Analysis of Breakdown Voltages in AlGaN/GaN HEMTs with Low-Double Passivation Layers

2-D analysis of off-state drain current-drain voltage characteristics in AlGaN/GaN HEMTs is performed; where three cases with single passivation layers (SiN or high-{k} dielectric) and double passivation layers (first layer: SiN, second layer: high-{k} dielectric) are compared. The passivation layer's thicknesses and relative permittivity of high-{k} dielectric are varied as parameters. It is shown that in the case of double passivation layers, the breakdown voltage is enhanced significantly compared to the case of the SiN single passivation layer when the second high-{k} layer becomes thick. This occurs because the electric field at the drain edge of the gate is reduced. However, in the case of a relatively thin second high-{k} layer, the breakdown voltage can be lowered remarkably compared to the case with a high-{k} single passivation layer even if the first SiN layer is rather thin. Also, when the first SiN layer is thick ( {\sim } 0.1~{\mu }\text{m} ), the improvement of the breakdown voltage by increasing the thickness of the second layer is rather limited. However, it is also shown that in the case of double passivation layers, the breakdown voltage becomes close to the case of the high-{k} single passivation layer when the relative permittivity of the second passivation layer becomes high and the SiN layer is relatively thin.