Patent Publication Number: US-8116117-B2

Title: Method of driving multi-level variable resistive memory device and multi-level variable resistive memory device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part application of U.S. application Ser. No. 11/855,525, filed Sep. 14, 2007, which claims priority from Korean Patent Application No. 10-2006-0119148 filed on Nov. 29, 2006, the subject matter of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Field 
     The present invention relates to a method of driving a multi-level variable resistive memory device and a multi-level variable resistive memory device. 
     2. Description of the Related Art 
     Memory devices using resistance materials to store data in a non-volatile manner include; phase change random access memory (PRAM), resistive RAM (RRAM), and ferroelectric RAM (FRAM). In contrast, dynamic RAM (DRAM) and contemporary flash memory store data using electrical charge to store data, although DRAM is not nonvolatile in its operating nature. Instead of variations in electrical charge non-volatile memory devices using resistance materials store data in relation to change of a phase state change for a material like a chalcogenide alloy (PRAM), change in resistance of variable resistance materials (RRAM), and change in a polarization phenomenon of ferroelectric materials (FRAM). 
     Contemporary memory devices regardless of data storage mechanism, are characterized by continuing attempts to shrink memory cell size, increase integration density, and/or increase data storage density per unit area occupied by the memory device. Among other techniques used to increase data storage density, memory cells capable of storing multiple data bits are increasingly common. 
     SUMMARY 
     In one embodiment, the invention provides a method of driving a write data value to a multi-level variable resistive memory device, the method comprising; reading resistance of reference cells, applying a write current to a variable resistive memory cell to change the actual resistance of the variable resistive memory cell, verifying whether the actual resistance resides in an intended resistance window associated with the write data value and generating verification results, the intended resistance window depending on the resistance of the reference cells, and varying the amount of the write current in accordance with the verification results, and then re-applying the write current to the variable resistive memory cell. 
     In another embodiment, the invention provides a multi-level variable resistive memory device, comprising; a memory cell array including a variable resistive memory cell, a temperature compensation circuit comprising one or more reference cells and reading resistance of the reference cells, a verification sense amplifier verifying whether the actual resistance of the variable resistive memory cell resides in an intended resistance window and generating verification results, the intended resistance window depending on the resistance of the reference cells, a write control circuit supplying a control signal to increase or decrease the amount of write current applied to the variable resistive memory cell on the basis of the verification results, and a write driver supplying the write current to the variable resistive memory cell, and increasing or decreasing the amount of the write current in response to the control signal. 
     In another embodiment, the invention provides a memory system comprising, a multi-level variable resistive memory device; and a memory controller accessing the variable resistive memory device in response to an external command, wherein the variable resistance memory device comprises: a memory cell array including a variable resistive memory cell; a temperature compensation circuit comprising one or more reference cells and reading resistance of the reference cells; a verification sense amplifier verifying whether the actual resistance of the variable resistive memory cell resides in an intended resistance window and generating verification results, the intended resistance window depending on the resistance of the reference cells; a write control circuit supplying a control signal to increase or decrease the amount of write current applied to the variable resistive memory cell on the basis of the verification results; and a write driver supplying the write current to the variable resistive memory cell, and increasing or decreasing the amount of the write current in response to the control signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be described with reference to the attached drawings in which: 
         FIG. 1  is a graph illustrating a resistance window of a multi-level phase change random access memory device according to an embodiment of the present invention; 
         FIG. 2  is a flowchart illustrating a method of driving a multi-level phase change random access memory device according to an embodiment of the present invention; 
         FIGS. 3A and 3B  are conceptual timing charts illustrating the method of driving a multi-level phase change random access memory device according to the embodiment of the present invention; 
         FIGS. 4 to 6  are graphs illustrating the method of driving a multi-level phase change random access memory device according to the embodiment of the present invention; 
         FIG. 7  is a block diagram showing a multi-level phase change random access memory device according to the embodiment of the present invention; 
         FIG. 8  is an exemplary block diagram showing a verification sense amplifier shown in  FIG. 7 ; and 
         FIG. 9  is an exemplary block diagram showing a first sense amplifier shown in  FIG. 8 . 
         FIGS. 10A through 12  are diagrams for explaining a nonvolatile memory device according to a fifth embodiment of the present invention. 
         FIG. 13  is a block diagram of the nonvolatile memory device according to the fifth embodiment of the present invention. 
         FIG. 14  is an exemplary block diagram of the temperature compensation circuit  2100  illustrated in  FIG. 13 . 
         FIG. 15  is an exemplary circuit diagram of the reference write driver  2140  illustrated in  FIG. 14  and a portion related to a write operation. 
         FIG. 16  is an exemplary circuit diagram of the reference read circuit  2180  illustrated in  FIG. 14  and a portion related to a read operation. 
         FIG. 17  is another exemplary circuit diagram of the reference read circuit  180  illustrated in  FIG. 14  and a portion related to a read operation. 
         FIG. 18  is another exemplary circuit diagram of the reference read circuit  180  illustrated in  FIG. 14  and a portion related to a read operation. 
         FIG. 19  is a diagram for explaining a nonvolatile memory device according to a sixth embodiment of the present invention. 
         FIGS. 20 and 21  are diagrams for explaining nonvolatile memory devices according to sixth through eighth embodiments of the present invention. 
         FIG. 22  is a perspective view of a nonvolatile memory device according to a ninth embodiment of the present invention. 
         FIG. 23  is a layout diagram of a second resistor  188  included in the nonvolatile memory device according to the ninth embodiment of the present invention. 
         FIG. 24  is a layout diagram of a nonvolatile memory device according to a tenth embodiment of the present invention. 
         FIG. 25  is a perspective view of the nonvolatile memory device according to the tenth embodiment of the present invention. 
         FIG. 26A  is a diagram for explaining a variable resistance memory device according to an eleventh embodiment of the present invention. 
         FIG. 26B  is a flowchart illustrating a method of driving the nonvolatile memory devices according to the fifth and seventh embodiments of the present invention. 
         FIGS. 27 through 31  are diagrams for explaining storage systems according to first through fifth embodiments of the present invention. 
         FIG. 32  is a schematic block diagram of a portable media system including phase-change random access memory device according to embodiments of the present invention. 
         FIG. 33  through  FIG. 40  illustrate an example embodiment of an application of the semiconductor device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Advantages and features of the present invention and methods of accomplishing the same may be understood more readily by reference to the following detailed description of embodiments with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to only the illustrated embodiments. Rather, these embodiments are presented as teaching examples. Throughout the written description and drawings, like reference numerals refer to like or similar elements. 
     Hereinafter, embodiments of the invention will be described using a phase change random access memory device (PRAM) example. However, it will be apparent to those skilled in the art that other embodiments of the invention may be applied to others forms of nonvolatile memory using resistance materials, such as a resistive RAM (RRAM), and a ferroelectric RAM (FRAM). 
       FIG. 1  is a graph illustrating a series of resistance window for a multi-level PRAM according to an embodiment of the invention. In  FIG. 1 , the exemplary PRAM is assumed to store two bits of data. However, it will be apparent to those skilled in the art that a memory cell storing three or more bits might be used equally well as an example. 
     Referring to  FIG. 1 , the multi-level PRAM has four resistance windows W 1 , W 2 , W 3 , and W 4 . A first resistance window W 1  is characterized by a lowest resistance level and a fourth resistance window W 4  is characterized by a highest resistance level with a second resistance window W 2  and a third resistance window W 3  being sequentially intermediate thereto. As shown in  FIG. 1 , the first resistance window W 1  is defined as being less than a first reference resistance RL 1 . The second resistance window W 2  is defined as being greater than a second reference resistance RH 1  and less than a third reference resistance RL 2 . The third resistance window W 3  is defined as being greater than a fourth reference resistance RH 2  and less than a fifth reference resistance RL 3 . The fourth resistance window W 4  is defined as being greater than a sixth reference resistance RH 3 . The separate four resistance windows W 1 , W 2 , W 3 , and W 4  are ascribed to respective two-bit data states 00, 01, 10, and 11. 
     Resistance margins M 1 , M 2 , and M 3  are respectively disposed between the four resistance windows W 1 , W 2 , W 3 , and W 4 . In particular, a first resistance margin M 1  separates the first resistance window W 1  from the second resistance window W 2 . A second resistance margin M 2  separates the second resistance window W 2  from the third resistance window W 3 , and the third resistance margin M 3  separates the third resistance window W 3  from the fourth resistance window W 4 . 
       FIG. 2  is a flowchart illustrating a method of driving a multi-level PRAM according to the embodiment of the invention. The term “driving” in this context refers to the process(es) by which a memory cell in the multi-level PRAM is ascribed a particular resistance value. 
