The relation between Schottky gate leakage current and the breakdown voltage of AlGaN-GaN high-electron mobility transistors (HEMTs) is discussed based on the newly introduced simple, yet useful, surface defect charge model. This model represents the leakage current caused by the positive charge in the surface portion of AlGaN layer induced by process damage such as nitrogen vacancies. The new model has been implemented into a two-dimensional device simulator, and the relationship between the gate leakage current and the breakdown voltage was simulated. The simulation results reproduced the relationship obtained experimentally between the leakage current and the breakdown voltage. Further simulation and experiment results show that the breakdown voltage is maintained even if the defect charge exists up to the defect charge density of 2.5×1012 cm-2, provided the field plate structure is adopted, while the breakdown voltage shows a sudden drop for the defect density over 5×1011 cm-2 without the field plate. This result shows that the field plate structure is effective for suppressing the surface charge influence on breakdown voltage due to the relaxation of the electric field concentration in the surface portion of the AlGaN layer.