On the Physical Mechanism of Transient Negative Capacitance Effect in Deep Subthreshold Region

Abstract

We have investigated the physical mechanism of steep subthreshold slope (SS) in ferroelectric FET (FeFET) based on a dynamic ferroelectric (FE) model without traversing the negative capacitance (NC) region of the S-shaped polarization-voltage predicted by Landau theory. The dynamic FE model is applied to an FE-dielectric (FE-DE) series capacitor as well as FeFET after calibration and verification by transient measurement of an FE-HfO 2 capacitor. By investigating current through the FE-DE series capacitor and the gate capacitor of FeFET, we find that incomplete screening of spontaneous polarization charge results in transient NC and sub-60 mV/dec SS. Also, it should be noted that, for FeFET, small depletion layer capacitance has an important role to cause strong depolarization effect and thus steep SS. Moreover, reverse drain induced barrier lowering happens even with this FE model. The model presented in this paper provides a reasonable interpretation for the previously reported steep SS of NC FETs. INDEX TERMS Steep subthreshold slope (SS), ferroelectric, negative capacitance (NC).

Physical Mechanisms of Reverse DIBL and NDR in FeFETs With Steep Subthreshold Swing

“…In other words, the depolarization effect caused by the switched spontaneous polarization results in TNC, if the screening by free charge at the gate electrode cannot response fast enough. Moreover, both |dP/dt| and |I g | = |dQ/dt| are suppressed by small depletion capacitance in subthreshold region [10], which is consistent with the experimental result in [31]. The extracted Q fe − V fe also exhibits TNC in certain region, as shown in Fig.…”

“…In certain V g region, |dP/dt| is larger than |I g | = |dQ/dt| (green shade in Fig. 9 (a)), leading to TNC and SS improvement [10], [17], [30]. This can be better understood by considering the charge balance condition: Q = ε 0 ε r V fe /t fe +P.…”

“…Finally, polarization switching delay is introduced to simulate the dynamic behavior of ferroelectric [10], [14]. The delay is determined by a first order differential equation (the equation in Fig.…”

“…In our previous work, both a ferroelectric-dielectric (FE-DE) series capacitor and FeFET were simulated to investigate the physical mechanism of sub-60mV/dec SS based on the TNC theory [10]. We found that TNC and sub-60mV/dec SS are induced under the condition of |dP/dt| > |dQ/dt| (depolarization effect) in transient case.…”

“…1 (b)) without traversing the QSNC region of the S-curve [10]- [15]. Even with the conventional multi-domain model in which the quasi-static capacitance of ferroelectric is always positive, NC may happen in transient conditions due to the incomplete screening of spontaneous polarization charge [10], [11], [16]- [18]. Note that polarization switching dynamics is responsible for NC effect in transient conditions according to the TNC theory.…”