U.S. patent application number 09/814855 was filed with the patent office on 2001-10-11 for semiconductor memory device having normal and standby modes, semiconductor integrated circuit and mobile electronic unit.
This patent application is currently assigned to MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.. Invention is credited to Yamauchi, Hiroyuki.
Application Number | 20010028591 09/814855 |
Document ID | / |
Family ID | 18609752 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010028591 |
Kind Code |
A1 |
Yamauchi, Hiroyuki |
October 11, 2001 |
Semiconductor memory device having normal and standby modes,
semiconductor integrated circuit and mobile electronic unit
Abstract
In a standby mode, a word driver supplies a negative voltage
onto its associated word line, and a precharge circuit electrically
disconnects its associated pair of bit lines from power supply
nodes receiving a supply voltage. In this manner, access
transistors, connected to L- or H-level data retention nodes, can
have their source-drain voltage lowered to such a level as
eliminating a GIDL current. As a result, the amount of current
dissipated in the standby mode can be reduced without causing the
GIDL current problem.
Inventors: |
Yamauchi, Hiroyuki; (Osaka,
JP) |
Correspondence
Address: |
Jack Q. Lever, Jr.
McDERMOTT, WILL & EMERY
600 Thirteenth Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
MATSUSHITA ELECTRIC INDUSTRIAL CO.,
LTD.
|
Family ID: |
18609752 |
Appl. No.: |
09/814855 |
Filed: |
March 23, 2001 |
Current U.S.
Class: |
365/227 |
Current CPC
Class: |
G11C 8/08 20130101; G11C
5/145 20130101; G11C 11/417 20130101 |
Class at
Publication: |
365/227 |
International
Class: |
G11C 005/14 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2000 |
JP |
2000-094757 |
Claims
What is claimed is:
1. A semiconductor memory device having a normal mode and a standby
mode, the device comprising: an array of memory cells arranged in
columns and rows; a plurality of word lines, each being associated
with one of the rows; a plurality of bit lines, each being
associated with one of the columns; a plurality of access
transistors, each being provided for an associated one of the
memory cells, connected between a data retention node of the
associated memory cell and one of the bit lines that is associated
with the memory cell, and receives, at its gate, a voltage on one
of the word lines that is associated with the memory cell; and
potential difference generating means for generating a negative
potential difference between the gate and source of one of the
access transistors while the device is in the standby mode, the
access transistor being connected to a data retention node storing
logical-one-level or logical-zero-level data thereon.
2. The device of claim 1, wherein if the potential difference
between the gate and source of the access transistors is 0 V, a
current of 100 pA/.mu.m or more flows between the drain and source
of the access transistor.
3. The device of claim 1, wherein the potential difference
generating means comprises potential holding means for holding a
potential on the bit lines at a predetermined positive level while
the device is in the standby mode.
4. The device of claim 1, wherein the potential difference
generating means comprises means for allowing the bit lines to be
floating while the device is in the standby mode.
5. The device of claim 1, wherein the potential difference
generating means comprises word line driving means for supplying a
negative voltage onto the word lines while the device is in the
standby mode.
6. A semiconductor memory device having a normal mode and a standby
mode, the device comprising: an array of memory cells arranged in
columns and rows; a plurality of word lines, each being associated
with one of the rows; a plurality of bit lines, each being
associated with one of the columns; a plurality of access
transistors, each being provided for an associated one of the
memory cells, connected between a data retention node of the
associated memory cell and one of the bit lines that is associated
with the memory cell, and receives, at its gate, a voltage on one
of the word lines that is associated with the memory cell; word
line driving means for activating one of the word lines, the word
line being associated with one of the memory cells that is to be
accessed; and precharge means for precharging the bit lines to a
supply voltage level during a predetermined period before the
memory cell is accessed, wherein while the device is in the standby
mode, the word line driving means supplies a negative voltage onto
the word lines, and the precharge means electrically disconnects
the bit lines from a power supply node receiving a supply
voltage.
7. The device of claim 6, further comprising means for holding a
potential on the bit lines at a predetermined level while the
device is in the standby mode.
8. The device of claim 7, wherein the predetermined level is equal
to or lower than an intermediate potential level.
9. The device of claim 6, wherein the word line driving means
comprises: means for supplying a ground voltage onto the word lines
while the device is in the standby mode; and means for supplying
the negative voltage onto the word lines after the ground voltage
has been supplied thereto.
10. A semiconductor integrated circuit comprising a semiconductor
memory device as recited in claim 6.
11. The circuit of claim 10, further comprising: a logic circuit
section; and supply switching means, which applies the supply
voltage to the logic circuit section while the device is in the
normal mode but does not apply the supply voltage to the logic
circuit section while the device is in the standby mode, and
wherein the precharge means of the semiconductor memory device
precharges the bit lines to the supply voltage level when the
device switches its mode of operation from the standby mode into
the normal mode.
12. A mobile electronic unit comprising a semiconductor integrated
circuit as recited in claim 10.
13. The unit of claim 12, further comprising means for supplying a
mode switching signal to the semiconductor integrated circuit to
instruct the circuit to switch the mode of operation from the
normal mode into the standby mode, or vice versa.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a semiconductor memory
device having normal and standby modes and also relates to a
semiconductor integrated circuit and a mobile electronic unit
including a device of that type.
[0002] A semiconductor memory device called a static random access
memory (SRAM) is basically made up of flip-flops, and is easy to
use because such a device needs no refreshing. An SRAM can also
operate at a high speed and allows a large operation margin. By
taking advantage of all these beneficial features, SRAMs are often
used as memories for mobile electronic units, for example. In
addition, as transistors have their sizes further reduced over the
last couple of years, mobile electronic units have also been
downsized.
[0003] However, the smaller a transistor, the lower its breakdown
voltage. Accordingly, a transistor of a very small size should be
operated with its operating voltage lowered. Furthermore, to enable
a transistor to operate at a low voltage without sacrificing its
operating speed, the threshold voltage of the transistor needs to
be reduced. For that reason, small-sized mobile electronic units,
including cell phones, which are normally driven by battery, use
transistors with a low threshold voltage. However, if the threshold
voltage of a transistor is too low, then the transistor with that
low threshold voltage cannot be cut off completely, thus allowing
some leakage current to flow. In that case, an increased amount of
current is dissipated in vain in a standby mode.
[0004] A battery-driven mobile electronic unit of a small size is
required to operate at a low voltage and with low power
dissipation. As for a cell phone, in particular, it is one of the
key features determining its market value how long the cell phone
can hold its standby state. And to make the standby state as long
as possible, the amount of current dissipated in the standby mode
should be minimized.
SUMMARY OF THE INVENTION
[0005] It is therefore an object of the present invention to
provide a semiconductor memory device that dissipates a reduced
amount of current in its standby mode.
