Abstract: A AlGaN/GaN HEMT device structure with polarization modulation layer is proposed for the low breakdown voltage of the conventional GaN HEMT, which is caused by the leakage of the GaN buffer and the concentration effect of electric field at closing to the drain under the gate. The structure contains two characteristics. First, Al_0.05Ga_0.95N with wider energy band gap is used instead of GaN as buffer layer material to reduce the leakage of the buffer layer and increase breakdown voltage. Second, a polarization modulation layer is epitaxially grown on the AlGaN barrier layer between the gate and the drain, which is formed by Al_xGa_1-xN with reverse gradient of Al composition(0<x < 0.26). According to the theory of polarization doping, AlGaN materials with a graded Al composition will produce the hole due to the uneven polarization charge. The hole in the polarization modulation layer expands the area of the depletion region, raises the electric field between the gate and the drain, relieves the electric field concentration near the gate. The proposed structure is based on the standard of power device figure of merit, and is verified by Sentaurus TCAD. The breakdown voltage, specific on-resistance and FOM value of the proposed structure are 645 V, 1.09 mΩ ·cm² and 382 MW ·cm^-2.Compared with the double heterojunction structure, the breakdown voltage is increased by 339%, while the specific on-resistance is only increased by 0.38 mΩ ·cm².