The concept of ubiquitous wireless sensor networks has been proposed to provide persistent and high spatial/temporal solution sensing in a variety of applications. Although technology has progressed to result in ever smaller and more capable sensors and processors, the realization of small unattended sensor nodes with long lifetime remains an open problem. State-of-the-art (SOA) sensors use active electronics to monitor the environment for the external trigger, consuming power continuously and limiting the sensor lifetime to months or less.
A UC Davis team lead by Prof.~David Horsley was funded by DARPA to design novel persistent wireless sensor front-ends that are nearly passive, i.e. consuming extremely low power ($<$10\,nW, lower than the leakage power of a battery). Our role in the project was to develop extremely sensitive passive RF power detectors and energy conversion circuitry. We proposed to use a high-Q piezoelectric MEMS resonators as a high-Q inductor to achieve passive voltage amplification, therefore boosting the efficiency of the rectifiers. We demonstrated the effectiveness of the idea through theoretical derivation and experimental validation. In particular, we showed >10x rectification efficiency boost in a MEMS-CMOS integrated fabrication process.