Skip to Main Content
A statistical threshold voltage VTH shift variation of the pass gate (PG) transistor in local electron injected asymmetric PG transistor 6T-SRAM is investigated. Measurements show that the positive correlation between the PG transistor VTH shift (VTHPG shift) and its original VTH of the PG transistor (VTHPG) before injection due to VD effect is self-compensated by the negative correlation between those by ID effect. As a result, the measured VTHPG shift is less correlated with the VTHPG before electron injection. Therefore, near VTH word-line (WL) voltage injection self-convergence scheme is proposed to avoid VTHPG shift in the high VTHPG cell and enhance in the low VTHPG cell. By the proposed scheme, VD effect is reduced and ID effect is enhanced. The improved negative correlation factor is observed between the VTHPG shift and the forward VTHPG before injection. R2 is increased by 21 times by the proposed scheme. As a result, excess write margin degradation is suppressed. Furthermore, the fabricated 64 kb SRAM macro demonstrates 3 times larger WL operation voltage window, 41% less read margin variation and 80 mV lower VCCMIN after the local electron injection.
Circuits and Systems I: Regular Papers, IEEE Transactions on (Volume:59 , Issue: 8 )
Date of Publication: Aug. 2012