A Thermodynamic Perspective of Negative-Capacitance Field-Effect Transistors

Physical phenomena, underlying operation of negative capacitance field-effect transistors (NCFETs), are treated within a unified simulation framework. The framework incorporates the Landau mean-field treatment of free energy of a ferroelectric (FE) and the polarization dynamics according to Landau-Khalatnikov (LK) equation. These equations are self-consistently solved with the 1-D metal-oxide- semiconductor structure electrostatics and the drift-diffusion solution for currents in the semiconductor channel. Numerical simulations show that, depending on the strength of depolarization field, both regimes of hysteresis switching and of higher on-currents and steeper subthreshold slope with a negligible hysteresis can be delivered by NCFETs under the steady-state condition due to either multiple local energy minima or a single global energy minimum in the free energy landscape. The transient behavior of NCFETs is also studied by the viscosity coefficient in LK equation, and it is found that when the FE response becomes too slow, a significant hysteresis effect on currents shows up even for a free energy landscape with a single global energy minimum. Finally, recent experimental measurements on transient NC in an FE capacitor are discussed. It is shown that those results on transient NC can be due to the mismatch between free charge and polarization driven by the negative curvature of FE thermodynamic energy profile during polarization switching and cannot lead to a transient charge-boost in a hysteresis-free NCFETs.