Abstract:
Memory systems can include a memory device having an array of nonvolatile memory cells therein, which is electrically coupled to a plurality of bit lines and a plurality of word lines. The nonvolatile memory cells may include respective nonvolatile resistive devices electrically coupled in series with corresponding cell transistors. A controller is also provided, which may be coupled to the memory device. The controller can be configured to drive the memory device with signals that support dual programming of: (i) the nonvolatile resistive devices; and (ii) interface states within the cell transistors, during operations to write data into the memory device.

Description:
REFERENCE TO PRIORITY APPLICATION 
       [0001]    The present application is a divisional application of U.S. patent application Ser. No. 14/337,313, filed on Jul. 22, 2014, and claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2013-0149391 filed on Dec. 3, 2013, the disclosure of which is hereby incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    Embodiments of the inventive concept relate to semiconductor memory devices and, more particularly, to resistive memory devices. 
         [0004]    2. Description of Related Art 
         [0005]    Semiconductor memory devices, which are used to store data, may be classified into volatile semiconductor memory devices and nonvolatile semiconductor memory devices. A volatile semiconductor memory device may store data through charge or discharge of a capacitor, for example. A volatile semiconductor memory device, such as a random access memory (RAM), may store or read data when power supply is applied and lose data when power supply is interrupted. The volatile semiconductor memory device may be mainly used as a main memory device of a computer. 
         [0006]    A nonvolatile semiconductor memory device may store data even if power supply is interrupted. The nonvolatile semiconductor memory device may be used to store programs and data for a wide range of applications, such as computers and portable communication devices. 
         [0007]    Owing to the demand for fabrication of high-capacity low-power semiconductor memory devices, research has been conducted on advanced nonvolatile memory devices that do not require refresh operations. Currently, phase-change RAMs (PRAMs) using phase-change materials, resistive RAMs (RRAMs) using variable resistive materials, such as transition metal oxides, and magnetic RAMs (MRAMs) using ferromagnetic materials have attracted considerable attention as advanced memory devices. All of the advanced memory devices may have variable resistances according to applied currents or voltages and may not need refresh operations due to nonvolatile characteristics so that even if the currents or voltages are no longer applied, the advanced memory devices can retain the resistance value as it is. 
       SUMMARY 
       [0008]    Memory systems according to embodiments of the invention can include a memory device having an array of nonvolatile memory cells therein, which is electrically coupled to a plurality of bit lines and a plurality of word lines. The nonvolatile memory cells may include respective nonvolatile resistive devices electrically coupled in series with corresponding cell transistors. A controller is also provided, which may be coupled to the memory device. The controller can be configured to drive the memory device with signals that support dual programming of the nonvolatile resistive devices and interface states within the cell transistors, during operations to write data into the memory device. 
         [0009]    According to some of these embodiments of the invention, the operations to write data into the memory device may include writing equivalent data into first and second nonvolatile memory cells by programming first and second cell transistors within the first and second nonvolatile memory cells, respectively, with unequal levels of interface states. In particular, the operations to write equivalent data into first and second nonvolatile memory cells can include programming first and second nonvolatile resistive devices within the first and second nonvolatile memory cells, respectively, in advance of the operations to program the first and second cell transistors with unequal levels of interface states. The operations to program the first and second nonvolatile resistive devices can also be followed by operations to determine whether the first and second nonvolatile resistive devices have been programmed to equivalent target values. Moreover, in the event the first and second cell transistors are MOS transistors, then they may be programmed with unequal levels of interface states via at least one of hot-carrier injection and Fowler-Nordheim (F-N) tunneling. 
