Physics of ferroelectric differential capacitance based upon free energy, and implications for use in electronic devices

In this paper, it is shown that for stable, steady state operation of devices typical of microwave and millimeter wave electronics, no negative differential capacitance is possible with conventional thinking. However, it may be possible, with strain engineering of materials, to obtain some if not all elements of the differential capacitance tensor which are negative. Rigorous derivations are provided based upon analyzing the physics using thermodynamic phenomenological free energy. It should be emphasized that, even with strain engineering, and possible discovery of some negative capacitive elements, stable operation will not be obtained because the thermodynamics precludes it

Nanowire and Nanocable Intrinsic Quantum Capacitances and Junction Capacitances: Results for Metal and Semiconducting Oxides

Here we calculate the intrinsic quantum capacitance of RuO2 nanowires and RuO2/SiO2 nanocables (filled interiors of nanotubes, which are empty), based upon available ab initio density of states values, and their conductances allowing determination of transmission coefficients. It is seen that intrinsic quantum capacitance values occur in the aF range. Next, expressions are derived for Schottky junction and p-n junction capacitances of nanowires and nanocables. Evaluation of these expressions for RuO2 nanowires and RuO2/SiO2 nanocables demonstrates that junction capacitance values also occur in the aF range. Comparisons are made between the intrinsic quantum and junction capacitances of RuO2 nanowires and RuO2/SiO2 nanocables, and between them and intrinsic quantum and junction capacitances of carbon nanotubes. We find that the intrinsic quantum capacitance of RuO2-based nanostructures dominates over its junction capacitances by an order of magnitude or more, having important implications for energy and charge storage

Evaluation of the differential capacitance for ferroelectric materials using either charge-based or energy-based expressions

Differential capacitance is derived based upon energy, charge or current considerations, and determined when it may go negative or positive. These alternative views of differential capacitances are analyzed, and the relationships between them are shown. Because of recent interest in obtaining negative capacitance for reducing the subthreshold voltage swing in field effect type of devices, using ferroelectric materials characterized by permittivity, these concepts are now of paramount interest to the research community. For completeness, differential capacitance is related to the static capacitance, and conditions when the differential capacitance may go negative in relation to the static capacitance are shown