Physics-based Full-band GaN High-Electron-Mobility Transistor Simulation Suggests Upper Bound of LO Phonon Lifetime

Intrinsic limits to device performance arise from fundamental material properties that define the best achievable operation, independent of engineering constraints. In GaN high-electron-mobility transistors (HEMTs), hot longitudinal optical (LO) phonons can act as an intrinsic performance bottleneck by reducing electron saturation velocity, output current, and transconductance, which are key device metrics. While bulk GaN studies report LO phonon lifetimes of approximately 1 ps, leading to strong nonequilibrium phonon populations, ungated heterostructures show much shorter lifetimes of only tens of femtoseconds. Because direct measurement inside a HEMT channel is challenging, the true impact of hot phonons remains uncertain. Using full-band transport simulations of a fabricated GaN HEMT, we show that LO phonon lifetimes must be less than about 40 fs to reproduce measured I-V characteristics, consistent with ultrafast decay observed in GaN heterostructures. We further demonstrate that even these ultrafast lifetimes are not sufficient to eliminate hot phonon effects: the residual nonequilibrium LO population continues to limit the current density at high bias. Moreover, when the LO phonon lifetime exceeds a few tens of femtoseconds, a pronounced hot phonon bottleneck emerges, leading to substantial current-density suppression that is inconsistent with experiment.