Semiconductor devices with self-feedback mechanisms are considered as a promising alternative to traditional CMOS, in order to achieve faster operation and lower switching energy. Examples include IMOS and FBFET that are operated in a non-equilibrium condition to rapidly generate mobile carriers [1-2]. More recently, Fe-FET was proposed to improve the switching by integrating a ferroelectric material as gate insulator in a MOSFET structure [3-5]. Under particular circumstance, ferroelectric capacitance is effectively negative, due to the negative slope of its polarization-electrical field (P-E) curve. This property makes the ferroelectric layer a voltage amplifier to boost surface potential, achieving fast transition. In this paper: (1) A new threshold voltage model is developed to capture the feedback of negative capacitance and IV characteristics of Fe-FET; (2) It is further revealed that the impact of random dopant fluctuation (RDF) on leakage variability can be significantly suppressed in Fe-FET, by tuning the thickness of the ferroelectric layer.