Hot LO Phonon-Induced RF Nonlinearity in GaN High-Electron-Mobility Transistors

TL;DR

This study demonstrates that hot LO phonons significantly impair the RF linearity of GaN HEMTs, establishing intrinsic phonon-induced limits on their RF performance through full-band transport simulations.

Contribution

It reveals the impact of ultrafast LO phonons on RF linearity and clarifies that $g_m$ flatness alone cannot predict RF linearity in GaN HEMTs.

Findings

LO phonons degrade 1-dB compression point by ~3 dB

LO phonons reduce third-order intercept power by ~3 dB

Improvements in $g_m$ flatness do not necessarily enhance RF linearity

Abstract

Hot longitudinal optical (LO) phonons in GaN have recently been identified as a major factor degrading the DC performance of GaN high-electron-mobility transistors (HEMTs) by 30-60%, despite their ultrafast decay. However, their impact on large-signal RF performance, particularly RF linearity, remains poorly understood. Using full-band transport simulations of a fabricated GaN HEMT, we show that even ultrafast LO phonons with a lifetime of 30 fs degrade the output 1-dB compression point and the third-order output intercept power by ~3 dB compared to the case without LO phonon heating. Furthermore, our analysis reveals that improvements in transconductance ($g_\textrm{m}$) flatness do not necessarily translate into improved RF linearity because multiple nonlinear mechanisms contribute to the transistor response, and their combined effect cannot be captured by $g_\mathrm{m}$ flatness alone. This work clarifies a persistent ambiguity in the literature regarding using $g_\mathrm{m}$ flatness as a proxy for RF linearity and establishes intrinsic phonon-induced limits on the RF performance of GaN HEMTs.