     Referring to  FIG. 2 , a write-verification operation is used when two-bit data is written in the memory cell of the multi-level PRAM. That is, a method of writing two-bit data in a particular memory cell includes; supplying a predetermined write current to the memory cell, verifying whether or not the proper data value has been written (i.e., whether the memory cell exhibits the appropriate resistance state for the data intended to be written to the memory cell), and in view of the verification results increasing or decreasing the applied write current. 
     The example of  FIG. 2  assumes that a data value of 10 is to be written to the memory cell. However, other data values might be used as alternate examples. 
     Initially, write current is supplied to the phase change memory cell in order to change the resistance R exhibited by the phase change memory cell (S 10 ). The write current applied to the phase change memory cell may be set to a constant level. Alternately, a sequentially increasing or decreasing stepped current may be applied. Alternately, an increasing then decreasing or deceasing then increasing current may be applied. Thus, the present invention is not limited to any particular form of applied write current. 
     Following the initial application of the write current, one or more a data verification operation(s) is performed to determine whether the actual resistance R exhibited by the phase change memory cell corresponds to a resistance window properly indicating the write data (e.g., “10” in the illustrated example which corresponds to resistance window W 3 ) (S 20  and S 40 ). 
     That is, actual resistance R of the phase change memory cell is compared with the fourth reference resistor RH 2  (S 20 ). If the actual resistance R exhibited by the phase change memory cell is less than fourth reference resistance RH 2 , the level of the applied write current should be increased (S 30 ) to increase the resistance R of the phase change memory cell. Application of write current (S 10 ) and comparison of actual resistance R to the fourth reference resistance RH 2  (S 30 ) continues until the actual resistance is greater than the fourth reference resistance RH 2 . Exemplary methods of applying an increased write current within this control loop are illustrated in  FIGS. 3A and 3B . 
     Once the actual resistance R of the phase change memory cell is determined to be greater than the fourth reference resistance RH 2  (S 20 =no), it is compared to the fifth reference resistance RL 3  (S 40 ). 
     If the actual resistance R of the phase change memory cell is greater than the fifth reference resistance RL 3 , it should be decreased. This may be accomplished by applying a reduced write current to the phase change memory cell (S 50 ). Application of write current (S 10 ) and comparison of actual resistance R to the fifth reference resistance RL 3  (S 50 ) continues until the actual resistance is less than the fifth reference resistance RL 3 . Exemplary methods of applying a decreased write current useful within this control loop are illustrated in  FIGS. 3A and 3B . 
     Once the actual resistance R exhibited by the phase change memory cell enters resistance window W 3  and properly corresponds to a data value of 10 using the above-described methods, application of the write current to the phase change memory cell is topped and the write operation ends. 
       FIGS. 3A and 3B  are conceptual timing charts illustrating exemplary methods of driving a multi-level PRAM designed in accordance with an embodiment of the invention. 
     Referring to  FIGS. 3A and 3B , the multi-level PRAM according to an embodiment of the invention writes data during a plurality of write loops L=1 through 11. Before each write loop begins, a verification operation (VERIFY READ) is performed. During each write loop, data may be written to the phase change memory cell. 
     In  FIG. 3A , the amplitude of an applied write current (I_data 10 ) adapted to write a data value of 10 will vary in accordance with the results of the verification operation. For example, a first applied write current having an amplitude of 0.5 mA is supplied during the first write loop L=1. The following verification result shows that the actual resistance R of the phase change memory cell has not entered the intended resistance window W 3  corresponding to a data value of 10. Thus, a write current having an amplitude of 1.0 mA is applied to the phase change memory cell during the second write loop L=2. 
     In contrast and as shown in  FIG. 3B , the pulse width of an applied write current (I_data 10 ) adapted to write a data value of 10 varies in accordance to the verification result. For example, if the actual resistance R exhibited by the phase change memory cell does not enter the intended resistance window W 3  corresponding to a data value of 10 in response to the application of an initial write current supplied during the first write loop L=1, the pulse width of the write current applied during the second write loop L=2 is increased over the pulse width of the initial write current supplied during the first write loop L=1. 
     As shown in  FIGS. 2 ,  3 A, and  3 B, according to an embodiment of the invention, the amount of the write current (I_data 10 ) may vary over a series of write loops L=1 to 11. Within the illustrated embodiments, the term “amount” as used to characterize the applied write current indicates a particular quantity of electrical charge supplied by the write current. This supplied quantity of electrical charge may be varied, for example, by increasing the amplitude of the write current or the applied period of the write current. 
     Thus, if the actual resistance R of the phase change memory cell is less than the fourth reference resistance RH 2 , the amount of the write current increases and, if the actual resistance R of the phase change memory cell is greater than the fifth reference resistance RL 3 , the amount of the write current decreases. Of course, according to the flowchart of  FIG. 2 , as each of the write loops L=1 to 11 progresses, the amount of the write current may continuously increase or decrease. As described above in relation to the illustrated embodiment, since actual resistance R of the phase change memory cell can be controlled to place it within an intended resistance window W 3 , it is possible to improve reliability of the write operation. Further, since it is possible to ensure sufficient resistance margins M 2  and M 3  in the working example, it is possible to minimize read errors associated with the stored data. 
       FIGS. 4 ,  5  and  6  are graphs illustrating exemplary methods for driving a multi-level PRAM according to embodiments of the invention.  FIGS. 4 ,  5  and  6  show a case where a write data value of 10 is written to a phase change memory cell currently exhibiting an actual resistance R indicating a data value of 00. That is, the initial data is 00 and the target data is 10.  FIGS. 4   5 , and  6  show examples of variations in the actual resistance R of the phase change memory cell through a number of write loops. These are, however, merely selected examples further illustrating embodiments of the invention. The arrows shown in  FIGS. 4 ,  5  and  6  indicate respective changes in the actual resistance R exhibited by the phase change memory cell having the target data  10  written to each through a series of write loops. 
     Referring to  FIG. 4 , the actual resistance R of the phase change memory cell is increased by a write current supplied during a first write loop L=1. However, since the actual resistance R of the phase change memory cell is still less than the fourth reference resistance RH 2 , the amount of the write current is increased and then the increased write current is again supplied during the second write loop L=2. As the verification results show, since the actual resistance R of the phase change memory cell following the second write loop L=2 resides in the intended resistance window corresponding to a data value of 10, the write operation ends. 
     Referring to  FIG. 5 , the actual resistance of the phase change memory cell is increased by a write current supplied during the first write loop L=1. However, since the actual resistance of the phase change memory cell becomes greater than the fifth reference resistance RL 3 , the amount of the write current is decreased and then the decreased write current is supplied during a second write loop L=2. As the verification results show, since the actual resistance of the phase change memory cell following the second write loop L=2 resides in the intended resistance window corresponding to a data value of 10, the write operation ends. 
     Referring to  FIG. 6 , the actual resistance of the phase change memory cell is increased by a write current supplied during a first write loop L=1. However, since the actual resistance of the phase change memory cell is greater than the fifth reference resistance RL 3 , the amount of the write current must be decreased during a second write loop L=2. However, the verification results show that following the second write loop L=2, the actual resistance of the phase change memory cell is now less than the fourth reference resistance RH 2 . Hence, the amount of the write current must be increased and supplied during a third write loop L=3. Following the third write loop L=3, the verification results show that the actual resistance of the phase change memory cell reside in the intended resistance window corresponding to a data value of 10, and the write operation ends. 
     In particular and as illustrated in  FIGS. 3A and 3B , the amount of the write current applied during the third write loop L=3 should be greater than the amount of write current applied during the second write loop L=2, but less than the amount of write current applied during the first write loop. Therefore, as shown in  FIG. 6 , it can be seen that the magnitude of change in the actual resistance of the phase change memory cell following the third write loop is less than the magnitude of change in actual resistance of the phase change memory cell following the second write loop. Thus, in a multi-level PRAM according to an embodiment of the invention, the step increase or decrease in the amount of applied write current may be reduced over a series of write loops. 
       FIG. 7  is an exemplary block diagram showing a multi-level PRAM according to an embodiment of the invention, and capable of implementing the above-described or similar driving methods. 
     Referring to  FIG. 7 , the multi-level PRAM generally includes a memory cell array  110 , a row decoder  120 , a column decoder  130 , a verification sense amplifier  140 , a write control circuit  170 , and a write driver  180 . 
