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.