         [0010]    According to still further embodiments of the invention, methods of programming a nonvolatile memory device may include programming first and second nonvolatile memory elements within first and second nonvolatile memory cells, respectively, with equivalent data, and then reading data from the first and second nonvolatile memory cells to verify whether the first and second nonvolatile memory elements have been programmed to equivalent target values. Thereafter, first and second cell transistors within the first and second nonvolatile memory cells, respectively, may be programmed with unequal levels of interface states. In particular, the programming of the first and second cell transistors can be performed in response to determining that the first and second nonvolatile memory elements have been programmed to unequal target values. According to further embodiments of the invention, the programming of the first and second cell transistors can include programming the first and second cell transistors so that a combined series resistance of the first nonvolatile memory element and the first cell transistor is about equal to a combined series resistance of the second nonvolatile memory element and the second cell transistor. In this manner, the unequal series resistance values provided by the first and second cell transistors may be used to compensate for the fact that the series resistances provided by the first and second nonvolatile memory elements are unequal, notwithstanding the fact that the first and second nonvolatile memory cells were to be programmed with equivalent data during write operations. 
         [0011]    Additional embodiments of the inventive concept provide a resistive memory device capable of compensating resistance distribution and increasing sensing margin of data and a method of compensating resistance distribution of a resistive memory device capable of compensating resistance distribution and increasing sensing margin of data. In accordance with an aspect of the inventive concept, a resistive memory device may include a memory cell array and an input/output (I/O) sense amplifier. 
         [0012]    The memory cell array includes a plurality of unit memory cells. The memory cell array operates in response to a word line driving signal and a column selecting signal. Each of the unit memory cells includes a resistive device and a cell transistor connected in series. The I/O sense amplifier senses and amplifies data output from the memory cell array to output the amplified data, generates a program current based on input data received from an exterior, and provides the program current to the memory cell array. The resistive memory device reads out output data of the I/O sense amplifier, and adjusts interface states of cell transistors included in the memory cell array based on a voltage level of the output data in a test mode. 
         [0013]    In an embodiment, resistance distributions of the resistive device and a parasitic resistor may be compensated by resistance distribution of the cell transistor. 
         [0014]    In an embodiment, resistance values of the cell transistors may be changed by adjusting interface states of the cell transistors. 
         [0015]    In an embodiment, the interface states may be generated because of stress-related degradation by Fowler-Nordheim (F-N) tunneling. 
         [0016]    In an embodiment, a high voltage may be applied to each gate terminal of the cell transistors, and a ground voltage may be applied to each source terminal and each drain terminal of the cell transistors. 
         [0017]    In an embodiment, the interface states may be generated by hot-carrier injection. 
         [0018]    In an embodiment, a first high voltage may be applied to each gate terminal of the cell transistors, a ground voltage may be applied to each source terminal of the cell transistors, and a second high voltage may be applied to each drain terminal of the cell transistors. The second high voltage may have a higher voltage than the first high voltage. 
         [0019]    In an embodiment, a first high voltage may be applied to each gate terminal of the cell transistors, a ground voltage may be applied to each drain terminal of the cell transistors, and a second high voltage may be applied to each source terminal of the cell transistors. The second high voltage may have a higher voltage than the first high voltage. 
         [0020]    In an embodiment, the resistive memory device may further include an address input buffer, a row decoder and a column decoder. 
         [0021]    The address input buffer generates a row address signal and a column address signal based on an external address. The row decoder decodes the row address signal and generates the word line driving signal based on the decoded row address signal. The column decoder decodes the column address signal and generates the column selecting signal based on the decoded column address signal. 
         [0022]    In accordance with another aspect of the inventive concept, a resistive memory device may include a memory cell array, an address input buffer, a row decoder, a column decoder, and an input/output (I/O) sense amplifier. 
         [0023]    The memory cell array includes a plurality of unit memory cells, and operates in response to a word line driving signal and a column selecting signal. Each of the unit memory cells includes a resistive device and a cell transistor connected in series. The address input buffer generates a row address signal and a column address signal based on an external address. The row decoder decodes the row address signal and generates the word line driving signal based on the decoded row address signal. The column decoder decodes the column address signal and generates the column selecting signal based on the decoded column address signal. The I/O sense amplifier senses and amplifies data output from the memory cell array to output the amplified data, generates a program current based on input data received from an exterior, and provides the program current to the memory cell array. The memory device reads out output data of the I/O sense amplifier, and adjusts interface states of cell transistors included in the memory cell array based on a voltage level of the output data in a test mode. 
