We make a 2-D analysis of breakdown characteristics of field-plate AlGaN/GaN HEMTs with a high-k passivation layer, and the results are compared with those having a normal SiN passivation layer. As a result, it is found that the breakdown voltage is enhanced particularly in the cases with relatively short field plates because the reduction in the electric field at the drain edge of gate effectively improves the breakdown voltage in the case with the high-k passivation layer. In the case with the moderate-length field plate, the enhancement of breakdown voltage due to the high-k passivation layer occurs because the electric field profiles between the field-plate edge and the drain become more uniform. It is also studied how the breakdown voltage depends on a deep-acceptor density in the Fe-doped semiinsulating buffer layer when a high-k passivation layer is used. It is shown that the breakdown voltage increases with increasing the relative permittivity of the passivation layer ε r and with increasing the deep-acceptor density N DA . When ε r = 60 and N DA = 2-3 × 10 17 cm −3 at the gate length of 0.3 μm, the breakdown voltage becomes about 500 V at a gate-to-drain distance of 1.5 μm, which corresponds to an average electric field of about 3.3 MV/cm between the gate and the drain. Index Terms-2-D analysis, breakdown characteristics, buffer layer, GaN HEMT, high-k passivation layer. I. INTRODUCTION N OWADAYS, AlGaN/GaN HEMTs are attractive for applications to high-power microwave devices and highpower switching devices [1], [2]. It is well known that introducing a field plate enhances the power performance of AlGaN/GaN HEMTs as well as GaAs FETs [3]-[5]. This occurs because by introducing a field plate, the current collapse is reduced [6], [7], and the breakdown voltage increases [8]-[10]. The increase in breakdown voltage occurs because the electric field at the drain edge of gate is reduced by introducing a field plate.