U.S. patent application number 10/810291 was filed with the patent office on 2005-02-24 for program voltage generation circuit for stably programming flash memory cell and method of programming flash memory cell.
This patent application is currently assigned to Samsung Electronics Co., Ltd.. Invention is credited to Park, Jae-Kwan.
Application Number | 20050041475 10/810291 |
Document ID | / |
Family ID | 34192189 |
Filed Date | 2005-02-24 |
United States Patent
Application |
20050041475 |
Kind Code |
A1 |
Park, Jae-Kwan |
February 24, 2005 |
PROGRAM VOLTAGE GENERATION CIRCUIT FOR STABLY PROGRAMMING FLASH
MEMORY CELL AND METHOD OF PROGRAMMING FLASH MEMORY CELL
Abstract
Provided are a program voltage generation circuit for achieving
stable programming of a flash memory cell, and a method of
programming a flash memory cell. In the program voltage generation
circuit, a program wordline voltage to be applied to the gate of a
flash memory cell is generated in response to a sink current
provided by a constant current source and the result of a
comparison between a reference voltage and a bitline voltage. The
bitline voltage is generated according to a program current flowing
to the first flash memory cell. A bitline current control voltage
is generated in response to the program current that flows to a
second flash memory cell in response to the program wordline
voltage. Accordingly, even when the characteristics of the flash
memory cell vary due to a change of a manufacturing process
thereof, a constant program wordline voltage, a constant bitline
voltage, and a constant bitline current control voltage are
generated, and thus the flash memory cell is stably programmed.
Inventors: |
Park, Jae-Kwan; (Seoul,
KR) |
Correspondence
Address: |
Steven M. Mills
MILLS & ONELLO LLP
Suite 605
Eleven Beacon Street
Boston
MA
02108
US
|
Assignee: |
Samsung Electronics Co.,
Ltd.
|
Family ID: |
34192189 |
Appl. No.: |
10/810291 |
Filed: |
March 26, 2004 |
Current U.S.
Class: |
365/185.18 |
Current CPC
Class: |
G11C 16/0425 20130101;
G11C 8/08 20130101; G11C 16/12 20130101 |
Class at
Publication: |
365/185.18 |
International
Class: |
G11C 011/34; G11C
016/04 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 22, 2003 |
KR |
03-58253 |
Claims
What is claimed is:
1. A program voltage generation circuit in a flash memory, for
achieving stable programming of flash memory cells, the program
voltage generation circuit comprising: a constant current source
which provides a sink current; a program wordline voltage
generation unit which generates a program wordline voltage applied
to the gate of a flash memory cell, in response to the sink current
and the result of a comparison between a reference voltage and a
bitline voltage; a bitline voltage generation unit which generates
the bitline voltage according to a program current flowing to the
first flash memory cell; and a bitline current control voltage
generation unit which generates a bitline current control voltage
in response to the program current that flows to a second flash
memory cell in response to the program wordline voltage.
2. The program voltage generation circuit of claim 1, wherein the
program wordline voltage generation unit comprises: a first PMOS
transistor which has a source coupled to a power supply voltage and
a gate and a drain that are coupled to each other; a first NMOS
transistor which has a source coupled to a ground voltage and a
gate and a drain that are coupled to each other, and is coupled to
the drain of the PMOS transistor; a second NMOS transistor which
has a source coupled to the ground voltage, a gate coupled to the
gate of the first NMOS transistor to form a current mirror, and a
drain coupled to the program wordline voltage; a second PMOS
transistor which is connected between the power supply voltage and
the drain of the second NMOS transistor and has a gate coupled to
an output of a voltage comparator; and the voltage comparator which
compares the bitline voltage with the reference voltage and
provides the output corresponding to the result of the comparison
to the gate of the second PMOS transistor.
3. The program voltage generation circuit of claim 1, wherein the
bitline voltage generation unit comprises: the first flash memory
cell which has a source coupled to a source voltage and a gate
coupled to the program wordline voltage; and a resistor which is
coupled between the drain of the first flash memory cell and the
ground voltage and generates the bitline voltage.
