On the Validity and Applicability of Models of Negative Capacitance and Implications for MOS Applications

TL;DR

This paper critically examines models of negative capacitance in ferroelectric MOSFETs, highlighting unphysical assumptions and proposing a clarified model that explains observed effects without implying true negative capacitance, impacting device applications.

Contribution

It provides a thorough analysis of negative capacitance models, identifies unphysical assumptions, and proposes a realistic model clarifying conditions for apparent negative capacitance in ferroelectric-dielectric stacks.

Findings

QS negative capacitance is unlikely in series capacitor models.

Strong interface coupling is necessary for apparent NC effects.

Observed sub-60 mV/dec slopes are not due to true negative capacitance.

Abstract

The observation of room temperature sub-60 mV/dec subthreshold slope (SS) in MOSFETs with ferroelectric (FE) layers in the gate stacks or in series with the gate has attracted much attention. Recently, we modeled this effect in the framework of a FE polarization switching model. However, there is a large amount of literature attributing this effect to a stabilization of quasi-static (QS) negative capacitance (NC) in the FE. The technological implications of a stabilized non-switching (NS) QSNC model vs a FE switching model are vastly different; the latter precluding applications to sub-60 mV/dec SS scaled CMOS due to speed limitations and power dissipated in switching. In this letter, we provide a thorough analysis assessing the foundations of models of QSNC, identifying which specific assumptions (ansatz) may be unlikely or unphysical, and analyzing their applicability. We show that it is not reasonable to expect QSNC for two separate capacitors connected in series (with a metal plate between dielectric (DE) and FE layers). We propose a model clarifying under which conditions a QS "apparent NC" for a FE layer in a FE-DE bi-layer stack may be observed, quantifying the requirements of strong interface polarization coupling in addition to capacitance matching. In this regime, our model suggests the FE layer does not behave as a NC layer, simply, the coupling leads to both the DE and FE behaving as high-k DE with similar permittivities. This may be useful for scaled EOT devices but does not lead to sub-60 mV/dec SS.