Polarization‐Graded High‐Electron‐Mobility Transistors for Improved Johnson's Figure of Merit

Abstract

Polarization grading in AlGaN/GaN‐based high‐electron‐mobility transistors (HEMTs) is a promising device design option that can improve linearity, speed, power, and noise performance for use in millimeter‐wave applications. This work investigates the potential of compositionally graded HEMT heterostructures to enhance device breakdown through lateral electric field engineering while maintaining a high device cutoff frequency (fT) due to reduced longitudinal optical (LO) phonon scattering. The impact of polarization grading on electric field profile is compared with conventional gate‐integrated mini‐field plates (mini‐FPs). It is also observed that polarization grading can augment the efficacy of gate‐connected FPs, further enhancing performance. Using physics‐based 2D device simulations, it is demonstrated that polarization engineering via polarization grading enhances breakdown (VBD) while preserving high fT, resulting in a Johnson's figure of merit (JFOM = fT × VBD), that is, ≈2.4× that of a conventional abrupt‐junction HEMT. This improvement represents a significant advancement over the ≈1.25× to ≈2× increase achieved with the use of mini‐FPs alone in HEMTs.