Researchers from IBM developed a graphene transistor with a record cut-off frequency of 155 Ghz and the shortest gate length ever (just 40nm). They used a diamond-like carbon as the top layer of the substrate on which the Graphene is deposited. This material is a great substrate for Graphene. It's a non-polar dielectric material - so it does not 'trap' or scatter charges, doesn't absorb a lot of water and has excellent thermal conductivity. It's also cheap to make and widely used today in the semiconductor industry.
Just over a year ago, IBM developed a 100Ghz RF graphene transistor - so the recent development is a 50% improvement over the previous design.