A temperature sensor was developed that runs on 113 picowatts of power — about 10 billion times smaller than a Watt. The technology could enable devices that can be powered by harvesting energy from low-power sources, such as the body or the surrounding environment. Potentially, the system could run for years on a tiny battery.
The approach involves minimizing power in two domains: the current source and the conversion of temperature to a digital readout. An ultra-low-power current source was built using gate leakage transistors — transistors in which tiny levels of current leak through the electronic barrier, or the gate. Transistors typically have a gate that can turn the flow of electrons on and off. But as the size of modern transistors continues to shrink, the gate material becomes so thin that it can no longer block electrons from leaking through — a phenomenon known as the quantum tunneling effect. Gate leakage is considered problematic in systems such as microprocessors or precision analog circuits. In the new technology, minuscule levels of electron flow are used to power the circuit.
Using these current sources, a less power-hungry way to digitize temperature was developed. This process normally requires passing current through a resistor — its resistance changes with temperature — then measuring the resulting voltage and converting that voltage to its corresponding temperature using a high-power analog-to-digital converter. Instead of this conventional process, the new system digitizes temperature directly and saves power. The 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, and faster at higher temperatures.
As the temperature changes, the system adapts so the temperature-dependent current source charges in the same amount of time as the fixed current source. A built-in digital feedback loop equalizes the charging times by reconnecting the temperature-dependent current source to a capacitor of a different size. The size of this capacitor is directly proportional to the actual temperature; for example, when the temperature falls, the temperature-dependent current source will charge more slowly, and the feedback loop compensates by switching to a smaller capacitor, which dictates a particular digital readout.
The temperature sensor is integrated into a small chip measuring 0.15 × 0.15 square millimeters in area. It operates at temperatures ranging from –20 to 40 °C. One tradeoff is that the sensor has a response time of approximately one temperature update per second, which is slightly slower than existing temperature sensors. However, this response time is sufficient for devices that operate in the human body, and in other environments where temperatures do not fluctuate rapidly.