Current leakage reduction for loaded bit-lines in on-chip...

Details Current leakage reduction for loaded bit-lines in on-chip... Current leakage reduction for loaded bit-lines in on-chip...

2002-06-13

2003-09-02

Mai, Son (Department: 2818)

Static information storage and retrieval

Systems using particular element

Flip-flop

C365S156000

Reexamination Certificate

active

06614680

ABSTRACT:

FIELD
Embodiments of the present invention relate to circuits, and more particularly, to memory circuits.
BACKGROUND
As semiconductor process technology provides for smaller and smaller device size, sub-threshold leakage current in MOSFETs (Metal-Oxide-Semiconductor-Field-Effect-Transistor) may increase. Sub-threshold leakage current in a nMOSFET may occur when the gate-to-source voltage of the nMOSFET is less than its threshold voltage, V
T
. Sub-threshold leakage current may present design challenges for various on-chip memory structures, such as, for example, register files, CAMs (Content-Addressable-Memory), caches, SRAM (Static-Random-Access-Memory), and DRAM (Dynamic-RAM).
Shown in
FIG. 1
is a portion of an on-chip SRAM, or cache memory. For simplicity, only four cells are indicated. The content of the stored data is read through complementary bit-lines
102
and
104
by sense amplifier
114
. The cells are accessed by bringing one of word lines
106
,
108
,
110
, and
112
HIGH. In the particular embodiment of
FIG. 1
, word line
106
is HIGH and word lines
108
,
110
, and
112
are LOW. By bringing word line
106
, access nMOSFETs
116
and
118
are turned ON, and the state of memory element
120
may be sensed by sense amplifier
114
via bit-lines
102
and
104
. The solid arrows nearby access nMOSFETs
116
and
118
indicate that conduction current flows through access nMOSFETs
116
and
118
to charge or discharge bit lines
102
and
104
.
With word lines
108
,
110
,
112
LOW, access nMOSFETs
121
are OFF because their gate-to-source voltages are less than their threshold voltages. However, there may be sub-threshold leakage current, as indicated by the dashed arrows nearby nMOSFETs
121
. In the particular embodiment of
FIG. 1
, assume that memory element
120
is such that node
122
is HIGH, and memory elements
124
are such that nodes
126
are HIGH. Assume that bit-lines
102
and
104
are pre-charged to HIGH. When memory cell
120
is read, memory cell
120
will keep bit-line
102
HIGH and will bring bit-line
104
from HIGH to LOW. However, there will be contention with the sub-threshold leakage currents through access nMOSFETs
121
, which try to charge bit-line
104
and discharge bit-line
102
, opposite the effect of the conduction current through access nMOSFETs
116
and
118
.
Shown in
FIG. 2
is a portion of an on-chip register file. The state stored in memory element
202
is accessed by bringing read select line
204
HIGH so that pass nMOSFET
206
is ON, and keeping the other read select lines LOW. Assume that the state of memory element
202
is such that node
208
is LOW so that pass nMOSFET
210
is OFF. Assume that bit line
212
is pre-charged HIGH. Then, with read select line
204
brought HIGH, bit-line
212
will not be discharged by conduction current. However, there may be sub-threshold leakage current through pass nMOSFET
210
as indicated by the dashed arrow nearby nMOSFET
210
. Assume also that nodes
214
are HIGH. Then, with read select lines
216
LOW, there may be sub-threshold leakage current flowing through pass nMOSFETs
218
. Consequently, the sub-threshold leakage currents depicted in
FIG. 2
will tend to discharge bit-line
212
, and may increase the noise margin.
As seen above, sub-threshold leakage current in memory structures may cause undesired voltage level changes in bit-lines, which may lead to incorrect read operations. One approach to mitigating this problem is to partition the bit-lines so as to reduce the number of memory cells connected to any one bit-line. However, this leads to an increase in the number of sense amplifiers, which increases die area and may reduce performance.


REFERENCES:
patent: 5477489 (1995-12-01), Wiedmann
patent: 5790461 (1998-08-01), Holst
patent: 6307805 (2001-10-01), Andersen et al.

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