  Transistors operating at high frequencies are the basic building blocks of
millimeter-wave communication and sensor systems. The high velocity and
mobility of carriers in graphene can open way for ultra-fast group IV
transistors with similar or even better performance than can be achieved with
III-V based semiconductors. However, the progress of high-speed graphene
transistors has been hampered due to fabrication issues, influence of adjacent
materials, and self-heating effects. Here, we report a graphene field-effect
transistor (FET) on a diamond substrate, with a $f_{max}$ up to 54 GHz for a
gate length of 500 nm. The high thermal conductivity of diamond provides an
efficient heat-sink, and its relatively high optical-phonon energy improves
saturation velocity of carriers in the graphene channel. Moreover, we show that
graphene FETs on diamond, with different gate lengths, exhibit excellent
scaling behavior. These results indicate that graphene FETs on diamond
technology can reach sub-terahertz frequency performance.
