Two-Dimensional Tunable Reactance Element Free from Electromagnetic Coupling

A capacitor modeled as a parallel combination of a resistance (R) and a capacitance (C) exhibits three distinct operating regimes when both parameters depend on the applied voltage (V): a positive-capacitance regime (dR/R>dV/V), an Ohmic regime (dR/R=dV/V), and a negative-capacitance regime (dR/R<dV/V). In the limit (R→∞), the device behaves as a conventional permittivity-based capacitor, whereas in the limit (R→0), negative capacitance emerges due to nonlinear current–voltage characteristics. To verify this mechanism, we fabricated nanometer-spaced two-electrode structures using multi-walled carbon nanotubes (MWCNTs) and Si crystals. The measurements confirmed negative capacitance consistent with theoretical predictions. Unlike ferroelectric negative capacitance, the effect demonstrated here arises solely from the nonlinear I–V characteristics at the electrode interfaces, without involving any ferroelectric polarization dynamics. This negative capacitance can be interpreted as an equivalent inductance, enabling a two-dimensional tunable reactance element (TDTRE) that operates without electromagnetic coupling and is compatible with conventional IC technologies.