  Ambipolar black phosphorus MOSFETs with record n-channel extrinsic
transconductance are reported. The devices consist of multi-layer black
phosphorus aligned to a local back-gate electrode with 10-nm-thick HfO2 gate
dielectric. Before passivation, devices with 0.3-um gate length behaved as
p-MOSFETs with peak extrinsic transconductance, gm, of 282 uS/um at VDS = -2 V.
After passivation, the same devices displayed ambipolar behavior, and when
tested as n-MOSFETs, had peak gm = 66 uS/um at VDS = +2 V, and similar devices
on the same wafer had gm as high as 80 uS/um. These results are an important
step toward realization of high-performance black phosphorus complementary
logic circuits.
