Low-power temperature sensor could turn wearables into “unawareables”

The sensor, developed by a team led by Patrick Mercier of the Jacobs School of Engineering at the University of California San Diego, works by minimising power consumption in two domains: the current source of the device and the conversion of temperature to a digital readout. It takes advantage of a phenomenon generally seen as a disadvantage in electronics, known as gate leakage. The gate is the part of a semiconductor device that is capable of turning on or off the flow of electrons, but as electronics become smaller, the gate material has become so thin that electrons can pass through it even when it is closed, via the quantum tunnelling effect.

Rather than trying to eliminate leakage, the San Diego team has used the leaking electrons to power the sensor. To digitise the temperature, the team used another innovative approach: rather than passing the current through a resistor whose resistance changes with temperature, measuring the voltage across it, and converting that voltage to give a reading, they use the current to charge two capacitors. One capacitor charges in a fixed amount of time regardless of temperature, and the other charges faster at higher temperatures.

As the temperature changes, the system adapts so that both capacitors charge in the same time via additional feedback loop that switches between fixed-charging capacitors of different sizes. The size of this capacitor is therefore directly related to the temperature, and this dictates the digital readout.

The sensor consumes 113 picowatts of power: 628 times lower than the current state-of-the-art, the team claims. It is incorporated onto a chip measuring 0.15mm² in area, and operates at temperatures from -20° C to +40°C. The trade-off with its low-power consumption is its response time, the team said: it can deliver one temperature update per second, which is slightly slower than conventional sensors. However, it is still useful for measuring body temperatures and environments where temperatures do not fluctuate quickly, they added.

In a paper in Nature Scientific Reports, the team explains how the system could form part of an energy harvester that would work at ambient temperatures or from the heat of the human body. "Our vision is to make wearable devices that are so unobtrusive, so invisible that users are virtually unaware that they're wearing their wearables, making them 'unawearables.' Our new near-zero-power technology could one day eliminate the need to ever change or recharge a battery," said Mercier.