At this year's Consumer Electronics Show in Las Vegas, the big theme was the "Internet of things" -- the idea that everything in the human environment, from kitchen appliances to industrial equipment, could be equipped with sensors¹ and processors that can exchange data, helping² with maintenance and the coordination³ of tasks. Realizing that vision, however, requires transmitters that are 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 vibrations⁴.

 

"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," explains 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 leakage⁵ power."

 

This week, 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 protocol⁶.

 

"The trick is that we borrow techniques that we use to reduce the leakage power in digital circuits," Chandrakasan explains. The basic element of a digital circuit is a transistor⁷, in which two electrical leads are connected by a semiconducting material, such as silicon⁸. In their native states, semiconductors¹⁰ are not particularly good conductors. But in a transistor, the semiconductor⁹ has a second wire sitting on top of it, which runs perpendicularly¹¹ 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 insulators¹². Even when no charge is applied¹³ 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.

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