# Patent

## Title
Memory cell array biasing method and a semiconductor memory device

## Abstract

A method of biasing a memory cell array during a data writing operation and a semiconductor memory device, in which the semiconductor memory device includes: a memory cell array including a plurality of memory cells in which a first terminal of a memory cell is connected to a corresponding first line of a plurality of first lines and a second terminal of the memory cell is connected to a corresponding second line of a plurality of second lines; a bias circuit for biasing a selected second line of the second lines to a reference voltage and a non-selected second line to a first voltage; and a local word line address decoder applying the reference voltage or a pumping voltage corresponding to the first voltage to the bias circuit.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of application Ser. No. 11/969,326 filed Jan. 4, 2008, which is a continuation-in-part of application Ser. No. 11/327,967, filed Jan. 9, 2006 now U.S. Pat. No. 7,317,655, which claims priority to Korean Patent Application No. 10-2005-0006581, filed on Jan. 25, 2005, in the Korean Intellectual Property Office, the disclosures of which are incorporated by reference herein in their entirety.


BACKGROUND OF THE INVENTION
1. Technical Field
The present disclosure relates to a semiconductor memory device, and more particularly, to a semiconductor memory device and a data writing method for controlling a biasing level of a memory cell array.
2. Discussion of Related Art
Phase change random access memories (PRAMs) are nonvolatile memory devices that store data using a phase change material, e.g., Ge—Sb—Te (GST), whose resistance changes in accordance with a phase transition due to a change in temperature.
FIG. 1 illustrates an equivalent circuit of a unit cell C of a PRAM. Referring to FIG. 1, the unit cell C consists of a P-N diode D and a phase change material GST. The phase change material GST is connected to a bit line BL and a P-junction of the diode D. A word line WL is connected to an N-junction of the diode D.
The phase change material GST of the PRAM unit cell C, goes into a crystalline state or an amorphous state depending on a temperature applied thereto and a heating time. This enables data to be stored in the PRAM cell. In general, a temperature higher than 900° C. is needed for a phase transition of the phase change material GST. Such temperatures are obtained by Joule heating, which uses a current flowing through the PRAM cell to increase or decrease the temperature thereof.
A write operation for the phase change material GST will now be described. First, the phase change material GST is heated above its melting temperature by a current and then it is rapidly cooled. The phase change material GST then goes into the amorphous state and stores a data “1”. This state is referred to as a reset state. The phase change material GST is then heated above its crystallization temperature for a predetermined period of time and cooled. Next, the phase change material GST goes into the crystalline state and stores a data “0”. This state is referred to as a set state.
A read operation for the phase change material GST will now be described. After a bit line and a word line are used to select a memory cell, an external current is provided to the selected memory cell. It is then determined whether data to be stored in the selected memory cell is “1” or “0” based on a change in voltage according to a resistance value of the phase change material GST of the selected memory cell.
FIG. 2 illustrates a semiconductor memory device 200 comprising a memory cell array MAY including a plurality of the PRAM unit cells C shown in FIG. 1. An exemplary structure of the memory cell array MAY is disclosed in U.S. Pat. Nos. 6,667,900 and 6,567,296.
Referring to FIG. 2, the semiconductor memory device 200 comprises the memory cell array MAY and a word line driver 210. The memory cell array MAY comprises a plurality of unit cells C connected to corresponding bit lines BL0 ̃BLk−1 and word lines WL0, WL1, and WL2. Although only k bit lines BL0 ̃BLk−1 and three word lines WL0, WL1, and WL2 are shown in FIG. 2, the number of bit lines and word lines is not limited thereto.
For a data write operation, if one of the bit lines BL0 ̃BLk−1 is first selected, the word line driver 210 selects one of the word lines WL0, WL1, and WL2. The selected word line is then set to a low level. If a first bit line BL0 and a first word line WL0 are sequentially selected, a write current applied to the first bit line BL0 flows through a unit cell connected between the first bit line BL0 and the first word line WL0. The state of the phase change material of the unit cell is then changed to store data.
Each of the word lines WL0, WL1, and WL2 has its own resistance R_WL. Because the word lines WL0, WL1, and WL2 pass current when writing data, the resistance R_WL should be minimized. Because the word lines WL0, WL1, and WL2 have a high resistance, however, the number of unit cells connected to the word lines WL0, WL1, and WL2 is limited. Further, the word line driver 210 should be powerful enough to drive the word lines WL0, WL1, and WL2.
When data is written to the unit cell connected between the first bit line BL0 and the first word line WL0, the write current is applied to the first bit line BL0, and the first word line WL0 is set to a low level by the word line driver 210. The second and third word lines WL1 and WL2 are then in a floating state. The first bit line BL0 maintains a relatively high voltage due to the applied write current, and the second and third word lines WL1 and WL2 maintain a relatively low level in the floating state. Therefore, current flows through unit cells connected between the first word line WL0 and the second and third word lines WL1 and WL2, which can change the state of the phase change material in those unit cells.
