Delay Time Analysis of AlGaN/GaN Heterojunction Field-Effect Transistors with AlN or SiN Surface Passivation

Abstract

We have carried out delay time analysis for the high-frequency characteristics of AlGaN/GaN heterojunction field-effect transistors (HFETs) on sapphire substrates, focusing on a comparison between the effects of AlN and SiN surface passivation. HFETs with AlN passivation, in comparison with those with SiN passivation, exhibit high current gain cut-off frequencies fT with good flatness, in accordance with high intrinsic transconductance due to the self-heating reduction effect of AlN. From the delay time analysis, we find that the effective saturation electron velocity for AlN and SiN passivation is similar, while the electron mobility under the gate for AlN passivation is higher. This implies that the higher fT is mainly due to the higher electron mobility under the gate owing to the self-heating reduction by AlN passivation. This is consistent with the fact that the saturation velocity has a weak temperature dependence, while the mobility has a strong dependence.