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Ultra-low-power transistor solves battery woes for wearables and IoT

Wearable devices and the Internet of Things (IoT) could see huge benefits from a newly-developed form of ultra-low-power transistor.

Developed by engineers at the University of Cambridge, devices based on the new transistor could function for months or even years without a battery by ‘scavenging’ energy from their environment.

Using a similar principle to a computer in sleep mode, the transistor harnesses a tiny ‘leakage’ of electrical current, known as a near-off-state current, for its operations.

Although this leak is a characteristic of all transistors, this is the first time that it has been effectively captured and used functionally.

The transistors can be produced at low temperatures and can be printed on almost any material, from glass and plastic to polyester and paper.

They are based on a unique geometry that uses a ‘non-desirable’ characteristic, namely the point of contact between the metal and semiconducting components of a transistor, a so-called ‘Schottky barrier’.

“We’re challenging conventional perception of how a transistor should be,” said Professor Arokia Nathan of Cambridge’s Department of Engineering. “We’ve found that these Schottky barriers, which most engineers try to avoid, actually have the ideal characteristics for the type of ultra-low-power applications we’re looking at, such as wearable or implantable electronics for health monitoring.”

The new design gets around one of the main issues preventing the development of ultra-low-power transistors, namely the ability to produce them at very small sizes.

As transistors get smaller, their two electrodes start to influence the behaviour of one another, and the voltages spread, meaning that below a certain size, transistors fail to function as desired.

By changing the design of the transistors, the Cambridge researchers were able to use the Schottky barriers to keep the electrodes independent of each other, so that the transistors can be scaled down to very small geometries.

The design also achieves a very high level of gain, or signal amplification. The transistor’s operating voltage is less than a volt, with power consumption below a billionth of a watt.

This ultra-low power consumption makes them most suitable for applications where function is more important than speed, such as in IoT devices.

“If we were to draw energy from a typical AA battery based on this design, it would last for a billion years,” said Dr Sungsik Lee. “Using the Schottky barrier allows us to keep the electrodes from interfering with each other in order to amplify the amplitude of the signal even at the state where the transistor is almost switched off.”

“This will bring about a new design model for ultra-low-power sensor interfaces and analogue signal processing in wearable and implantable devices, all of which are critical for the Internet of Things,” said Nathan.

“This is an ingenious transistor concept,” said Professor Gehan Amaratunga, head of electronics at Cambridge’s Engineering Department. “This type of ultra-low-power operation is a prerequisite for many of the new ubiquitous electronics applications, where what matters is function - in essence ‘intelligence’ - without the demand for speed.

“In such applications the possibility of having totally autonomous electronics now becomes a possibility. The system can rely on harvesting background energy from the environment for very long term operation, which is akin to organisms such as bacteria in biology.”

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