U.S. patent application number 14/096091 was filed with the patent office on 2014-04-03 for memory device and writing method thereof.
This patent application is currently assigned to Elpida Memory, Inc.. The applicant listed for this patent is Elpida Memory, Inc.. Invention is credited to Kenji Mae, Kiyoshi Nakai, Koji Sato.
Application Number | 20140092679 14/096091 |
Document ID | / |
Family ID | 42171960 |
Filed Date | 2014-04-03 |
United States Patent
Application |
20140092679 |
Kind Code |
A1 |
Sato; Koji ; et al. |
April 3, 2014 |
MEMORY DEVICE AND WRITING METHOD THEREOF
Abstract
A write amplifier for driving a bit line connected to a selected
phase change memory cell drives the bit line with a first current
driving capability and then drives the bit line with a second
current driving capability lower than the first current driving
capability.
Inventors: |
Sato; Koji; (Tokyo, JP)
; Nakai; Kiyoshi; (Tokyo, JP) ; Mae; Kenji;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Elpida Memory, Inc. |
Tokyo |
|
JP |
|
|
Assignee: |
Elpida Memory, Inc.
Tokyo
JP
|
Family ID: |
42171960 |
Appl. No.: |
14/096091 |
Filed: |
December 4, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12591295 |
Nov 16, 2009 |
8605494 |
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14096091 |
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Current U.S.
Class: |
365/163 |
Current CPC
Class: |
G11C 7/1078 20130101;
G11C 13/0069 20130101; G11C 13/0004 20130101; G11C 7/1096 20130101;
G11C 2013/0078 20130101 |
Class at
Publication: |
365/163 |
International
Class: |
G11C 13/00 20060101
G11C013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 20, 2008 |
JP |
2008-296848 |
Claims
1. A semiconductor device comprising a write amplifier adapted to
drive a bit line, connected to a selected phase change memory cell,
with a first current driving capability and then to drive said bit
line to a write voltage, corresponding to data to be written, with
a second current driving capability lower than said first current
driving capability.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This Application is a Continuation Application of U.S.
patent application Ser. No. 12/591,295, filed on Nov. 16, 2009.
[0002] This application is based on and claims priority from
Japanese Patent Application No. 2008-296848 filed on Nov. 20, 2008.
The disclosure thereof is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] This invention relates to a memory device and a writing
method thereof.
[0005] 2. Description of the Related Art
[0006] The phase state of a phase change material such as
chalcogenide can be changed between a crystalline state and an
amorphous state by heating and cooling and the phase change
material exhibits a different resistance value depending on its
phase state. A memory device configured to store information using
such a phase change material (variable resistance element) is
called a phase change memory device.
[0007] In the phase change memory device, the phase state of the
variable resistance element is changed by controlling the magnitude
and duration of current supplied to the variable resistance element
to heat and cool it. Information can be stored as a phase state of
the variable resistance element and can be read as a resistance
value thereof.
[0008] A related phase change memory device comprises a memory cell
array having a plurality of memory cells arranged in rows and
columns and each using a phase change material as a variable
resistance element, and a write driver connected to the memory cell
array through a bit line selection circuit. Such a phase change
memory device is described in, for example, Japanese Unexamined
Patent Application Publication (JP-A) No. 2007-87568 (Patent
Document 1).
SUMMARY
[0009] The technique described in Patent Document 1 achieves a
reduction in program time (i.e. speed-up) by reducing the setup
time of a pump circuit serving to enhance the current supply
capability of the write driver.
[0010] Following the speed-up of a computer system, however, a
phase change memory device for use therein is required to be
further speeded up. The present inventors have discovered that when
driving a bit line, it takes time to raise the voltage of the bit
line to a predetermined value due to a relatively large capacitance
of the bit line, thus resulting in a long write time.
[0011] In one embodiment, there is provided a semiconductor device
which includes a write amplifier adapted to drive a bit line,
connected to a selected phase change memory cell, with a first
current driving capability and then to drive the bit line to a
write voltage, corresponding to data to be written, with a second
current driving capability lower than the first current driving
capability.
