  Gallium nitride (GaN), a wide band-gap semiconductor, has been broadly used
in power electronic devices due to its high electron mobility and high
breakdown voltage. Its relatively high thermal conductivity makes GaN a
favorable material for such applications, where heat dissipation is a major
concern for device efficiency and long-term stability. However, in GaN-based
transistors, where the active region can withstand extremely strong electric
fields, the field effect on the thermal transport properties has drawn little
attention so far. In this work, we apply first-principles methods to
investigate phonon properties of wurtzite GaN in the presence of a
near-breakdown electric field applied along different crystallographic
directions. We find that the electric field changes thermal conductivity
considerably via impacting the bond stiffness and ionicity as well as the
crystal symmetry, although it has little effect on phonon dispersions. The
presence of an out-of-plane electric field increases (decreases) the thermal
conductivity parallel (perpendicular) to the electric field, which is
attributed to different changes of the Ga-N bond stiffness and ionicity. When
an in-plane electric field is applied, the sizable decrease of thermal
conductivities along all directions is attributed to the crystal symmetry
breaking that enhances the phonon-phonon scattering. Our study provides
insights into the effect of extreme external electric fields on phonon
transport properties in wide-gap semiconductors.
