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 epsilon(r) is changed from 1 to 60, and L-GD is changed from 1.5 to 10 mu m. It is shown that, in all cases with different L-GD, the breakdown voltage V-br increases as epsilon(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 mu m, V-br is determined by buffer leakage current at epsilon(r) >= 30 before impact ionization dominates. Hence, V-br is similar at L-GD = 3-10 mu m, and the increase rate in V-br from L-GD = 1.5 mu m is about 50% even at epsilon(r) = 60. However, when NDA is 2 x 10(17) cm(-3), V-br is determined by impact ionization of carriers even at epsilon(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 mu m, respectively, at epsilon(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 mu m, V-br is about the same as that (1650 V) of L-GD = 7 mu m, 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 mu m, the combination of field plate or using a higher epsilon(r) material may be effective because both of them decrease the electric field at the drain edge of the gate. Index