Comparative Analysis of Sapphire and β-Ga2O3 Substrates for AlN/GaN HEMTs with Graded β-(AlxGa1 – x)2O3 Buffer Layers

Abstract— This work presents a comparative investigation of substrate-dependent electrical and RF characteristics of β-Ga2O3 back-barrier AlN/GaN HEMTs incorporating a graded β-(AlxGa1 – x)2O3 buffer layer. Two device configurations employing sapphire and β-Ga2O3 substrates were designed and analyzed using TCAD simulations. The energy-band analysis confirms the formation of a quantum well and enhanced carrier confinement in the β-Ga2O3-substrate-based structure. The proposed device demonstrates improved electron transport characteristics, achieving a maximum drain current of 2.75 A/mm and a peak transconductance of 3.55 S/mm at VDS = 2 V. Furthermore, the cutoff frequency reaches 190 GHz, indicating excellent high-frequency capability. The β-Ga2O3-substrate-based device also exhibits superior switching performance with an ultra-low off-state current of approximately 10–16 A/mm and an Ion/Ioff ratio exceeding 1015. In addition, the breakdown voltage is enhanced from 331 V for the sapphire-substrate structure to 357 V for the β-Ga2O3-substrate structure. The results demonstrate that substrate engineering using β-Ga2O3 significantly improves carrier confinement, leakage suppression, RF performance, and voltage-handling capability, making the proposed HEMT for high-power electronic applications.