     The memory cell array  110  includes a plurality of phase change memory cells divided into a plurality of cell groups. Though not shown in  FIG. 7 , the phase change memory cell includes a variable resistance element that includes a phase change material having at least two different resistance values according to a crystal status or an amorphous status, and an access element that controls a current flowing to the variable resistance element. Here, the access element may be a diode or a transistor connected to the resistance element in series. Further, as the phase change material, various materials, such as, GaSb, lnSb, InSe, Sb2Te3, and GeTe containing two atoms, GeSbTe, GaSeTe, InSbTe, SnSb2Te4, and InSbGe containing three atoms, and AgInSbTe, (GeSn)SbTe, GeSb(SeTe), and Te81Ge15Sb2S2 containing four atoms may be used. 
     The row decoder  120  receives and decodes a row address and designates a row of the plurality of phase change memory cells to be written. The column decoder  130  receives and decodes a column address and designates a column of the plurality of phase change memory cells to be written. 
     The verification sense amplifier  140  verifies whether or not resistance of the phase change memory cell enters a predetermined resistance window. 
     Specifically, the verification sense amplifier  140  receives first to sixth reference voltages VRL 1  to VRL 3  and VRH 1  to VRH 3  corresponding to first to sixth reference resistances RL 1  to RL 3  and RH 1  to RH 3 , respectively, and receives write data (WDATA) to be written in the phase change memory cell. The reference voltage to be used for sensing is selected among the first to sixth reference voltages VRL 1  to VRL 3  and VRH 1  to VRH 3  on the basis of the write data WDATA. For example, in a case where the write data WDATA to be written is 10, since the resistance window corresponding to the data  10  is greater than fourth reference resistance RH 2  and less than fifth reference resistance RL 3 , the reference voltages to be used for sensing are the fourth reference voltage VRH 2  corresponding to fourth reference resistance RH 2  and the fifth reference voltage VRL 3  corresponding to fifth reference resistance RL 3 . For another example, in a case where the write data WDATA to be written is 01, the reference voltages to be used for sensing are the second reference voltage VRH 1  and the third reference voltage VRL 2 . With the selected reference voltages, it is determined whether the actual resistance of the phase change memory cell resides in the intended resistance window. As the determination result, first and/or second comparison signals PASS 1  and PASS 2  are supplied to the write control circuit  170 . 
     In a case where the write data WDATA to be written is 10, the first comparison signal PASS 1  may indicate a determination result using the fourth reference voltage VRH 2  and the second comparison signal PASS 2  may indicate a determination result using the fifth reference voltage VRL 3 . An exemplary write driver  180  will be described in some additional detail with reference to  FIGS. 8 and 9 . 
     The write control circuit  170  supplies a control signal CON so as to increase or decrease the amount of the write current on the basis of the verification result of the verification sense amplifier  140 . 
     For example, when the first comparison signal PASS 1  is logically low (e.g., indicating that the actual resistance of the phase change memory cell is less than fourth reference resistance RH 2 ), the write control circuit  170  supplies the control signal CON to increase the amount of the write current. When the second comparison signal PASS 2  is high (e.g., indicating that the actual resistance of the phase change memory cell is greater than fifth reference resistance RL 3 ), the write control circuit  170  supplies the control signal CON to decrease the amount of the write current. Further, when the first comparison signal PASS 1  is high and the second comparison signal PASS 2  is low (e.g., indicating that the actual resistance of the phase change memory cell resides in the intended resistance window corresponding to the data  10 ), write control circuit  170  supplies the control signal CON, such that the write driver  180  discontinues the supply of write current. 
     Thus, the write driver  180  supplies the write current to the phase change memory cell while increasing or decreasing the amount of the write current in response to the control signal CON. 
       FIG. 8  is an exemplary block diagram showing the verification sense amplifier of  FIG. 7  in some additional detail.  FIG. 9  is an exemplary block diagram showing the first sense amplifier of  FIG. 8  in some additional detail. In  FIG. 9 , a row selection circuit  121 , a column selection circuit  131 , and a phase change memory cell  111  are described as well as the first sense amplifier for convenience of explanation. 
     Referring to  FIG. 8 , the verification sense amplifier  140  includes first and second sense amplifiers  150  and  160  that receive the first to sixth reference voltages VRL 1  to VRL 3  and VRH 1  and VRH 3  and the write data WDATA. In a case where the write data WDATA to be written is 01, the first sense amplifier  150  performs sensing using the fourth reference voltage VRH 2  and supplies the first comparison signal PASS 1 , and the second sense amplifier  160  performs sensing using the fifth reference voltage VRL 3  and supplies the second comparison signal PASS 2 . In the exemplary block diagram of  FIG. 8 , the number of sense amplifiers is determined according to the number of reference voltages necessary for sensing. This will be true for all embodiments of the invention. 
     Referring to  FIG. 9 , the column selection circuit  131  receives a column selection signal YSEL to select a bit line BL and the row selection circuit  121  receives a row selection signal XSEL to select a word line WL, thereby selecting the phase change memory cell  111  to be written. 
     The first sense amplifier  150  applies a current to the selected phase change memory cell  111  and senses a change in level of a sensing node NS that occurs due to a current flowing through the selected phase change memory cell  111  so as to read data. 
     The above-described sense amplifier  150  may include a precharge unit  152 , a compensation unit  154 , a compensation voltage selection unit  155 , a clamping unit  156 , and a comparison unit  158 . 
     The precharge unit  152  precharges a sensing node with a predetermined level, for example, a power supply voltage VDD during a precharge period before the sensing operation. The precharge unit  152  may be a PMOS transistor that is coupled between the power supply voltage VDD and the sensing node NS, and has a gate to which a precharge control signal VPRE is applied. 
     The compensation unit  154  supplies a compensation current to the sensing node NS so as to compensate a reduction in level of the sensing node NS that occurs due to a current flowing through the selected phase change memory cell  111 . In particular, when the data  00  is stored in the phase change memory cell  111 , since resistance of the phase change material is small, the amount of the current flowing through the selected phase change memory cell  111  is large. Further, when the data  11  is stored in the phase change memory cell  111 , since resistance of the phase change material is large, the amount of the current flowing through the selected phase change memory cell  111  (i.e., “cell current”) is small. When it is expressed by an inequality, it can be expressed as “the amount of the cell current in case of data  00 ”&gt;“the amount of the cell current in case of data  01 ”&gt;“the amount of the cell current in case of data  10 ”&gt;“the amount of the cell current in case of data  11 ”. For example, when the amount of the current supplied from the compensation unit  154  corresponds to a cell current in cases of data  10  and data  11 , the levels at the sensing node NS for the data  10  and data  11  are kept unchanged, but the levels at the sensing node NS for the data  00  and data  01  are reduced. Therefore, it is possible to distinguish the data  00 , data  01 , data  10 , and data  11  by adjusting the amount of the compensation current supplied from the compensation unit  154 . The compensation unit  154  may be a PMOS transistor coupled between the power supply voltage VDD and the sensing node NS, and having a gate receiving the compensation voltage. In this case, the amount of the compensation current may be adjusted by adjusting the compensation voltage. 
     The compensation voltage selection unit  155  receives the first to sixth reference voltages VRL 1  to VRL 3  and VRH 1  to VRH 3  and the write data WDATA to be written, and supplies the reference voltage corresponding to the write data WDATA to be written. The example of  FIG. 9  shows a case where the fourth reference voltage VRH 2  is selected as the compensation voltage. 
     The clamping unit  156  performs clamping with respect to a level of the bit line BL within an appropriate range to be read. In particular, the clamping unit  156  performs clamping with respect to the level of the bit line BL to a predetermined level lower than a threshold voltage Vth of the phase change material. This is because a phase of the phase change material of the selected phase change memory cell  111  may change when the level of the bit line BL becomes equal to or greater than the threshold voltage Vth. The clamping unit  156  may be an NMOS transistor coupled between the bit line BL and the sensing node NS, and having a gate receiving a clamping control signal VCMP. 
     The comparison unit  158  compares the level of the sensing node NS and the reference level REF and outputs the comparison signal PASS 1 . 
     When the fourth reference voltage VRH 2  is selected as the compensation voltage, if the actual resistance of the phase change memory cell is less than fourth reference resistance RH 2 , the level of the sensing node NS goes down and the comparison signal PASS 1  is output as a logically low value. If resistance of the phase change memory cell is greater than fourth reference resistance RH 2 , the level of the sensing node NS is remains unchanged and the comparison signal PASS 1  is output as a logically high value. 
     From here, the reference voltage VRL 1 ˜VRL 3 , VRH 1 ˜VRH 3  and clamping control signal VCMP are explained. From here, a compensation control signal VBIAS may be at least one of the reference voltage VRL 1 ˜VRL 3 , VRH 1 ˜VRH 3 . 
