Direct relationship between sub-60 mV/dec subthreshold swing and internal potential instability in MOSFET externally connected to ferroelectric capacitor

Steep subthreshold swing (SS) in ferroelectric (FE) FETs have been intensively discussed these years in terms of the negative capacitance(NC) effect, but still under debate. This paper demonstrates the direct correlation between sub-60 mV/dec SS and internal potential $(V_{int})$ enhancement in MOSFET externally connected to FE capacitor in DC mode through systematic experiments. It is shown that $V_{int}$ enhancement only occurs in a limited voltage window, and that hysteresis-free steep SS is achievable by tuning the paraelectric capacitance. The present results support that the steep SS values so far reported are tightly related to FE domain switching around the coercive field rather than the ideal NC effect.