Miniature sensor nodes have unique feature sets that include wireless communication, energy harvesting, and a small form-factor, thus enabling non-invasive, secure placement for biomedical, ecological, surveillance, and infrastructure applications, among others. There has been substantial research on the miniaturization of sensing systems. The size of the bare die is often only 1-2 mm; however, the associated systems are typically much larger than just the die, resulting in centimeter-size systems due to included peripherals such as batteries and casings. This leads to a design challenge for the electronics of miniaturized systems because the maximum physical battery size and battery storage capacity are severely limited. For a system with a millimeter-scale battery to survive for several days (up to a month), its average power consumption must be within the 2-200 nW range.

To successfully meet this energy limitation, I was a key contributor to the development of a millimeter-scale sensing platform called the Michigan Micro Mote in 2011. To optimize circuit performance, the system is constructed from dies fabricated in different technologies, which are then stacked and wire-bonded together. The stacked structure increases the silicon area per unit volume and also makes it easy to swap layers in and out for flexibility in the system configuration.

For this type of system, I designed low-power analog and mixed-signal circuits that included an energy harvester, power management unit, timer, sensor interface, battery voltage management, optical receiver, and voltage/current reference. I also developed miniaturized systems for specific applications in fields such as ecology, energy exploration, and biomedical study. I have collaborated with scientists and engineers in the following areas and successfully achieved useful outputs from this interdisciplinary research:

  • Solid-state devices in electrical engineering for photovoltaic energy device
  • Chemical and mechanical engineering for battery for use at high temperatures
  • Chemistry for sensor encapsulation
  • Biology and ecology for snail study and field test
  • Biomedical engineering for implantable sensor development and in vivo test

I published 1 book chapter, 18 journal articles (3 first-author JSSC papers and 2 first-author TCAS I papers), and 31 conference papers (1 first-author ISSCC paper, 3 first-author VLSI papers). I am currently preparing several more papers for publication.