Patent Publication Number: US-7710767-B2

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

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Korean Patent Application No. 10-2005-0006581, filed on Jan. 25, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference herein in its entirety. 
     This application is a continuation-in-part of application Ser. No. 11/327,967 filed Jan. 9, 2006, now U.S. Pat. No. 7,317,655. 
    
    
     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 BL 0 ˜BLk- 1  and word lines WL 0 , WL 1 , and WL 2 . Although only k bit lines BL 0 ˜BLk- 1  and three word lines WL 0 , WL 1 , and WL 2  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 BL 0 ˜BLK- 1  is first selected, the word line driver  210  selects one of the word lines WL 0 , WL 1 , and WL 2 . The selected word line is then set to a low level. If a first bit line BL 0  and a first word line WL 0  are sequentially selected, a write current applied to the first bit line BL 0  flows through a unit cell connected between the first bit line BL 0  and the first word line WL 0 . The state of the phase change material of the unit cell is then changed to store data. 
     Each of the word lines WL 0 , WL 1 , and WL 2  has its own resistance R_WL. Because the word lines WL 0 , WL 1 , and WL 2  pass current when writing data, the resistance R_WL should be minimized. Because the word lines WL 0 , WL 1 , and WL 2  have a high resistance, however, the number of unit cells connected to the word lines WL 0 , WL 1 , and WL 2  is limited. Further, the word line driver  210  should be powerful enough to drive the word lines WL 0 , WL 1 , and WL 2 . 
     When data is written to the unit cell connected between the first bit line BL 0  and the first word line WL 0 , the write current is applied to the first bit line BL 0 , and the first word line WL 0  is set to a low level by the word line driver  210 . The second and third word lines WL 1  and WL 2  are then in a floating state. The first bit line BL 0  maintains a relatively high voltage due to the applied write current, and the second and third word lines WL 1  and WL 2  maintain a relatively low level in the floating state. Therefore, current flows through unit cells connected between the first word line WL 0  and the second and third word lines WL 1  and WL 2  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 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; and bias circuits for biasing a selected second line among the second lines to a first voltage and a non-selected second line to a second voltage. 
     The first voltage may be connected to a local WL address decoder and the second voltage may be a pumping voltage higher than a power supply voltage. The bias circuits may comprise: NMOS transistors connected between the second lines and the address decoder; and PMOS transistors connected between the second lines and the pumping voltage. 
     Drains of the NMOS transistors may be connected to the second lines, sources are connected to the 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 pumping voltage, 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. 
     The semiconductor memory device may further comprise: diode transistors connected between the pumping voltage and the PMOS transistors. The second voltage may be obtained by subtracting a threshold voltage of the diode transistors from the pumping voltage. 
     The bias circuits may comprise inverters for biasing the selected second line to the first voltage in response to the main word line signals. 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. 
     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 first voltage; and basing non-selected second lines to a second 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; and 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 the memory cell is connected to a corresponding one of the second lines; wherein a selected second line is biased to a first voltage; and non-selected second lines are biased to a second voltage. 
     The semiconductor memory device may further comprise: bias circuits which control the voltage of the second lines: wherein the bias circuits comprise: NMOS transistors which bias the selected second line to the first voltage; and PMOS transistors which bias non-selected second lines to the second 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 among a plurality of first lines and a second terminal of a phase change memory cell is connected to a corresponding second line among a plurality of second lines, and inverters for biasing a selected second line to a first voltage and non-selected second lines to a second voltage in response to main word line signals. Input terminals of the inverters may receive the main word line signals, and output terminals of the inverters are connected to the second lines. 
    
