Effects of Undoped GaN Capping Layer on p-GaN Gate AlGaN/GaN Heterostructure Field-Effect Transistors

Abstract

We investigated the effects of an undoped GaN (u-GaN) capping layer in normally-off p-GaN gate AlGaN/GaN heterostructure field-effect transistors (HFETs). Three types of AlGaN/GaN HFETs were fabricated: without a u-GaN capping layer, with a u-GaN capping layer, and with a stack of a u-GaN capping layer and a SiN X passivation layer. The turn-on voltages were nearly identical for all the devices, but the drain current of the device without a capping layer was significantly lower than those of the devices with a capping layer. We also observed a lower current collapse and a considerably improved electrical stability under the OFF-state stress in the devices with a u-GaN capping layer compared with the device without a u-GaN capping layer. The experimental results show that a u-GaN capping layer can effectively suppress the surface trap states in p-GaN gate AlGaN/GaN HFETs.