Transient negative differential capacitance (NC), the dynamic reversal of transient capacitance in an electrical circuit is of highly technological and scientific interest since it probes the foundation of ferroelectricity. In this letter, we study a resistor-ferroelectric capacitor (R-FeC) network through a series of coupled equations based on Kirchhoffs law, Electrostatics, and Landau theory. We show that transient NC in a R-FeC circuit originates from the mismatch between rate of free charge change on the metal plate and that of bound charge change in a ferroelectric (FE) capacitor during polarization switching. This transient charge dynamic mismatch is driven by the negative curvature of the FE free energy landscape. It is also analytically shown that a free energy profile with the negative curvature is the only physical system that can describe transient NC during the two-state switching in a FE capacitor. Furthermore, this transient charge dynamic mismatch is justified by the dependence of external resistance and intrinsic FE viscosity coefficient. The depolarization effect on FE capacitors also shows the importance of negative curvature to transient NC. The relation between transient NC and negative curvature provides a direct insight into the free energy landscape during the FE switching.

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A ferroelectric semiconductor junction is a promising two-terminal ferroelectric device for nonvolatile memory and neuromorphic computing applications. In this work, we propose and report the experimental demonstration of asymmetric metal/α-In2Se3/Si crossbar ferroelectric semiconductor junctions (c-FSJs). The depletion in doped Si is used to enhance the modulation of the effective Schottky barrier height through the ferroelectric polarization. A high-performance α-In2Se3 c-FSJ is achieved with a high on/off ratio > 104 at room temperature, on/off ratio > 103 at an elevated temperature of 140 °C, retention > 104 s, and endurance > 106 cycles. The on/off ratio of the α-In2Se3 asymmetric FSJs can be further enhanced to >108 by introducing a metal/α-In2Se3/insulator/metal structure.

In this paper, a theoretical approach comprising the nonequilibrium Green's function method for electronic transport and the Landau-Khalatnikov equation for electric polarization dynamics is presented to describe polarization-dependent tunneling electroresistance (TER) in ferroelectric tunnel junctions. Using appropriate contact, interface, and ferroelectric parameters, the measured current-voltage characteristic curves in both inorganic (Co=BaTiO 3 =La 0.67 Sr 0.33 MnO 3 ) and organic (Au=PVDF=W) ferroelectric tunnel junctions can be well described by the proposed approach. Furthermore, under this theoretical framework, the controversy of opposite TER signs observed experimentally by different groups in Co=BaTiO 3 =La 0.67 Sr 0.33 MnO 3 systems is addressed by considering the interface termination effects using the effective contact ratio defined through the effective screening length and dielectric response at the metal-ferroelectric interfaces. Finally, our approach is extended to investigate the role of a CoO x buffer layer at the Co=BaTiO 3 interface in a ferroelectric tunnel memristor. It is shown that in order to have a significant memristor behavior not only the interface oxygen vacancies but also the CoO x layer thickness may vary with the applied bias.

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Physical phenomena underlying operation of ferroelectric field-effect transistors (FeFETs) is treated within a unified simulation framework. The framework incorporates the Landau mean-field treatment of free energy of a ferroelectric and the polarization dynamics according to Landau-Khalatnikov (LK) equation. These equations are self-consistently solved with the one-dimensional metal-oxide-semiconductor (MOS) structure electrostatics and the drift-diffusion solution for the current in the semiconductor channel. Numerical simulations demonstrate, depending on the ferroelectric (FE) thickness, both regimes of hysteresis switching (relevant for a non-volatile memory) and of higher on-currents and steeper subthreshold slope (SS) with a negligible hysteresis (relevant for logic) via the negative capacitance effect.

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