Gate-Tunable Semiconductor Heterojunctions from 2D/3D van der Waals Interfaces

van der Waals (vdW) semiconductors are attractive for highly scaled devices and heterogeneous integration as they can be isolated into self-passivated, two-dimensional (2D) layers that enable superior electrostatic control. These attributes have led to numerous demonstrations of field-effect devices ranging from transistors to triodes. By exploiting the controlled, substitutional doping schemes in covalently bonded, three-dimensional (3D) semiconductors and the passivated surfaces of 2D semiconductors, one can construct devices that can exceed performance metrics of "all-2D" vdW heterojunctions. Here, we demonstrate 2D/3D semiconductor heterojunctions using MoS2 as the prototypical 2D semiconductor laid upon Si and GaN as the 3D semiconductor layers. By tuning the Fermi levels in MoS2, we demonstrate devices that concurrently exhibit over 7 orders of magnitude modulation in rectification ratios and conductance. Our results further suggest that the interface quality does not necessarily affect Fermi level tuning at the junction, opening up possibilities for novel 2D/3D heterojunction device architectures.