Two types of ultra-fast diode are fabricated, characterized, and simulated for use in optical rectennas. A rectenna consists of an antenna connected to a diode in which the electromagnetic radiation received by the antenna is rectified in the diode. I have investigated metal/insulator/metal (MIM) tunnel diodes and a new, geometric diode for use in rectenna-based infrared detectors and solar cells. Factors influencing the performance of a rectenna are analyzed. These include DC and optical-frequency diode-characteristics, circuit parameters, signal amplitude, and coherence of incoming radiation.
To understand and increase the rectification response of MIM-based rectennas, I carry out an in-depth, simulation-based analysis of MIM diodes and design improved multi-insulator tunnel barriers. MIM diodes are fundamentally fast. However, from a small-signal circuit model the operating frequency of a rectenna is found to be limited by the diode\u27s RC time constant. To overcome this limitation, I have designed and simulated a distributed rectifier that uses the MIM diode in a traveling-wave configuration. High-frequency characteristics of MIM diodes are obtained from a semiclassical theory for photon-assisted tunneling. Using this theory, the dependence of rectenna efficiency on diode characteristics and signal amplitude is evaluated along with the maximum achievable efficiency. A correspondence is established between the first-order semiclassical theory and the small-signal circuit model.
The RC time constant of MIM diodes is too large for efficient operation at near-infrared-to-visible frequencies. To this end, a new, planar rectifier that consists of an asymmetrically-patterned thin-film, is developed. The diode behavior in this device is attributed to the geometric asymmetry of the conductor. Geometric diodes are fabricated using graphene and measured for response to infrared illumination. To model the I(V) curve of geometric diodes, I have implemented a quantum mechanical simulation based on the tight-binding Hamiltonian. The simulated and the measured current-voltage characteristics are consistent with each other. I have also derived a semiclassical theory, analogous to the one for MIM diodes, for analyzing the optical response of geometric diodes
