  Two key subjects stand out in the pursuit of semiconductor research: material
quality and contact technology. The fledging field of atomically thin
transition metal dichalcogenides (TMDCs) faces a number of challenges in both
efforts. This work attempts to establish a connection between the two by
examining the gate-dependent conductance of few-layer (1-5L) WSe2 field effect
devices. Measurements and modeling of the subgap regime reveal Schottky barrier
transistor behavior. We show that transmission through the contact barrier is
dominated by thermionic field emission (TFE) at room temperature, despite the
lack of intentional doping. The TFE process arises due to a large number of
subgap impurity states, the presence of which also leads to high mobility edge
carrier densities. The density of states of such impurity states is
self-consistently determined to be approximately 1-2x10^13 /cm^2/eV in our
devices. We demonstrate that substrate is unlikely to be a major source of the
impurity states and suspect that lattice defects within the material itself are
primarily responsible. Our experiments provide key information to advance the
quality and understanding of TMDC materials and electrical devices.
