  Analytical calculations corroborated by the finite element modelling show
that thin films of Van der Waals ferrielectrics covered by a 2D-semiconductor
are promising candidates for the controllable reduction of the dielectric layer
capacitance due to the negative capacitance (NC) effect emerging in the
ferrielectric film. The NC state is conditioned by energy-degenerated
poly-domain states of the ferrielectric polarization induced in the films under
incomplete screening conditions in the presence of a dielectric layer.
Calculations performed for the FET-type heterostructure "ferrielectric CuInP2S6
film - 2D-MoS2 single-layer - SiO2 dielectric layer" reveal the pronounced size
effect of the multilayer capacitance. Derived analytical expressions for the
electric polarization and multilayer capacitance allow to predict the thickness
range of the dielectric layer and ferrielectric film for which the NC effect is
the most pronounced in various Van der Waals ferrielectrics, and the
corresponding subthreshold swing becomes much less than the Boltzmann's limit.
Obtained results can be useful for the size and temperature control of the NC
effect in the steep-slope ferrielectric FETs.