         [0024]    In an embodiment, each of the unit memory cells may include a cell transistor having a gate connected to a word line, and the resistive device connected between a drain of the cell transistor and a bit line. 
         [0025]    In an embodiment, the I/O sense amplifier may include a selection circuit, a sense amplifier, a verifying circuit, a program driver and a write driver. 
         [0026]    The selection circuit selectively connects the bit lines to a first node in response to the column selecting signal. The sense amplifier amplifies a difference between a voltage signal of the first node and a reference voltage to generate first data. The verifying circuit compares the first data with a target value, and determines whether the first data equals to the target value to generate a first control signal in the test mode. The program driver drives a high voltage circuit based on the first control signal to generate a word line control signal and a bit line control signal. The write driver generates a first program current based on write data, provides the program current to the first node in a normal operating mode, generates a second program current based on the bit-line control signal, and provides the second program current to the first node in the test mode. 
         [0027]    In an embodiment, the row decoder may change a voltage level of the a word line driving signal based on the word line control signal in the test mode. 
         [0028]    In an embodiment, the I/O sense amplifier may further include a switch configured to selectively connect the first node to the sense amplifier and the write driver. 
         [0029]    In an embodiment, a magnitude of the second program current may be changed according to a voltage level of the first control signal. 
         [0030]    In an embodiment, the resistive memory device may further include an input/output (I/O) circuit configured to determine an output order of the first data and perform a parallel-serial conversion to generate output data, and configured to buffer the input data to provide the buffered data to the I/O sense amplifier. 
         [0031]    In accordance with still another aspect of the inventive concept, a method of compensating resistance distribution of a resistive memory device including a plurality of resistive devices and a plurality of cell transistors connected to each of the resistors in series may include writing data of “0” or “1” with respect to all the memory cells included in a memory cell array; selecting a memory cell from which data is read out; reading out data from the selected memory cell; determining whether the read data equals to a target value; ending a process when the data is equal to the target value, and adjusting an interface states of cell transistors when the data is not equal to the target value. 
         [0032]    In an embodiment, the adjusting of the interface states of cell transistors may include driving a program driver, selecting a memory cell for which interface state is to be adjusted, and giving stress with respect to a cell transistor of the selected memory cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The foregoing and other features and advantages of the inventive concepts will be apparent from the more particular description of preferred embodiments of the inventive concepts, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the inventive concepts. In the drawings: 
           [0034]      FIG. 1  is a block diagram illustrating a resistive memory device in accordance with an embodiment of the inventive concept; 
           [0035]      FIG. 2  is a circuit diagram illustrating an example of a memory cell array and an input/output (I/O) sense amplifier included in the resistive memory device of  FIG. 1 ; 
           [0036]      FIG. 3  is a circuit diagram illustrating an example of an I/Q sense amplifier included in  FIG. 2 ; 
           [0037]      FIG. 4  is a circuit diagram illustrating an example of a unit memory cell constituting the memory cell array of  FIG. 2 ; 
           [0038]      FIG. 5  is a diagram illustrating an example of a resistive device constituting the unit memory cell of  FIG. 4 ; 
           [0039]      FIG. 6  is a diagram illustrating another example of a resistive device constituting the unit memory cell of  FIG. 4 ; 
           [0040]      FIG. 7  is a diagram illustrating still another example of a resistive device constituting the unit memory cell of  FIG. 4 ; 
           [0041]      FIGS. 8 and 9  are diagrams illustrating magnetization directions of a magnetic tunnel junction (MTJ) element according to written data; 
           [0042]      FIG. 10  is a diagram illustrating a write operation of the resistive memory device of  FIG. 1  when a unit memory cell is a magnetoresistive memory cell; 
           [0043]      FIG. 11  is a conceptual diagram illustrating a method of compensating resistance distribution of the resistive memory device of  FIG. 1 ; 