[0006] A semiconductor memory device according to an aspect of the
present invention has normal and standby modes and includes: array
of memory cells; a plurality of word lines; a plurality of bit
lines; a plurality of access transistors; and potential difference
generating means. In the memory array, multiple memory cells are
arranged in columns and rows. Each of the word lines is associated
with one of the rows. Each of the bit lines is associated with one
of the columns. Each of the access transistors is provided for an
associated one of the memory cells, connected between a data
retention node of the associated memory cell and one of the bit
lines that is associated with the memory cell, and receives, at its
gate, a voltage on one of the word lines that is associated with
the memory cell. And the potential difference generating means
generates a negative potential difference between the gate and
source of one of the access transistors while the device is in the
standby mode. The access transistor is connected to a data
retention node storing logical-one-level or logical-zero-level data
thereon.
[0007] In one embodiment of the present invention, if the potential
difference between the gate and source of the access transistors is
0 V, a current of 100 pA /.mu.m or more flows between the drain and
source of the access transistors.
[0008] While the inventive semiconductor memory device is in the
standby mode, a negative potential difference is generated between
the gate and source of one of the access transistors that is
connected to a data retention node storing logical-one-level or
logical-zero-level data thereon. As a result, the amount of leakage
current, flowing from the logical-one-level data retention node
into one of a pair of bit lines via the access transistor or from
the other bit line into the logical-zero-level data retention node
via the access transistor, can be reduced.
[0009] In another embodiment of the present invention, the
potential difference generating means preferably includes potential
holding means for holding a potential on the bit lines at a
predetermined positive level while the device is in the standby
mode.
[0010] In the semiconductor memory device of this embodiment, the
potential level on the bit lines is higher than that on the word
lines during the standby mode. Accordingly, a negative potential
difference is generated between the gate and source of one of the
access transistors that is connected to the logical-one-level data
retention node. As a result, the amount of leakage current, flowing
from the logical-one-level data retention node into the bit line
via the access transistor, can be reduced. In addition, by holding
the potential on the bit lines at such a level as not causing the
problem of gate-induced-drain-leakage (GIDL) current, the GIDL
current problem is avoidable.
[0011] In still another embodiment, the potential difference
generating means preferably includes means for allowing the bit
lines to be floating while the device is in the standby mode.
[0012] In the semiconductor memory device of this embodiment, the
leakage current, flowing from the logical-one-level data retention
node into the bit line via the access transistors, precharges the
bit lines during the standby mode. As a result, the potential on
the bit lines can be held at a positive level.
[0013] In yet another embodiment, the potential difference
generating means preferably includes word line driving means for
supplying a negative voltage onto the word lines while the device
is in the standby mode.
[0014] In the semiconductor memory device of this embodiment, the
potential level on the word lines is lower than the potential level
at the logical-zero-level data retention node during the standby
mode. Accordingly, a negative potential difference is generated
between the gate and source of one of the access transistors that
is connected to the logical-zero-level data retention node. As a
result, the amount of leakage current, flowing from the bit line
into the logical-zero-level data retention node via the access
transistor, can be reduced.
[0015] A semiconductor memory device according to another aspect of
the present invention has normal and standby modes and includes:
array of memory cells; a plurality of word lines; a plurality of
bit lines; a plurality of access transistors; word line driving
means; and precharge means. In the memory array, multiple memory
cells are arranged in columns and rows. Each of the word lines is
associated with one of the rows. Each of the bit lines is
associated with one of the columns. Each of the access transistors
is provided for an associated one of the memory cells, connected
between a data retention node of the associated memory cell and one
of the bit lines that is associated with the memory cell, and
receives, at its gate, a voltage on one of the word lines that is
associated with the memory cell. The word line driving means
activates one of the word lines, which is associated with one of
the memory cells that is to be accessed. And the precharge means
precharges the bit lines to a supply voltage level during a
predetermined period before the memory cell is accessed. While the
device is in the standby mode, the word line driving means supplies
a negative voltage onto the word lines, and the precharge means
electrically disconnects the bit lines from a power supply node
receiving a supply voltage.
[0016] While the inventive semiconductor memory device is in the
standby mode, no power is supplied from the power supply node to
the bit lines that are electrically disconnected from the power
supply node. Accordingly, the potential level on the bit lines is
lower than the supply voltage level, and is normally stabilized at
around an intermediate potential level, which is half as high as
the supply voltage level. As a result, the source-drain voltage of
the access transistors, connected to the data retention nodes, can
be lowered to such a level as not causing the GIDL current
problem.
[0017] Thus, the inventive semiconductor memory device can reduce
the amount of current dissipated in the standby mode while
eliminating the GIDL current problem, because the device supplies a
negative voltage onto the word lines and electrically disconnects
the bit lines from the power supply node during the standby
mode.
[0018] In one embodiment of the present invention, the inventive
device preferably further includes means for holding a potential on
the bit lines at a predetermined level while the device is in the
standby mode.
[0019] In this particular embodiment, the predetermined level is
preferably equal to or lower than an intermediate potential
level.
[0020] In the semiconductor memory device of this embodiment, a
potential on the bit lines is held at a predetermined level during
the standby mode. Accordingly, a precharge period, which is
necessary for the device to return from the standby mode into the
normal mode, can have a constant length.
[0021] In another embodiment of the present invention, the word
line driving means preferably includes: means for supplying a
ground voltage onto the word lines while the device is in the
standby mode; and means for supplying the negative voltage onto the
word lines after the ground voltage has been supplied thereto.
[0022] In the semiconductor memory device of this embodiment, the
ground voltage supplying means once drops the potential on the word
lines to the ground level at a high speed. Accordingly, the power
dissipated by the negative voltage supplying means can be
reduced.
[0023] A semiconductor integrated circuit according to still
another aspect of the present invention includes a semiconductor
memory device according to the second aspect of the present
invention.
[0024] In one embodiment of the present invention, the
semiconductor integrated circuit preferably further includes a
logic circuit section and supply switching means. The supply
switching means applies the supply voltage to the logic circuit
section while the device is in the normal mode but does not apply
the supply voltage to the logic circuit section while the device is
in the standby mode. The precharge means of the semiconductor
memory device precharges the bit lines to the supply voltage level
when the device switches its mode of operation from the standby
mode into the normal mode.
[0025] A mobile electronic unit according to yet another aspect of
the present invention includes the inventive semiconductor
integrated circuit.
[0026] In one embodiment of the present invention, the mobile
electronic unit preferably further includes means for supplying a
mode switching signal to the semiconductor integrated circuit to
instruct the circuit to switch the mode of operation from the
normal mode into the standby mode, or vice versa.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a circuit diagram illustrating an overall
configuration for an SRAM according to a first embodiment of the
present invention.