    
     
       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 exemplary embodiment of the present invention; 
         FIG. 4  illustrates another semiconductor memory device according to an exemplary embodiment of the present invention; and 
         FIG. 5  illustrates still another semiconductor memory device according to an exemplary 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 BS 0 , BS 1 , and BS 2 , and a word line driver  310 . The semiconductor memory device  300  is a PRAM, which includes a memory cell having a phase change materials 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 time 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 TR 0 , TR 1 ˜TRn- 1  which are turned on or off in response to column selection signals Y 0 ˜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 BL 0 ˜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 LWL 0 , LWL 1 , and LWL 2 . The plurality of first lines BL 0 ˜BLn- 1  are bit lines and the plurality of second lines LWL 0 , LWL 1  and LWL 2  are word lines. 
     Even though only three second lines LWL 0 , LWL 1 , and LWL 2  are shown in  FIG. 3  the number of second lines is not limited thereto. 
     The bias circuits BS 0 , BS 1 , and BS 2  bias a selected second line to a first voltage and a non-selected second line to a second voltage. The first voltage is a voltage provided by decoding the local word line address ADD from the local word line address decoder  340  and the second 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 BS 0 , BS 1 , and BS 2  comprise NMOS transistors NTR 01 ˜NTR 23  connected between each of the second lines LWL 0 , LWL 1 , and LWL 2  and the word line voltage from the local address word line decoder  340 , and PMOS transistors PTR 0 , PTR 1 , PTR 2  connected between each of the second lines LWL 0 , LWL 1 , and LWL 2  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 LWL 0 , LWL 1 , and LWL 2 , 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 LWL 0 , LWL 1 , and LWL 2  and the length of the second lines LWL 0 , LWL 1 , and LWL 2 . 
     The structure and operation of the bias circuits BS 0 , BS 1 , and BS 2  will now be described in more detail. The drains of the NMOS transistors NTR 01 ˜NTR 23  are connected to the second lines LWL 0 , LWL 1 , and LWL 2 , and the sources are connected to the local address word line voltage, and main word line signals SWL 0 , SWL 1 , and SWL 2  are applied to the gates of the NMOS transistors NTR 01 ˜NTR 23 . 
     The word line driver  310  generates the main word line signals SWL 0 , SWL 1 , and SWL 2  in response to word line enable signals X 0 , X 1 , and X 2  and block addresses BLK 0 , BLK 1 , and BLK 2 . 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 Y 0  of the column decoder  320  is activated to turn on a first transistor TR 0 , a write current is applied to a first bit line BL 0 . Data is then written to a memory cell connected between the first bit line BL 0  and a first word line LWL 0  (which may be a local word line). 
     If the word line enable signal X 0  and the first block address BLK 0  are input at a high level, a NAND gate N 0  and an inverter I 0  generate the first main word line signal SWL 0  at a high level. The first main word line signal SWL 0  is then generated at a high level and the other main word line signals SLW 1  and SWL 2  are generated at a low level to select the first word line LWL 0 . 
     The NMOS transistors NTR 01 ˜NTR 23  of the first bias circuit BS 0  are turned on in response to the high level of the first main word line signal SWL 0 , and the first word line LWL 0  goes to a low level. 
     The PMOS transistor PTR 0  of the first bias circuit BS 0  is turned off. The write current applied to the first bit line BL 0  flows to the word line decoder  340  through a memory cell and the first word line LWL 0  so that data is stored in the memory cell. 
     The PMOS transistors PTR 1  and PTR 2  of the second and third bias circuits BS 1  and BS 2  are turned on by the second and third main word line signals SWL 1  and SWL 2  at a low level, and the voltage of the second and third word lines LWL 1  and LWL 2  becomes the pumping voltage VPP_SA. 
     The write current applied to the first bit line BL 0  does not flow to the second and third word lines LWL 1  and LWL 2  since the second and third word lines LWL 1  and LWL 2  are at the pumping voltage VPP_SA. Thus, memory cells other than that connected to the first bit line BL 0  and the first word line LWL 0  can stably hold data since they are not influenced by the write current of the first bit line BL 0 . 
     As a further operational example, assuming that the word line LWL 0  and the bit line BL 0  are selected, local word line address decoder EPDEC applies 0V to the source of NMOS transistor NTR 01  that is connected to the selected word line LWL 0 . In this case, a current path is generated from the selected bit line BL 0  to the source of selected NMOS transistor NTR 01 . 
