A two-dimensional analysis of off-state breakdown characteristics in AlGaN/GaN HEMTs is performed by considering a deep donor and a deep acceptor in a buffer layer, with the relative permittivity of passivation layer εr as a parameter. The analysis is made with and without impact ionization of carriers to study how the buffer leakage current affects the breakdown performance. It is shown that when εr is low, the breakdown voltage is determined by the impact ionization of carriers, and when εr becomes high, it is determined by the buffer leakage current. This buffer leakage current decreases as εr increases because the electric field at the drain edge of the gate is weakened, and hence the breakdown voltage increases as εr increases. It is also shown that when the gate voltage is more negative, the breakdown voltage in the high εr region becomes higher because the buffer leakage current becomes smaller.
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