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 10μm. 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 10¹⁷ 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 × 10¹⁷ 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 10μm , 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. [ABSTRACT FROM AUTHOR]