Ambipolar insulator-to-metal transition in black phosphorus by ionic-liquid gating.

We report ambipolar transport properties in black phosphorus using an electric-double-layer transistor configuration. The transfer curve clearly exhibits ambipolar transistor behavior with an ON-OFF ratio of ∼5 × 10(3). The band gap was determined as ≅0.35 eV from the transfer curve, and Hall-effect measurements revealed that the hole mobility was ∼190 cm(2)/(V s) at 170 K, which is 1 order of magnitude larger than the electron mobility. By inducing an ultrahigh carrier density of ∼10(14) cm(-2), an electric-field-induced transition from the insulating state to the metallic state was realized, due to both electron and hole doping. Our results suggest that black phosphorus will be a good candidate for the fabrication of functional devices, such as lateral p-n junctions and tunnel diodes, due to the intrinsic narrow band gap.