Because current may flow through unselected word lines that are floating, it is difficult to increase the operating speed of the semiconductor memory device and perform stable sensing. As such, a need exists for a semiconductor memory device that is capable of performing a stable sensing operation while increasing its operating speed.
SUMMARY OF THE INVENTION
A semiconductor memory device and a data writing method are provided that prevent current from flowing from a selected bit line to a non-selected word line by maintaining a constant voltage in the non-selected word line, thereby enabling stable sensing and increasing the operating speed of the semiconductor memory device.
According to an aspect of the present invention, there is provided a semiconductor memory device comprising: a memory cell array including a plurality of memory cells in which a first terminal of a memory cell is connected to a corresponding first line of a plurality of first lines and a second terminal of the memory cell is connected to a corresponding second line of a plurality of second lines; a bias circuit for biasing a selected second line of the second lines to a reference voltage and a non-selected second line to a first voltage; and a local word line address decoder applying the reference voltage or a pumping voltage corresponding to the first voltage to the bias circuit.
The bias circuit may comprise NMOS transistors and PMOS transistors connected between the second lines and the local word line address decoder.
The first voltage may have a voltage level obtained by subtracting a threshold voltage of the NMOS transistors from the pumping voltage.
The pumping voltage may have a voltage level greater than a sum of a voltage applied to one selected from the first lines and the threshold voltage of the NMOS transistors.
The local word line address decoder may apply the pumping voltage to one end of the PMOS transistors, and applies the pumping voltage or the reference voltage to one end of the NMOS transistors.
The drains of the NMOS transistors may be connected to the second lines, sources are connected to the local word line address decoder, and main word line signals are applied to gates of the NMOS transistors; and drains of the PMOS transistors are connected to the second lines, sources are connected to the local word line address decoder, and the main word line signals are applied to gates of the PMOS transistors.
The semiconductor memory device may further comprise a word line driver which generates the main word line signals in response to a word line enable signal and a block address.
Each memory cell may comprise a phase change material connected to one of the first lines; and a diode connected between the phase change material and one of the second lines.
The first lines may be bit lines and the second lines are word lines.
The reference voltage may be a ground voltage.
According to another aspect of the present invention, there is provided a method of writing data to a selected memory cell connected to a selected first line and a selected second line of a semiconductor memory device including a plurality of memory cells in which a first terminal of a memory cell is connected to a corresponding first line among a plurality of first lines and a second terminal of a memory cell is connected to a corresponding second line among a plurality of second lines, the method comprising: biasing the selected first line to a predetermined voltage; biasing the selected second line to a reference voltage; and basing non-selected second lines to a first voltage, wherein the first voltage is a pumping voltage.
According to still another aspect of the present invention, there is provided a semiconductor memory device comprising: a plurality of first lines and a plurality of second lines; a memory cell array including a plurality of memory cells in which a first terminal of a memory cell is connected to a corresponding one of the first lines and a second terminal of a memory cell is connected to a corresponding one of the second lines; and a local word line address decoder applying the reference voltage or a pumping voltage corresponding to the first voltage to the bias circuit, wherein a selected second line is biased to a reference voltage; and non-selected second lines are biased to a first voltage, wherein the first voltage is a pumping voltage.
According to yet another aspect of the present invention, there is provided a semiconductor memory device comprising: a memory cell array including a plurality of phase change memory cells in which a first terminal of a phase change memory cell is connected to a corresponding first line of a plurality of first lines and a second terminal of a phase change memory cell is connected to a corresponding second line of a plurality of second lines; inverters for biasing a selected second line to a reference voltage and non-selected second lines to a first voltage in response to main word line signals; and a local word line address decoder applying the reference voltage or a pumping voltage corresponding to the first voltage to the inverters; wherein the first voltage is a pumping voltage.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:
FIG. 1 illustrates an equivalent circuit of a unit cell of a PRAM;
FIG. 2 illustrates a semiconductor memory device comprising a memory cell array including a plurality of the unit cells shown in FIG. 1;
FIG. 3 illustrates a semiconductor memory device according to an embodiment of the present invention;
FIGS. 4A and 4B are circuit diagrams for explaining the operation of the semiconductor memory device of FIG. 3 to embodiments of the present invention;
FIG. 5 is a block diagram of a computing system of the semiconductor memory device of FIG. 3 to an embodiment of the present invention; and
FIG. 6 is a block diagram of a memory card of the semiconductor memory device of FIG. 3 to an embodiment of the present invention.



DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. Like reference numerals represent like elements throughout the drawings.
FIG. 3 illustrates a semiconductor memory device 300 according to an exemplary embodiment of the present invention. Referring to FIG. 3, the semiconductor memory device 300 includes a memory cell array MAY, bias circuits BS0, BS1, and BS2, and a word line driver 310. The semiconductor memory device 300 is a PRAM, which includes a memory cell having a phase change material, e.g., GST, connected to a first line, and a diode connected between the phase change material and a second line. The PRAM may be similar to or the same as that shown in FIG. 1.
As shown in FIG. 3, the semiconductor memory device 300 is connected to a peripheral circuit 330, a column decoder 320, and a local word line address decoder 340. The peripheral circuit 330 may comprise a write driver (not shown) and a sense amplification circuit (not shown). The column decoder 320 includes transistors TR0, TR1,  ̃TRn−1 which are turned on or off in response to column selection signals Y0 ̃Yn−1. The local word line address decoder 340 receives the local word line address ADD.
The memory cell array MAY includes a plurality of memory cells in which a first terminal of a memory cell is connected to a corresponding first line among a plurality of n first lines BL0 ̃BLn−1, where n is a natural number, and a second terminal of the memory cell is connected to a corresponding second line among a plurality of second lines LWL0, LWL1, and LWL2. The plurality of first lines BL0 ̃BLn−1 are bit lines and the plurality of second lines LWL0, LWL1, and LWL2 are word lines.
Even though only three second lines LWL0, LWL1, and LWL2 are shown in FIG. 3, the number of second lines is not limited thereto.
The bias circuits BS0, BS1, and BS2 bias a selected second line to a reference voltage and a non-selected second line to a first voltage. The reference voltage is a voltage provided by decoding the local word line address ADD from the local word line address decoder 340 and the first voltage is pumping voltage VPP-SA. The term “selected” means that the corresponding second line is activated by the word line driver 310 to write data to the memory cell connected to the second line.
The semiconductor memory device 300 maintains the voltage of a selected word line at the word line voltage, and prevents non-selected word lines from floating, thereby preventing a current from flowing from a selected bit line to the non-selected word line.
In this way, the semiconductor memory device 300 can perform stable sensing, and increase its operating speed.
As further shown in FIG. 3, the bias circuits BS0, BS1, and BS2 comprise NMOS transistors NTR01 ̃NTR23 connected between each of the second lines LWL0, LWL1, and LWL2 and the word line voltage from the local address word line decoder 340, and PMOS transistors PTR0, PTR1, PTR2 connected between each of the second lines LWL0, LWL1, and LWL2 and the pumping voltage VPP_SA. The pumping voltage VPP_SA is selected to be higher than the typical power supply voltage VDD used in the other exemplary embodiments described hereinafter.
Even though only three NMOS transistors are connected to each of the second lines LWL0, LWL1, and LWL2, the number of NMOS transistors is not limited thereto. In addition, the number of NMOS transistors can vary according to the number of memory cells connected to the second lines LWL0, LWL1, and LWL2 and the length of the second lines LWL0, LWL1, and LWL2.
The structure and operation of the bias circuits BS0, BS1, and BS2 will now be described in more detail. The drains of the NMOS transistors NTR01 ̃NTR23 are connected to the second lines LWL0, LWL1, and LWL2, and the sources are connected to the local address word line voltage VPP_SA, and main word line signals SWL0, SWL1, and SWL2 are applied to the gates of the NMOS transistors NTR01 ̃NTR23.
The word line driver 310 generates the main word line signals SWL0, SWL1, and SWL2 in response to word line enable signals X0, X1, and X2 and block addresses BLK0, BLK1, and BLK2. A main word line signal corresponding to a selected second line has a high level, and a main word line signal corresponding to a non-selected second line has a low level.
