  We report the fabrication of both n-type and p-type WSe2 field effect
transistors with hexagonal boron nitride passivated channels and ionic-liquid
(IL)-gated graphene contacts. Our transport measurements reveal intrinsic
channel properties including a metal-insulator transition at a characteristic
conductivity close to the quantum conductance e2/h, a high ON/OFF ratio of >107
at 170 K, and large electron and hole mobility of ~200 cm2V-1s-1 at 160 K.
Decreasing the temperature to 77 K increases mobility of electrons to ~330
cm2V-1s-1 and that of holes to ~270 cm2V-1s-1. We attribute our ability to
observe the intrinsic, phonon limited conduction in both the electron and hole
channels to the drastic reduction of the Schottky barriers between the channel
and the graphene contact electrodes using IL gating. We elucidate this process
by studying a Schottky diode consisting of a single graphene/WSe2 Schottky
junction. Our results indicate the possibility to utilize chemically or
electrostatically highly doped graphene for versatile, flexible and transparent
low-resistance Ohmic contacts to a wide range of quasi-2D semiconductors.
  KEYWORDS: MoS2, WSe2, field-effect transistors, graphene, Schottky barrier,
ionic-liquid gate
