This paper clarifies for the first time the origin of ferroelectricity in the Negative Capacitance Field-Effect Transistors (NCFETs) by molecular dynamics (MD) simulation. MD simulation considering atomic interactions between all atoms enables accurate predictions for the microstructure even at all interfaces. By incorporating the results from MD simulations into a kinetic model, it is able to predict the conditions of crystallization and phase transition during RTP and cooling processes that govern ferroelectricity in FETs. Our simulation reveals that the comparable interfacial energy between o-and t-phase, and in-plane tensile stress from metal capping or interfacial layers (ILs) enable more phase transition from t-to o-phase, and more ferroelectricity in NCFETs. Finally, design methodology to maintain the electric variation of NCFETs is also proposed.