Energy Harvesting Using Substrate Photodiodes

A circuit that is able to harvest energy from substrate/well photodiodes fabricated in standard complimentary metal-oxide-semiconductor (CMOS) processes is presented. The motivation for using substrate/well photodiodes over diffusion/well photodiodes for energy harvesting are the increased quantum efficiency due to deeper junctions, the elimination of the parasitic diode effect, and better silicon area utilization. Moreover, since substrate photodiodes also work as light sensors, they provide a new solution to self-powered image sensors. The proposed circuit uses an inductor to transfer energy from the photodiode to the load and, in the process, boosts the relatively low photodiode voltage to high levels. An analytical model of the proposed energy harvesting circuit is developed. The model is used to gain insight into the factors limiting the conversion efficiency. As a proof of concept, a test microchip was fabricated in the 0.5-μm CMOS technology. Measurements from the test microchip validate the proposed circuit and the developed analytical model.