Non-equilibrium phonon effects on high-field electron transport in β -Ga2O3

Electron transport in β-Ga2O3 is limited by polar optical phonon scattering, yet every Monte Carlo study of this material to date evaluates that scattering with the phonon population held at its equilibrium Bose–Einstein value. β-Ga2O3 combines the strongest Fröhlich coupling of the wide-bandgap semiconductors with the lowest optical phonon energies and a thermal conductivity 5–25 times below that of GaN. The carrier-driven phonon population consequently reaches twice its equilibrium value at device-relevant bias and up to 20 times it for the most strongly coupled mode at the wavevectors where emission concentrates. Using ensemble Monte Carlo simulations in which the phonon occupation is resolved in wavevector and evolved self-consistently with the carrier ensemble, we find the steady-state drift velocity at n = 1018 cm−3 and 150 kV/cm is reduced by 39.3%±0.7% relative to an equilibrium-phonon calculation with identical parameters. The reduction grows with carrier density and saturates above approximately 100 kV/cm. The excess accumulates at the wavevectors where the polar coupling is strongest, so the phonon occupation sampled by the carriers is 2.4 times the zone average. Evaluating the scattering rate from the resolved distribution while retaining a uniform-occupation angular distribution overestimates the reduction by a factor of 1.6, so both must be taken from the same distribution. The reduction changes by at most 0.6 points under free-carrier screening, exceeds 25% even for a 0.5 ps optical phonon lifetime, and remains above 26% when the polar coupling is partitioned into as many as 16 independently evolving mode groups.