Influence of Metal–Graphene Contact on the Operation and Scalability of Graphene Field-Effect Transistors

We explore the effects of metal contacts on the operation and scalability of 2-D graphene field-effect transistors (GFETs) using detailed numerical device simulations based on the nonequilibrium Green´s function formalism self-consistently solved with the Poisson equation at the ballistic limit. Our treatment of metal-graphene (M-G) contacts captures the following: 1) the doping effect due to the shift of the Fermi level in graphene contacts and 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 (I_min) 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 I_min 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.