Transient nature of negative capacitance in ferroelectric field-effect transistors

Abstract

Abstract-Negative capacitance (NC) in ferroelectrics, which stems from the imperfect screening of polarization, is considered a viable approach to lower voltage operation in the field-effect transistors (FETs) used in logic switches. In this paper, we discuss the implications of the transient nature of negative capacitance for its practical application. It is suggested that the NC effect needs to be characterized at the proper time scale to identify the type of circuits where functional NC-FETs can be used effectively.Index Terms-Fe-FET, ferroelectric, MOSFET, NC-FET, negative capacitance, screening, steep subthreshold, transient.Recently, there has been much interest in the negative capacitance (NC) phenomenon in ferroelectrics, which could potentially reduce the energy consumption in field-effect transistors (FETs) due to the enhanced subthreshold slope [1][2][3]. While there is much debate on the physics, interpretation, and applications of this NC effect, there is a general consensus that the phenomenon is metastable in nature [4]. In order to measure negative capacitance, as well as to put this phenomenon to use in a real device, particular constraints have to be considered. In particular, Catalan et al [5] pointed out the transient effects associated with the dynamics of screening processes in ferroelectric capacitors. We follow up on that discussion and address the transient nature of NC-FET functionality that must follow.One of the main problems in transistor scaling is the power consumption, which turns into heat of the integrated circuits. In MOSFETs, it is critical to maximize the subthreshold slope so that a certain ON/OFF current ratio can be obtained with the minimum supply voltage. However, semiconductor physics places a fundamental theoretical limit on the maximum slope at 60 mV/decade at room temperature [6]. Exploiting negative capacitance in NC-FET can presumably overcome this limit to achieve a class of low-power transistors with a steep subthreshold slope sharper than 60 mV/decade.In the ferroelectric-based NC-FET, the starting assumption is the negative capacitance from the section with a negative slope in the P-E (polarization-field) curve shown in Fig. 1(a), which arises from the thermodynamic consideration of electrically induced transition from one polarization state to the opposite . one. In this paper, we discuss the transient electrical behavior of the ferroelectric-based NC-FET.A conventional P-E hysteresis curve typically measured in ferroelectric capacitors (Fig. 1(b)) does not exhibit a negative slope, and hence, in contrast to the loop in Fig. 1(a), has no negative capacitance. The schematics shown in Fig. 2 help illustrate this difference and the benefits of negative capacitance. The NC-FET structure comprises a ferroelectric layer added on top of the gate dielectric of a regular MOSFET. The gate voltage Vg applied in the direction of turning the transistor ON drops partially across the ferroelectric, VFE, and partially across the MOSFET, VFET, so that Vg = VFE + VFET. With an ...

Compensation Charge Control and Modeling for Reproducing Negative Capacitance Effect of Hafnia Ferroelectric Thin Films

“…That is, in order to observe a voltage drop, a voltage higher than the coercive voltage must be applied to the ferroelectric in a short time to maximize the change in the capacitance due to the polarization switching, while simultaneously, the magnitude of the current must be small enough to not compensate for polarization switching. This is consistent with the claim made by Ng et al in 2007 [33] that the rate and amount of the compensating charge (screening charge) in ferroelectric capacitors greatly influence the improvement of the SS value. To apply a high voltage to the ferroelectric for a short period of time, a low series resistor must be used, but in this case, since the amount of current increases, it is difficult to satisfy the condition of lowering the rate of the compensating charge (or screening charge).…”

“…This is consistent with the claim made by Ng et al. in 2007 [ 33 ] that the rate and amount of the compensating charge (screening charge) in ferroelectric capacitors greatly influence the improvement of the SS value. To apply a high voltage to the ferroelectric for a short period of time, a low series resistor must be used, but in this case, since the amount of current increases, it is difficult to satisfy the condition of lowering the rate of the compensating charge (or screening charge).…”

Ferroelectric negative capacitance

“…Similar behaviour has been reported by many groups; however, its interpretation as a signature of the traversal of the negative ∂Q/∂V part of the ferro electric Sshaped curve remains controversial. It is gen erally accepted that a realistic model should include a domainmediated process 13 and that the observed volt age reduction originates from the mismatch between the rate at which the ferroelectric polarization switches and the delivery of the charge to the metallic plates of the capacitor 38,39 . The question remains whether the negative curvature of the free energy has to be involved in this process 37,39,[46][47][48] .…”

“…However, the ferroelectric response can also enable a temporary reduction in the corresponding voltage V(t) by increasing the charge Q(t) as a function of time t dur ing ferroelectric switching. This behaviour has long been discussed in the literature 7,36 and has recently been called transient NC [37][38][39][40][41] .…”

Thermodynamic equilibrium theory revealing increased hysteresis in ferroelectric field-effect transistors with free charge accumulation

“…A second strategy exploits a positive feedback mechanism that amplifies the electrostatic control of the gate over the channel surface potential, such as in the nanoelectromechanical FET [11][12][13] and the steep-slope ferroelectric FET (SS-FeFET) [14][15][16][17] . In a SS-FeFET, the internal voltage amplification is realized either by means of abrupt (non-stabilized SS-FeFET) [18][19][20][21][22][23] , or gradual polarization switching (stabilized SS-FeFET) [24][25][26][27][28][29][30][31] of the ferroelectric.…”