       FIGS. 10A through 12  are diagrams for explaining a nonvolatile memory device according to a fifth embodiment of the present invention. Specifically,  FIGS. 10A and 11  are diagrams related to single-level cells (i.e., 1 bit-level cells), and  FIGS. 10B and 12  are diagrams related to multi-level cells (i.e., n bit-level cells where “n” is a natural number equal to or greater than two). 
     In the nonvolatile memory device according to the fifth embodiment of the present invention, each nonvolatile memory cell has any one of a plurality of first resistance distributions. Here, the first resistance distributions do not overlap each other, that is, are separated from each other to secure a margin. 
     For example, when the nonvolatile memory cells are single-level cells, the first resistance distributions may include two (=21) kinds of resistance distributions, i.e., a set resistance distribution SET and a reset resistance distribution RESET, as illustrated in  FIG. 10A . Here, each of the nonvolatile memory cells may have resistance corresponding to any one of the first resistance distributions. 
     When the nonvolatile memory cells are multi-level cells, each of the nonvolatile memory cells may have any one of 2 n  different kinds of resistance distributions. For example, when the nonvolatile memory cells are 2 bit multi-level cells, the first resistance distributions may include four (=2 2 ) kinds of resistance distributions, i.e.,  00 ,  01 ,  10 , and  11 , as illustrated in  FIG. 10B . Here, each of the nonvolatile memory cells may have resistance corresponding to any one of the first resistance distributions. In addition, each of the resistance distributions  00 ,  01 ,  10 , and  11  may be determined by the amount of amorphous material contained in a phase-change material. For example, the resistance distribution  00  may be determined when the phase-change material is crystalline, and the amount of amorphous material contained in the phase-change material may increase in the order of the resistance distributions  01 ,  10 , and  11 . 
     The nonvolatile memory device according to the fifth embodiment of the present invention may also include one or more reference cells for adjusting the amount of compensation current or clamping current to enhance the reliability of a read operation. The reference cells may be identical to the nonvolatile memory cells. That is, like the nonvolatile memory cells, the reference cells each may include a variable resistance element and an access element, and the variable resistance element may be a phase-change material. 
     Each of the reference cells has any one of a plurality of second resistance distributions. The second resistance distributions may not overlap the first resistance distributions of the nonvolatile memory cells. 
     When the nonvolatile memory cells are single-level cells, the reference cells have one resistance distribution Ref 1 , as illustrated in  FIG. 10A . On the other hand, when the nonvolatile memory cells are multi-level cells (i.e., n bit-level cells), the reference cells have 2 n −1 resistance distributions. For example, when the nonvolatile memory cells are 2 bit multi-level cells, three different resistance distributions Ref 1 , Ref 2 , and Ref 3  may be generated as illustrated in  FIG. 10B . Consequentially, when the nonvolatile memory cells are m bit-level cells (“m” is a natural number), the reference cells have 2 m −1 resistance distributions. 
     Referring to  FIGS. 10A and 10B , any one of the second resistance distributions may be located between two adjacent ones of the first resistance distributions. For example, referring to  FIG. 10B , the resistance distribution Ref 2  is located between the resistance distributions  00  and  01 , the resistance distribution Ref 1  is located between the resistance distributions  01  and  10 , and the resistance distribution Ref 3  is located between the resistance distributions  10  and  11 . 
     Referring to  FIG. 11 , the resistance distributions of the nonvolatile memory cells and the resistance distributions of the reference cells change as temperature rises. That is, as temperature rises, the reset resistance distribution RESET is reduced more significantly than the set resistance distribution SET. Therefore, as temperature rises, a margin between the set resistance distribution SET and the reset resistance distribution RESET is reduced. 
     However, the arrangement of the set resistance distribution SET, the resistance distribution Ref 1  of the reference cells, and the reset resistance distribution RESET remains unchanged before and after temperature rises. In addition, the set resistance distribution SET, the resistance distribution Ref 1  of the reference cells, and the reset resistance distribution RESET still do not overlap each other. Therefore, the resistance of the reference cells may be used to clearly discriminate between set resistance and reset resistance. 
     Referring to  FIG. 12 , the resistance distributions of the nonvolatile memory cells and the resistance distributions of the reference cells change as temperature rises. That is, as temperature rises, the degree by which the resistance distribution is reduced increases in the order of the resistance distributions  00 ,  01 ,  10 , and  11 . Therefore, as temperature rises, margins between the resistance distributions  00 ,  01 ,  10 , and  11  are reduced. 
     However, the arrangement of the resistance distributions  00 ,  01 ,  10 , and  11 , and the resistance distributions Ref 1 , Ref 2 , and Ref 3  of the reference cells remains unchanged before and after temperature rises. In addition, the resistance distributions  00 ,  01 ,  10 , and  11 , and the resistance distributions Ref 1 , Ref 2 , and Ref 3  of the reference cells still do not overlap each other. Therefore, the resistances of the reference cells may be used to clearly discriminate among resistances corresponding to the resistance distributions  00 ,  01 ,  10 , and  11 . 
     In some embodiments of the present invention, the resistances of the reference cells are used to adjust the amount of compensation current or clamping current. As described above with reference to  FIGS. 11 and 12 , the resistance change of the reference cells according to temperature may fully reflect the resistance change of the nonvolatile memory cells according to temperature. That is, a change in the resistance of the reference cells according to temperature may be equal to a change in the resistance of the nonvolatile memory cells according to temperature. Therefore, the amount of compensation current or clamping current may fully reflect temperature changes. 
     The amount of compensation current or clamping current can be adjusted more accurately by using the resistance change of the reference cells. This is because the resistance change of the reference cells can accurately reflect the resistance change of the nonvolatile memory cells since the reference cells and the nonvolatile memory cells are the same type of memory cells. 
     A specific configuration (block or circuit) for adjusting the amount of compensation current or clamping current using the reference cells will now be described. 
       FIG. 13  is a block diagram of the nonvolatile memory device according to the fifth embodiment of the present invention. In  FIG. 13 , a case where a verification read operation is performed using the resistance of the reference cells will be described based on the description made with reference to  FIGS. 10A through 12 . 
     Referring to  FIG. 13 , the nonvolatile memory device according to the fifth embodiment of the present invention includes a memory cell array  2007 , a read circuit  2010 , and a temperature compensation circuit  2100 . The read circuit  2010  may be verification sense amplifier  140  of the  FIG. 7 . 
     The memory cell array  2007  includes a plurality of nonvolatile memory cells. Each of the nonvolatile memory cells has resistance corresponding to any one of the first resistance distributions. That is, when the nonvolatile memory cells are single-level cells, each of the nonvolatile memory cells has the set resistance or the reset resistance. When the nonvolatile memory cells are multi-level cells, each of the nonvolatile memory cells has resistance corresponding to any one of the resistance distributions  00 ,  01 ,  10 , and  11 . 
     The temperature compensation circuit  2100  includes one or more reference cells. Each of the reference cells has resistance corresponding to any one of the second resistance distributions. That is, each of the reference cells may have the resistance distribution Ref 1  as illustrated in  FIG. 10A  or may have any one of the resistance distributions Ref 1  through Ref 3  as illustrated in  FIG. 10B . 
     Meanwhile, one or more reference cells may form a reference set. For example, the number of resistance distributions that the reference cells can have may be equal to the number of reference cells included in a reference set. When the nonvolatile memory cells are single-level cells, the reference cells may have one reference level Ref 1 . When the nonvolatile memory cells are multi-level cells (n bit-level cells), the reference cells may have 2 n −1 resistance levels. Therefore, when the nonvolatile memory cells are single-level cells, one reference cell is included in a reference set. When the nonvolatile memory cells are multi-level cells (n bit-level cells), 2 n −1 reference cells are included in the reference set. For example, referring to  FIG. 10B , three (=2 2 −1) reference cells may be included in a reference set. Consequently, when the nonvolatile memory cells are m bit-level cells (“m” is a natural number), 2 m −1 reference cells are included in the same reference set. 
     The 2 m −1 reference cells included in the same reference set may respectively have resistances corresponding to different resistance distributions. For example, when the nonvolatile memory cells are 2 bit-level cells, three reference cells may be included in the same reference set. Here, the three reference cells may respectively have resistances corresponding to different resistance distributions, i.e., Ref 1  through Ref 3 . 
