Effects of buffer leakage current on breakdown voltage in AlGaN/GaN HEMTs with a high-k passivation layer

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 ε_r 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 ε_r is low, the breakdown voltage is determined by the impact ionization of carriers, and when ε_r becomes high, it is determined by the buffer leakage current. This buffer leakage current decreases as ε_r increases because the electric field at the drain edge of the gate is weakened, and hence the breakdown voltage increases as ε_r increases. It is also shown that when the gate voltage is more negative, the breakdown voltage in the high ε_r region becomes higher because the buffer leakage current becomes smaller.