           [0044]      FIGS. 12 to 14  are diagrams illustrating methods of generating interface states of a cell transistor in the resistive memory device of  FIG. 1 ; 
           [0045]      FIG. 15  is a conceptual diagram for explaining a method of compensating resistance distribution of the resistive device included in the memory cell array of  FIG. 2 ; 
           [0046]      FIG. 16  is a diagram illustrating a sensing margin of a resistive device included in the memory cell array of  FIG. 2  before and after resistance distribution is compensated; 
           [0047]      FIG. 17  is a diagram illustrating range of a magnitude of a write current for programming a resistive device to compensate distribution of the resistive device included in the memory cell array of  FIG. 2 ; 
           [0048]      FIG. 18  is a flowchart illustrating a method of compensating resistance distribution of a resistive memory device, in accordance with an embodiment of the inventive concept; 
           [0049]      FIG. 19  is a flowchart illustrating a method of adjusting the interface states of cell transistors in the method of compensating resistance distribution of the resistive memory device of  FIG. 18 ; 
           [0050]      FIG. 20  is a diagram illustrating a memory module including resistive memory devices according to embodiments of the inventive concept; 
           [0051]      FIG. 21  is a simplified perspective view of a semiconductor device having a stack structure including a resistive memory device according to embodiments of the inventive concept; 
           [0052]      FIG. 22  is a block diagram of an example of a memory system including a resistive memory device according to embodiments of the inventive concept; 
           [0053]      FIG. 23  is a block diagram of an example of a memory system including a resistive memory device and an optical link according to an embodiment of the inventive concept; and 
           [0054]      FIG. 24  is a block diagram of an example of an information processing system including a resistive memory device according to an embodiment of the inventive concept. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0055]    Embodiments of the inventive concept are described below in sufficient detail to enable those of ordinary skill in the art to embody and practice the inventive concept. It is important to understand that the inventive concept may be embodied in many alternate forms and should not be construed as limited to the embodiments set forth herein. 
         [0056]    Accordingly, while the inventive concept is susceptible to various modifications and may take on various alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the inventive concept to the particular forms disclosed. On the contrary, the inventive concept is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the claims. 
         [0057]    It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the inventive concept. 
         [0058]    It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (i.e., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
         [0059]    The terminology used herein to describe embodiments of the inventive concept is not intended to limit the scope of the inventive concept. The articles “a,” “an,” and “the” are singular in that they have a single referent, however the use of the singular form in the present document should not preclude the presence of more than one referent. In other words, elements of the inventive concept referred to in the singular may number one or more, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, items, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, items, steps, operations, elements, components, and/or groups thereof. 
         [0060]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. It should also be noted that in some alternative implementations, the functions/acts noted in the blocks may occur out of the order noted in the flowcharts. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. The inventive concept will now be described more fully with reference to the accompanying drawings, in which embodiments of the inventive concept are shown. 
         [0061]      FIG. 1  is a block diagram of a resistive memory device  1000  in accordance with an embodiment of the inventive concept. Referring to  FIG. 1 , the resistive memory device  1000  may include a command decoder  1050 , an address input buffer  1100 , a row decoder  1200 , a column decoder  1300 , a memory cell array  1400 , an input/output (I/O) sense amplifier  1500  and an I/O circuit  1600 . The command decoder  1050  decodes a chip selecting signal CSB, a row address strobe signal RASB, a column address strobe signal CASB, a write enable signal WEB, and a clock enable signal CKE to generate various control signals and controls circuit blocks included in the resistive memory device  1000 . 