4. The program voltage generation circuit of claim 1, wherein the
bitline current control voltage generation unit comprises: a second
flash memory cell which has a source coupled to a source voltage
and a gate coupled to the program wordline voltage; and an NMOS
transistor which has a source coupled to a ground voltage and a
gate and a drain that are coupled to the drain of the second flash
memory cell and generate the bitline current control voltage.
5. The program voltage generation circuit of claim 4, wherein the
flash memory device comprises: a flash memory cell in a flash
memory cell core array to be programmed, which has a gate coupled
to the program wordline voltage, a source coupled to the source
voltage, and a drain coupled to the bitline voltage; and an NMOS
transistor gated to the bitline current control voltage between the
drain of the flash memory cell and the ground voltage.
6. A program voltage generation circuit in a flash memory, for
achieving stable programming of flash memory cells, the program
voltage generation circuit comprising: a constant current source
which includes a first PMOS transistor and first and second NMOS
transistors, wherein the first PMOS transistor has a source coupled
to a power supply voltage and a gate and a drain that are coupled
to each other, the first NMOS transistor has a source coupled to a
ground voltage and a gate and a drain that are coupled to each
other, and is coupled to the drain of the first PMOS transistor,
and the second NMOS transistor has a source coupled to the ground
voltage, a gate coupled to the gate of the first NMOS transistor to
form a current mirror, and a drain coupled to the program wordline
voltage; a second PMOS transistor which is connected between the
power supply voltage and the drain of the second NMOS transistor
and has a gate coupled to an output of a voltage comparator; the
voltage comparator which compares the bitline voltage with a
reference voltage and provides the output corresponding to the
result of the comparison to the gate of the second PMOS transistor;
a first flash memory cell which has a source coupled to a source
voltage and a gate coupled to the program wordline voltage; a
resistor which is coupled between the drain of the first flash
memory cell and the ground voltage and generates the bitline
voltage according to a program current flowing to the first flash
memory cell; a second flash memory cell which has a source coupled
to a source voltage and a gate coupled to the program wordline
voltage; and an NMOS transistor which has a source coupled to a
ground voltage and a gate and a drain that are coupled to the drain
of the second flash memory cell and generate a bitline current
control voltage according to a program current flowing to the
second flash memory cell.
7. The program voltage generation circuit of claim 6, wherein the
flash memory device comprises: a flash memory cell in a flash
memory cell core array to be programmed, which has a gate coupled
to the program wordline voltage, a source coupled to the source
voltage, and a drain coupled to the bitline voltage; and an NMOS
transistor gated to the bitline current control voltage between the
drain of the flash memory cell and the ground voltage.
8. A method of programming a flash memory cell, the method
comprising: supplying a sink current from a constant current
source; generating a program wordline voltage applied to the gate
of a flash memory cell, in response to the sink current and the
result of a comparison between a reference voltage and a bitline
voltage; generating the bitline voltage to be applied to the drain
of the first flash memory cell, according to a program current
flowing to the first flash memory cell; generating a bitline
current control voltage in response to the program current by
applying the program wordline voltage to the gate of a second flash
memory cell; and programming the flash memory cell by flowing the
program current to the flash memory cell by applying the program
wordline voltage to the gate of the flash memory cell, the bitline
voltage to the drain of the flash memory cell, and the bitline
current control voltage to the gate of an NMOS transistor coupled
between the flash memory cell and a ground voltage.
Description
BACKGROUND OF THE INVENTION
[0001] This application claims the priority of Korean Patent
Application No. 2003-58253, filed on Aug. 22, 2003, in the Korean
Intellectual Property Office, the contents of which are
incorporated herein in their entirety by reference.
[0002] 1. Field of the Invention
[0003] The present invention relates to a semiconductor memory
device, and more particularly, to a program voltage generation
circuit for stably programming a flash memory cell and a method of
programming a flash memory cell.
