2.7.2 Threshold Control

As the supply voltages are decreased a tight control of the threshold voltage becomes a primary concern. Some authors, most important [42, chapter 3], advocate a well-biasing approach, which is somewhat controversial. As this approach is neither the ideal solution to the problem, nor is it unfeasible, a list of pros and cons is given at this point. The most important arguments in favor are:

• The requirements on process parameter control are released and processes which are actually not qualified can be used to fabricate ULP circuits.
• Standard processes can be made ULP simply by masking out the threshold-adjust implant and adjusting with a well-bias.
• The well-biasing circuitry provides also for a power-down mode which is needed in virtually all portable applications.
• The routing overhead is smaller than compared to a complete second power bus, because the currents needed are much smaller.
• In some sensitive (analog and mixed analog-digital) applications well-biasing will be unavoidable anyway.

The most severe objections, on the other hand, are:

• In high-performance technologies the DIBL effect ``eats up'' the body effect to some extent, or, a reverse bulk bias actually enhances the DIBL as it depletes the region underneath the channel. Thus, well-biasing of such devices is less effective and will degrade the device behavior.
• Tuning both n and p-wells requires an expensive triple-well technology.
• The overhead for the additional well-routing can be considerable in advanced technologies, especially, as local straps cannot be used. Furthermore, the routing programs must be adapted to support this technique.
• An equivalent technique for SOI is still unknown.
• Power-down modes can partly be integrated into the power-supply scheme, especially, when on-chip voltage converters are used. Or, they are provided implicitly by the logic style, e.g., when pseudo-NMOS circuits are used strategically.