For example, when a first column selection signal Y0 of the column decoder 320 is activated to turn on a first transistor TR0, a write current is applied to a first bit line BL0. Data is then written to a memory cell connected between the first bit line BL0 and a first word line LWL0 (which may be a local word line).
If the word line enable signal X0 and the first block address BLK0 are input at a high level, a NAND gate N0 and an inverter I0 generate the first main word line signal SWL0 at a high level. The first main word line signal SWL0 is then generated at a high level and the other main word line signals SLW1 and SWL2 are generated at a low level to select the first word line LWL0.
The NMOS transistors NTR01 ̃NTR23 of the first bias circuit BS0 are turned on in response to the high level of the first main word line signal SWL0, and the first word line LWL0 goes to a low level.
The PMOS transistor PTR0 of the first bias circuit BS0 is turned off. The write current applied to the first bit line BL0 flows to the word line decoder 340 through a memory cell and the first word line LWL0 so that data is stored in the memory cell.
The PMOS transistors PTR1 and PTR2 of the second and third bias circuits BS1 and BS2 are turned on by the second and third main word line signals SWL1 and SWL2 at a low level, and the voltage of the second and third word lines LWL1 and LWL2 becomes the pumping voltage VPP_SA.
The write current applied to the first bit line BL0 does not flow to the second and third word lines LWL1 and LWL2 since the second and third word lines LWL1 and LWL2 are at the pumping voltage VPP_SA. Thus, memory cells other than that connected to the first bit line BL0 and the first word line LWL0 can stably hold data since they are not influenced by the write current of the first bit line BL0.
The reason why the write current does not flow to the word lines that are not selected in the semiconductor memory device of the present embodiment will now be in more detail described. Referring to FIGS. 4A and 4B which are circuit diagrams for explaining the operation of the semiconductor memory device of FIG. 3, if the local word line LWL0 and the bit line BL0 are selected, local word line address decoder EPDEC applies 0V to the source of NMOS transistor NTR01 that is connected to the selected word line LWL0 (FIG. 4A). In this case, a current path is generated from the selected bit line BL0 to the source of selected NMOS transistor NTR01.
Assuming that the word line LWL1 is not selected, however, and the bit line BL0 is selected, local word line address decoder EPDEC applies the pumping voltage VPP_SA to the source of NMOS transistor NTR11 that is connected to the un-selected word line LWL1 (FIG. 4B). In this case, a voltage drop corresponding to the threshold voltage of the un-selected NMOS transistor NTR11 is generated, and accordingly the reference voltage that is obtained by subtracting the threshold voltage of the un-selected NMOS transistor NTR11 from the pumping voltage VPP_SA is applied to the un-selected word line LWL1.
That is, the reference voltage is applied to one terminal of the diode that is connected to the un-selected word line LWL1, and the voltage of the selected bit line BL0 that is applied to the other terminal of the diode is the voltage of the selected bit line BL0.
In this exemplary embodiment, it is the pumping voltage VPP_SA that is greater than the threshold voltage of the un-selected NMOS transistor NTR11 plus the voltage of the selected bit line BL0. Therefore, the diode of the PRAM cell is in the off state, and accordingly leakage current in the diode is not generated.
Assuming that the power supply voltage VDD of subsequently described embodiments, which is lower than the pumping voltage VPP_SA, is used, however, the diode of the PRAM cell may be in the on state. Accordingly, leakage current in the diode may be generated.
