On the Physical Mechanism of Transient Negative Capacitance Effect in Deep Subthreshold Region

We have investigated the physical mechanism of steep subthreshold slope (SS) in ferroelectric FET (FeFET) based on a dynamic ferroelectric (FE) model without traversing the negative capacitance (NC) region of the S-shaped polarization-voltage predicted by Landau theory. The dynamic FE model is applied to an FE-dielectric (FE-DE) series capacitor as well as FeFET after calibration and verification by transient measurement of an FE-HfO2 capacitor. By investigating current through the FE-DE series capacitor and the gate capacitor of FeFET, we find that incomplete screening of spontaneous polarization charge results in transient NC and sub-60 mV/dec SS. Also, it should be noted that, for FeFET, small depletion layer capacitance has an important role to cause strong depolarization effect and thus steep SS. Moreover, reverse drain induced barrier lowering happens even with this FE model. The model presented in this paper provides a reasonable interpretation for the previously reported steep SS of NC FETs.