[0004] 2. Description of the Related Art
[0005] As flash memory is used in, for example, portable products
and built-in products both having increased storage capacities. The
demand for flash memory is sharply increasing. Flash memory can
replace large storage media such as a hard disk and is used in, for
example, digital cameras, voice mail systems, and the like.
Compared with nonvolatile memory devices that can perform
electrical programming and erasure, NOR flash memory devices
perform exceedingly fast programming and reading, so that they are
very popular to users who want a fast operation.
[0006] FIG. 1 is a schematic diagram of a flash memory cell.
Referring to FIG. 1, the flash memory cell has a structure in which
a floating gate and a control gate are formed over a channel region
between a source and a drain. The flash memory cell is programmed
using a Channel Hot Electron Injection (CHEI) method, where channel
hot electrons are formed on the side of the drain and injected into
the floating gate. Also, the flash memory cell performs erasure by
erasing the electrons stored in the floating gate using a
Fowler-Nordheim tunneling technique.
[0007] FIG. 2 is a circuit diagram of a core cell array of a NOR
flash memory. Referring to FIG. 2, zeroth through j-th word lines
are arrayed in rows, and zeroth through i-th bit lines are arrayed
in columns, thereby forming a matrix. In this matrix, flash memory
cells as shown in FIG. 1 are formed at intersection points between
the zeroth through j-th word lines and the zeroth through i-th bit
lines. Word line voltages Vwl(j) are applied to control gates of
the flash memory cells, source voltages Vs(k) are applied to
sources thereof, and bit line voltages Vbl(i) are applied to drains
thereof.
[0008] Reading, programming, and erasure of flash memory cells are
performed using operating voltages shown in FIG. 3. Referring to
FIG. 3, the flash memory cells are read out using a word line
voltage Vwl of about 1.5V, a source voltage Vs of 0V, and a bit
line voltage Vbl of about 0.7V. The flash memory cells are
programmed using a word line voltage Vwl of about 1.4V, a source
voltage Vs of about 8V, and a bit line voltage Vbl of about 0.4V.
The flash memory cells are erased using a word line voltage Vwl of
about 11V, a source voltage Vs of 0V, and a bit line voltage Vbl of
0V.
[0009] Particularly, a flash memory cell is programmed by
increasing its threshold voltage while channel hot electrons
generated by a big potential difference applied to the drain and
source of the flash memory cell are moving to the floating gate.
Upon such programming, a predetermined amount of operating current
is consumed. The performance of flash memory depends on how much
the operation current consumption is reduced. Also, when a flash
memory cell is programmed, the bit line voltage Vbl must be applied
to the drain of the flash memory cell in order to prevent its
threshold voltage from being changed due to unintended stress, that
is, punch through disturbing caused by unselected flash memory
cells because of the structure of a flash memory cell array in
which a plurality of flash memory cells share a bit line.
[0010] FIG. 4 is a schematic circuit diagram of a conventional
program wordline voltage generation circuit. Referring to FIG. 4, a
constant current generator 410 generates a program current lpgm. A
first PMOS transistor P1 transfers the program current lpgm to a
second PMOS transistor P2. Here, the first and second PMOS
transistors P1 and P2 constitute a current mirror. The program
current lpgm flows along a path from the second PMOS transistor P2
to a resistor R via a cell capacitor C1. The cell transistor C1 is
formed of a diode type in which a control gate and a drain are
coupled to each other. A voltage for a connection node between the
second PMOS transistor P2 and the cell transistor C is generated as
a wordline voltage Vwl and provided to a core cell array.
[0011] FIG. 5 is a graph showing a distribution of the program
wordline voltage Vwl generated by the conventional program wordline
voltage generation circuit of FIG. 4. Referring to FIG. 5, the
program wordline voltage Vwl is changed to first, second, and third
program wordline voltages Vwl1, Vwl2, and Vwl3 as the program
current lpgm is changed to first, second, and third program
currents lpgm1, lpgm2, and lpgm3. That is, a variation in the
program current lpgm directly affects the generation of the program
wordline voltage Vwl. The program current lpgm varies with a change
in a process of manufacturing a flash memory device, and the
variation of the program current lpgm changes the level of the
wordline voltage Vwl of FIG. 4 desired upon programming. The
variation of the program current lpgm also changes the bitline
voltage Vbl applied to both ends of the resistor R of the
conventional program wordline voltage generation circuit of FIG.
