Ultra-low voltage CMOS logic circuits

The operation of digital circuits from power supply voltages of the order of 200 mV or less imposes that, in general, MOSFETs are biased in the subthreshold regime, characterized by the exponential relation between the control voltages and the current. In this tutorial paper we analyze some of the basic building blocks of digital circuits operating in the subthreshold region. We analyze the basic CMOS inverter as regards the voltage transfer characteristic, dynamic behavior, and power dissipation. To reduce the dependence of the drain current on the process parameters we show some compensation circuits that adjust the body voltage, with a small silicon area penalty. Some properties of the static random access memory (SRAM) are reviewed. Finally, the Schmitt Trigger inverter, which is well suited to replace the standard inverter as a basic building block for ultra-low-voltage operation, is briefly analyzed.