  Van der Waals (vdW) semiconductors are attractive for highly scaled devices
and heterogeneous integration since 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
seven 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.