4.
[0012] FIG. 6 is a schematic circuit diagram of a conventional
circuit for controlling a bitline current that is applied to
bitlines of a flash memory cell upon programming. Referring to FIG.
6, a constant current source 610 generates a program current lpgm.
A third PMOS transistor P3 transfers the program current lpgm to a
fourth PMOS transistor P4. Here, the third and fourth PMOS
transistors P3 and P4 constitute a current mirror. The program
current lpgm flows along a path of from the fourth PMOS transistor
P4 to an NMOS transistor N1 whose gate and drain are coupled to
each other. Referring to FIG. 7, which is a graph showing the
operation of the NMOS transistor N1, if the constant program
current lpgm flows along a path between the drain and source of the
NMOS transistor N1, a voltage flowing between the gate and source
of the NMOS transistor N1 is a bitline current control voltage
Vgc.
[0013] The bitline current control voltage Vgc is applied to the
gate of an NMOS transistor N2 coupled to a flash memory cell C2 of
FIG. 8. The program wordline voltage Vwl and the bitline voltage
Vbl are respectively applied to the gate and drain of the flash
memory cell C2. While the program current lpgm is flowing to the
flash memory cell C2 provided with the aforementioned voltages, the
flash memory cell C2 is programmed.
[0014] To program the flash memory cell C2, the wordline voltage
generation circuit of FIG. 4 and the bitline current control
circuit of FIG. 6 must be exactly aware of the current
characteristics of the flash memory cell C2 in order to set an
ideal program current lpgm. If the program current lpgm varies with
a process change, the program wordline voltage Vwl, which causes
the program current lpgm to flow into the flash memory cell C2,
also varies, which impedes generation of an accurate program
current lpgm to flow to the flash memory cell C2. Consequently, the
flash memory cell C2 is unstably programmed.
[0015] Therefore, a flash memory device that can program flash
memory cells by providing a program wordline voltage Vwl and a
bitline current control voltage Vgc that are stable against a
process change is required.
SUMMARY OF THE INVENTION
[0016] One aspect of the present invention provides a program
voltage generation circuit for generating a program wordline
voltage, a bitline voltage, and a bitline current control voltage
that are used to stably program flash memory cells regardless of a
change in a manufacturing process thereof.
[0017] Another aspect of the present invention provides a method of
programming flash memory cells.
[0018] According to one aspect of the present invention, there is
provided a program voltage generation circuit for achieving stable
programming of flash memory cells. This program voltage generation
circuit includes a constant current source, a program wordline
voltage generation unit, a bitline voltage generation unit, and a
bitline current control voltage generation unit. The constant
current source provides a sink current. The program wordline
voltage generation unit generates a program wordline voltage
applied to the gate of a flash memory cell, in response to the sink
current and the result of a comparison between a reference voltage
and a bitline voltage. The bitline voltage generation unit
generates the bitline voltage according to a program current
flowing to the first flash memory cell. The bitline current control
voltage generation unit generates a bitline current control voltage
in response to the program current that flows to a second flash
memory cell in response to the program wordline voltage.
[0019] In one embodiment, the program wordline voltage generation
unit comprises: a first PMOS transistor which has a source coupled
to a power supply voltage and a gate and a drain that are coupled
to each other; a first NMOS transistor which has a source coupled
to a ground voltage and a gate and a drain that are coupled to each
other, and is coupled to the drain of the PMOS transistor; a second
NMOS transistor which has a source coupled to the ground voltage, a
gate coupled to the gate of the first NMOS transistor to form a
current mirror, and a drain coupled to the program wordline
voltage; a second PMOS transistor which is connected between the
power supply voltage and the drain of the second NMOS transistor
and has a gate coupled to an output of a voltage comparator; and
the voltage comparator which compares the bitline voltage with the
reference voltage and provides the output corresponding to the
result of the comparison to the gate of the second PMOS
transistor.
