This is not the first time chips designers have looked at using transistors below their operating voltages to conserve power, says Chandrakasan. In the 1970s, the technique was used to make chips for watches. “They were doing it primarily to reduce the energy of digital circuits, where the speed of the circuit is not so important,” he says.
Now the time has come to start delving into transistor thresholds again, says Hicks. “The circuits have gotten fast enough so that even when they are operating slowly it’s fast enough to do what we want them to do.” While processing speed is still the dominant factor in chip design for desktop computers, for mobile or embedded devices the concern has shifted. “Power is the new barrier,” says Dennis Buss, vice president of Silicon Technology Development at Texas Instruments (TI) in Dallas.
One field where the new technique might have a huge impact is radio frequency identification (RFID) tags, says Buss. These tags usually have no power source of their own. Instead, they harness the radio waves of the scanning device in order to create the power to emit a radio signal. Under these constraints, every power-saving trick goes a long way, he says. TI has already worked with Chandrakasan to produce prototypes of memory chips that enable blocks of transistors to be shifted to subthreshold voltages.
Another area where power is an issue is in medical devices, says Kaushik Roy, an electrical engineer at Purdue University in West Lafayette, IN. Roy’s group was the first to revive this technique in 1999 and is now looking at using low-power, subthreshold transistors in hearing aids. The biggest challenge, he says, is overcoming the manufacturing variations, because slight variations at the input can produce dramatic differences at the output.
One unique quality of Chandrakasan’s work is that it targets an application that requires both performance and high power, says Buss. “With cell phones there are bursts of very high performance, but then very long periods of very low activity.” Using subthreshold transistors allows one to switch back and forth, he says.