Nishit Harshad Shah
From Murmann Mixed-Signal Group
Research: Energy Harvesting for CMOS Imagers
Email: nishits AT stanford DOT edu
As the form factor and weight of the portable devices shrink, the battery technology has to keep up with the scaling of integrated circuits. Though several thin film printed batteries and super capacitors have made considerable improvements, the energy densities of these devices is considerably lower. To circumvent this situation and proceed with lighter and smaller devices, energy harvesting has become an unavoidable resort to recharge the batteries.
This project focuses on harvesting energy from ambient light. It finds applications in all image sensors that are not actively capturing images. The photodiodes of image sensors can work as miniature solar cells and deliver photoelectric energy [1-2]. This harvested energy is initially stored in on-chip capacitors and later boosted to battery voltage or to on-chip power supply voltage.
There are several trade-offs in the design of photo-electric energy harvesting circuits for imaging applications. As the energy harvested will vary from few nanowatts to several microwatts, the energy harvesting circuit needs to be extremely efficient and support wide input range. Techniques like using back-surface illumination or micro-lens used in traditional multi-junction solar cells cannot be employed for a CMOS image sensor. The power generating area in this application is also much smaller than that of a solar cell.
A test chip has been taped out where energy harvesting capability of different photodiodes is going to be evaluated. To implement maximum power point tracking (MPPT) algorithms, a scheme to change the open circuit voltage is being explored.
 A. Fish, S. Hamami and O. Yadid-Pecht, “CMOS Image Sensors With Self-Powered Generation Capability”, IEEE Trans. Circuits Syst. II, Express Briefs, Vol. 53, No. 11, November 2006.
 S. U. Ay, “A CMOS Energy Harvesting and Imaging (EHI) Active Pixel Sensor (APS) Imager for Retinal Prosthesis”, IEEE Trans, on Biomedical Circuit and Systems, Vol.. 5, No. 6, December 2011.