A data writing method according to an embodiment of the present invention will now be described. The data writing method may be used by one of the semiconductor memory device 300. In addition, the data writing method may be used by a semiconductor memory device other than the semiconductor memory device 300 that includes a plurality of memory cells, in which a first terminal of a memory cell is connected to a corresponding first line among a plurality of first lines, and a second terminal connected to a corresponding second line among a plurality of second lines.
In the data writing method, data is written to a selected memory cell connected to a selected first line and a selected second line of a semiconductor memory device. In more detail, the selected first line is biased to a predetermined voltage and the first line is a bit line and the selected first line is a bit line connected to a memory cell for applying data.
A write current for writing data is then applied to a bit line and the selected second line is biased to a reference voltage. The selected second line is a word line connected to a memory cell for writing data. The reference voltage lowers the voltage of the word line connected to the memory cell for writing data.
Non-selected second lines are biased to a first voltage and the non-selected second lines are word lines other than the word line connected to the memory cell for writing data. The first voltage lowers the voltage of non-selected second lines other than the word line connected to the memory cell for writing data. In this way, a current is prevented from flowing from a selected bit line to a non-selected word line.
FIG. 5 is a block diagram of a computing system 500 having the semiconductor memory device of FIG. 3 to an embodiment of the present invention. Referring to FIG. 5, the computing system 500 includes a CPU 530 electrically connected to the bus 560, a user interface 550, a memory module 510. The memory module 510 comprises a memory controller 512 and the semiconductor memory device 300. N-bit data (N is 1 or an integer greater than 1) that has been processed and is to be processed will be stored in the semiconductor memory device 300 by the CPU 530. The computing system 500 of the present embodiment may further include a power supply device 520.
If the computing system 500 of the present embodiment is a mobile device, a battery for supplying an operational voltage to the computing system 500 and a modem such as a baseband chipset may be further included in the computing system 500. It would have been obvious to one of ordinary skill in the art that an application chipset, a camera image processor (CIS), a mobile DRAM, and the like may be further included in the computing system 500 of the present embodiment and thus the detailed description thereof will not be repeated here.
The semiconductor memory device 300 of the present embodiment may be form in a memory card 600 as shown in FIG. 6 with a controller 620. In this case, the controller 620 communicates with an external device (e.g. a host) through one of various interface protocols such as USB, MMC, PCI-E, SATA, PATA, SCSI, ESDI, IDE, and the like.
The structures and operations of a bus 626, a CPU 622, a host interface 623, an ECC 624 and a memory interface 625 included in the controller 620 would have been obvious to one of ordinary skill in the art and thus the detailed descriptions thereof will not be repeated here.
The semiconductor memory device 300 of the present embodiment may be packaged using various types of packages. For example, the semiconductor memory device 300 of the present embodiment may be packaged using packages as follows: PoP (Package on Package), Ball grid arrays (BGAs), Chip scale packages (CSPs), Plastic Leaded Chip Carrier (PLCC), Plastic Dual In-Line Package (PDIP), Die in Waffle Pack, Die in Wafer Form, Chip On Board (COB), Ceramic Dual In-Line Package (CERDIP), Plastic Metric Quad Flat Pack (MQFP), Thin Quad Flatpack (TQFP), Small Outline (SOIC), Shrink Small Outline Package (SSOP), Thin Small Outline (TSOP), Thin Quad Flatpack (TQFP), System In Package (SIP), Multi Chip Package (MCP), Wafer-level Fabricated Package (WFP), or Wafer-Level Processed Stack Package (WSP).
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.

