  We explore the effects of metal contacts on the operation and scalability of
2D Graphene Field-Effect-Transistors (GFETs) using detailed numerical device
simulations based on the non-equilibrium Green's function formalism
self-consistently solved with the Poisson equation at the ballistic limit. Our
treatment of metal-graphene (M-G) contacts captures: (1) the doping effect due
to the shift of the Fermi level in graphene contacts, (2) the density-of-states
(DOS) broadening effect inside graphene contacts due to Metal-Induced-States
(MIS). Our results confirm the asymmetric transfer characteristics in GFETs due
to the doping effect by metal contacts. Furthermore, at higher M-G coupling
strengths the contact DOS broadening effect increases the on-current, while the
impact on the minimum current (Imin) in the off-state depends on the source to
drain bias voltage and the work-function difference between graphene and the
contact metal. Interestingly, with scaling of the channel length, the MIS
inside the channel has a weak influence on Imin even at large M-G coupling
strengths, while direct source-to-drain (S -> D) tunneling has a stronger
influence. Therefore, channel length scalability of GFETs with sufficient gate
control will be mainly limited by direct S -> D tunneling, and not by the MIS.
