Conductance of graphene: Role of metal contact, charge puddles and differential gating

We demonstrate how several experiments on graphene transport can be explained semi-quantitatively within a Non-Equilibrium Green´s Function (NEGF) formalism. The key features are controlled by-(i) the boundary potential established at a metal-graphene interface,(ii) charge puddles that help the conductivity in the ballistic limit and hurt in the diffusive limit and (iii) alignment of the local Dirac points in a multiply gated segment. Simulations reveal that at the ballistic limit, the conductance depends on the aspect ratio which controls tunneling from source to drain and across the metal-graphene interface. We show that the boundary potential V_B at the interface together with Metal Induced Doping (MID) are critical to graphene transport-specifically, the maximum conductance achievable with a given metal contact, the electron-hole asymmetry (EHA) and the peak device resistance. The boundary potential is formed due to in-plane charge transfer from metal covered graphene to graphene on substrate [1] and may produce an additional smooth pn junction, typically ignored in existing models. In the experiments however, the contact resistance heavily depends on the fabrication procedure [2], varying from hundreds of Ω- μm to several thousands of Ω- μm [3]. A rigorous model of the performance limits of several contacts and change of carrier transport from ballistic to diffusive regime is lacking. We report the upper limit of the performance of various metal-graphene contacts and compare with the best available experimental values. To reach experimental dimensions, we use tight-binding real space calculations as well as the powerful KSF-RGFA approach (combination of K Space Formalism (KSF) and Recursive Green´s Function Algorithm (RGFA) [4]), which allows us to simulate devices as large as microns in size.