  Integration of electrical contacts into van der Waals (vdW) heterostructures
is critical for realizing electronic and optoelectronic functionalities.
However, to date no scalable methodology for gaining electrical access to
buried monolayer two-dimensional (2D) semiconductors exists. Here we report
viable edge contact formation to hexagonal boron nitride (hBN) encapsulated
monolayer MoS$_2$. By combining reactive ion etching, in situ Ar$^+$ sputtering
and annealing, we achieve a relatively low edge contact resistance, high
mobility (up to ~30 cm$^2$/Vs) and high on-current density (>50 uA/um at
V$_{\rm DS}$ = 3V), comparable to top contacts. Furthermore, the atomically
smooth hBN environment also preserves the intrinsic MoS$_2$ channel quality
during fabrication, leading to a steep subthreshold swing of 116 mV/dec with a
negligible hysteresis. Hence, edge contacts are highly promising for
large-scale practical implementation of encapsulated heterostructure devices,
especially those involving air sensitive materials, and can be arbitrarily
narrow, which opens the door to further shrinkage of 2D device footprint.