[0012] According to this invention, by providing a write amplifier
adapted to drive a bit line, connected to a selected phase change
memory cell, with a first current driving capability and then to
drive the bit line to a write voltage, corresponding to data to be
written, with a second current driving capability lower than the
first current driving capability, it is possible to quickly drive
the bit line to the required voltage and thus to shorten the write
time for the selected memory cell.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The above features and advantages of the present invention
will be more apparent from the following description of certain
preferred embodiments taken in conjunction with the accompanying
drawings, in which:
[0014] FIG. 1 is a circuit diagram showing a main portion of a
phase change memory device according to a first embodiment of this
invention;
[0015] FIG. 2 is a time chart for explaining the operation of the
phase change memory device of FIG. 1;
[0016] FIG. 3 is a waveform diagram showing time-dependent changes
in voltage and current at the time of driving a bit line in the
phase change memory device of FIG. 1;
[0017] FIG. 4 is a circuit diagram showing a main portion of a
phase change memory device according to a second embodiment of this
invention; and
[0018] FIG. 5 is a time chart for explaining the operation of the
phase change memory device of FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] The invention will be now described herein with reference to
illustrative embodiments. Those skilled in the art will recognize
that many alternative embodiments can be accomplished using the
teachings of the present invention and that the invention is not
limited to the embodiments illustrated for explanatory
purposes.
[0020] FIG. 1 is a circuit diagram showing a main portion of a
phase change memory device (or a semiconductor device) according to
a first embodiment of this invention. FIG. 1 only illustrates a
single memory cell 11 and a portion relating to a write operation
for this memory cell 11. Actually, the phase change memory device
includes a plurality of memory cells arranged in rows and columns.
Hereinafter, it is assumed that the memory cell 11 is a selected
memory cell in which data are written.
[0021] The phase change memory device of FIG. 1 comprises a cell
array (not illustrated) including the memory cell 11, Yj switches
(bit line selection switches) 13 for hierarchizing bit lines 12 of
the cell array, respectively, write amplifiers (write drivers) 14
for driving the bit lines 12 selected by the Yj switches 13,
respectively, and voltage drop circuits 15 each for dropping the
voltage of the corresponding bit line 12 to a low-potential side
power supply VSS level. In this specification, a "bit line"
includes not only a line connecting between the memory cell 11 and
the Yj switch 13, but also a line connecting between the Yj switch
13 and the write amplifier 14.
[0022] The memory cell 11 comprises a variable resistance element
111 made of, for example, GST (GeSbTe:
germanium.antimony.tellurium) and cell transistors (herein a pair
of NMOSs (n-channel MOS transistors), which, however, may be a
single NMOS if it has a predetermined current performance) 112.
[0023] The Yj switch 13 comprises a pair of an NMOS 131 and a PMOS
(p-channel MOS transistor) 132 both connected to the bit line
12.
[0024] The write amplifier 14 comprises a first write amplifier
section 141 and a second write amplifier section 142 adapted to
jointly provide a first current driving capability. The second
write amplifier section 142 provides a second current driving
capability lower than the first current driving capability in a
state where the first write amplifier section 141 stops driving the
bit line 12. The first write amplifier section 141 serves as a
write amplifier for high-speed bit line activation.
[0025] The first write amplifier section 141 comprises a PMOS 1411
with its drain and gate commonly connected to the bit line 12, a
PMOS 1412 connected between the PMOS 1411 and a high-potential side
power supply VPS, and a PMOS 1413 connected between the PMOS 1411
and a high-potential side (boost) power supply VPP. The PMOSs 1412
and 1413 each correspond to a first switch and the PMOS 1411
corresponds to a first current trimming portion.
[0026] A SET signal (SET pulse) and a RESET signal (RESET pulse)
are input to the gates of the PMOSs 1412 and 1413, respectively.
The SET signal is an input signal for changing the variable
resistance element 111 into a crystalline state (set state), i.e. a
SET write signal, and the RESET signal is an input signal for
changing the variable resistance element 111 into an amorphous
state (reset state), i.e. a RESET write signal. The PMOSs 1412 and
1413 are turned on when the SET signal and the RESET signal are at
an inactive level, respectively, and are turned off when the SET
signal and the RESET signal are at an active level, respectively.