## Claims

1. A semiconductor memory device comprising:
a memory cell array including a plurality of memory cells in which a first terminal of a memory cell is connected to a corresponding first line of a plurality of first lines and a second terminal of the memory cell is connected to a corresponding second line of a plurality of second lines;
a bias circuit for biasing a selected second line of the second lines to a reference voltage and a non-selected second line to a first voltage; and
a local word line address decoder applying the reference voltage or a pumping voltage corresponding to the first voltage to the bias circuit.

2. The semiconductor memory device of claim 1, wherein the bias circuit comprises:
NMOS transistors and PMOS transistors connected between the second lines and the local word line address decoder.

3. The semiconductor memory device of claim 2, wherein the first voltage has a voltage level obtained by subtracting a threshold voltage of the NMOS transistors from the pumping voltage.

4. The semiconductor memory device of claim 2, wherein the pumping voltage has a voltage level greater than a sum of a voltage applied to one selected from the first lines and the threshold voltage of the NMOS transistors.

5. The semiconductor memory device of claim 2, wherein the local word line address decoder applies the pumping voltage to one end of the PMOS transistors, and applies the pumping voltage or the reference voltage to one end of the NMOS transistors.

6. The semiconductor memory device of claim 2, wherein drains of the NMOS transistors are connected to the second lines, sources are connected to the local word line address decoder, and main word line signals are applied to gates of the NMOS transistors; and
drains of the PMOS transistors are connected to the second lines, sources are connected to the local word line address decoder, and the main word line signals are applied to gates of the PMOS transistors.

7. The semiconductor memory device of claim 6, further comprising:
a word line driver which generates the main word line signals in response to a word line enable signal and a block address.

8. The semiconductor memory device of claim 1, wherein each memory cell comprises: a phase change material connected to one of the first lines; and
a diode connected between the phase change material and one of the second lines.

9. The semiconductor memory device of claim 1, wherein the first lines are bit lines and the second lines are word lines.

10. The semiconductor memory device of claim 1, wherein the reference voltage is a ground voltage.

11. A method of writing data to a selected memory cell connected to a selected first line and a selected second line of a semiconductor memory device including a plurality of memory cells in which a first terminal of a memory cell is connected to a corresponding first line of a plurality of first lines and a second terminal of the memory cell is connected to a corresponding second line of a plurality of second lines, the method comprising:
biasing the selected first line to a predetermined voltage;
biasing the selected second line to a reference voltage; and
biasing non-selected second lines to a first voltage, wherein the first voltage is a pumping voltage.

12. A semiconductor memory device, comprising:
a plurality of first lines and a plurality of second lines;
a memory cell array including a plurality of memory cells in which a first terminal of a memory cell is connected to a corresponding one of the first lines and a second terminal of a memory cell is connected to a corresponding one of the second lines; and
a local word line address decoder applying a reference voltage or a pumping voltage corresponding to a first voltage to a bias circuit,
wherein a selected second line is biased to a reference voltage; and
non-selected second lines are biased to a first voltage, wherein the first voltage is a pumping voltage.

13. The semiconductor memory device of claim 12, further comprising:
bias circuits which control voltages of the second lines;
wherein the bias circuits comprise:
NMOS transistors and PMOS transistors connected between the second lines and the local word line address decoder.

14. The semiconductor memory device of claim 13, wherein drains of the NMOS transistors are connected to the second lines, sources are connected to the local word line decoder, and main word line signals are applied to gates of the NMOS transistors; and
drains of the PMOS transistors are connected to the second lines, sources are connected to the pumping voltage, and the main word line signals are applied to gates of the PMOS transistors.

15. The semiconductor memory device of claim 13, wherein the first voltage has a voltage level obtained by subtracting a threshold voltage of the NMOS transistors from the pumping voltage.

16. The semiconductor memory device of claim 13, wherein the local word line address decoder applies the pumping voltage to one end of the PMOS transistors, and applies the pumping voltage or the reference voltage to one end of the NMOS transistors.

17. The semiconductor memory device of claim 13, further comprising:
a word line driver which generates main word line signals in response to a word line enable signal and a block address.

18. A semiconductor memory device comprising:
a memory cell array including a plurality of phase change memory cells in which a first terminal of a phase change memory cell is connected to a corresponding first line of a plurality of first lines and a second terminal of a phase change memory cell is connected to a corresponding second line of a plurality of second lines;
inverters for biasing a selected second line to a reference voltage and non-selected second lines to a first voltage in response to main word line signals; and
a local word line address decoder applying the reference voltage or a pumping voltage corresponding to the first voltage to the inverters;
wherein the first voltage is a pumping voltage.

19. The semiconductor memory device of claim 18, wherein input terminals of the inverters receive the main word line signals, and output terminals of the inverters are connected to the second lines, a main word line signal corresponding to the selected second line is at a high level; and main word line signals corresponding to non-selected second lines are at a low level.

20. The semiconductor memory device of claim 19, further comprising:
a word line driver which generates the main word line signals in response to a word line enable signal and a block address.