Conventional metallic contacts can be replicated by quantum two dimensional charge (of Fermion) systems (2DFS). Unlike metals, the particle concentration of these ``unconventional'' systems can be accurately controlled in an extensive range and by means of external electronic or optical stimuli. A 2DFS can, hence, transition from a high-density kinetic liquid into a dilute-but highly correlated-gas state, in which inter-particle Coulombic interactions are significant. Such interactions contribute negatively, by so-called exchange-correlation energies Eex-corr, to the overall energetics of the system, and are manifested as a series negative quantum capacitance.

This dissertation investigates the capacitive performance of a class of unconventional devices based on a planar metal-semiconductor-metal structure with an embedded 2DFS. They constitute an opto-electronically controlled variable capacitor, with record breaking merits in capacitance tuning ranges of up to 7000 and voltage sensitivities as large as 400. Internal field manipulations by localized depletion of a dense 2DFS account for the enlarged maximum and reduced minimum capacitances. The capacitance-voltage characteristics of these devices incur an anomalous ``Batman'' shape capacitance enhancement (CE) of up to 200% that may be triggered optically. The CE is attributed to the release and storage of Eex-corr, from the ``unconventional'' plate and in the dielectric, respectively. This process is enforced by density manipulation of the 2DFS by a hybrid of an external field and light-generated carriers. Under moderate optical powers, the capacitance becomes 43 times greater than the dark value. This new capacitance-based photodetection method has a range of applications in optoelectronics, particularly in the next generation of photonic integrated systems.