  The two-dimensional transition metal dichalcogenides (TMDs) have been
proposed as candidates for the channel material in future field effect
transistor designs. The heterophase design which utilizes the metallic T- or T'
phase of the TMD as contacts to the semiconducting H phase channel has shown
promising results in terms of bringing down the contact resistance of the
device. In this work, we use ab-initio calculations to demonstrate how
atomic-scale and quantum effects influence the ballistic transport properties
in such heterophase transistors with channel lengths up to 20 nm. We
investigate how the charge transfer depends on the carrier density both in T'-H
MoTe$_2$ Schottky contacts and planar T'-H-T' MoTe$_2$ transistors. We find
that the size of the Schottky barrier and the charge transfer is dominated by
the local atomic arrangements at the interface and the doping level.
Furthermore, two types of quantum states have a large influence on the charge
transport; interface states and standing waves in the semiconductor due to
quantum confinement. We find that the latter can be associated with rises in
the current by more than an order of magnitude due to resonant tunneling. Our
results demonstrate the quantum mechanical nature of these 2D transistors and
highlight several challenges and possible solutions for achieving a competitive
performance of such devices.
