Analysis of Dependence of Breakdown Voltage on Gate–Drain Distance in AlGaN/GaN HEMTs With High-k Passivation Layer

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 L _GD . The relative permittivity of the passivation layer ε _r is changed from 1 to 60, and L _GD is changed from 1.5 to 10 μ\textm. It is shown that, in all cases with different L _GD , the breakdown voltage V _br increases as ε _r increases. When a deep-acceptor density in an Fe-doped buffer layer N _DA is 10 ¹⁷ cm ^-3 and the gate length is 0.3 μm, V _br is determined by buffer leakage current at ε _r ≥ 30 before impact ionization dominates. Hence, V _br is similar at L _GD = 3-10 μm, and the increase rate in V _br from L _GD = 1.5 μm is about 50% even at ε _r = 60. However, when N _DA is 2×10 ¹⁷ cm ^-3 , V _br is determined by impact ionization of carriers even at ε _r ≥ 30 because the buffer leakage current is reduced. V _br becomes about 500, 930, 1360, and 1650 V for L _GD = 1.5, 3, 5, and 7 μm, respectively, at ε _r = 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 L _GD , the electric field profiles between the gate and the drain are rather uniform. However, in the case of L _GD = 10 μm, V _br is about the same as that (1650 V) of L _GD = 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 V _br by using a high- k passivation layer in this case. However, it can be said that, to improve V _br further at long L _GD , such as 10 μm, the combination of field plate or using a higher ε _r material may be effective because both of them decrease the electric field at the drain edge of the gate.