  Bilayer graphene has the very interesting property of an energy gap tunable
with the vertical electric field. We propose an analytical model for a
bilayer-graphene field-effect transistor, suitable for exploring the design
parameter space and to find a device structure with promising performance in
terms of transistor operation. Our model, based on the effective mass
approximation and ballistic transport assumptions, takes into account
bilayer-graphene tunable gap and self polarization, and includes all
band-to-band tunneling current components, which are shown to represent the
major limitation to transistor operation, because the limited achievable energy
gap is not sufficient to obtain a large Ion/Ioff ratio.
