  Since many years, sub-60mV/decade switching has been reported in
ferroelectric FETs. However, thus far these reports have lacked full physical
explanation since they typically use a negative capacitance in the
ferroelectric layer to be able to explain the experimental observations.
Because negative capacitance as such is not a physical concept, we propose an
alternative model that relies on the non-linear and non-equilibrium behavior of
the ferroelectric layer. It is shown that a steep subthreshold slope can be
obtained by a 2-step switching process, referred to as nucleation and
propagation. Making use of the concept of domain wall motion as known also from
fracture dynamics, we are able to explain the steep slope effect. A simple
mathematical model is added to further describe this phenomenon, and to further
investigate its eventual benefit for obtaining steep slope transistors in the
sub-10nm era.
