A recent article in Semiconductor Engineering announced The Return of Biasing (read the whole thing here). It’s back because of the quest to build more powerful mobile devices that support long battery life. And with FD-SOI designers can once again easily use what is essentially an old design trick for controlling threshhold voltage (Vt). (In the simplest terms, Vt is the point at which a transistor turns on or off.)
This piece is a really good read if you want to know why body biasing is back in the game, and when and how it’s used. It gets fairly technical, but it’s also very clear. SemiEngineering’s Ann Steffora Mutschler really explores the advantages and issues in interviews with experts at ARM and ST, among others. They explained, for example, the differences between leveraging body biasing on bulk and FD-SOI.
By way of background, btw, for much of the history of chip design, body biasing was standard operating procedure. But, as ST Marketing Director Giorgio Cesana noted, body biasing effectively ended at the 40nm node for bulk, and is unworkable in FinFETs. But with FD-SOI, you can not only lower the Vt, but greatly expand the Vdd (supply voltage) range.
ARM Fellow Rob Aitken notes that, “If you are using an FD-SOI type of process, then biasing the substrate is fairly straightforward because the insulator is just sitting there. There are some mechanics to doing it, but the process is tuned to do it easily.”
You really will want to read the whole piece to get a fuller understanding of why and how the use of body biasing is once again on the rise.