     Assuming that the word line LWL 1  is not selected, however, and the bit line BL 0  is selected, local word line address decoder EPDEC applies the pumping voltage VPP_SA to the source of NMOS transistor NTR 11  that is connected to the un-selected word line LWL 1 . In this case, a voltage drop corresponding to the threshold voltage of the un-selected NMOS transistor NTR 11  is generated, and accordingly the first voltage that is obtained by subtracting the threshold voltage of the un-selected NMOS transistor NTR 11  from the pumping voltage VPP_SA is applied to the un-selected word line LWL 1 . 
     That is, the first voltage is applied to one terminal of the diode that is connected to the un-selected word line LWL 1 , and the voltage of the selected bit line BL 0  that is applied to the other terminal of the diode is the voltage of the selected bit line BL 0 . 
     In this exemplary embodiment, it is the pumping voltage VPP_SA that is greater than the threshold voltage of the un-selected NMOS transistor NTR 11  plus the voltage of the selected bit line BL 0 . 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. 
       FIG. 4  illustrates a semiconductor memory device  400  according to another embodiment of the present invention. The semiconductor memory device  400  has the same or similar structure and operation as the semiconductor memory device  300  shown in  FIG. 3 . However, the structure of bias circuits BS 0 , BS 1 , and BS 2  of the semiconductor memory device  400  is different than that of the bias circuits BS 0 , BS 1 , and BS 2  of the semiconductor memory device  300 . 
     For example, the bias circuits BS 0 , BS 1 , and BS 2  of the semiconductor memory device  400  include diode transistors DTR 0 , DTR 1 , and DTR 2  connected between the power voltage VDD and the sources of the PMOS transistors PTR 0 , PTR 1 , and PTR 2 . Word lines (e.g. the second and third word lines LWL 1  and LWL 2 ) which are not selected by the diode transistors DTR 0 , DTR 1 , and DTR 2  are at a voltage obtained by subtracting the threshold voltage of the diode transistors DTR 0 , DTR 1 , and DTR 2  from the power voltage VDD. 
     In  FIG. 4 , the voltage applied to a bit line to create the write current is lower than the power voltage VDD and non-selected word lines maintain a voltage lower than the power voltage VDD. 
       FIG. 5  illustrates still another semiconductor memory device  500  according to an embodiment of the present invention. The semiconductor memory device  500  has the same or similar structure as the semiconductor memory device  300  shown in  FIG. 3 . However, the structure of bias circuits BS 0 , BS 1  and BS 2  of the semiconductor memory device  500  is different than that of the bias circuits BS 0 , BS 1 , and BS 2  of the semiconductor memory device  300 . 
     As shown in  FIG. 4 , the bias circuits BS 0 , BS 1 , and BS 2  of the semiconductor memory device  500  include inverters I 01 ˜I 23  for biasing a selected second line to a first voltage in response to the main word line signals SWL 0 , SWL 1 , and SWL 2 . The inverters I 01 ˜I 23  perform the functions of the NMOS transistors NTR 01 ˜NTR 23  and the PMOS transistors PTR 0 , PTR 1 , PTR 2  included in the bias circuits BS 0 , BS 1 , and BS 2  shown in  FIG. 3 . For example, the main word line signal SWL 0  is generated at a high level, and other main word line signals SWL 1  and SWL 2  are generated at a low level. 
     The inverters I 01 , I 02  and I 03  of the first bias circuit BS 0  invert the voltage of the first word line LWL 0  to a low level. The inverters I 11 ˜I 23  of the second and third bias circuits BS 1  and BS 2  invert the voltage of the second and third word lines LWL 2  and LWL 3  to a high level. The bias circuits BS 0 , BS 1 , and BS 2  of the semiconductor memory device  500  perform the same or similar function as the bias circuits BS 0 , BS 1 , and BS 2  of the semiconductor memory device  300 . 
     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 devices  300 ,  400  or  500 . In addition, the data writing method may be used by a semiconductor memory device other than the semiconductor memory devices  300 ,  400  or  500  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 first voltage. The selected second line is a word line connected to a memory cell for writing data. The first voltage lowers the voltage of the word line connected to the memory cell for writing data. 
     Non-selected second lines are biased to a second voltage and the non-selected second lines are word lines other than the word line connected to the memory cell for writing data. The second 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 tine. 
     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.