  The metal-graphene contact resistance is a technological bottleneck for the
realization of viable graphene based electronics. We report a useful model to
find the gate tunable components of this resistance determined by the
sequential tunneling of carriers between the 3D-metal and 2D-graphene
underneath followed by Klein tunneling to the graphene in the channel. This
model quantifies the intrinsic factors that control that resistance, including
the effect of unintended chemical doping. Our results agree with experimental
results for several metals.