         [0062]    The memory cell array  1400  includes a plurality of unit memory cells including a resistive device  1402  and a cell transistor  1404 , and operates in response to a word line driving signal WL and a column selecting signal CSL. The address input buffer  1100  generates a row address signal ADDR_X and a column address signal ADDR_Y based on an external address ADDR. The row decoder  1200  decodes the row address signal ADDR_X and generates the word line driving signal WL based on the decoded row address signal. Further, the row decoder  1200  may change a voltage level of the a word line driving signal WL in the test mode. The column decoder  1300  decodes the column address signal ADDR_Y and generates the column selecting signal CSL based on the decoded column address signal. 
         [0063]    The I/O sense amplifier  1500  senses and amplifies data output from the memory cell array  1400  to generate a first data, generates a program current based on input data DIN received from an exterior, and provides the program current to the memory cell array  1400 . The I/O circuit  1600  determines an output order of the first data, and performs a parallel-serial conversion to generate output data DOUT, buffers the input data DIN and provides the buffered input data to the I/O sense amplifier  1500 . The resistive memory device  1000  reads out output data of the I/O sense amplifier, and adjusts interface states of cell transistors included in the memory cell array based on a voltage level of the output data in a test mode. 
         [0064]    Resistance distributions of the resistor and a parasitic resistive device may be compensated by resistance distribution of the cell transistor. Resistance values of the cell transistors may be changed by adjusting interface states of the cell transistors. 
         [0065]      FIG. 2  is a circuit diagram of an example of a memory cell array  1400  and an I/O sense amplifier  1500  included in the resistive memory device  1000  of  FIG. 1 . Referring to  FIG. 2 , the memory cell array  1400  may include a plurality of word lines WL 1  to WLm and a plurality of bit lines BL 1  to BLn, and a unit memory cell may be provided between each of the word lines WL 1  to WLm and the corresponding one of the bit lines BL 1  to BLn. 
         [0066]    The memory cell array  1400  may include cell transistors MN 11  to MN 1   n  having gates connected to the word line WL 1 , and resistive devices R 11  to R 1   n  connected between each of the cell transistors MN 11  to MN 1   n  and each of the bit lines BL 1  to BLn. A source of each of the cell transistors MN 11  to MN 1   n  may be connected to the source line SL. Further, the memory cell array  1400  may include cell transistors MN 21  to MN 2   n  having gates connected to the word line WL 2 , and resistive devices R 21  to R 2   n  connected between each of the cell transistors MN 21  to MN 2   n  and each of the bit lines BL 1  to BLn. A source of each of the cell transistors MN 21  to MN 2   n  may be connected to the source line SL. Further, the memory cell array  1400  may include cell transistors MNm 1  to MNmn having gates connected to the word line WLm, and resistive devices Rm 1  to Rmn connected between each of the cell transistors MNm 1  to MNmn and the corresponding one of the bit lines BL 1  to BLn. Sources of the cell transistors MNm 1  to MNmn may be connected to the source line SL. The I/O sense amplifier  1500  may be connected to bit lines BL 1  to BLn, and senses and amplifies data output from the memory cell array  1400  and output the amplified data, generates a program current based on input data DI received from an exterior, and provides the program current to the memory cell array  1400 . 
         [0067]      FIG. 3  is a circuit diagram illustrating an example of an I/O sense amplifier  1500  included in  FIG. 2 . The I/O sense amplifier  1500  may include a selection circuit  1520 , a sense amplifier  1540 , a write driver  1530 , a switch  1550 , a verifying circuit  1560  and a program driver  1570 . The selection circuit  1520  may selectively connect the bit lines BL 1 -BLn to a first node N 1  in response to column selection signals CSL 1  to CSLn. The sense amplifier  1540  may amplify a difference between a voltage signal of the first node N 1  and a reference voltage VREF, and generate first data DO. The verifying circuit  1560  compares the first data DO with a target value, determines whether the first data DO equals to the target value, and generates a first control signal in the test mode. The program driver  1570  drives a high voltage circuit based on the first control signal to generate a word line control signal CON_WL and a bit line control signal CON_BL. The write driver  1530  generates a first program current based on write data DI and provides the first program current to the first node N 1  during a normal operation mode. Further, the write driver  1530  generates a second program current based on the bit line control signal CON_BL, and provides the second program current to the first node N 1  in the test operation. The switch  1550  may selectively connect the first node N 1  to the sense amplifier  1540  and the write driver  1530 . 
