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

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 fundamental phonon-induced limits on the RF performance of GaN HEMTs.

Figures (11)

• Figure 1: Schematic cross section of the AlGaN/GaN HEMT (top) and the meshed structure (bottom). Gate Schottky barrier height: 1.2 eV; polarization sheet charge: $1.115\times 10^{13}$ cm$^{-2}$; surface trap density in the source (drain) access region: $1.8\times 10^{12}$ cm$^{-2}$ ($5.2\times 10^{12}$ cm$^{-2}$); acoustic phonon deformation potential: 8.3 eV; LO phonon energy: 92 meV; LO phonon deformation potential: $10^9$ eV/cm.
• Figure 2: Drain current $I_{\rm D}$ versus drain voltage $V_{\rm DS}$ with different gate biases $V_{\rm GS}$ for the GaN HEMT structure in Fig. \ref{['fig:device_sch']}. Measurements reported by Marino et al.marinoDislocationsGaNHEMT2010
• Figure 3: Small signal current gain versus frequency for the GaN HEMT in Fig. \ref{['fig:device_sch']}. Measured data reported by Palacios et al.palaciosEmodeGaN2006
• Figure 4: (a) The simulated steady-state current at $V_\mathrm{GS} = 1$ V and $V_\mathrm{GS} = -1$ V highlights the impact of LO phonon heating on DC response under high bias conditions. The symbol marks the quiescent (Q)-point: $I_\mathrm{D} = 900$ mA/mm, $V_\mathrm{DS} = 6$ V. (b) Output curves at the Q-point for various cases of phonon heating. For each case, the gate bias is adjusted to arrive at the Q point as indicated in the inset table.
• Figure 5: Two-dimensional heat maps of the peak (a) electron, (b) LO phonon, and (c) acoustic phonon temperatures in the GaN channel. The symbol indicates the Q-point.