Ultralow-Voltage Retention SRAM With a Power Gating Cell Architecture Using Header and Footer Power-Switches

Abstract

An ultralow-voltage retention SRAM (ULVR-SRAM) cell using header and footer power-switches (HFPSs) is investigated for power-gating (PG) applications. The cell can change its operational mode depending on the cell voltage ( ${V} _{\mathrm{ cell}}$ ) controlled by the HFPSs: When the ordinary supply voltage is applied, the cell can act as a high-performance SRAM cell. When ${V} _{\mathrm{ cell}}$ is reduced to an ultralow voltage, the cell can transition to the ULVR mode and dramatically reduce the leakage power without losing its data, i.e., the substantive PG can be achieved using the ULVR. The ability of leakage power reduction is enhanced by introducing the body biases that are automatically induced only during the ULVR mode. The design methodology is developed based on quasi-static noise margins, where the transistor sizes and the bias condition for ${V} _{\mathrm{ cell}}$ are determined so as to minimize the leakage power with keeping a sufficiently high noise margin in the ULVR mode. An optimally designed ULVR-SRAM cell shows excellent PG ability: The leakage power can be reduced by ~98% using the ULVR and a minimally short break-even time of $1.5\mu \text{s}$ can be achieved for the 8KB macro. The ULVR-SRAM can provide a new class of energy efficient PG architecture.