.　©　2017,　Science　Press.　All　right　reserved.