  Semiconductors require stable doping for applications in transistors,
optoelectronics, and thermoelectrics. However, this has been challenging for
two-dimensional (2D) materials, where existing approaches are either
incompatible with conventional semiconductor processing or introduce
time-dependent, hysteretic behavior. Here we show that low temperature (<
200$^\circ$ C) sub-stoichiometric AlO$_x$ provides a stable n-doping layer for
monolayer MoS$_2$, compatible with circuit integration. This approach achieves
carrier densities > 2x10$^{13}$ 1/cm$^2$, sheet resistance as low as ~7
kOhm/sq, and good contact resistance ~480 Ohm.um in transistors from monolayer
MoS$_2$ grown by chemical vapor deposition. We also reach record current
density of nearly 700 uA/um (>110 MA/cm$^2$) in this three-atom-thick
semiconductor while preserving transistor on/off current ratio > $10^6$. The
maximum current is ultimately limited by self-heating and could exceed 1 mA/um
with better device heat sinking. With their 0.1 nA/um off-current, such doped
MoS$_2$ devices approach several low-power transistor metrics required by the
international technology roadmap