[0028] FIG. 2 is a circuit diagram illustrating a configuration for
the memory cell shown in FIG. 1.
[0029] FIG. 3 is a circuit diagram illustrating a configuration for
the negative voltage generator shown in FIG. 1.
[0030] FIG. 4 illustrates a capacitor and a diode associated with a
node receiving a negative voltage.
[0031] FIG. 5 is a circuit diagram illustrating a configuration for
the word driver shown in FIG. 1.
[0032] FIG. 6 is a timing diagram illustrating how the word driver
shown in FIG. 5 operates.
[0033] FIG. 7 is a timing diagram illustrating how the SRAM shown
in FIG. 1 operates.
[0034] FIG. 8 is a circuit diagram illustrating how the amount of
current dissipated can be reduced in a standby mode.
[0035] FIG. 9 is a graph illustrating a GIDL current.
[0036] FIG. 10 illustrates how the amount of current dissipated can
be reduced in a standby mode.
[0037] FIG. 11 is a diagram illustrating a configuration for an
SRAM according to a second embodiment of the present invention.
[0038] FIG. 12 is a block diagram illustrating a configuration for
a mobile electronic unit according to a third embodiment of the
present invention.
[0039] FIG. 13 is a timing diagram illustrating how the mobile
electronic unit shown in FIG. 12 operates.
[0040] FIG. 14 is a circuit diagram illustrating an overall
configuration for an SRAM according to a fourth embodiment of the
present invention.
[0041] FIG. 15 is a circuit diagram illustrating where a leakage
current flows in a standby mode.
[0042] FIG. 16 is a graph illustrating a GIDL current.
[0043] FIG. 17 illustrates how the amount of current dissipated can
be reduced in a standby mode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] Hereinafter, preferred embodiments of the present invention
will be described in detail with reference to the accompanying
drawings, in which components having like or the same function will
be identified by the same reference numeral for the sake of
simplicity of description.
EMBODIMENT 1
SRAM Configuration
[0045] FIG. 1 is a circuit diagram illustrating an overall
configuration for an SRAM according to a first embodiment of the
present invention. As shown in FIG. 1, the SRAM includes memory
array 1, row decoder 2, column decoder 3, column selector 4,
input/output circuit 5, precharge circuits 6 and 7, word drivers 8
and 9, negative voltage generator 10 and NAND gate 11.
[0046] The memory array 1 includes memory cells MC1 through MC4,
word lines WL0 and WL1, two pairs of complementary bit lines BL0,
/BL0 and BL1, /BL1, and access transistors NT1a through NT4a and
NT1b through NT4b. The memory cells MC1 through MC4 are arranged in
matrix, or in columns and rows. The word line WL0 is associated
with the memory cells MC1 and MC3, while the word line WL1 is
associated with the memory cells MC2 and MC4. One pair of bit lines
BL0, /BL0 is associated with the memory cells MC1 and MC2, while
the other pair of bit lines BL1, /BL1 is associated with the memory
cells MC3 and MC4. Each of the access transistors NT1a through NT4a
and NT1b through NT4b has a low threshold voltage. Specifically, in
each of these access transistors NT1a through NT4a and NT1b through
NT4b, when there is a potential difference of 0 V between the gate
and source of the transistor, a current of 100 pA/.mu.m or more
flows between its drain and source. The access transistor NT1a is
connected between a data retention node (not shown in FIG. 1) of
the associated memory cell MC1 and the bit line BL0 and receives a
voltage on the word line WL0 at its gate. The access transistor
NT1b is connected between another data retention node (not shown in
FIG. 1) of the associated memory cell MC1 and the bit line /BL0 and
receives the voltage on the word line WL0 at its gate. The access
transistor NT2a is connected between a data retention node (not
shown in FIG. 1) of the associated memory cell MC2 and the bit line
BL0 and receives a voltage on the word line WL1 at its gate. The
access transistor NT2b is connected between another data retention
node (not shown in FIG. 1) of the associated memory cell MC2 and
the bit line /BL0 and receives the voltage on the word line WL1 at
its gate. The access transistor NT3a is connected between a data
retention node (not shown in FIG. 1) of the associated memory cell
MC3 and the bit line BL1 and receives the voltage on the word line
WL0 at its gate. The access transistor NT3b is connected between
another data retention node (not shown in FIG. 1) of the associated
memory cell MC3 and the bit line /BL1 and receives the voltage on
the word line WL0 at its gate. The access transistor NT4a is
connected between a data retention node (not shown in FIG. 1) of
the associated memory cell MC4 and the bit line BL1 and receives
the voltage on the word line WL1 at its gate. The access transistor
NT4b is connected between another data retention node (not shown in
FIG. 1) of the associated memory cell MC4 and the bit line /BL1 and
receives the voltage on the word line WL1 at its gate.
[0047] The NAND gate 11 outputs a negated logical product of a mode
signal MD and a precharge control signal PRO as a precharge signal
PR1.
[0048] The row decoder 2 includes an inverter IV21 and two NAND
gates ND21 and ND22. The inverter IV21 inverts an address signal
A1. The NAND gate ND21 outputs a negated logical product of the
precharge signal PR1 and the address signal A1 as a word line
select signal SW0. The NAND gate ND22 outputs a negated logical
product of the precharge signal PR1 and the output of the inverter
IV21 as a word line select signal SW1.
[0049] The negative voltage generator 10 generates a negative
voltage Vng.
[0050] The word drivers 8 and 9 together makes word line driving
means as defined in the appended claims. Responsive to the word
line select signal SW0 or SW1, each of these word drivers 8 and 9
selectively outputs supply voltage VDD, ground voltage Vss or
negative voltage Vng to the associated word line WL0 or WL1.
[0051] The column decoder 3 includes an inverter IV31 and two AND
gates AD31 and AD32. The inverter IV31 inverts an address signal
A0. The AND gate AD31 outputs a logical product of the address
signal A0 and an access signal R/W. The AND gate AD32 outputs a
logical product of the output of the inverter IV31 and the access
signal R/W.
[0052] The column selector 4 includes inverters IV41 and IV42 and
transfer gates TG41 through TG44. The inverters IV41 and IV42
invert the outputs of the AND gates AD31 and AD32, respectively.
The transfer gates TG41 and TG42 are connected between the bit line
BL0 and an input/output line IO and between the bit line /BL0 and
an input/output line /IO, respectively. Responsive to the output of
the AND gate AD31, the transfer gate TG41 connects or disconnects
the bit line BL0 to/from the input/output line IO and the transfer
gate TG42 connects or disconnects the bit line /BL0 to/from the
input/output line /IO. In the same way, responsive to the output of
the AND gate AD32, the transfer gate TG43 connects or disconnects
the bit line BL1 to/from the input/output line IO and the transfer
gate TG44 connects or disconnects the bit line /BL1 to/from the
input/output line /IO.
