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Power switch transistors are very effective in cutting the leakage currents of digital circuits in a deep-freeze mode, by de-supplying unused blocks. Among existing power switch transistors, Super Cut-off CMOS (SCCMOS) is the most suited to a low supply voltage environment since it uses a low threshold voltage transistor. This power switch type achieves good leakage reduction results, provided that an optimal voltage is applied on its gate in order to maximize the leakage gain. This optimal voltage value, depending on the operating conditions (process, voltage, temperature), cannot be determined at the design level. A polarization circuit, that automatically finds the optimal bias voltage whatever the environment conditions, was therefore designed and fabricated. This circuit, made in Bulk 65 nm technology, achieves more than two decades leakage current reduction at the power switch level, for a power dissipation overhead of 45 nW at ambient temperature. A very simple scheme is also presented to alleviate the voltage stress applied on the dielectric in case of an ageing of the latter, increasing its time-to-breakdown by several orders of magnitude.
Date of Publication: July 2008