It never happens that the SET signal and the RESET signal are
simultaneously set to the inactive level. The pulse width of the
SET signal is set to be greater than that of the RESET signal. The
PMOS 1411 supplies current to the bit line 12 when either one of
the PMOSs 1412 and 1413 is on and further the voltage of the bit
line 12 is relatively low.
[0027] As will be described later, while the first write amplifier
section 141 supplies the current to the bit line 12, the second
write amplifier section 142 constantly supplies current to the bit
line 12 so that the bit line 12 is driven with the first current
driving capability higher than the second current driving
capability possessed by the second write amplifier section 142.
When the current supply from the first write amplifier section 141
is stopped, the bit line 12 is driven only by the second write
amplifier section 142 with its second current driving capability
lower than the first current driving capability.
[0028] The second write amplifier section 142 comprises a SET write
amplifier section 142a and a RESET write amplifier section 142b.
The SET write amplifier section 142a and the RESET write amplifier
section 142b may have different current driving capabilities.
However, since these amplifier sections 142a and 142b do not
simultaneously drive the bit line 12 (the SET signal and the RESET
signal are not simultaneously set to the inactive level), it is
expressed in this specification that each of them has the second
current driving capability.
[0029] The SET write amplifier section 142a comprises a switching
PMOS 1421 and a current trimming PMOS 1422 connected in series
between the bit line 12 and the high-potential side power supply
VPS. Likewise, the RESET write amplifier section 142b comprises a
switching PMOS 1423 and a current trimming PMOS 1424 connected in
series between the bit line 12 and the high-potential side (boost)
power supply VPP. The switching PMOSs 1421 and 1423 each correspond
to a second switch and the current trimming PMOSs 1422 and 1424
each correspond to a second current trimming portion.
[0030] The SET signal and the RESET signal are input to the gates
of the switching PMOSs 1421 and 1423, respectively. Like the PMOSs
1412 and 1413, the PMOSs 1421 and 1423 are turned on when the SET
signal and the RESET signal are at the inactive level,
respectively, and are turned off when the SET signal and the RESET
signal are at the active level, respectively.
[0031] Constant voltages VWEREF1 and VWEREF2 are input to the gates
of the current trimming PMOSs 1422 and 1424, respectively. The
constant voltages VWEREF1 and VWEREF2 are determined so that the
appropriate (i.e. the second) current driving capabilities are
obtained for SET writing and RESET writing, respectively. For
example, the current driving capability is set to be smaller for
SET writing than for RESET writing.
[0032] The voltage drop circuit 15 comprises a coincidence circuit
151 and an NMOS 152.
[0033] Referring now also to a waveform diagram of FIG. 2, the
write operation of the phase change memory device of FIG. 1 will be
described.
[0034] When a word line drive signal (SW) and a Yj selection signal
(Yj) are changed to an active level at timing Act, the cell
transistors 112 and the Yj switch 13 are both turned on. In the
initial state, the NMOS 152 of the voltage drop circuit 15 is on so
that the bit line 12 is at the same potential as the low-potential
side power supply VSS (=0V).
[0035] Then, when a Write pulse (SET signal stcl or RESET signal
rstcl) is changed to an inactive level at timing Write, the write
amplifier 14 starts supplying write current and, simultaneously,
the NMOS 152 of the voltage drop circuit 15 is turned off. Thus,
the voltage of the bit line 12 rises to a predetermined value.
[0036] Thereafter, when the Write pulse (stcl or rstcl) is changed
to an active level, the write amplifier 14 stops supplying the
current to the bit line 12 and, simultaneously, the NMOS 152 of the
voltage drop circuit 15 is turned on. Thus, the voltage of the bit
line 12 becomes equal to the low-potential side power supply
voltage (=0V).
[0037] In the manner described above, the write current is supplied
to the variable resistance element 111 for a time corresponding to
the pulse width of the Write pulse (i.e. the SET signal stcl or the
RESET signal rstcl). The variable resistance element 111 is heated
according to the magnitude of the supplied current and then cooled
(heat dissipation) so that the phase state thereof is
determined.
