  The two-dimensional (2D) layered semiconductors such as MoS2 have attracted
tremendous interest as a new class of electronic materials. However, there is
considerable challenge in making reliable contacts to these atomically thin
materials. Here we present a new strategy by using graphene as back electrodes
to achieve Ohmic contact to MoS2. With a finite density of states, the Fermi
level of graphene can be readily modified by gate potential to ensure a nearly
perfect band alignment with MoS2. We demonstrate, for the first time, a
transparent contact can be made to MoS2 with essentially zero contact barrier
and linear output behaviour at cryogenic temperatures (down to 1.9 K) for both
monolayer and multilayer MoS2. Benefiting from the barrier-free transparent
contacts, we show that a metal-insulator-transition (MIT) can be observed in a
two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky
barrier and only seen in four-terminal devices in conventional metal-contacted
MoS2 system. With further passivation y born nitride encapsulation, we
demonstrate a record high extrinsic (two-terminal) field effect mobility over
1300 cm2/Vs in MoS2.
