We　propose　a　specialized　gate-drain　separation　structure　for　use　in　investigation　of　the　dynamic　behavior　of　the　thermal　transport　characteristics　in　AlGaN/AlN/GaN　heterojunction　transistors.　Using　this　structure,　the　influence　of　the　two-dimensional　electron　gas　(2DEG)　on　the　horizontal　heat　transfer　in　these　transistors　was　identified　experimentally.　A　temperature　delay　(i.e.,　the　difference　in　temperature　at　the　same　time)　of　1.3　degrees　C,　which　accounts　for　8.6%　of　the　total　temperature　rise,　was　calculated　to　be　caused　by　a　0.3　pm　x　150　mu　m　2DEG　depleted　region.　In　addition,　the　temperature　transients　show　that　the　2DEG　can　accelerate　the　temperature　to　the　steady　state.　Infrared　measurements　were　also　carried　out　to　benchmark　the　results.　In　particular,　the　sample　structure　allows　the　measurement　of　a　complete　thermal　process　that　includes　both　heating　and　cooling　without　interruption.　Based　on　a　comparison　of　the　two　processes,　we　demonstrated　and　corrected　an　error　in　the　temperature　rise　that　occurred　when　the　Schottky　junction　voltage　was　used　as　the　temperature-sensitive　parameter.