Simulation of buffer current effects on breakdown voltage in AIGaN/GaN HEMTs having passivation layers with different permittivity

A two-dimensional simulation of off-state breakdown characteristics in AIGaN/GaN HEMTs is performed, with the relative permittivity of passivation layer ϵ_r as a parameter. The simulation is made with and without impact ionization of carriers to study how the buffer leakage current affects the breakdown chracteristics. 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.