  We report the fabrication of ionic liquid (IL) gated field-effect transistors
(FETs) consisting of bilayer and few-layer MoS2. Our transport measurements
indicate that the electron mobility about 60 cm2V-1s-1 at 250 K in ionic liquid
gated devices exceeds significantly that of comparable back-gated devices.
IL-FETs display a mobility increase from about 100 cm2V-1s-1 at 180 K to about
220 cm2V-1s-1 at 77 K in good agreement with the true channel mobility
determined from four-terminal measurements, ambipolar behavior with a high
ON/OFF ratio >107 (104) for electrons (holes), and a near ideal sub-threshold
swing of about 50 mV/dec at 250 K. We attribute the observed performance
enhancement, specifically the increased carrier mobility that is limited by
phonons, to the reduction of the Schottky barrier at the source and drain
electrode by band bending caused by the ultrathin ionic-liquid dielectric
layer.
