Assessing the role of quantum effects in 2D heterophase MoTe$_2$ field effect transistors

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.