Designing digital subthreshold CMOS circuits using parallel transistor stacks

A method for designing faster digital CMOS circuits operating in the subthreshold mode is proposed. Since the threshold voltage may be lower at narrower widths owing to the inverse-narrow-width effect in modern nanometre MOSFETs, the subthreshold current may be higher than expected at these narrow widths. It is shown that using only transistor widths that maximise the current-to-capacitance ratio, either individually or in parallel stacks, as appropriate, leads to faster circuits. Speed increases of up to 2.85 times have been demonstrated in ring oscillator simulations.