  Metal contacts have been identified to be a key technological bottleneck for
the realization of viable graphene electronics. Recently, it was observed that
for structures that possess both a top and a bottom gate, the electron-hole
conductance asymmetry can be modulated by the bottom gate. In this letter, we
explain this observation by postulating the presence of an effective thin
interfacial dielectric layer between the metal contact and the underlying
graphene. Electrical results from quantum transport calculations accounting for
this modified electrostatics corroborate well with the experimentally measured
contact resistances. Our study indicates that the engineering of metal-
graphene interface is a crucial step towards reducing the contact resistance
for high performance graphene transistors.
