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A 2-D analysis of OFF-state breakdown characteristics of AlGaN/GaN HEMTs with a high-k passivation layer is performed as a function of gate-to-drain distance LGD. The relative permittivity of the passivation layer εr is changed from 1 to 60, and LGD is changed from 1.5 to 10μm. It is shown that, in all cases with different LGD, the breakdown voltage Vbr increases as εr increases. When a deep-acceptor density in an Fe-doped buffer layer NDA is 1017 cm−3 and the gate length is 0.3 μm , Vbr is determined by buffer leakage current at εr ≥ 30 before impact ionization dominates. Hence, Vbr is similar at LGD = 3 – 10 μm , and the increase rate in Vbr from LGD = 1.5 μm is about 50% even at εr =60. However, when NDA is 2 × 1017 cm−3 , Vbr is determined by impact ionization of carriers even at εr ≥ 30 because the buffer leakage current is reduced. Vbr becomes about 500, 930, 1360, and 1650 V for LGD = 1.5 , 3, 5, and 7 μm, respectively, at εr = 60. These voltages correspond to gate-to-drain average electric fields of about 3.3, 3.1, 2.7, and 2.3 MV/cm, respectively. Particularly, for short LGD , the electric field profiles between the gate and the drain are rather uniform. However, in the case of LGD = 10 μm , Vbr is about the same as that (1650 V) of LGD = 7 μm , suggesting that the electric field at the drain edge of the gate becomes a critical value before the high electric field region extends to the drain enough. This may be a limitation to increase Vbr by using a high- k passivation layer in this case. However, it can be said that, to improve Vbr further at long LGD , such as 10μm , the combination of field plate or using a higher εr material may be effective because both of them decrease the electric field at the drain edge of the gate. [ABSTRACT FROM AUTHOR]