     The temperature compensation circuit  2100  may generate a compensation control signal VBIAS or a clamp control signal VCMP that changes according to the resistance of the reference cells. As described above with reference to  FIGS. 11 and 12 , the resistance of the reference cells changes according to temperature. Accordingly, the compensation control signal VBIAS or the clamp control signal VCMP also changes according to temperature. The detailed configuration and operation of the temperature compensation circuit  2100  will be described later with reference to  FIGS. 14 through 18 . 
       FIG. 14  is an exemplary block diagram of the temperature compensation circuit  2100  illustrated in  FIG. 13 . 
     Referring to  FIG. 14 , the temperature compensation circuit  2100  includes a reference cell  2110 , a reference write driver  2140 , a reference read circuit  2180 , and an output node NOUT. 
     The reference write driver  2140  is connected to the output node NOUT, supplies a write current to the reference cell  2110 , and determines the resistance distribution of the reference cell  2110 . The detailed structure of the reference write driver  2140  will be described later with reference to  FIG. 15 . 
     The reference read circuit  2180  is connected to the output node NOUT, supplies a read current to the reference cell  2110 , and reads the resistance distribution of the reference cell  2110 . That is, the reference read circuit  2180  identifies whether the resistance (i.e., resistance distribution) of the reference cell  2110  is located at a desired position. 
     When the resistance distribution of the reference cell  2110  is not located at the desired position, the reference write driver  2140  supplies a write current again to the reference cell  2110  and determines the resistance distribution of the reference cell  2110 . 
     When the resistance distribution of the reference cell  2110  is located at the desired position, the compensation control signal VBIAS or the clamp control signal VCMP, which changes according to the resistance level of the reference cell  2110 , is output through the output node NOUT. The resistance distribution of the reference cell  2110  depends on the temperature. 
     Changing the compensation control signal VBIAS according to the temperature means changing the reference voltage VRL 1 ˜VRL 3 , VRH 1 ˜VRH 3  according to the temperature. Thus, changing the reference voltage VRL 1 ˜VRL 3 , VRH 1 ˜VRH 3  according to the temperature changing the resistance window according to the temperature. That is, the resistance window depends on the resistance of the reference cells. 
     The detailed structure of the reference read circuit  2180  will be described later with reference to  FIGS. 16 through 18 . 
     The reference write driver  2140  may be a replica circuit of a write driver which determines resistance distributions of the nonvolatile memory cells by supplying a write current to the nonvolatile memory cells. The reference read circuit  2180  may be a replica circuit of a read circuit (see  FIG. 3 ) which reads the resistance distributions of the nonvolatile memory cells by supplying a read circuit to the nonvolatile memory cells. Since the reference cell  2110  is identical to each nonvolatile memory cell as described above, a change in the resistance of the reference cell  2110  according to temperature can fully reflect a change in the resistance of each nonvolatile memory cell according to temperature. Here, if the reference write driver  2140  and the reference read circuit  2180  are replica circuits, the resistance change of the reference cell  2110  according to temperature can more accurately reflect the resistance change of each nonvolatile memory cell according to temperature. However, the reference write driver  2140  and the reference read circuit  2180  are not limited to the replica circuits. 
       FIG. 15  is an exemplary circuit diagram of the reference write driver  2140  illustrated in  FIG. 14  and a portion related to a write operation. 
     Referring to  FIG. 15 , the reference write driver  2140  supplies a write current Icell 2  to write set data or reset data. 
     As illustrated in  FIG. 15 , the write current Icell 2  is output to a ground source via a first transistor  2154 , a second transistor  2155 , the reference cell  2110 , a first resistor  2151 , a third transistor  2152 , and then an inverter  2153 . 
     Here, a global column select replica signal GYR 1  is transmitted to the first transistor  2154 , a row column select replica signal LYR 1  is transmitted to the second transistor  2155 , and a main word line replica signal MWLR is transmitted to the third transistor  2152 . That is, the first transistor  2154  is a replica of a global column select circuit, the second transistor  2155  is a replica of a row column select circuit, and the third transistor  2152  is a replica of a main word line select circuit. In addition, the first resistor  2151  is a replica of a resistor in an active region of a substrate. A path of the write current Icell 2  used to write the reference cell  2110  may be made identical to a path of a write current used to write a nonvolatile memory cell by configuring the first transistor  2154 , the second transistor  2155 , the first resistor  2151 , the third transistor  2152 , and the inverter  2153  as described above. 
       FIG. 16  is an exemplary circuit diagram of the reference read circuit  2180  illustrated in  FIG. 14  and a portion related to a read operation. 
     Referring to  FIG. 16 , the reference read circuit  2180  includes a fourth transistor  2181 , a fifth transistor  2182 , a sense amplifier  2185 , a resistor string  2186 , and a second resistor  2188 . 
     The fourth transistor  2181  and the fifth transistor  2182  are connected in series between a step-up voltage source VPPSA and an output node NOUT. A plurality of resistors in the resistance string  2186  are connected in series between the step-up voltage source VPPSA and a ground source and provides a first voltage and a second voltage. The first voltage is applied to a gate of the fourth transistor  2181 , and the second voltage is applied to a gate of the fifth transistor  2182 . Here, the fourth transistor  2181  is a replica of the compensation unit  1014  (see  FIG. 9B ), and the fifth transistor  2182  is a replica of the clamping unit  1016  (see  FIG. 9B ). The second resistor  2188  is, but not limited to, a replica of a bit line. 
     As illustrated in  FIG. 16 , a read current Icell 3  is output to the ground voltage source via the step-up voltage source VPPSA, the fourth transistor  2181 , the fifth transistor  2182 , a sixth transistor  2156 , a seventh transistor  2157 , the second resistor  2188 , the reference cell  2110 , the first resistor  2151 , the third transistor  2152 , and then the inverter  2153 . 
     Here, a global column select replica signal GYR 2  is transmitted to the sixth transistor  2156 , and a local column select replica signal LYR 2  is transmitted to the seventh transistor  2157 . That is, the sixth transistor  2156  is a replica of a global column select circuit, and the seventh transistor  2157  is a replica of a local column select circuit. A path of the read current Icell 3  used to read the reference cell  2110  may be made identical to a path of a read current used to read a nonvolatile memory cell by configuring the fourth transistor  2181 , the fifth transistor  2182 , the sixth transistor  2156 , the seventh transistor  2157 , the first resistor  2151 , the third transistor  2152 , and the inverter  2153  as described above. 
     The sense amplifier  2185  outputs reference data RDATA by comparing a node N 1  and a reference voltage VREF. The reference data RDATA is read to determine whether the resistance of the reference cell  2110  has a desired value. 
     When the resistance of the reference cell  2110  does not have the desired value, a write operation is performed again. 
     When the resistance of the reference cell  2110  has the desired value, the compensation control signal VBIAS or the clamp control signal VCMP may be output through the output node NOUT. For example, the voltage of the output node NOUT may also be output as the compensation control signal VBIAS or the clamp control signal VCMP. 
     Alternatively, as will be described with reference to  FIG. 18 , the voltage of the output node NOUT may be adjusted through trimming, and the adjusted voltage of the output node NOUT may be output as the compensation control signal VBIAS or the clamp control signal VCMP. 
     Alternatively, the resistance of the second resistor  2188  may be controlled to adjust a voltage that is output to the output node NOUT. This is because the voltage output to the output node NOUT may be a voltage divided by the resistance of the second resistor  2188  and that of the reference cell  2110 . Here, it is assumed that the resistances of the fourth transistor  2181 , the fifth transistor  2182 , the sixth transistor  2156 , the seventh transistor  2157 , and the third transistor  2152  are significantly small. Since the resistance of the reference cell  2110  will be determined by a write operation, if the resistance of the second resistor  2188  is adjusted in a manufacturing process, the voltage output to the output node NOUT can be easily adjusted. 
       FIG. 17  is another exemplary circuit diagram of the reference read circuit  2180  illustrated in  FIG. 14  and a portion related to a read operation. 
     Referring to  FIG. 17 , the circuit of  FIG. 17  is different from that of  FIG. 16  in that it further includes a bypass circuit  2190 . When a reference cell  2110  is read, the bypass circuit  2190  is turned on in response to a control signal A 1 . Therefore, while a read current Icell 3  passes through a variable resistance element, it bypasses an access element (that is, a diode). 
     After the reference cell  2110  is read, if it is found out that the resistance of the reference cell  2110  does not have a desired value, the reference cell  2110  must be re-programmed. Thus, it is desirable to minimize the time required to read the reference cell  2110 . The bypass circuit  2190  is implemented to reduce a period of time required by the read current Icell 3  to pass through an n-type semiconductor of the diode, thereby reducing the total read time. 
     On the other hand, when a write operation is performed, the bypass circuit  2190  is turned off so that a current flows toward the access element (the diode). 
