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Can Transistors Be Made To Work When They're Off?
An anonymous reader writes "Engineers at the Belgian research institute IMEC are looking at the use of silicon transistors in the sub-threshold region of their operation as a way of pursuing ultra-low power goals. A chip the engineers are designing for biomedical applications could have blocks designed to operate at 0.2 or 0.3 volts, researchers said, according to EE Times. The threshold voltage is the point at which the transistor nominally switches off. Operating a transistor when it is 'off' would make use of the leakage conduction that is normally seen as wasted energy, according to the article."
As we've scaled deep into the submicron region, it's been getting harder and harder to turn the devices really "off". Leakage current has been rising and has been quite noticable for several generations now.
So the idea of doing useful work with subthreshold current sounds neat
(OK, I just went and read TFA.)
Still sounds neat, but...
In deep submicron part of the reason for the subthreshold leakage problems is control of Leff. (The effective channel length of the FETs.) There's a thing called "line length variation" which means that channel lengths in different parts of the chip will be different, sometimes subtly, sometimes not so subtly. Threshold voltage (Vt) is a strong function of channel length, making subthreshold leakage also a strong function of channel length. Performance characteristics will vary widely across the chip, likely much more than conventional transistor operation.
This will make it tough to scale down, (in feature size) scale up, (in chip size) and make manufacturable.
The living have better things to do than to continue hating the dead.
As far as I know (i.e., according to some professors I've spoken to), transistors in devices with extremely long battery lives, such as hearing aids and watches, are typically operated in sub-threshold in order to conserve power. Of course, these devices are also typically not speed-critical. A lot of biomedical applications probably fall under the umbrella of requiring low power (for battery life and/or thermal reasons) and not requiring high speed, making the application a natural fit for sub-threshold operation.