  We investigate the design of steep-slope metal-oxide-semiconductor
field-effect transistors (MOSFETs) exploiting monolayers of transition metal
dihalides as channel materials. With respect to other previously proposed
steep-slope transistors, these devices require simplified manufacturing
processes, as no confinement of the 2D material is needed, nor any tunneling
heterojunction or ferroelectric gate insulators, and only n- or p-type contacts
are demanded. We demonstrate their operation by studying an implementation
based on monolayer CrI$_2$ through quantum transport simulations. We show that
the subthermionic capabilities of the device originate from a cold-source
effect, intrinsically driven by the shape of the band structure of the 2D
material and robust against the effects of thermalization induced by the
electron-phonon interactions. Due to the absence of a tunneling barrier when
the device is switched on, current levels can be achieved that are typically
out of reach for tunnel FETs. The device also exhibits excellent scaling
properties, maintaining a subthermionic subthreshold swing (SS) up to channel
lengths as short as 5 nm.