       FIG. 18  is another exemplary circuit diagram of the reference read circuit  2180  illustrated in  FIG. 14  and a portion related to a read operation. 
     Referring to  FIG. 18 , the circuit of  FIG. 18  is different from that of  FIG. 16  in that it further includes a trimming circuit  2195 . The trimming circuit  2195  may include a plurality of fuses  2191  through  2194 . 
     At least one of the fuses  2191  through  2194  may be cut using a mode resister set (MRS) to adjust the level of voltage applied to a fourth transistor  2181  and a fifth transistor  2182 . 
     A voltage value of a compensation control signal VBIAS or a clamp control signal VCMP output through an output node NOUT may be adjusted using the trimming circuit  2195 . That is, even when the resistance of a reference cell  2110  has a desired value, if the voltage value of the compensation control signal VBIAS or the clamp control signal VCMP needs to be precisely adjusted, the trimming circuit  2195  can be used. 
       FIG. 19  is a diagram for explaining a nonvolatile memory device according to a sixth embodiment of the present invention. 
     Referring to  FIG. 19 , a circuit of  FIG. 19  is different from the circuit of  FIG. 16  in that a plurality of reference sets are respectively disposed corresponding to a plurality of memory banks. As described above, when nonvolatile memory cells are 2 bit-level cells, there may be three (=22−1) reference cells, and each of the three reference cells may store a resistance value corresponding to one of resistance distributions Ref 1  through Ref 3 . Therefore, as illustrated in  FIG. 19 , three temperature compensation circuits  2100 _ 1  through  2100 _ 3  may corresponds to each of the memory banks. Each of the three temperature compensation circuits  2100 _ 1  through  2100 _ 3  includes a corresponding one of the three reference cells. 
       FIGS. 20 and 21  are diagrams for explaining nonvolatile memory devices according to sixth through eighth embodiments of the present invention. In  FIGS. 20 and 21 , a memory block BLK 0  (see  FIG. 1 ) is illustrated as an example, and the above-described reference cell is included in the memory block BLK 0 . 
     Referring to  FIG. 20 , normal memory cells NC are connected between word lines WLO through WLn and bit lines BLO through BLm. In addition, dummy memory cells DC are connected between dummy bit lines DBL 0  through DBL 2  and the word lines WLO through WLn. A reference cell RC may be connected between the dummy bit line DBL 0  and the word line WLk. 
     The normal memory cells NC are cells in which data is stored. The shape of the dummy memory cells DC may be identical to that of the normal memory cells NC. However, the dummy memory cells DC are cells in which no data is stored. As described above, the reference cell RC is a cell used to adjust the amount of compensation current or clamping current by reflecting temperature changes. 
     As illustrated in  FIG. 20 , the reference cell RC may be located approximately in the middle of the dummy bit line DBL 0  to prevent edge loading. 
     Referring to  FIG. 21 , the normal memory cells NC may not be connected to the word line WLk that is connected to the reference cell RC. That is, only the dummy memory cells DC in addition to the reference cell RC may be connected to the word line WLk. 
     Whenever data stored in all normal memory cells NC in the memory block BLK 0  is read, the reference cell RC may also be read. Accordingly, the word line WLk connected to the reference cell RC is very frequently connected to a ground source. Thus, if the normal memory cells NC are connected to the word line WLk, they may be adversely affected. For this reason, the normal memory cells NC may not be connected to the word line WLk that is connected to the reference cell RC. 
       FIG. 22  is a perspective view of a nonvolatile memory device according to a ninth embodiment of the present invention.  FIG. 23  is a layout diagram of a second resistor  2188  included in the nonvolatile memory device according to the ninth embodiment of the present invention. 
     Referring to  FIG. 22 , when a reference cell RC is disposed within a memory block BLK 0  as described above with reference to  FIGS. 20 and 21 , a reference write driver  2140  and a reference read circuit  2180  may be disposed in a peripheral region. 
     Specifically, an element isolation region  2412  is formed on a substrate  2410  of a first conductivity type (e.g., a P type) to define a plurality of active regions. For example, an active region formed in the memory block BLK 0  may extend in a first direction. The active region formed in the memory block BLK 0  may be implanted with impurities of a second conductivity type (e.g., an N type) to form a word line WLk. The substrate  2410  may be a silicon substrate, a silicon-on-insulator (SOI) substrate, a gallium arsenic substrate, a silicon germanium substrate, or the like. 
     A method of forming the word line WLk by implanting the substrate  2410  of the first conductivity type with the impurities of the second conductivity type has been described above. However, the present invention is not limited to this method. For example, the word line WLk may also be formed by epitaxial growth on the substrate  2410 . 
     A first semiconductor pattern  2432  and a second semiconductor pattern  2434  are formed on the word line WLk to form a diode D. To form the first and second semiconductor patterns  2432  and  2434 , for example, a mold pattern may be formed on the substrate  2410 , and then a semiconductor pattern may be formed along the mold pattern using selective epitaxial growth or solid phase epitaxial (SPE) growth. Then, an ion implantation process may be performed. 
     An ohmic contact layer  2436  is formed on the diode D. The ohmic contact layer  2436  may be made of metal such as tungsten. In addition, the ohmic contact layer  2436  may be formed by physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), or the like. 
     A bottom electrode contact  2438  is formed on the ohmic contact layer  2436 . The bottom electrode contact  2438  may be made of a material selected from TiN, TiAlN, TaN, WN, MoN, NbN, TiSiN, TiBN, ZrSiN, WSiN, WBN, ZrAIN, MoAIN, TaSiN, TaAIN, TiW, TiAl, TiON, TiAION, WON, and TaON. 
     A phase-change material  2442  is formed on the bottom electrode contact  2438 . 
     A top electrode contact  2446  is formed on the phase-change material  2442 . The top electrode contact  2446  may be omitted when necessary. 
     A dummy bit line DBL 0  is formed on the top electrode contact  2446  and extends in a second direction which is different from the first direction. That is, the word line WLk and the dummy bit line DBL 0  may intersect each other. The dummy bit line DBLO may extend up to the peripheral region. The dummy bit line DBL 0  may contain aluminum, copper, or the like. For example, the dummy bit line DBL 0  may be a lowest metal wiring layer. 
     A specified active region (a region corresponding to an output node NOUT) in the peripheral region may be connected to the dummy bit line DBLO by a contact  2456 . Unlike the illustration in  FIG. 22 , the contact  2456  may include a plurality of contacts that are stacked in a vertical direction. 
     The second resistor  2188  may be implemented in various ways. For example, referring to  FIG. 23 , the second resistor  2188  may be implemented by forming an active region in a zigzag pattern in the substrate  2410 . That is, the second resistor  2188  may be implemented in an active region between a region in which the output node NOUT is implemented and a region in which a seventh transistor  2157  is to be formed. When this zigzag pattern is repeated a number of times, a physical length of the active region can be increased. Thus, the resistance of the second resistor  2188  can be easily adjusted. 
       FIG. 24  is a layout diagram of a nonvolatile memory device according to a tenth embodiment of the present invention.  FIG. 25  is a perspective view of the nonvolatile memory device according to the tenth embodiment of the present invention. The illustrations in  FIGS. 24 and 25  may be for implementing the circuit (which further includes the bypass circuit  2190 ) illustrated in  FIG. 17 . 
     Referring to  FIG. 24 , a plurality of memory blocks BLK are arranged in a 3×3 matrix. Regions  2470 , in each of which a local column select circuit is disposed, may be located between the memory blocks BLK, and a conjunction region CNJ may be located between neighboring ones of the regions  2470 . 
     In the tenth embodiment of the present invention, a reference cell RC may be disposed in each memory block BLK, and a bypass circuit  2190  may be disposed in each region  2470  in which the local column select circuit is located or in the conjunction region CNJ. 
     In  FIG. 25 , the bypass circuit  2190  is implemented in each region  2470 . Referring to  FIG. 25 , an ohmic contact layer  2437  connected to the reference cell RC may extend from a memory block region to each of the regions  2470 . A node N 2  implemented in an active region, which is defined in each region  2470 , and the ohmic contact layer  2437  are connected to each other by a contact  2458 . 
     The bypass circuit  2190  is formed in each region  2470  and connected between the node N 2  and the word line WLk. 
       FIG. 26A  is a diagram for explaining a variable resistance memory device according to an eleventh embodiment of the present invention. 
     Referring to  FIG. 26A , the variable resistance memory device may include a cell array  2910 , a temperature compensation circuit  2940 , a data read circuit (for verification)  2930 , and a write circuit  2920 . 
