  High frequency performance limits of graphene field-effect transistors (FETs)
down to a channel length of 20nm are examined by using self-consistent quantum
simulations. The results indicate that although Klein band-to-band tunneling is
significant for sub-100nm graphene FET, it is possible to achieve a good
transconductance and ballistic on-off ratio larger than 3 even at a channel
length of 20nm. At a channel length of 20nm, the intrinsic cut-off frequency
remains at a couple of THz for various gate insulator thickness values, but a
thin gate insulator is necessary for a good transconductance and smaller
degradation of cut-off frequency in the presence of parasitic capacitance. The
intrinsic cut-off frequency is close to the LC characteristic frequency set by
graphene kinetic inductance and quantum capacitance, which is about 100GHz
\cdot {\mu}m divided by the gate length.
