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Elevating IoT: Reducing off-state leakage to boost efficiency

Posted: 24 Feb 2015 ?? ?Print Version ?Bookmark and Share

Keywords:MIT? IoT? transmitter? off-state leakage?

One pitfall of any Internet of Things (IoT) system is transmitter efficiency. Basically, the transmitter needs to be powerful enough to broadcast to devices dozens of yards away but energy-efficient enough to last for months, or even to harvest energy from heat or mechanical vibrations.

Circuit that reduces power leakage when transmitters are idle could greatly extend battery life, which could take IoT performance a few notches higher in terms of energy consumption.

"A key challenge is designing these circuits with extremely low standby power, because most of these devices are just sitting idling, waiting for some event to trigger a communication," said Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley professor in electrical engineering at MIT. "When it's on, you want to be as efficient as possible, and when it's off, you want to really cut off the off-state power, the leakage power."


Illustration: Jose-Luis Olivares/MIT

At the Institute of Electrical and Electronics Engineers' International Solid-State Circuits Conference, Chandrakasan's group will present a new transmitter design that reduces off-state leakage 100-fold. At the same time, it provides adequate power for Bluetooth transmission, or for the even longer-range 802.15.4 wireless-communication protocol.

"The trick is that we borrow techniques that we use to reduce the leakage power in digital circuits," Chandrakasan noted. The basic element of a digital circuit is a transistor, in which two electrical leads are connected by a semiconducting material, such as silicon. In their native states, semiconductors are not particularly good conductors. But in a transistor, the semiconductor has a second wire sitting on top of it, which runs perpendicularly to the electrical leads. Sending a positive charge through this wire, known as the gate, draws electrons toward it. The concentration of electrons creates a bridge that current can cross between the leads.

But while semiconductors are not naturally very good conductors, neither are they perfect insulators. Even when no charge is applied to the gate, some current still leaks across the transistor. It's not much, but over time, it can make a big difference in the battery life of a device that spends most of its time sitting idle.

Going negative

Chandrakasan, along with Arun Paidimarri, an MIT graduate student in electrical engineering and computer science and first author on the paper, and Nathan Ickes, a research scientist in Chandrakasan's lab, reduced the leakage by applying a negative charge to the gate when the transmitter is idle. That drove electrons away from the electrical leads, making the semiconductor a much better insulator.

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