[0053] Responsive to the access signal R/W, the input/output
circuit 5 transmits data, read out on the input/output line IO or
/IO, to an input/output terminal D or another data, externally
input to the input/output terminal D, to the input/output line IO
or /IO.
[0054] The precharge circuit 6 includes p-channel MOS transistors
PT61 through PT63. The p-channel MOS transistors PT61 and PT62 are
connected between a power supply node receiving the supply voltage
VDD and the pair of bit lines BL0 and /BL0, respectively, and turn
ON/OFF responsive to the precharge signal PR1. The p-channel MOS
transistor PT63 is connected between the pair of bit lines BL0 and
BL0 and also turns ON/OFF responsive to the precharge signal
PR1.
[0055] The precharge circuit 7 includes p-channel MOS transistors
PT71 through PT73. The p-channel MOS transistors PT71 and PT72 are
connected between the power supply node receiving the supply
voltage VDD and the pair of bit lines BL1 and /BL1, respectively,
and turn ON/OFF responsive to the precharge signal PR1. The
p-channel MOS transistor PT73 is connected between the pair of bit
lines BL1 and /BL1 and also turns ON/OFF responsive to the
precharge signal PR1.
[0056] FIG. 2 illustrates a specific configuration for the memory
cells MC1 through MC4 shown in FIG. 1. As shown in FIG. 2, the
memory cell MC1 includes p-channel MOS transistors MPia and MPb and
n-channel MOS transistors MNia and MNib (where
1.ltoreq.i.ltoreq.4).
[0057] The p-channel MOS transistor MPia is connected between the
power supply node, receiving the supply voltage VDD, and a data
retention node Nia. The n-channel MOS transistor MNia is connected
between the data retention node Nia and a ground node receiving the
ground voltage Vss. The gates of the p- and n-channel MOS
transistors MPia and MNia are connected to another data retention
node Nib. The p-channel MOS transistor MPib is connected between
the power supply node and the data retention node Nib. The
n-channel MOS transistor MNib is connected between the data
retention node Nib and the ground node. The gates of the p- and
n-channel MOS transistors MPib and MNib are connected to the data
retention node Nia.
[0058] In the memory cell MCi with such a configuration, a one-bit
complementary data signal is stored at each of the data retention
nodes Nia and Nib.
[0059] As shown in FIG. 2, each access transistor NTia (where
1.ltoreq.i.ltoreq.4) is connected between the bit line BL0 or BL1
and its associated data retention node Nia, while each access
transistor NTib is connected between the bit line /BL0 or /BL1 and
its associated data retention node Nib.
[0060] FIG. 3 illustrates a specific configuration for the negative
voltage generator 10 shown in FIG. 1. As shown in FIG. 3, the
negative voltage generator 10 includes ring oscillator 101,
inverter 102, capacitors C101 through C104 and p-channel MOS
transistors PT11 through PT106.
[0061] The ring oscillator 101 includes multiple inverters (not
shown), which are connected together to form an odd number of
stages, and outputs a signal with a predetermined oscillation
frequency. The inverter 102 inverts the output signal of the ring
oscillator 101. The capacitor C101 is connected between the output
node of the inverter 102 and a node N102. The capacitor C102 is
connected between the output node of the inverter 102 and a node
N104. The capacitor C103 is connected between the output node of
the ring oscillator 101 and a node N103. And the capacitor C104 is
connected between the output node of the ring oscillator 101 and a
node N105.
[0062] The p-channel MOS transistor PT101 is connected between a
node N101 and the node N102. The p-channel MOS transistor PT102 is
connected between the node N102 and the ground node receiving the
ground voltage Vss. The p-channel MOS transistor PT103 is connected
between the nodes N101 and N103. The p-channel MOS transistor PT104
is connected between the node N103 and the ground node. The
p-channel MOS transistor PT105 is connected between the node N104
and the ground node. And the p-channel MOS transistor PT106 is
connected between the node N105 and the ground node. The gates of
the p-channel MOS transistors PT101 and PT104 are coupled together
and also connected to the node N104. The gates of the p-channel MOS
transistors PT102 and PT103 are coupled together and also connected
to the node N105. The gate of the p-channel MOS transistor PT105 is
connected to the node N105. And the gate of the p-channel MOS
transistor PT106 is connected to the node N104.
[0063] The negative voltage generator with such a configuration
performs charge pumping and generates the negative voltage Vng at
the node N101 synchronously with the rise or fall of the output
signal of the ring oscillator 101.
[0064] The charge supplied to the node N101 when the negative
voltage Vng is generated is stored on the capacitor 104 shown in
FIG. 4. The capacitor 104 may be a capacitance associated with a
gate oxide film, an interconnect capacitance or a coupling
capacitance associated with a word line.
[0065] The potential level of the negative voltage Vng is clamped
to an intermediate level between a built-in voltage level and the
ground voltage level Vss by the pn junction diode 103 shown in FIG.
4. This intermediate potential level is controllable to a desired
level by a known analog technique (e.g., a combination of monitor
and reference circuits). It is expected that the level is often set
to a range from -0.3 V to -0.5 V in accordance with the
characteristic of a GIDL current (to be described later).
[0066] FIG. 5 illustrates a specific configuration for the word
drivers 8 and 9 shown in FIG. 1. Since the word drivers 8 and 9
have the same configuration, only the word driver 8 is illustrated
in FIG. 5. As shown in FIG. 5, the word driver 8 includes inverters
IV81 through IV92, NAND gate ND81, level shifters LS1 and LS2,
p-channel MOS transistor PT81 and n-channel MOS transistors NT81
and NT82.
[0067] The inverters IV81 through IV85 are connected in series
together. The word line select signal SW0 is supplied to the input
terminal of the inverter IV81. The output terminal of the inverter
IV85 is connected to one of the two input terminals of the NAND
gate ND81. The series of inverters IV81 through IV85 delays the
word line select signal SW0 for a predetermined amount of time and
then provides the delayed signal to one of the two input terminals
of the NAND gate ND81. The NAND gate ND81 outputs a negated logical
product of the output of the inverter IV85 and the word line select
signal SW0. The inverter IV86 inverts the output of the NAND gate
ND81. The inverter IV87 inverts the word line select signal SW0.
The inverter IV88 inverts the output of the inverter IV87. The
inverters IV89 through IV92 are also connected in series together.
The word line select signal SW0 is also supplied to the input
terminal of the inverter IV89. The series of inverters IV89 through
IV92 delays the word line select signal SW0 for another
predetermined amount of time and then outputs the delayed
signal.