[0020] In one embodiment, the bitline voltage generation unit
comprises: the first flash memory cell which has a source coupled
to a source voltage and a gate coupled to the program wordline
voltage; and a resistor which is coupled between the drain of the
first flash memory cell and the ground voltage and generates the
bitline voltage.
[0021] The bitline current control voltage generation unit can
include: a second flash memory cell which has a source coupled to a
source voltage and a gate coupled to the program wordline voltage;
and an NMOS transistor which has a source coupled to a ground
voltage and a gate and a drain that are coupled to the drain of the
second flash memory cell and generate the bitline current control
voltage. The flash memory device comprises: a flash memory cell in
a flash memory cell core array to be programmed, which has a gate
coupled to the program wordline voltage, a source coupled to the
source voltage, and a drain coupled to the bitline voltage; and an
NMOS transistor gated to the bitline current control voltage
between the drain of the flash memory cell and the ground
voltage.
[0022] According to one aspect of the present invention, there is
provided another program voltage generation circuit for achieving
stable programming of flash memory cells. This program voltage
generation circuit includes a constant current source, a second
PMOS transistor, a voltage comparator, first and second flash
memory cells, a resistor, and an NMOS transistor. The constant
current source includes a first PMOS transistor and first and
second NMOS transistors. The first PMOS transistor has a source
coupled to a power supply voltage and a gate and a drain that are
coupled to each other. The first NMOS transistor has a source
coupled to a ground voltage and a gate and a drain that are coupled
to each other, and is coupled to the drain of the first PMOS
transistor. The second NMOS transistor has a source coupled to the
ground voltage, a gate coupled to the gate of the first NMOS
transistor to form a current mirror, and a drain coupled to the
program wordline voltage. The second PMOS transistor is connected
between the power supply voltage and the drain of the second NMOS
transistor and has a gate coupled to an output of the voltage
comparator. The voltage comparator compares the bitline voltage
with a reference voltage and provides the output corresponding to
the result of the comparison to the gate of the second PMOS
transistor. The first flash memory cell has a source coupled to a
source voltage and a gate coupled to the program wordline voltage.
The resistor is coupled between the drain of the first flash memory
cell and the ground voltage and generates the bitline voltage
according to a program current flowing to the first flash memory
cell. The second flash memory cell has a source coupled to a source
voltage and a gate coupled to the program wordline voltage. The
NMOS transistor has a source coupled to a ground voltage and a gate
and a drain that are coupled to the drain of the second flash
memory cell and generate a bitline current control voltage
according to a program current flowing to the second flash memory
cell.
[0023] In one embodiment, the flash memory device comprises: a
flash memory cell in a flash memory cell core array to be
programmed, which has a gate coupled to the program wordline
voltage, a source coupled to the source voltage, and a drain
coupled to the bitline voltage; and an NMOS transistor gated to the
bitline current control voltage between the drain of the flash
memory cell and the ground voltage.
[0024] According to another aspect of the present invention, there
is provided a method of programming a flash memory cell. In this
method, a sink current is supplied from a constant current source.
Next, a program wordline voltage to be applied to the gate of a
flash memory cell is generated in response to the sink current and
the result of a comparison between a reference voltage and a
bitline voltage. The bitline voltage to be applied to the drain of
the first flash memory cell is generated according to a program
current flowing to the first flash memory cell. A bitline current
control voltage is generated in response to the program current by
applying the program wordline voltage to the gate of a second flash
memory cell. The program wordline voltage is applied to the gate of
the flash memory cell, the bitline voltage is applied to the drain
of the flash memory cell, and the bitline current control voltage
is applied to the gate of an NMOS transistor coupled between the
flash memory cell and a ground voltage, so that the program current
flows to the flash memory cell to thereby program the flash memory
cell.
