  Black phosphorus (BP) has recently emerged as a promising narrow band gap
layered semiconductor with optoelectronic properties that bridge the gap
between semi-metallic graphene and wide band gap transition metal
dichalcogenides such as MoS2. To date, BP field-effect transistors have
utilized a lateral geometry with in-plane transport dominating device
characteristics. In contrast, we present here a vertical field-effect
transistor geometry based on a graphene/BP van der Waals heterostructure. The
resulting device characteristics include high on-state current densities (>
1600 A/cm2) and current on/off ratios exceeding 800 at low temperature. Two
distinct charge transport mechanisms are identified, which are dominant for
different regimes of temperature and gate voltage. In particular, the Schottky
barrier between graphene and BP determines charge transport at high
temperatures and positive gate voltages, whereas tunneling dominates at low
temperatures and negative gate voltages. These results elucidate out-of-plane
electronic transport in BP, and thus have implications for the design and
operation of BP-based van der Waals heterostructures.