         [0068]      FIG. 4  is a circuit diagram illustrating an example of a unit memory cell constituting the memory cell array  1400  of  FIG. 2 . Referring to  FIG. 4 , a unit memory cell  1411  may include a cell transistor M 11  comprised of an NMOS transistor and a resistive device R 11 . The cell transistor Ml  1  may have a gate connected to the word line WL 1  and a source connected to the source line SL. The resistive device R 11  is connected between the drain of the cell transistor M 11  and the bit line BL 1 . 
         [0069]      FIG. 5  is a diagram illustrating an example of a resistive device R 11  constituting the unit memory cell of  FIG. 4 . Referring to  FIG. 5 , the resistive device R 11  includes a top electrode TE, a bottom electrode BE, and a transition metal oxide VR disposed between the top electrode TE and the bottom electrode BE. Platinum (Pt) may used as the top electrode TE and a cobalt oxide may be used as the transition metal oxide VR. 
         [0070]      FIG. 6  is a diagram illustrating another example of a resistive device R 11  constituting the unit memory cell  1411  of  FIG. 4 . Referring to  FIG. 6 , the resistive device R 11  includes a top electrode TE, a bottom electrode BE, and a phase change material GST disposed between the top electrode TE and the bottom electrode BE. The phase change material GST can transition between an amorphous state or a crystalline state, and a resistance value of the phase change material GST is varied according to temperature and heating time. An example of the phase change material GST is Ge x Sb y Te z . 
         [0071]      FIG. 7  is a diagram illustrating still another example of a resistive device R 11  constituting the unit memory cell  1411  of  FIG. 4 . Referring to  FIG. 7 , the resistive device R 11  may include a pinned layer (PL) having a predetermined pinned magnetization direction, a free layer (FL) magnetized in the direction of an externally applied magnetic field, and a tunnel barrier layer (BL) formed using an insulating film between the pinned layer PL and the free layer FL. To pin the magnetization direction of the pinned layer PL, the resistive device R 11  may further include an anti-ferromagnetic layer (not shown). The resistive device R 11  of  FIG. 7  may be an MTJ element constituting a spin-transfer torque magneto-resistive random access memory (STT-MRAM). 
         [0072]    To enable a write operation of the STT-MRAM, the cell transistor MN 11  may be turned on by applying a logic-high voltage to the word line WL 1 , and a write current may be applied between the bit line BL 1  and the source line SL. To enable a read operation of the STT-MRAM, the cell transistor MN 11  may be turned on by applying a logic-high voltage to the word line WL 1 , and a read current may applied from the bit line BL 1  toward a source line SL so that data stored in the MTJ cell may be determined according to a measured resistance value. 
         [0073]      FIGS. 8 and 9  are diagrams illustrating magnetization directions of a magnetic tunnel junction (MTJ) element relative to written data, where “I” designates the direction of a read current and “e” designates the direction of electron flow. A resistance value of the MTJ element may vary according to magnetization direction of the free layer FL. When a read current I is supplied to the MTJ element, a data voltage may be output according to the resistance value of the MTJ element. Since the read current I has a much lower intensity than a write current, the magnetization direction of the free layer FL may not be shifted by the read current I. Referring to  FIG. 8 , in the MTJ element, the magnetization direction of the free layer FL may be parallel to the magnetization direction of the pinned layer PL. Accordingly, the MTJ element may have a low resistance value. In this case, data ‘0’ may be read. Referring to  FIG. 9 , in the MTJ element, magnetization of the free layer FL may be anti-parallel to the magnetization direction of the pinned layer PL. In this case, the MTJ element may have a high resistance value. In this case, data ‘1’ may be read. Although  FIGS. 8 and 9  illustrate that each of the free layer FL and the pinned layer PL of the MTJ cell is a horizontal magnetic element, the inventive concept may provide other embodiments in which each of the free layer FL and the pinned layer PL is a vertical magnetic element. 