[0038] Now, the operation of the write amplifier 14 will be
described in further detail.
[0039] In the SET write amplifier section 142a (or the RESET write
amplifier section 142b) of the second write amplifier section 142,
when the SET signal stcl (or the RESET signal rstcl) is changed to
the inactive level, the PMOS 1421 (or 1423) is turned on in
response thereto. Thus, the second write amplifier section 142
drives the bit line 12 with the second current driving capability
determined by VWEREF1 (or VWEREF2).
[0040] On the other hand, in the first write amplifier section 141,
when the SET signal stcl (or the RESET signal rstcl) is changed to
the inactive level, the PMOS 1412 (or 1413) is turned on in
response thereto. In this event, since the voltage of the bit line
12 is relative low (0V in the initial state), the PMOS 1411 allows
current from the PMOS 1412 (or 1413) toward the bit line 12 to pass
therethrough. That is, the first write amplifier section 141 drives
the bit line 12 jointly with the second write amplifier section
142. As a result, the bit line 12 is driven with the first current
driving capability higher than the second current driving
capability possessed by the second write amplifier section 142.
Thus, the rise of the voltage of the bit line 12 becomes steep to
make it possible to shorten the write time for the memory cell
11.
[0041] Thereafter, when the voltage of the bit line 12 reaches
VPS-Vth (a voltage lower than VPS by Vth) (or VPP-Vth, i.e., a
voltage lower than VPP by Vth), almost no current flows through the
PMOS 1411. That is, the current supply to the bit line 12 from the
first write amplifier section 141 is stopped. On the other hand,
the current supply to the bit line 12 (i.e. the memory cell 11)
from the second write amplifier section 142 is continued. This
prevents an increase in power consumption and a write error due to
supplying an amount of current more than necessary.
[0042] The driving of the bit line 12 with the second current
driving capability using the second write amplifier section 142 is
carried out until the voltage of the bit line 12 reaches a voltage
(VPS or VPP) corresponding to data to be written, and is further
continued thereafter. After the voltage of the bit line 12 reaches
the voltage corresponding to the data to be written, a constant
current is supplied to the memory cell 11 through the bit line
12.
[0043] FIG. 3 shows, in solid lines, changes in bit line voltage
and current at the time of SET writing and RESET writing in the
phase change memory device according to this embodiment and further
shows, in broken lines, those in the related phase change memory
device. As clear from FIG. 3, in the phase change memory device
according to this embodiment, the voltage rise at the time of
driving the bit line 12 is steep.
[0044] As described above, according to this embodiment, it is
possible to shorten the write time for the memory cell 11 by, when
driving the bit line 12, starting to drive the bit line 12 with the
first current driving capability higher than the second current
driving capability necessary for SET writing or RESET writing, then
driving the bit line 12 with the second current driving capability
lower than the first current driving capability.
[0045] Referring now to FIG. 4, a phase change memory device (or a
semiconductor device) according to a second embodiment of this
invention will be described.
[0046] The phase change memory device of FIG. 4 differs from that
of FIG. 1 in that a write amplifier 41 is used instead of the write
amplifier 14.
[0047] The write amplifier 41 comprises a SET write amplifier
section 41a and a RESET write amplifier section 41b.
[0048] The SET write amplifier section 41a has a switching PMOS 411
and a current trimming PMOS 412 connected in series between a bit
line 12 and a high-potential side power supply VPS. Further, the
SET write amplifier section 41a has a control voltage changing
section 413 for controlling the control voltage of the current
trimming PMOS 412. The control voltage changing section 413
comprises a delay circuit 4131, and a PMOS 4132 and an NMOS 4133
each adapted to be turned on or off according to an output from the
delay circuit 4131.
[0049] A SET signal stcl is input, as an input signal, to the gate
of the switching PMOS 411 and is also input to the delay circuit
4131. Further, a constant voltage VWEREF1 is input to the drain of
the PMOS 4132.
