Hot LO phonon-induced RF nonlinearity in GaN high-electron-mobility transistors

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 (gm) 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 gm flatness alone. This work clarifies a persistent ambiguity in the literature regarding the use of gm flatness as a proxy for RF linearity and establishes intrinsic phonon-induced limits on the RF performance of GaN HEMTs.