I. Introduction
Power gating (PG) is one of the most effective methods of reducing the subthreshold leakage power of portable systems in the standby mode where a header and/or footer (called sleep transistor) with a high is added between the actual power/ground rail and the virtual power/ground. The subthreshold leakage power is significantly reduced by cutting off the high-threshold sleep transistor in the standby mode, while still keeping a high speed in the active mode [1], [2].