We measure the temperature distribution in a biased single-layer graphene
transistor using Raman scattering microscopy of the 2D-phonon band. Peak
operating temperatures of 1050 K are reached in the middle of the graphene
sheet at 210 KW cm^(-2) of dissipated electric power. The metallic contacts act
as heat sinks, but not in a dominant fashion. To explain the observed
temperature profile and heating rate, we have to include heat-flow from the
graphene to the gate oxide underneath, especially at elevated temperatures,
where the graphene thermal conductivity is lowered due to umklapp scattering.
Velocity saturation due to phonons with about 50 meV energy is inferred from
the measured charge density via shifts in the Raman G-phonon band, suggesting
that remote scattering (through field coupling) by substrate polar surface
phonons increases the energy transfer to the substrate and at the same time
limits the high-bias electronic conduction of graphene.Comment: The pdf-file contains the main manuscript (19 pages, 3 figures) and
the supplement (5 pages, 4 figures