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

TL;DR

The paper investigates intrinsic limits imposed by hot LO phonons in GaN HEMTs using a full-band, deterministic Boltzmann transport framework (FKT) that incorporates non-equilibrium LO phonon dynamics. It demonstrates that to reproduce the measured DC characteristics of a fabricated AlGaN/GaN HEMT, the LO phonon lifetime must be $\tau_{LO} \lesssim 40$ fs, consistent with ultrafast decay observed in GaN heterostructures, yet even these fast lifetimes do not eliminate the hot-phonon bottleneck, reducing current density by about 30% and peak transconductance by about 60%. Longer LO lifetimes produce predictions inconsistent with experiment, underscoring LO phonons as an intrinsic performance limiter rather than an engineering artifact. The results highlight the coupled interplay of electron transport, phonon dynamics, and heat flow, and provide a quantitative framework for phonon-engineering approaches in GaN HEMTs.

Abstract

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.

Figures (12)

• Figure 1: Workflow for preprocessing transport quantities: from (a) EPM bandstructure, (b) Brillouin-zone computation, and (c) iso-surface extraction to (d) polynomial fitting of transport integrals used in FKT simulations.
• Figure 2: $\Gamma$ valley flux isosurface integrals in the horizontal plane of GaN with varying LO phonon temperatures at 300 K lattice temperature and $\mathrm{10^{18} cm^{-3}}$ ionized defect and mobile electron densities.
• Figure 3: Bulk electron drift velocity versus field for intrinsic GaN at room temperature for different LO phonon lifetimes.
• Figure 4: Occupation numbers $n_q$ for LO phonons emitted into different ranges of phonon momentum $|\mathbf{q}|$ during the simulation of electron drift velocity in bulk GaN. The value of $n_q$ is plotted as a symbol in each range of $|\mathbf{q}|$ for different applied fields, where $|\mathbf{E}|_{\mathrm{max}}=70$ kV/cm (top) and 600 kV/cm (bottom). $\tau_{\mathrm{LO}}=0.3$ ps was used for each range of $|\mathbf{q}|$.
• Figure 5: Cross-section of the fabricated GaN HEMT (top), and the meshed structure (bottom).