Models and algorithms for bounds on leakage in CMOS circuits

Subthreshold leakage current in deep submicron MOS transistors is becoming a significant contributor to power dissipation in CMOS circuits as threshold voltages and channel lengths are reduced, Consequently, estimation of leakage current and identification of minimum and maximum leakage conditions are becoming important, especially in low power applications. In this paper we outline methods for estimating leakage at the circuit level and then propose heuristic and exact algorithms to accomplish the same task for random combinational logic. In most cases the heuristic is found to obtain bounds on leakage that are close and often identical to bounds determined by a complete branch and bound search. Methods are also demonstrated to show how estimation accuracy can be traded off against execution time. The proposed algorithms have potential application in power management applications or quiescent current (I(D)DQ) testing if one wished to control leakage by application of appropriate input vectors. For a variety of benchmark circuits, leakage was found to vary by as much as a factor of six over the space of possible input vectors.