Highly Tunable Carrier Tunneling in Vertical Graphene-WS-Graphene van der Waals Heterostructures.

Owing to the fascinating properties, the emergence of two-dimensional (2D) materials brings various important applications of electronic and optoelectronic devices from field-effect transistors (FETs) to photodetectors. As a zero-band-gap material, graphene has excellent electric conductivity and ultrahigh carrier mobility, while the ON/OFF ratio of the graphene FET is severely low. Semiconducting 2D transition metal chalcogenides (TMDCs) exhibit an appropriate band gap, realizing FETs with high ON/OFF ratio and compensating for the disadvantages of graphene transistors. However, a Schottky barrier often forms at the interface between the TMDC and metallic contact, which limits the on-state current of the devices. Here, we lift the two limits of the 2D-FET by demonstrating highly tunable field-effect tunneling transistors based on vertical graphene-WS-graphene van der Waals heterostructures. Our devices show a low off-state current below 1 pA and a high ON/OFF ratio exceeding 10 at room temperature. Moreover, the carrier transport polarity of the device can be effectively tuned from n-type under small bias voltage to bipolar under large bias by controlling the crossover from a direct tunneling region to the Fowler-Nordheim tunneling region. Further, we find that the effective barrier height can be controlled by an external gate voltage. The temperature dependence of carrier transport demonstrates that both tunneling and thermionic emission contribute to the operation current at elevated temperature, which significantly enhances the on-state current of the tunneling transistors.