     The memory cell array  2910  includes a plurality of resistance memory cells and is divided into a first area  2911  and a second area  2912  which are different from each other. That is, the memory cell array  2910  may be divided into a plurality of areas, depending on what it is used for. The first area  2911  may have a shorter response time and a faster access time than the second area  2912 . In addition, the first area  2911  may replace a main memory, and the second area  2912  may replace a hard disk drive (HDD). 
     As described above, the temperature compensation circuit  2940  may include one or more reference cells. 
     As described above, the data read circuit  2930  may perform a verifying-read operation by supplying a current, which changes according to the resistance of the reference cells, to one or more of the resistance memory cells. 
     The write circuit  2920  examines data provided by an external source and writes the data to the first area  2911  or the second area  2912  based on the examination result. The data provided by the external source may include a code, which indicates whether the data must be stored in the first area  2911  or the second area  2912 , in a data header. Therefore, the write circuit  2920  may examine the code and determine whether to write the data to the first area  2911  or the second area  2912 . 
     The second area  2912  may have a hierarchical structure described above in the first through fourth embodiments. The variable resistance memory device may include a global bit line, which is connected to a write circuit and a data read circuit, one or more local bit lines, which are coupled to the resistance memory cells disposed in the second area  2912 , and a plurality of column select transistors. Here, each of the column select transistors may selectively connect the global bit line to any one of the local bit lines. 
     The resistance of a column select transistor located far from the write circuit and the data read circuit may be smaller than that of a column select transistor located close to the write circuit and the data read circuit. Alternatively, the column select transistor located far from the write circuit and the data read circuit may be larger than the column select transistor located close to the write circuit and the data read circuit. The doping concentration of a channel region of the column select transistor located far from the write circuit and the data read circuit may be higher than that of a channel region of a column select transistor located close to the write circuit and/or the data read circuit. 
     The first area  2911  or the second area  2912  may include a temperature compensation circuit which includes a reference cell and which has been described above in the fifth through tenth embodiments. Therefore, a verifying-read operation may be performed by supplying a current, which changes according to the resistance of one reference cell, to one or more of the resistance memory cells. 
       FIG. 26B  is a flowchart illustrating a method of driving the nonvolatile memory devices according to the fifth and seventh embodiments of the present invention. 
     Referring to  FIGS. 14 and 26B , a memory cell array, which includes m bit-level cells (“m” is a natural number), and a reference set, which includes 2 m −1 reference cells, are provided. 
     The resistance of the reference cells is read. 
     The 2 m −1 reference cells included in one reference set are written such that the 2 m −1 reference cells respectively have different resistance distributions (operation S 2210 ). 
     For example, when the m bit-level cells are 2 bit-level cells, there may be three (=2 2 −1) reference cells, and the three reference cells may respectively store resistance values corresponding to resistance distributions Ref 1  through Ref 3 . 
     Data is written to the m bit-level cells (operation S 2220 ). 
     Then, a current, which changes according to the resistance of the 2 m −1 reference cells, is supplied to the m bit-level cells to verifying-read data from the m bit-level cells (operation S 2230 ). 
     For example, a compensation current, which changes according to a resistance value corresponding to each of the resistance distributions Ref 1  through Ref 3  sequentially, is supplied to the m bit-level cells in order to verifying-read data. 
     Application Embodiments 
       FIGS. 27 through 31  are diagrams for explaining storage systems according to first through fifth embodiments of the present invention. Specifically,  FIGS. 27 through 31  illustrate storage systems using the nonvolatile memory devices according to the fifth through tenth embodiments of the present invention. 
       FIG. 27  is a diagram illustrating a cellular phone system that uses nonvolatile memory devices according to embodiments of the present invention. 
     Referring to  FIG. 27 , the cellular phone system may include an adaptive differential pulse code modulation (ADPCM) codec circuit  2202  which compresses sound or decompresses compressed sound, a speaker  2203 , a microphone  2204 , a time division multiplex access (TDMA) circuit  2206  which time-division multiplexes digital data, a phase-locked loop (PLL) circuit  2210  which sets a carrier frequency of a wireless signal, and a radio frequency (RF) circuit  2211  which transmits or receives a wireless signal. 
     In addition, the cellular phone system may include various types of memory devices. For example, the cellular phone system may include a nonvolatile memory device  2207 , a read only memory (ROM)  2208 , and a static random access memory (SRAM)  2209 . The nonvolatile memory device  2207  may be any one of the nonvolatile memory devices according to the embodiments of the present invention and may store, for example, an identification (ID) number. The ROM  2208  may store programs, and the SRAM  2209  may serve as a work area for a system control microcomputer  2212  or temporarily store data. The system control microcomputer  2212  is a processor and may control a write operation and a read operation of the nonvolatile memory device  2207 . 
       FIG. 28  is a diagram illustrating a memory card which uses nonvolatile memory devices according to embodiments of the present invention. The memory card may be, for example, a multimedia card (MMC), a secure digital (SD) card, a multi-use card, a micro SD card, a memory stick, a compact SD card, an ID card, a personal computer memory card international association (PCMCIA) card, a solid-state drive (SSD) card, a chip card, a smart card, or a universal serial bus (USB) card. 
     Referring to  FIG. 28 , the memory card may include an interface part  2221  which interfaces with an external device, a controller part  2222  which includes a buffer memory and controls the operation of the memory card, and one or more nonvolatile memories  2207  according to embodiments of the present invention. The controller part  2222  is a processor and may control the write and read operations of the nonvolatile memories  2207 . Specifically, the controller part  2222  is coupled to each of the interface part  2221  and the nonvolatile memory devices  2207  by a data bus DATA and an address bus ADDRESS. 
       FIG. 29  is a diagram illustrating a digital still camera that uses nonvolatile memory devices according to embodiments of the present invention. 
     Referring to  FIG. 29 , the digital still camera includes a body  2301 , a slot  2302 , a lens  2303 , a display  2308 , a shutter button  2312 , and a strobe  2318 . In particular, a memory card  2307  may be inserted into the slot  2302  and include one or more nonvolatile memory devices PK 1  according to embodiments of the present invention. 
     If the memory card  2307  is of a contact type, it electrically contacts a specified electrical circuit on a circuit board when it is inserted into the slot  2302 . When the memory card  2331  is of a non-contact type, it communicates with the memory card  2307  using a wireless signal. 
       FIG. 30  is a diagram illustrating various systems that use the memory card of  FIG. 28 . 
     Referring to  FIG. 30 , a memory card  2331  may be used in (a) a video camera, (b) a television, (c) an audio device, (d) a game device, (e) an electronic music device, (f) a mobile phone, (g) a computer, (h) a personal digital assistant (PDA), (i) a voice recorder, and (j) a personal computer (PC) card. 
       FIG. 31  is a diagram illustrating an image sensor system that uses nonvolatile memory devices according to embodiments of the present invention. 
     Referring to  FIG. 31 , the image sensor system may include an imager  2332 , an input/output device  2336 , a random access memory (RAM)  2348 , a central processing unit (CPU)  2344 , and a nonvolatile memory device  2354  according to embodiments of the present invention. These components, i.e., the imager  2332 , the input/output device  2336 , the RAM  2348 , the CPU  2344 , and the nonvolatile memory device  2354  communicate with each other using a bus. The imager  2332  may include a photo sensing element such as a photogate or a photodiode. Each of the above components and a processor may be implemented as a single chip or separate chips. 
       FIG. 32  is a schematic block diagram of a portable media system including phase-change random access memory device according to embodiments of the present invention. In an embodiment of the present invention, a mobile phone as the portable media system is described by way of example, but the invention is not limited thereto. Rather, the invention may also be applied to a two-way communications system, a one-way pager, a two-way pager, a personal communications system, a portable computer, a personal data assistance (PDA), an MPEG audio layer-3 (MP3) player, a digital camera, and other electronic devices. 
     Referring to  FIG. 32 , the portable media system  2400  includes a controller  2410 , a memory unit  2420 , an I/O device  2430 , and a transceiver  2440 . 
     The controller  2410  may, for example, include microprocessors, digital signal processors, microcontrollers, and the like. 
     The memory  2420  stores messages transmitted to the portable media system  2400  or an external device. That is to say, the memory  2420  stores data or instructions executed by the controller while the portable media system  2400  is operating. The memory  2420  is composed of one or more different kinds of memories. For example, the memory  2420  may be a volatile memory device, or a nonvolatile memory device such as a flash memory device and/or a phase-change memory device. Here, usable examples of the phase-change memory device include PRAMs according to preferred embodiments of the present invention. 
