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.
R
²
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.