  We report ambipolar transport properties in black phosphorus using an
electric-double-layer transistor (EDLT) 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/Vs at
170 K, which is one order of magnitude larger than the electron mobility. By
inducing an ultra-high 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.