[0025] In the present invention, a flash memory cell is adopted in
the program wordline voltage generation circuit. Accordingly, even
when the characteristics of the flash memory cell vary due to a
change of a manufacturing process thereof, a constant program
wordline voltage Vwl, a constant bitline voltage Vbl, and a
constant bitline current control voltage Vgc are generated, and
thus the flash memory cell is stably programmed:
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The foregoing and other features and advantages of the
invention will be apparent from the more particular description of
embodiments of the invention, as illustrated in the accompanying
drawings. The drawings are not necessarily to scale, emphasis
instead being placed upon illustrating the principles of the
invention.
[0027] FIG. 1 is a schematic diagram of a flash memory cell.
[0028] FIG. 2 is a circuit diagram of a core cell array of a NOR
flash memory.
[0029] FIG. 3 is a table showing the levels of voltages according
to the operation modes of flash memory cells.
[0030] FIG. 4 is a schematic circuit diagram of a conventional
program wordline voltage generation circuit.
[0031] FIG. 5 is a graph showing a distribution of the program
wordline voltage Vwl generated by the conventional program wordline
voltage generation circuit of FIG. 4.
[0032] FIG. 6 is a schematic circuit diagram of a conventional
circuit for controlling a bitline current.
[0033] FIG. 7 is a graph showing the operation of the NMOS
transistor of FIG. 6.
[0034] FIG. 8 is a circuit diagram for illustrating programming of
a flash memory cell using a conventional program wordline voltage,
a conventional bitline voltage, and a conventional bitline current
control voltage.
[0035] FIG. 9 is a schematic circuit diagram of a program wordline
voltage generation circuit according to an embodiment of the
present invention.
[0036] FIG. 10 is a schematic circuit diagram of a bitline current
control circuit according to an embodiment of the present
invention.
[0037] FIG. 11 is a circuit diagram for illustrating programming of
a flash memory cell using a program wordline voltage according to
the present invention, a bitline voltage according to the present
invention, and a bitline current control voltage according to the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0038] Referring to FIG. 9, a program wordline voltage generation
circuit 900 according to an embodiment of the present invention
includes a program wordline voltage generation unit 910 and a
bitline voltage generation unit 920.
[0039] The program wordline voltage generation unit 910 includes
first and second PMOS transistors P91 and P92, first and second
NMOS transistors N91 and N92, and a voltage comparator 93. The
first PMOS transistor P91 and the first NMOS transistor N91 are
serially coupled to each other in a diode configuration between a
power supply voltage Vdd and a ground voltage Vss. The first and
second NMOS transistors N91 and N92 constitute a current mirror.
The second PMOS transistor P92 is coupled between a power supply
voltage Vdd and the second NMOS transistor N92. The voltage
comparator 93 compares a reference voltage Vref with a bitline
voltage Vbl and provides an output voltage Vp corresponding to the
result of the comparison to the second PMOS transistor P92.
[0040] The bitline voltage generation unit 920 is coupled between a
source voltage Vs and the bitline voltage Vbl and includes a flash
memory cell C91 and a resistor 94. The flash memory cell C91 is
gated to a program wordline voltage Vwl, which is a connection node
between the second PMOS transistor P92 and the second NMOS
transistor N92. The resistor 94 is connected between the bitline
voltage Vbl and a ground voltage Vss.
[0041] The program wordline voltage generation circuit 900 having
the above-described structure operates so that the bitline voltage
Vbl is finally equal to the reference voltage Vref. When the
bitline voltage Vbl is finally equal to the reference voltage Vref,
a program wordline voltage Vwl obtained at this time and the
bitline voltage Vbl are provided to a flash memory cell so that the
flash memory cell can be stably programmed.
[0042] To be more specific, an initial program wordline voltage Vwl
is determined by a current that flows to the first PMOS transistor
P92 and the second NMOS transistor N92 via the first PMOS
transistor P91 and the first NMOS transistor N91. An initial
program current lpgm flows to the flash memory cell C91 gated to
the initial program wordline voltage Vwl. An initial bitline
voltage Vbl to be applied to both ends of the resistor 94 is
generated by the initial program current lpgm.
