  Stacked van der Waals (vdW) heterostructures where semi-conducting
two-dimensional (2D) materials are contacted by overlayed graphene electrodes
enable atomically-thin, flexible electronics. We use first-principles quantum
transport simulations of graphene-contacted MoS2 devices to show how the
transistor effect critically depends on the stacking configuration relative to
the gate electrode. We can trace this behavior to the stacking-dependent
response of the contact region to the capacitive electric field induced by the
gate. The contact resistance is a central parameter and our observation
establish an important design rule for devices based on 2D atomic crystals.
