3. Body-Bias for Digital Designs

In this chapter we will detail the key advantages of Body-Bias for digital design. As Body-Bias modulates the transistors’ V _T, it enables higher I _on at the expense of higher I _off. This design tuning knob can be used to either increase yield (more dies within design specifications), or to extend the operating performance (with lower voltage, higher speed, or lower power). Increased yield can be brought directly by die acceleration, while indirect leakage gains can be obtained through increased transistor drive of slowest transistors at slowest process corner. Similarly, indirect dynamic power reduction can be obtained thanks to lower operating voltage. As some of these gains require trade-off, such as increased speed at low temperature vs. higher leakage at maximum temperature, the circuit architect can either find the best trade-off between these aspects and/or implement system operating modes. This chapter describes how to modulate the Body-Bias (i.e., the bias law and the bias limits) from a design perspective to take full advantage of this tuning capability.