  We report on the fabrication of field-effect transistors based on single and
bilayers of the semiconductor WS2 and the investigation of their electronic
transport properties. We find that the doping level strongly depends on the
device environment and that long in-situ annealing drastically improves the
contact transparency allowing four-terminal measurements to be performed and
the pristine properties of the material to be recovered. Our devices show
n-type behavior with high room-temperature on/off current ratio of ~106. They
show clear metallic behavior at high charge carrier densities and mobilities as
high as ~140 cm2/Vs at low temperatures (above 300 cm2/Vs in the case of
bi-layers). In the insulating regime, the devices exhibit variable-range
hopping, with a localization length of about 2 nm that starts to increase as
the Fermi level enters the conduction band. The promising electronic properties
of WS2, comparable to those of single-layer MoS2 and WSe2, together with its
strong spin-orbit coupling, make it interesting for future applications in
electronic, optical and valleytronic devices.
