In　this　work,　we　developed　the　voltage-transient　method　to　characterize　the　properties　of　traps　in　AlGaN/GaN　high-electron-mobility　transistors　(HEMTs)　in　the　OFF-state.　By　monitoring　the　drain　voltage　transients　of　the　HEMTs　at　various　temperatures,　three　types　of　trapping　mechanism　were　identified:　1)　buffer　charge　trapping,　which　occurred　on　a　timescale　of　approximately　1　ms;　2)　charge　trapping　in　the　AlGaN　layer　at　the　gate-drain　edge　with　the　energy　level　(E-a)　of　approximately　0.54　eV;　and　3)　surface　charge　trapping　with　E-a　of　approximately　0.28　eV.　In　particular,　we　extracted　accurate　amplitudes　for　the　first　two　trapping　behaviors　and　studied　the　dependence　of　the　trapping　effect　on　the　filling　bias　conditions.　The　results　showed　that　the　buffer　charge　trapping　was　primarily　affected　by　the　drain　voltage,　whereas　the　charge　trapping　on　the　drain　side　of　the　gate　was　affected　by　both　the　drain　and　gate　voltages;　these　results　were　verified　by　drift-diffusion　simulations.　In　addition,　we　observed　the　third　trapping　behavior,　which　was　apparent　in　the　measurement　window　beyond　308　K,　thus　demonstrating　the　advantages　of　our　method　for　correctly　and　effectively　monitoring　the　changes　in　the　peaks　in　the　time　constant　spectrum.