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New Temperature Sensor Could Extend Battery Life For Many Devices
October 19, 2017

As they strive to meet the ever growing demand for smaller electronic devices, researchers have been stymied by the challenge of providing adequate operational power. With batteries being the standard power source, the trick has been to design efficient operating systems to facilitate longer battery life.

According to information provided by the Jacobs School of Engineering at the University of California San Diego (UC San Diego), a team of electrical engineers has developed a temperature sensor that runs on only 113 picowatts of power — 628 times lower power than the state-of-the-art sensors and about 10 billion times smaller than a watt. This “near-zero-power” temperature sensor could extend the battery life of wearable or implantable devices that monitor body temperature, smart home monitoring systems, Internet of Things devices and environmental monitoring systems.

The temperature sensor is integrated into a small chip measuring 0.15 × 0.15 square millimeters in area. Credit: David Baillot/UC San Diego Jacobs School of Engineering.

The technology could also enable a new class of devices that can be powered by harvesting energy from low-power sources, such as the body or the surrounding environment, researchers said. With support provided by the Arnold and Mabel Beckman Foundation, details of the research were presented in a paper, “Near-Zero-Power Temperature Sensing via Tunneling Channels Through Complementary Metal-Oxide-Semiconductor Transistors,” published in the journal Scientific Reports.

“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 Patrick Mercier, an electrical engineering professor at UC San Diego and the study’s senior author.

“We’re building systems that have such low power requirements that they could potentially run for years on just a tiny battery,” said Hui Wang, an electrical engineering Ph.D. student in Prof. Mercier’s lab and the first author of the study.

The researcher’s new approach involves minimizing power in two domains: the current source and the conversion of temperature to a digital readout.

The research team built an ultra-low power current source using what are called “gate leakage” transistors — transistors in which tiny levels of current leak through the electronic barrier, or the gate, a phenomenon known as the quantum tunneling effect. Considered problematic in systems such as microprocessors or precision analog circuits, Prof. Mercier and his team are taking advantage of it by using these minuscule levels of electron flow to power the circuit.

Using these current sources, researchers developed a less power-intensive method to digitize temperature. Instead of the more conventional process, researchers developed an innovative system to digitize temperature directly and save power. Their system consists of two ultra-low power current sources: one that charges a capacitor in a fixed amount of time regardless of temperature, and one that charges at a rate that varies with temperature — slower at lower temperatures, faster at higher temperatures.

The temperature sensor is integrated into a small chip, 0.15 × 0.15 square millimeters in area, operating at temperatures ranging from minus 20°C to 40°C. According to the researchers, the new temperature sensor is fairly comparable in performance to that of current state-of-the-art sensors even at near-zero-power, however one trade-off is that the sensor has a response time of approximately one temperature update per second, slightly slower than existing sensors. Despite this slower update speed, the response time is sufficient for devices that operate in the human body, homes and other environments where temperatures do not fluctuate rapidly, researchers said.

With a provisional patent pending for this technology, the team is now working to improve the accuracy of the temperature sensor as well as optimizing the design so that it can be successfully integrated into commercial devices.

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