     In particular, the most challenging task of the portable media system  2400  is to minimize the amount of current consumed. As described in the foregoing embodiments of the present invention, the current and power consumption can be minimized by decreasing resistance of a column select transistor in a phase-change memory cell far away from a write and/or read circuit, thereby improving reliability during write and/or read operation. 
     The portable media system  2400  may transmit or receive messages in a wireless manner through the transceiver  2440  connected to an antenna (not shown). Here, the portable media system  2400  may transmit or receive messages using protocols such as CDMA (Code Division Multiple Access), GSM (Global System for Mobile communication), NADC (North 20 American Digital Cellular), TDMA (Time Division Multiple Access), ETDMA (Extended TDMA), third-generation WCDMA(Wideband CDMA), CDMA-2000, etc. 
     The I/O device  2430  generates a message by user&#39;s manipulation. The I/O device  2430  may comprise a keypad, a monitor, and the like. 
     A phase-change random access memory device of the present invention provides at least one of the following advantages. First, the phase-change random access memory device of the present invention can prevent failures during write and/or read operations. Second, the phase-change random access memory device provides a phase-change memory cell in which a reset state can be written surely, and whereby the resistance margin of the phase-change memory cells increase by reducing a distribution of set resistance, which improves the reliability of phase-change memory cells. Third, the phase-change random access memory device can decrease the level of write current and/or read current, thereby reducing current consumption. 
       FIG. 33  illustrates an example embodiment of an application of the semiconductor device. As shown, this embodiment includes a memory  3010  connected to a memory controller  3020 . The memory  3010  may be any of the semiconductor device embodiments described above. The memory controller  3020  supplies the input signals for controlling operation of the memory  3010 . For example, the memory controller  3020  supplies the command CMD and address signals. The memory controller  3020  may include a memory interface, a host interface, an ECC circuit, a central processing unit (CPU), and a buffer memory. The memory interface provides data transmitted from the buffer memory to the memory  3010  or transmits data read out of the memory  3010  to the buffer memory. Also the memory interface may provide a command or an address transmitted from an external host to the memory  3010 . 
     The host interface may communicate with an external host through USB (Universal Serial Bus), SCSI (Small Computer System Interface), PCI express, ATA, PATA (Parallel ATA), SATA (Serial ATA), SAS (Serial Attached SCSI). 
     A memory system according to embodiments of the present invention may include an ECC circuit, which generates a parity bit using data transmitted to a memory  3010 . The generated parity bit may be stored in a specific area of the memory  3010 , together with data. The ECC circuit detects an error of data read out of the memory  3010 . If the detected error is within a correction range, the ECC circuit corrects the detected error. 
     The CPU processes a signal input from the external host after analyzing the input signal. The CPU controls the external host or the memory  3010  through the host interface or the memory interface. The CPU may control write, read, and erase operations depending on firmware for driving a memory. 
     The buffer memory temporarily stores write data provided from the external host or data read out of the memory  3010 . Also the buffer memory may store meta data or cache data to stored in the memory  3010 . During a sudden power-off operation, meta data or cache data stored in the buffer memory may be stored in the memory  3010 . The buffer memory may include a DRAM and an SRAM. 
       FIG. 34  illustrates yet another embodiment. This embodiment is the same as the embodiment of  FIG. 33 , except that the memory  3010  and memory controller  3020  have been embodied as a card  3130 . For example, the card  3130  may be a memory card such as a flash memory card. Namely, the card  3130  may be a card meeting any industry standard for use with a consumer electronics device such as a digital camera, personal computer, etc. It will be appreciated that the memory controller  3020  may control the memory  3010  based on controls signals received by the card  3130  from another (e.g., external) device. 
       FIG. 35  illustrates a still further embodiment of the present invention. As shown, the memory  3010  may be connected with a host system  3210 . The host system  3210  may be a processing system such as a personal computer, digital camera, etc. The host system  3210  may use the memory  3010  as a removable storage medium. As will be appreciated, the host system  3210  supplies the input signals for controlling operation of the memory  3010 . For example, the host system  3210  supplies the command CMD and address signals. 
       FIG. 36  illustrates an embodiment of the present invention in which the host system  3210  is connected to the card  3130 . In this embodiment, the host system  3210  applies control signals to the card  3130  such that the memory controller  3020  controls operation of the memory  3010 . 
       FIG. 37  illustrates a further embodiment of the present invention. As shown, the memory  3010  may be connected to a central processing unit (CPU)  3120  within a computer system  3410 . For example, the computer system  3410  may be a personal computer, personal data assistant, etc. The memory  3010  may be directly connected with the CPU  3120 , connected via a bus, etc. It will be appreciated, that  FIG. 37  does not illustrate the full complement of components that may be included within a computer system  3410  for the sake of clarity. 
     A semiconductor device according to some embodiments of the present invention may be used as a storage class memory (SCM), which is the general concept of memories capable of simultaneously providing nonvolatile characteristics and access characteristics. The storage class memory may be utilized as not only a data storage region but also a program performing region. Also, the storage class memory may be utilized as not only a main memory but also a mass storage. 
     Also, the memory cell array of some embodiments of the present invention may be data storage region. 
     The above-described PRAM, FeRAM, and MRAM may be appropriate examples of a storage class memory. Such a storage class memory may be used as not only a data storage memory instead of a flash memory but also a main memory instead of an SRAM. Moreover, one storage class memory may be used instead of a flash memory and an SRAM. 
       FIG. 38  illustrates an exemplary configuration of a memory system  4100  in which a storage class memory (SCM) is used instead of a storage class memory. As illustrated, the memory system  4100  includes a CPU  4110 , an SDRAM  4120 , and an SCM  4130  used instead of a flash memory. 
     In the memory system  4100 , data access speed of the SCM  4130  is higher than that of a flash memory. For example, under a PC environment where the CPU  4110  runs at 4 GHz, data access speed of a PRAM which is a kind of the SCM  4130  is about 32 times higher than that of a flash memory. Thus, the memory system  4100  equipped with the SCM  4130  may attain higher-speed access gain than a memory system equipped with a flash memory. 
       FIG. 39  illustrates an exemplary configuration of a memory system  4200  in which an SCM is used instead of an SDRAM. As illustrated, the memory system  4200  includes a CPU  4210 , an SCM  4220 , and a flash memory  4230 . The SCM  4220  is used as a main memory instead of an SDRAM. 
     In the memory system  4200 , power dissipation of the SCM  4220  is lower than that of an SDRAM. Energy dissipated by a main memory of a computer system amounts to about 40 percent of total energy. Therefore, many efforts have been intensively made to reduce power dissipation of a main memory. An SCM may reduce dynamic energy dissipation to an average of about 53 percent and reduce energy dissipation caused by power leakage to an average of about 73 percent. As a result, the memory system  4200  equipped with the SCM  4220  may allow power dissipation to be reduced more than a memory system equipped with an SDRAM. 
       FIG. 40  illustrates an exemplary configuration of a memory system  4300  in which an SCM is substituted for an SDRAM as well as a flash memory. As illustrated, the memory system  4300  includes a CPU  4310  and an SCM  4320 . The SCM  4320  is used as a main memory instated of an SDRAM and as a data storage memory instead of a flash memory. This memory system  4300  is advantageous in data access speed, low power, space utilization, and costs. 
     A variable resistance memory device according to an embodiment of the present invention can be applied to a server SSD. U.S Patent Publication Nos. 2008/0256292, 2008/0256183, and 2008/0168304 disclose in  FIG. 1A  and the corresponding detailed description thereof that a solid-state storage  110  includes a PRAM, a flash, an MRAM, an NRAM, and a DRAM and that a solid-state storage device  102  includes a solid-state storage controller  104  and a solid-state storage  110 . In addition, U.S Patent Publication Nos. 2008/0256292, 2008/0256183, and 2008/0168304 disclose a solid-state memory and a controller, which process the speed of a high-speed interface, and a redundant array of independent drives (RAIDs) in a solid-state device, and are incorporated by reference herein in their entirety. 
     Meanwhile, U.S. Pat. No. 6,242,605, U.S Patent Publication Nos. 2007/0236987 and 2008/0123389, and U.S. patent application Ser. No. 12/461,036, and Korean Patent Application No. 2009-0066990 are incorporated and combined in the present specification. 
     Although the invention has been described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that various modifications and changes may be made thereto without departing from the scope of the present invention. Therefore, it should be understood that the above embodiments are not limitative, but illustrative in all aspects. The scope of the invention is defined by the appended claims, and all changes and modifications that fall within meets and bounds of the claims, or equivalents of such meets and bounds are therefore intended to be embraced by the claims.