Ionic-Liquid Gated Few-layer MoS$_2$ Field-Effect Transistors*

We report the electrical characterization of ionic-liquid-gated bilayer and few-layer MoS$_2$ field-effect transistors. The extrinsic mobility of our ionic-liquid-gated devices exceeds 70 cm$^{2}$V$^{-1}$S$^{-1}$ at 250 K, which is 1-2 orders of magnitude higher than that measured in the Si back-gate configuration (without ionic liquid). These devices also show ambipolar behavior with a high ON-OFF current ratio of \textgreater\ 10$^{7}$ for electrons and \textgreater\ 10$^6$ for holes, and a near ideal subthreshold swing (SS) of $\sim$ 50 mV/decade at 250 K for the electron channel. More significantly, we show that the mobility increases from $\sim$ 100 cm$^{2}$V$^{-1}$S$^{-1}$ at 180 K to $\sim$ 220 cm$^{2}$V$^{-1}$S$^{-1}$ at 77K as the temperature decreases following a $\mu $ $\sim$ T$^{-\gamma}$ dependence with $\gamma \approx $ 1, indicating that the intrinsic phonon-limited mobility can be achieved in few-layer MoS$_{2}$ FETs. We attribute the enhanced device performance to the drastic reduction of the Schottky barrier width (thus higher tunneling efficiency) via highly efficient band bending at the MoS$_{2}$/metal interface afforded by the extremely large electrical double layer capacitance of the ionic liquid.