Improved Carrier Mobility in Few-Layer MoS$_{2}$ Field-Effect Transistors with Ionic-Liquid Gating*

We report the fabrication of ionic liquid (IL) gated field-effect transistors (FETs) consisting of bilayer and few-layer MoS$_{2}$. Our transport measurements indicate that the electron mobility $\mu \approx $60~cm$^{2}$V$^{-1}$s$^{-1}$ at 250~K in ionic liquid gated devices exceeds significantly that of comparable back-gated devices. IL-FETs display a mobility increase from $\approx $100 cm$^{2}$V$^{-1}$s$^{-1}$ at 180~K to $\approx $220~cm$^{2}$V$^{-1}$s$^{-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 \textgreater 10$^{7}$ (10$^{4})$ for electrons (holes), and a near ideal sub-threshold swing of $\approx $50 mV/dec at 250 K. We attribute the observed performance enhancement, specifically the increased carrier mobility that is limited by phonons, to the reduction of the Schottky barrier at the source and drain electrode by band bending caused by the ultrathin ionic-liquid dielectric layer. In addition, graphene contacted MoS$_{2}$ FETs with IL-gating will also be discussed.