  We have developed metal-oxide graphene field-effect transistors (MOGFETs) on
sapphire substrates working at microwave frequencies. For monolayers, we obtain
a transit frequency up to ~ 80 GHz for a gate length of 200 nm, and a power
gain maximum frequency of about ~ 3 GHz for this specific sample. Given the
strongly reduced charge noise for nanostructures on sapphire, the high
stability and high performance of this material at low temperature, our MOGFETs
on sapphire are well suited for a cryogenic broadband low-noise amplifier.