[0068] The level shifter LS1 includes p-channel MOS transistors
PT91 and PT92, n-channel MOS transistors NT91 and NT92 and inverter
IV93.
[0069] The p-channel MOS transistor PT91 is connected between a
node N80 receiving the supply voltage VDD and a node N82 and
receives the output of the inverter IV86 at its gate. The n-channel
MOS transistor NT91 is connected between the node N82 and a node
N81 receiving the negative voltage Vng. The gate of the n-channel
MOS transistor NT91 is connected to a node N83. The inverter IV93
inverts the output of the inverter IV86. The p-channel MOS
transistor PT92 is connected between the nodes N80 and N83 and
receives the output of the inverter IV93 at its gate. The n-channel
MOS transistor NT92 is connected between the nodes N83 and N81. The
gate of the n-channel MOS transistor NT92 is connected to the node
N82.
[0070] The level shifter LS2 includes p-channel MOS transistors
PT93 and PT94, n-channel MOS transistors NT93 and NT94 and inverter
IV94.
[0071] The p-channel MOS transistor PT93 is connected between a
node N90 receiving the supply voltage VDD and a node N92 and
receives the output of the inverter IV92 at its gate. The n-channel
MOS transistor NT93 is connected between the node N92 and a node
N91 receiving the negative voltage Vng. The gate of the n-channel
MOS transistor NT93 is connected to a node N93. The inverter IV94
inverts the output of the inverter IV92. The p-channel MOS
transistor PT94 is connected between the nodes N90 and N93 and
receives the output of the inverter IV94 at its gate. The n-channel
MOS transistor NT94 is connected between the nodes N93 and N91. The
gate of the n-channel MOS transistor NT94 is connected to the node
N92.
[0072] The p- and n-channel MOS transistors PT81 and NT81 are
connected in series together between a power supply node receiving
the supply voltage VDD and a ground node receiving the ground
voltage Vss. The p-channel MOS transistor PT81 receives the output
of the inverter IV88 at its gate. The gate of the n-channel MOS
transistor NT81 receives a voltage Va at the node N83. A voltage at
an interconnect node between the p- and n-channel MOS transistors
PT81 and NT81 is supplied onto the word line WL0.
[0073] The n-channel MOS transistor NT82 is connected between the
interconnect node N84 and a node receiving the negative voltage
Vng. The gate of the n-channel MOS transistor NT82 receives a
voltage at the node N93.
[0074] Hereinafter, it will be described with reference to FIG. 6
how the word driver 8 with such a configuration operates.
[0075] As shown in FIG. 6, while the word line select signal SW0 is
in logical one state (which will be herein called "at H-level"),
the levels of the voltages Va and Vb at the nodes N83 and N93 are
equal to those of the negative voltage Vng and supply voltage VDD,
respectively. Accordingly, the n-channel MOS transistors NT81 and
NT82 are OFF and ON, respectively, and the p-channel MOS transistor
PT81 is OFF.
[0076] When the word line select signal SW0 falls from H-level
(i.e., logical one) to L-level (i.e., logical zero), the p-channel
MOS transistor PT81 turns ON. Also, the voltage Vb at the node N93
falls to the Vng level, thereby turning OFF the n-channel MOS
transistor NT82. As a result, the voltage at the node N84, i.e.,
the voltage on the word line WL0, rises from the Vng level to the
VDD level.
[0077] Thereafter, when the word line select signal SW0 rises from
L-level to H-level, the p-channel MOS transistor PT81 turns OFF.
Also, synchronously with the rise of the word line select signal
SW0, a one-shot pulse is applied as the voltage Va to the node N83.
Responsive to the one-shot pulse, the n-channel MOS transistor NT81
turns ON and will be kept ON for a certain period of time. As a
result, the node N84 is discharged. That is to say, the voltage on
the word line WL0 drops from the VDD level to the Vss level. After
the voltage Va at the node N83 has risen, the voltage Vb at the
node N93 rises to the VDD level, thereby turning ON the n-channel
MOS transistor NT82. Consequently, the voltage on the word line WL0
further drops from the Vss level to the Vng level.
[0078] As described above, the word driver 8 once gets the word
line WL0 rapidly discharged to the Vss level by the n-channel MOS
transistor NT81. Thereafter, the n-channel MOS transistor NT82 is
turned ON, thereby redistributing the charge that has been stored
on the capacitor 104 shown in FIG. 4 in such a manner that the
voltage on the word line WL0 drops from the Vss level to the Vng
level. In this manner, the voltage on the word line can be pulled
down at a high speed without dissipating charge in vain. That is to
say, the power dissipated by the negative voltage generator 10 can
be reduced.
SRAM Operation
[0079] Next, it will be described with reference to FIGS. 1 and 7
how the SRAM with such a configuration operates. FIG. 7 is a timing
diagram illustrating how the SRAM shown in FIG. 1 operates. In the
following description, the operation of the SRAM in (1) its normal
mode and (2) its standby mode will be detailed separately.
(1) Normal Mode Operation
[0080] While the mode signal MD is at H-level, the SRAM operates in
the normal mode. The "normal mode operation" herein refers to a
period in which a memory cell MCi is accessed. The SRAM is
controlled so that a memory cell MCi is accessed during the first
half of one cycle of the precharge signal PRO and that precharging
is carried out during the second half of one cycle of the precharge
signal PRO in preparation for the next cycle. The precharge signal
PRO is externally supplied to the SRAM synchronously with an
external clock signal CLK. The external clock signal CLK is used as
a reference signal for operation.
[0081] At a time t1, the precharge signal PR0 falls from H-level to
L-level. In response, the precharge signal PR1 rises to H-level,
and the access signal R/W also rises from L-level to H-level. To
access the memory cell MC1, for example, as shown in FIG. 1, the
address signals A0 and A1 both rise to H-level. Since both of the
address and precharge signals A1 and PR1 are now at H-level, the
word line select signal SW0 falls to L-level. In response, the word
driver 8 activates the word line WL0 and the voltage on the word
line WL0 reaches the VDD level (see FIG. 6). Then, the n-channel
MOS transistors NT1a and NT1b turn ON, thereby connecting the data
retention nodes N1a and N1b of the memory cell MCI to the bit lines
BL0 and /BL0, respectively (see FIG. 2).
[0082] On the other hand, since both of the address and access
signals A0 and R/W are now at H-level, the transfer gates TG41 and
TG42 turn ON. As a result, the bit lines BL0 and /BL0 are connected
to the input/output lines IO and /IO, respectively.
[0083] In reading out data from the memory cell MCi, the
complementary data, stored at the data retention nodes Nia and Nib,
is read out onto the bit lines BL0 and /BL0 and input/output lines
IO and /IO and then transmitted by the input/output circuit 5 to
the input/output terminal D.
