SRAM cell current in low leakage design

Abstract

This paper highlights the cell current characterization of a low leakage 6T SRAM by adjusting the threshold voltages of the transistors in the memory array to reduce the standby power. Experiments using a 0.25 mu m 2.5V standard CMOS process with and without the additional threshold voltage adjustment implant on a 1Mb test chip demonstrate the effectiveness. A substantial standby power reduction by an order of magnitude is achievable. However, it incurs a wider cell current variation, which is pronounced only at a lower supply voltage. As the supply voltage decreases, the percent deviation from the average value increases. This can be modeled by a simple power-law relationship. The result has important implications in both design and manufacturing of the low leakage SRAM. Comparing with the generic cell current without the additional threshold voltage adjustment, the crossover point of their percent deviations at 2V signifies two separate circuit strategies: operating at 1.5V requires larger sensing margin and operating at 2.5V enjoys better manufacturability. Hence, for the applications requiring low voltage operations, it favors a boosted supply voltage applied to a selected cell during the read access.