Near Threshold Voltage Word-Line Voltage Injection Self-Convergence Scheme for Local Electron Injected Asymmetric Pass Gate Transistor 6T-SRAM

A statistical threshold voltage V_TH 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 V_TH shift (V_THPG shift) and its original V_TH of the PG transistor (V_THPG) before injection due to VD effect is self-compensated by the negative correlation between those by ID effect. As a result, the measured V_THPG shift is less correlated with the V_THPG before electron injection. Therefore, near V_TH word-line (WL) voltage injection self-convergence scheme is proposed to avoid V_THPG shift in the high V_THPG cell and enhance in the low V_THPG cell. By the proposed scheme, VD effect is reduced and ID effect is enhanced. The improved negative correlation factor is observed between the V_THPG shift and the forward V_THPG 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 V_CCMIN after the local electron injection.