Inversion Charge Boost and Transient Steep-Slope Induced by Free-Charge-Polarization Mismatch in a Ferroelectric-Metal–Oxide–Semiconductor Capacitor

Abstract

In this letter, the transient behavior of a ferroelectric (FE) metal-oxide-semiconductor (MOS) capacitor is theoretically investigated with a series resistor. It is shown that compared to a conventional high-k dielectric MOS capacitor, a significant inversion charge-boost can be achieved by a FE MOS capacitor due to a steep transient subthreshold swing (SS) driven by the free chargepolarization mismatch. It is also shown that the observation of steep transient SS significantly depends on the viscosity coefficient under Landau's mean field theory, in general representing the average FE time response associated with domain nucleation and propagation. Therefore, this letter not only establishes a theoretical framework that describes the physical origin behind the inversion charge-boost in a FE MOS capacitor, but also shows that the key feature of depolarization effect on a FE MOS capacitor should be the inversion-charge boost, rather than the steep SS (e.g., sub-60mV/dec at room temperature), which cannot be experimentally observed as the measurement time is much longer than the FE response. Finally, we outlines the required material targets for the FE response in field-effect transistors to be applicable for next-generation high-speed and low-power digital switches.The relentless pursuit of Moore's law in the past four decades leads to a significant improvement in the computing power of modern microprocessors [1]. However, as the scaling of CMOS transistors continues, the on-off current ratio is dramatically reduced, and thus the increasing static power makes the circuit design more and more difficult for high energy-efficient applications [2]. In 2008, a transistor structure using FE materials in the gate stack was proposed to improve SS through stabilizing FE negative capacitance in the steady state [3]. However, the central idea of this proposal is all based on the S-shape of polarization-electric field relation given by Landau's free energy functional, which has been under debate if it can physically describe the polarization switching in the FE oxide [4]. In 2015, a transient differential negative capacitance was reported from a circuit composed of a resistor and a FE capacitor in series [5]. Our recent work theoretically proved that this differential negative capacitance is driven by the free charge-polarization mismatch as qualitatively pointed out in Ref. [6], which can be well described under Landau's mean field theory [7]. In 2018, the transient differential negative capacitance has been also experimentally observed in a FE MOS capacitor [8], indicating the existence of transient mismatch between bound charge and free charge in a MOS structure. The effect of free charge-polarization mismatch on transient behavior of a FE transistor was qualitatively discussed in Ref. [9], where the importance of measurement time in FE transistor characterization is emphasized but no inversion charge-boost in the steady-state from a FE transistor is concluded. Hence, in this letter, a numerical model is developed to invest...

Differential Electrically Insulated Magnetoelectric Spin-Orbit Logic Circuits

“…By subtracting V(c2) from V(u1), the voltage across CFE2 capacitor in MESO #2, V(cap2), can be obtained. Because of the transient negative capacitance effect in the FE [17], the absolute value of V(cap2) increases, decreases and again increases versus time during the FE switching. The voltage across the FE goes through the S-shaped polarization charge versus voltage curve of the ME material in this transient manner.…”

Device and circuit level analysis of negative capacitance hybrid CMOS: a prospect for low power/low voltage applications

“…However, the transient operation of NCFETs is strongly dependent upon the viscosity factor (ρ) [25,26] which accounts for the polarization reversal in ferroelectrics. It offers the resistance in charge switching [27] and thus, restricts the use of NCFET for transient applications. According to the studies [25,28], the value of ρ must be below 0.1 Ω m for ferroelectric operation in MHz-GHz frequency range.…”

“…According to the studies [25,28], the value of ρ must be below 0.1 Ω m for ferroelectric operation in MHz-GHz frequency range. It has been reported that Hafnium based ferroelectric materials have the value of ρ in the range of 0.1 Ω m [27,29,30] which makes doped Hf ferroelectric materials suitable for high frequency applications. Therefore, taking into account the advantages of HCMOS and NCFETs, we have incorporated ferroelectric gate stack in p-GeOIFET (Ge-on-insulator FET) and n-SiGeOIFET (SiGe-oninsulator FET) and have implemented NCHCMOS (negative capacitance HCMOS) in the present paper.…”