This work experimentally demonstrates, for the first time, that by integrating a thin ferroelectric layer into a gate stack of a standard MOS transistor one, it is possible to overcome the 60 mV/decade subthreshold swing limit at room temperature of MOSFET. We find sub-threshold swings as low as 13 mV/decade in Fe-FETs with 40 nm P(VDF-TrFE)/SiO₂ gate stack. The mechanism governing the low subthreshold swing in Fe-FET transistors is the negative capacitance of the ferroelectric layer that provides voltage amplification; with our particular ferroelectric gate stack we report for the first time negative capacitance at room temperature.