[0043] The initial bitline voltage Vbl is provided to the voltage
comparator 93 and compared with the reference voltage Vref. The
reference voltage Vref is set to about 0.4V, which is the bitline
voltage Vbl used upon the programming of FIG. 3. If the initial
bitline voltage Vbl is lower than the reference voltage Vref, the
output voltage Vp of the voltage comparator 93 has a logic low
level. The amount of current flowing in the second PMOS transistor
P92 increases in response to the output voltage Vp of the logic low
level. Accordingly, the level of the initial program wordline
voltage Vwl increases. An increased program wordline voltage Vwl
increases the amount of the program current lpgm flowing to the
flash memory cell C91. Hence, the initial bitline voltage Vbl
applied to both ends of the resistor 94 is increased.
[0044] An increased bitline voltage Vbl is provided to the voltage
comparator 93 and compared with the reference voltage Vref. If the
increased bitline voltage Vbl is higher than the reference voltage
Vref, the output voltage Vp of the voltage comparator 93 has a
logic high level. The amount of current supplied from the second
PMOS transistor P92 increases in response to the output voltage Vp
of the logic high level. In this case, the level of the increased
program wordline voltage Vwl is lowered due to an operation of the
current mirror constituted with the first and second NMOS
transistors N91 and N92. A lowered program wordline voltage Vwl
decreases the amount of program current lpgm flowing to the flash
memory cell C91. Hence, the increased bitline voltage Vbl applied
to both ends of the resistor 94 is decreased.
[0045] By repeating these operations, the program wordline voltage
generation circuit 900 generates a program wordline voltage Vwl
having the same level as a reference voltage Vref. A program
current lpgm determined at this time determines a final bitline
voltage Vbl.
[0046] FIG. 10 is a schematic circuit diagram of a bitline current
control circuit 1000 according to an embodiment of the present
invention. The bit line current control voltage generation circuit
1000 includes a flash memory cell C100 and an NMOS diode transistor
N100 that is serially connected to the flash memory cell C100. The
flash memory cell C100 is gated to a program wordline voltage Vwl
connected between a source voltage Vs and a ground voltage Vss. In
the flash memory cell C100, a program current lpgm flows in
response to the program wordline voltage Vwl generated by the
program wordline voltage generation circuit 900 of FIG. 9. Because
of the program current lpgm, a bitline current control voltage Vgc
is applied between the drain and source of the NMOS transistor
N100.
[0047] The program wordline voltage Vwl, the bitline voltage Vbl,
and the bitline current control voltage Vgc that are finally
generated in the embodiment of the present invention are
respectively applied to the gate and drain of a flash memory cell
C110 of FIG. 11 and an NMOS transistor N110 of FIG. 11, thereby
programming the flash memory cell C110. When the flash memory cell
C110 is programmed, a program current lpgm flowing to the flash
memory cell C110 is equal to the program current lpgm flowing in
the flash memory cell C91 of the program voltage generation circuit
900 of FIG. 9 and to the program current lpgm flowing in the flash
memory cell C100 of the bitline current control voltage generation
circuit 1000.
[0048] Hence, in the present invention, a program wordline voltage
Vwl has the same level as a predetermined reference voltage Vref. A
program current lpgm flowing to the flash memory cell C91 of FIG. 9
in response to the program wordline voltage Vwl is used upon
generation of a bitline current control voltage Vgc. Upon program
of a flesh memory cell, the program wordline voltage Vwl and the
bitline current control voltage Vgc are used so that the program
current lpgm can flow as a current used for programming the flash
memory cell. Accordingly, even when the characteristics of the
flash memory cell vary due to a change of a manufacturing process
thereof, a constant program wordline voltage Vwl, a constant
bitline voltage Vbl, and a constant bitline current control voltage
Vgc are generated, and thus the flash memory cell is stably
programmed.
[0049] While the present invention has been particularly shown and
described with reference to exemplary embodiments thereof, it will
be understood by those of ordinary skill in the art that various
changes in form and details may be made therein without departing
from the spirit and scope of the present invention as defined by
the following claims.
* * * * *