Graphene microwave transistors on sapphire substrates
  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. 
E. Pallecchi, C. Benz, A.C. Betz, H.v. L\"ohneysen, B. Pla\c{c}ais,   and R. Danneau
cond-mat.mes-hall