We　experimentally　investigated　how　self-heating　affected　the　transient　response　of　drain　current　in　AlGaN/GaN　high-electron-mobility　transistors.　Since　drain　current　depends　on　temperature,　we　measured　the　drain　current　to　assess　the　transient　temperature　rise　of　the　active　area.　Based　on　the　transient　temperature　rise,　we　used　the　structure　function　method　to　analyze　the　resistance　to　temperature　rise.　This　can　be　used　to　extract　the　chip　temperature　rise　even　for　a　packaged　device.　We　verified　the　proposed　electrical　method　by　comparing　its　results　to　those　measured　by　the　forward-Schottky-junction-characteristic　method,　revealing　good　agreement.