  Two-dimensional (2D) semiconductors are promising material candidates for
next-generation nanoelectronics. However, there are fundamental challenges
related to their metal-semiconductor (MS) contacts which limit the performance
potential for practical device applications. In this work, we exploit 2D
monolayer hexagonal boron nitride (h-BN) as an ultrathin decorating layer to
form a metal-insulator-semiconductor (MIS) contact, and demonstrate a novel
diode-like selective enhancement of the carrier transport through it. Compared
to the conventional MS contact, the MIS contact dominated by both thermionic
emission and quantum tunneling can significantly reduce the contact resistance
and boost the electron transport from the semiconductor to the metal, but has
negligible effects on the electron transport oppositely. We also investigate
the negative barrier height with the concept of carrier collection barrier, and
show its critical role at the drain end for determining the overall transistor
performance.
