  To develop effective thermal management strategies for GaN transistors, it is
essential to accurately predict the device junction temperature. Since the
width of the heat generation in the devices is comparable to phonon mean free
paths of GaN, phonon ballistic transport exists and can significantly affect
the heat transport process, which necessitates a thorough understanding of the
influence of the phonon ballistic effects in GaN transistors. In this paper,
the ballistic-diffusive phonon transport in GaN-on-SiC devices is examined by
measuring the hotspot temperature using the thermoreflectance thermal imaging
TTI combined with the hybrid phonon Monte Carlo-diffusion simulations. A series
of Au heaters are fabricated on the top of the GaN layer to quantitatively
mimic the different heat source distributions during device operation. The
experimental and simulation results show a good consistency and both indicate
that the phonon ballistic effects can significantly increase the hotspot
temperature. With the size of the heat source decreasing, the errors of
Fourier's law-based predictions increase, which emphasizes the necessity to
carefully consider the phonon ballistic transport in device thermal
simulations.
