  Atomically thin layers of van der Waals (vdW) crystals offer an ideal
material platform to realize tunnel field effect transistors (TFETs) that
exploit the tunneling of charge carriers across the forbidden gap of a vdW
heterojunction. This type of device requires a precise energy band alignment of
the different layers of the junction to optimize the tunnel current. Amongst
two-dimensional (2D) vdW materials, black phosphorus (BP) and indium selenide
(InSe) have a Brillouin zone-centered conduction and valence bands, and a type
II band offset, both ideally suited for band-to-band tunneling. Here, we
demonstrate TFETs based on BP/InSe heterojunctions with diverse electrical
transport characteristics: forward rectifying, Zener-tunneling and backward
rectifying characteristics are realized in BP/InSe junctions with different
thickness of the BP layer or by electrostatic gating of the junction.
Electrostatic gating yields a large on/off current ratio of up to 108 and
negative differential resistance at low applied voltages (V ~ 0.2V). These
findings illustrate versatile functionalities of TFETs based on BP and InSe,
offering opportunities for applications of these 2D materials beyond the device
architectures reported in the current literature.