[0084] In writing data on the memory cell MCI, the input/output
circuit 5 transmits the data, which has been input to the
input/output terminal D, through the input/output lines IO and /IO
and then bit lines BL0 and /BL0. As a result, the data signal,
which has been read out from the memory cell MCi onto the bit line
BL0 and /BL0, is rewritten.
[0085] Next, at a time t2, the precharge signal PRO rises to
H-level. In response, the precharge, access and address signals
PR1, R/W, A1 and A0 all fall to L-level, and the transfer gates
TG41 and TG42 turn OFF. Also, the word line select signal SW0 rises
to H-level so that the voltage on the word line WL0 drops to the
Vng level (see FIG. 6). As a result, the n-channel MOS transistors
NT1a and NT1b turn OFF.
[0086] When the precharge signal PR1 falls to L-level, the
p-channel MOS transistors PT61 through PT63 and PT71 through PT73
in the precharge circuits 6 and 7 all turn ON. As a result, the two
pairs of bit lines BL0, /BL0 and BL1, /BL1 are connected to the
power supply nodes receiving the supply voltage VDD so as to be
precharged to the VDD level. Furthermore, the p-channel MOS
transistor PT63 equalizes the potential levels on the bit lines BL0
and /BL0 with each other, while the p-channel MOS transistor PT73
equalizes the potential levels on the bit lines BL1 and /BL1 with
each other. In this manner, the precharge operation is completed
and the SRAM is ready to the access that will be performed between
times t3 and t4. As for the next one cycle between the times t3 and
t5, the access and precharge operations will also be performed in a
similar manner.
(2) Standby Mode Operation
[0087] While the mode signal MD is at L-level, the SRAM is in the
standby mode. As used herein, the "standby mode" refers to a period
in which memory cells are accessed one-tenth or less as frequent as
the normal mode.
[0088] At a time t5, the mode signal MD falls from H-level to
L-level and the SRAM enters the standby mode.
[0089] Once the mode signal MD has fallen to L-level, the precharge
signal PR1 is always at H-level no matter whether the precharge
signal PRO is zero or one. Synchronously with the rise of the
precharge signal PR1, the p-channel MOS transistors PT61 through
PT63 and PT71 through PT73 in the precharge circuits 6 and 7 turn
OFF. As a result, the two pairs of bit lines BL0, /BL0 and BL1,
/BL1 are electrically disconnected from the power supply nodes
receiving the supply voltage VDD. That is to say, the precharge
operation is aborted.
[0090] Also, the word line select signals SW0 and SW1 are both at
H-level, and the voltages on the word lines WL0 and WL1 are at the
Vng level.
[0091] Furthermore, since the access signal R/W is at L-level, the
transfer gates TG41 through TG44 are all OFF. As a result, the two
pairs of bit lines BL0, /BL0 and BL1, /BL1 are electrically
disconnected from the pair of input/output lines IO and /IO.
[0092] The SRAM will be kept in this standby mode until a time
t6.
[0093] Normally, an SRAM is controlled in such a manner as to
perform an access operation during the first half of one cycle and
a precharge operation during the second half thereof in preparation
for the next cycle. Accordingly, if the precharge operation has
been aborted halfway to get the SRAM to enter the standby mode
compulsorily, then the SRAM cannot start its next normal mode
operation at the beginning of the first half of the next one cycle,
i.e., so soon as the standby mode operation is over. However, it
usually takes a several milliseconds' delay (i.e., a power
stabilizing period to be described later) for a mobile electronic
unit including an SRAM to return from the standby state (or standby
mode) to the normal operation state (or normal mode). Accordingly,
if several dummy cycles are provided for this interval, then there
will be no problem because all the bit lines can be precharged
completely. For that reason, dummy cycles are also provided for the
SRAM of the present invention between the times t6 and t7.
[0094] If no such dummy cycles were allowable, the above problem
could be avoidable by changing the method of controlling the memory
in such a manner that precharge is performed during the first half
of one cycle and that access is performed during the second half
thereof. In such a case, however, it would take a longer time after
an access request has been issued and before the desired data is
actually output. Accordingly, the operation like that is effective
only in low-speed applications.
[0095] How current dissipated can be reduced in standby mode Next,
it will be described how the current dissipated can be reduced in
the standby mode. This effect will be described only on the memory
cells MC1 and MC2 for the sake of simplicity.
[0096] In a known SRAM, the p-channel MOS transistors PT61 through
PT63 in the precharge circuit 6 are turned ON during its standby
mode operation, thereby precharging the bit lines BL0 and /BL0 to
the VDD level shown in FIG. 8. Also, an L-level voltage (i.e., 0 V)
is supplied onto the word lines WL0 and WL1. Accordingly, a leakage
current II flows from the power supply nodes, receiving the supply
voltage VDD, into the ground nodes of the memory cells MC1 and MC2
by way of the access transistors NT1b and NT2a, respectively.
[0097] This leakage current II flows from the power supply nodes
into the L-level data retention nodes of all memory cells.
Accordingly, a total leakage current I1, flowing through the entire
SRAM, is obtained by multiplying together the number of memory
cells and a leakage current flowing through each access transistor.
Only two memory cells MC1 and MC2 are illustrated in FIG. 8.
However, supposing a leakage current of 0.1 .mu.A flows through
each access transistor, for example, a current of 100 mA in total
flows through an SRAM including one million memory cells. If an
SRAM dissipates that great amount of current in its standby mode,
such an SRAM is far from being qualified for a battery-driven
mobile electronic unit of a small size.
[0098] According to a technique of reducing the leakage current I1,
a negative voltage of -0.3 V, for example, is applied to the gates
of the access transistors NT1b and NT2a. In this method, the source
(i.e., the L-level data retention node N1b or N2a) and the gate of
the access transistor NT1b or NT2a is reverse biased, thus reducing
the leakage current I1.
[0099] However, since transistors have been further downsized
recently, a different type of problem, or a
gate-induced-drain-leakage (GIDL) current, newly arises. As shown
in FIG. 9, if the gate voltage Vgs is negative and if the drain
voltage Vds is approximately equal to the supply voltage VDD, then
a considerable amount of GIDL current flows. To avoid this problem,
it is effective to reduce the drain voltage Vds.
[0100] If a negative voltage of -0.3 v, for example, is applied to
the gate of the access transistor NT1b or NT2a, then a negative
potential difference between the gate and drain thereof increases
its magnitude. This is because the bit lines BL0 and /BL0 have been
precharged to the VDD level. For example, where the supply voltage
VDD is 1.5 V, the gate-drain voltage Vgd=-0.3-1.5=-1.8 V.
