As already mentioned, the power gating scheme can be implementedfor the peripheral circuits of SRAMs to reduce theleakage in the standby state, but this scheme cannot be implementedin the SRAM cell arrays, because memory cell datamust be kept in the standby state. For mobile applications, alow leakage standby mode, a sleep mode, where the leakage isreduced without losing memory cell data, has been introduced.In order to reduce gate-leakage as well as off-leakage in SRAMcells, the cell bias voltage is decreased in the sleep mode [4]–[7].Fig. 1 shows an SRAM cell schematic. As shown in Fig. 1, themajor leakages in an SRAM cell in the sleep mode are off-leakages,, in an nMOS driver, an nMOS transfer and a pMOSload, and gate-leakages, , in an nMOS driver and a pMOSload. Fig. 2 shows measured off-leakage and gate-leakage foreach transistor in an SRAM cell. Fig. 2(a) and (c) shows the caseof raising VSSC (the source terminals for the nMOS drivers),and Fig. 2(b) and (d) shows the case of lowering VDDC (thesource terminals for the pMOS loads). The gate-leakages fornMOS transfers are omitted, because they are negligible in theoff state. As shown in Fig. 2(a), by raising VSSC, the off-leakagesfor nMOS driver and nMOS transfer are reduced owing toback bias effects by fixing their substrate levels to the ground,as well as their drain-source bias reduction.
đang được dịch, vui lòng đợi..