[0050] The RESET write amplifier section 41b is configured in the
same manner as the SET write amplifier section 41a. That is, the
RESET write amplifier section 41b comprises a switching PMOS 414
and a current trimming PMOS 415 connected in series between the bit
line 12 and a high-potential side (boost) power supply VPP, and a
control voltage changing section 416. The control voltage changing
section 416 comprises a delay circuit 4161, a PMOS 4162, and an
NMOS 4163.
[0051] A RESET signal rstcl is input, as an input signal, to the
gate of the PMOS 414 and is also input to the delay circuit 4161.
Further, a constant voltage VWEREF2 is input to the drain of the
PMOS 4162.
[0052] Hereinbelow, referring also to FIG. 5, the operation of the
phase change memory device of FIG. 4 will be described.
[0053] When a word line drive signal (SW) and a Yj selection signal
(Yj) are changed to an active level at timing Act, cell transistors
112 and a Yj switch 13 are both turned on.
[0054] Then, when a Write pulse (SET signal stcl or RESET signal
rstcl) is changed to an inactive level at timing Write, the PMOS
411 (or 414) serving as a switch is turned on so that the bit line
12 is driven.
[0055] In this event, the delay circuit 4131 (or 4161) supplies a
delay Write pulse, obtained by delaying the Write pulse by a fixed
time At, to the gates of the PMOS 4132 and the NMOS 4133 (or the
PMOS 4162 and the NMOS 4163). Accordingly, the PMOS 4132 (or 4162)
remains off and the NMOS 4133 (or 4163) remains on until the lapse
of .DELTA.t from a time point when the Write pulse is changed to
the inactive level. In this manner, the PMOS 4132 and the NMOS 4133
(or the PMOS 4162 and the NMOS 4163) serve as a switching portion.
As a result, a low-potential side voltage VSS is applied to the
gate of the PMOS 412 (or 415). In this manner, the voltage VSS
lower than the constant voltage VWEREF1 (or VWEREF2) is applied to
the gate of the PMOS 412 (or 415) serving as a current trimming
portion. Consequently, the on-state resistance of the PMOS 412 (or
415) is reduced so that the current supplied to the bit line 12
becomes large. That is, the bit line 12 is driven with a relatively
high first current driving capability. As a result, the voltage
rise of the bit line 12 becomes steep (high speed). FIG. 5 shows,
in broken line, a change in bit line voltage when the constant
voltage VWEREF1 (or VWEREF2) is applied to the gate of the PMOS 412
(or 415).
[0056] After the lapse of the fixed time At, the delay Write pulse
is changed to an inactive level so that the PMOS 4132 (or 4162) is
turned on and the NMOS 4133 (or 4163) is turned off. Thus, the
constant voltage VWEREF1 (or VWEREF2) is applied to the gate of the
PMOS 412 (or 415) so that the amount of current supplied to the bit
line 12 is limited to a predetermined value or less. As a result,
the bit line 12 is driven with a second current driving capability
lower than the first current driving capability. The driving of the
bit line 12 with the second current driving capability is carried
out until the voltage of the bit line 12 reaches a voltage
corresponding to data to be written, and is further continued
thereafter.
[0057] As described above, in this embodiment, the control voltage
changing sections 413 and 416 each serve as current driving
capability changing section for changing the current driving
capability of the write amplifier 41 by changing the gate voltage
(control voltage) of the current trimming PMOS 412 or 415.
[0058] In the phase change memory device according to this
embodiment, by changing the control voltage of the current trimming
PMOS 412 or 415 to change the current limit value (current driving
capability), it is possible to drive the bit line with the first
current driving capability higher than the second current driving
capability immediately after the start of driving the bit line and
to drive the bit line with the second current driving capability
lower than the first current driving capability after the lapse of
the fixed time. This makes it possible to shorten the write time
and to prevent an increase in power consumption and the occurrence
of a write error.
[0059] It is apparent that the present invention is not limited to
the above embodiments, but may be modified and changed without
departing from the scope and spirit of the invention.
[0060] For example, the drains of the NMOSs 4133 and 4163 are each
connected to the low-potential side power supply VSS in the phase
change memory device of FIG. 4, but a lower voltage (negative
voltage) may be applied thereto to further enhance the current
driving capability.
* * * * *