Anomalous Capacitance Enhancement Triggered by Light

Capacitance of capacitors in which one or both plates are made of a two-dimensional charge system (2DCS) can be increased beyond their geometric structural value. This anomalous capacitance enhancement (CE) is a consequence of manipulation of quantum mechanical exchange and correlation energies in the ground state energy of the 2DCS. Macroscopically, it occurs at critical charge densities corresponding to transition from an interacting “metallic” to a noninteracting “insulator” mode in the 2-D system. Here, we apply this concept to a metal-semiconductor-metal capacitor with an embedded two-dimensional hole system (2DHS) underneath the plates for realization of a capacitance-based photodetector. Under sufficient illumination, and at critical voltages the device shows a giant CE of 200% and a peak-to-valley ratio of over 4 at probe frequencies larger than 10 kHz. Remarkably, the light-to-dark capacitance ratio due to CE at this critical voltage is well over 40. Transition of the 2DHS from insulator to metallic, enforced by charge density manipulation due to light-generated carriers, accounts for this behavior, which may be used in optical sensing, photo capacitors, and photo transistors.