Accordingly, a GIDL current I2 flows unintentionally and the
current dissipated in the stand-by mode cannot be reduced.
[0101] To solve this GIDL current problem, the SRAM of the first
embodiment turns OFF the p-channel MOS transistors PT61 through
PT63 and PT71 through PT73 of the precharge circuits 6 and 7 in the
standby state so that the bit lines BL0, /BL0 and BL1, /BL1 are
electrically disconnected from the power supply nodes receiving the
supply voltage VDD.
[0102] The potential level on these pairs of bit lines BL0, /BL0
and BL1, /BL1, which are electrically disconnected from the power
supply nodes, is lower than the VDD level, because no power is
supplied thereto from the power supply nodes. Usually, the
potential level is stabilized at around an intermediate potential
level (=1/2 VDD). Hereinafter, this phenomenon will be described
with reference to FIG. 10. FIG. 10 illustrates the p-channel MOS
transistor MP1a, access transistors NT1a and NT2a and n-channel MOS
transistor MN2a shown in FIG. 8. When the precharge operation is
aborted, the voltage VBN on the bit line BL0 is stabilized at
around the intermediate potential level (e.g., about 0.75 V when
VDD=1.5 V). Accordingly, the drain voltage Vds2 of the access
transistor NT2a becomes about 0.75 V. As a result, the current I2b,
flowing through the access transistor NT2a, decreases from I2 to I3
as shown in FIG. 9. Also, the drain voltage Vds1 of the access
transistor NT1a, which is connected to the H-level data retention
node N1a, also becomes about 0.75 V. As a result, the current I2a,
flowing through the access transistor NT1a, also decreases from I2
to I3 as shown in FIG. 9.
[0103] In this manner, by electrically disconnecting the bit lines
BL0, /BL0 and BL1, /BL1 from the power supply nodes, the access
transistors, connected to the L- and H-level data retention nodes,
respectively, can have their source-drain voltages decreased to
such a level as not causing the GIDL current problem.
[0104] As described above, according to the first embodiment, the
negative voltage Vng is supplied onto the word line WL0 or WL1 and
the bit lines BL0, /BL0 and BL1, /BL1 are electrically disconnected
from the power supply nodes in the standby state. Thus, the current
dissipated in the standby mode can be reduced while eliminating the
GIDL current.
EMBODIMENT 2
[0105] As shown in FIG. 11, an SRAM according to a second
embodiment of the present invention further includes a 1/2 VDD
generator 12 and level holders 13 and 14 in addition to all the
components shown in FIG. 1.
[0106] The 1/2 VDD generator 12 is a well-known circuit.
Specifically, the circuit 12 receives a supply voltage VDD and
generates 1/2 VDD, which is half as high as the supply voltage
VDD.
[0107] The level holder 13 includes p-channel MOS transistors PT131
through PT133. The p-channel MOS transistor PT131 is connected
between a node receiving the 1/2 VDD and a node N131 and turns ON
or OFF responsive to the mode signal MD. The p-channel MOS
transistor PT132 is connected between a node receiving the 1/2 VDD
and a node N132 and also turns ON or OFF responsive to the mode
signal MD. The nodes N131 and N132 are connected to the bit lines
BL0 and /BL0, respectively. The p-channel MOS transistor PT133 is
connected between these nodes N131 and N132 and turns ON or OFF
responsive to the mode signal MD.
[0108] The level holder 14 includes p-channel MOS transistors PT141
through PT143. The p-channel MOS transistor PT141 is connected
between a node receiving the 1/2 VDD and a node N141 and turns ON
or OFF responsive to the mode signal MD. The p-channel MOS
transistor PT142 is connected between a node receiving the 1/2 VDD
and a node N142 and also turns ON or OFF responsive to the mode
signal MD. The nodes N141 and N142 are connected to the bit lines
BL1 and /BL1, respectively. The p-channel MOS transistor PT143 is
connected between these nodes N141 and N142 and turns ON or OFF
responsive to the mode signal MD.
[0109] While this SRAM is in the standby mode, the p-channel MOS
transistors PT131 through PT133 and PT141 through PT143 are ON,
thereby holding the potential on the pairs of bit lines BL0, /BL0
and BL1, /BL1 at the 1/2 VDD level. As a result, the current
dissipated in the standby mode can also be reduced as in the first
embodiment. In addition, the following effects are also
attainable.
[0110] According to the first embodiment, the pairs of bit lines
BL0, /BL0 and BL1, /BL1 are floating, and therefore the voltage
levels thereof are not constant. For that reason, the precharge
period needed to make the SRAM return from the standby mode to the
normal mode (i.e., the dummy cycle period shown in FIG. 7) cannot
have a fixed length.
[0111] In contrast, according to the second embodiment, the
potential on the pairs of bit lines BL0, /BL0 and BL1, /BL1 is held
at a constant level (i.e., 1/2 VDD level) in the standby mode. As a
result, the precharge period needed to make the SRAM return from
the standby mode to the normal mode (i.e., the dummy cycle period
shown in FIG. 7) can have a fixed length.
[0112] In this embodiment, the potential on the bit lines is fixed
at the 1/2 VDD level. Alternatively, any other constant level may
be adopted so long as the level is lower than the VDD level. But
the level is preferably equal to or lower than the 1/2 VDD level,
i.e., the intermediate voltage level.
EMBODIMENT 3
[0113] FIG. 12 illustrates a configuration for a mobile electronic
unit 200 according to a third embodiment of the present invention.
As shown in FIG. 12, the unit 200 includes a microprocessor 210 and
a system LSI 220. Examples of mobile electronic units like this
include cell phones.
[0114] The microprocessor 210 is always ON to function as a system
controller for the electronic unit 200. Also, the microprocessor
210 supplies a mode switching signal CTA to the system LSI 220 to
instruct the LSI 220 to switch the mode of operation from the
normal mode into the standby mode, or vice versa.
[0115] The system LSI 220 includes control circuit 221, SRAMs 222
and 223, logic circuit 224 and switch 225.
[0116] Responsive to the mode switching signal CTA supplied from
the microprocessor 210, the control circuit 221 supplies the mode
signal MD to the SRAM 222 and a switching signal CTB to the switch
225, respectively. The SRAM 222 has the same configuration as the
counterpart shown in FIG. 1 and can also reduce the amount of
current dissipated in the standby mode. The supply voltage VDD is
always supplied to the SRAM 222 even while the SRAM 222 is in the
standby mode. On the other hand, to block the leakage current from
flowing in the standby mode, the SRAM 223 is supplied with no power
while in the standby mode. The switch 225 is connected between a
power supply node receiving the supply voltage VDD and a power
supply node for the SRAM 223 and logic circuit 224, and turns ON or
OFF responsive to the switching signal CTB. Specifically, while the
switch 225 is ON, the supply voltage VDD is supplied to the SRAM
223 and logic circuit 224. On the other hand, while the switch 225
is OFF, the supply voltage VDD is not supplied thereto.
[0117] That is to say, in this system LSI 220, power is always
supplied to only the control circuit 221 and SRAM 222 that
communicate with the microprocessor 210.
[0118] Hereinafter, it will be described how the mobile electronic
unit with such a configuration operates.
[0119] When the system should change its mode of operation from
normal mode into standby mode (e.g., when a cell phone enters a
standby state), the microprocessor 210 supplies the mode switching
signal CTA to the system LSI 220 to instruct the LSI 220 to change
into the standby mode. That is to say, as shown in FIG. 13, the
mode switching signal CTA falls from H-level to L-level at a time
t11.
[0120] Synchronously with this fall of the mode switching signal
CTA, the control circuit 221 negates the mode signal MD and
switching signal CTB to L-level. Then, responsive to the L-level
switching signal CTB, the switch 225 turns OFF. As a result, the
power that has been supplied to the SRAM 223 and logic circuit 224
is interrupted. On the other hand, the SRAM 222 aborts the
precharge operation in response to the L-level mode signal MD (see
FIG. 7).
[0121] When the system returns from the standby mode to the normal
mode, the mode switching signal CTA rises to H-level at a time t12
as shown in FIG. 13. In response, the switching signal CTB rises to
H-level to turn the switch 225 ON. The mode signal MD also rises to
H-level, thereby allowing the SRAM 222 to start its precharge
operation. During the standby mode, the bit lines of the SRAM 222
are electrically disconnected from the power supply nodes, and have
their potential levels dropped. Accordingly, it is expected that if
all the bit lines were precharged at a time at the beginning of the
precharge operation, a large peak current would flow. For that
reason, precharging of the bit lines should preferably be started
stepwise at some intervals. For example, multiple bit lines may be
classified into several groups and the precharge operation on one
of those groups may be started some time after the precharge
operation on another has been started. It would take several
milliseconds (i.e., the interval between times t12 and t13 shown in
FIG. 13) for the voltage Vint to be stabilized at the VDD level
after the switch 225 has turned ON. That is to say, it takes some
time for the system LSI 220 to start operating stably enough.
Accordingly, there is a sufficient amount of time for the SRAM 222
to return to the precharge state (i.e., the dummy cycles shown in
FIG. 7 are available), and the precharge operation can be started
step by step.
EMBODIMENT 4
[0122] FIG. 14 illustrates an overall configuration for an SRAM
according to a fourth embodiment of the present invention. As shown
in FIG. 14, the SRAM includes memory array 1, row decoder 2, column
decoder 3, column selector 4, input/output circuit 5, precharge
circuits 6 and 7, word drivers 1401 and 1402 and NAND gate 11.
Responsive to an H-level word line select signal SW0 or SW1, each
of the word drivers 1401 and 1402 supplies the supply voltage VDD
to its associated word line WL0 or WL1. And responsive to an
L-level word line select signal SW0 or SW1, each of the word
drivers 1401 and 1402 supplies the ground voltage Vss (=0 V) to its
associated word line WL0 or WL1.
[0123] Next, it will be described how the SRAM shown in FIG. 14 can
reduce the current dissipated in the standby mode. This effect will
be described only on the memory cells MC1 and MC2 for the sake of
simplicity.
[0124] In the standby mode, the ground voltage Vss (=0 V) is
supplied onto the word lines WL0 and WL1 to turn the access
transistors NT1a, NT1b, NT2a and NT2b OFF. The transfer gates TG41
through TG44 and p-channel MOS transistors PT61 through PT63 and
PT71 through PT73 also turn OFF to allow the bit lines BL0, /BL0
and BL1, /BL1 to be floating. However, as shown in FIG. 15, a
leakage current Ix flows along a path running from a power supply
node for the memory cell MC1 to a ground node for the memory cell
MC2 by way of the p-channel MOS transistor MP1a, H-level data
retention node N1a, access transistor NT1a, bit line BL0, access
transistor NT2a, L-level data retention node N2a and n-channel MOS
transistor MN2a. The leakage current Ix also flows along a path
running from a power supply node for the memory cell MC2 to a
ground node for the memory cell MC1 by way of the p-channel MOS
transistor MP2b, H-level data retention node N2b, access transistor
NT2b, bit line /BL0, access transistor NT1b, L-level data retention
node N1b and n-channel MOS transistor MN1b. This is because the
access transistors NT1a, NT1b, NT2a and NT2b have a low threshold
voltage. The leakage current Ix holds the potential on the bit
lines BL0 and /BL0 at a positive level, which is higher than the
ground voltage Vss (=0 V) and lower than the supply voltage VDD. As
a result, a negative potential difference is generated between the
gate and source of the access transistors NT1a and NT2b.
Accordingly, although the leakage current Ix flows through the
access transistors NT1a and NT2b, the negative gate-source
potential difference reduces the amount of the leakage current Ix
flowing.
[0125] As described above, the leakage current Ix holds the
potential on the bit lines BL0 and /BL0 at a positive level.
However, the level is not necessarily constant, but may be close to
the ground voltage Vss (=0 V). For example, the level may be 0.1 V.
In that situation, each of the access transistors NT1a and NT2b has
a negative gate-source voltage Vgs (i.e., -0.1 V) and a
drain-source voltage Vds approximately equal to the supply voltage
VDD (which is herein 1.5 V). Accordingly, a GIDL current
(.apprxeq.I11) is unintentionally allowed to flow as shown in FIG.
16. To avoid this GIDL current problem, the level holders 13 and 14
and 1/2 VDD generator 12 shown in FIG. 11 may be provided
additionally. Then, as shown in FIG. 17, the potential level VNB on
the bit lines BL0 and /BL0 during the standby mode gets equal to
the 1/2 VDD level, and the drain-source voltage Vds of the access
transistors NT1a and NT2b gets approximately equal to the 1/2 VDD
level (i.e., 0.75 V). As a result, the GIDL current can be reduced
to a negligible level 112 as shown in FIG. 16. Alternatively,
instead of providing the level holders 13 and 14 and 1/2 VDD
generator 12, the bit lines BL0 and /BL0 may be precharged to the
1/2 VDD level by the precharge circuits 6 and 7. Furthermore, the
potential on the bit lines BL0 and /BL0 does not have to held at
the 1/2 VDD level. Rather, the potential on the bit lines BL0 and
/BL0 may be held at any level higher than the Vss (ground voltage)
level (=0 V) and lower than the VDD (supply voltage) level so long
as the GIDL current problem is avoidable.
* * * * *