         [0074]      FIG. 10  is a diagram illustrating a write operation of the resistive memory device of  FIG. 1  when a unit memory cell is a magnetoresistive memory cell. Referring to  FIG. 10 , the magnetization direction of a free layer FL may be determined according to directions of write currents WC 1  and WC 2  flowing through an MTJ element. For example, when the first write current WC 1  is applied, free electrons having the same spin direction as a pinned layer PL may apply torque to the free layer FL. Thus, the free layer FL may be magnetized from anti-parallel (AP) to parallel (P) (i.e., AP→P) to the pinned layer PL. When the second write current WC 2  is applied, electrons having an opposite spin direction to the pinned layer PL may return to the free layer FL and apply torque. As a result, the free layer FL may be magnetized from parallel to anti-parallel (i.e., P→AP) to the pinned layer PL. That is, in an MTJ cell, the magnetization direction of the free layer FL may be shifted due to spin-transfer torque (STT). 
         [0075]      FIG. 11  is a conceptual diagram illustrating a method of compensating resistance distribution of the resistive memory device  1000  of  FIG. 1 , where the y-axis label “CUM. PROB.” designates “Cumulative Probability,” the abbreviation “RES. DISTRIBUTION” designates “Resistance Distribution,” and the abbreviation “MTJ+PARA” designates “Magnetic Tunnel Junction with Parasitic Resistor”. Referring to  FIG. 11 , the resistive memory device according to embodiments of the inventive concept may compensate for resistance distributions of the resistive device (MTJ) and a parasitic resistor using resistance distribution of the cell transistor. The resistance distribution of the resistive memory device may be nearly “0” after the compensation is completed. 
         [0076]      FIGS. 12 to 14  are diagrams illustrating methods of generating interface states of a cell transistor in the resistive memory device of  FIG. 1 . Referring to  FIG. 12 , the interface states of the cell transistor may be generated because of stress deterioration by Fowler-Nordheim (F-N) tunneling (i.e., Fowler-Nordheim stress (FN STRESS)). A high voltage may be applied to a gate terminal G of the cell transistor, and a ground voltage may be applied to a source terminal S and a drain terminal D of the cell transistor. In the example of  FIG. 12 , a voltage of 5V may be applied to the gate terminal G and the ground voltage may be applied to the source terminal S and the drain terminal D of the cell transistor. 
         [0077]    Referring to  FIG. 13 , the interface states of the cell transistor may be generated by hot-carrier injection. A first high voltage may be applied to a gate terminal G of the cell transistor, the ground voltage may be applied to a source terminal S of the cell transistor, and a second high voltage may be applied to a drain terminal D of the cell transistor. The second high voltage may have higher voltage level than the first high voltage. In the example of  FIG. 13 , a voltage of 2 to 3V may be applied to the gate terminal G and the ground voltage may be applied to the source terminal S and a voltage of 5V may be applied to the drain terminal D of the cell transistor. 
         [0078]    Referring to  FIG. 14 , the interface states of the cell transistor may be generated (or adjusted) by hot-carrier injection, and a first high voltage may be applied to a gate terminal G of the cell transistor, the ground voltage may be applied to a drain terminal D of the cell transistor, and a second high voltage having higher voltage level than the first high voltage may be applied to a source terminal S of the cell transistor. In the example of  FIG. 14 , a voltage of 2 to 3V may be applied to the gate terminal G and the ground voltage may be applied to the drain terminal D and a voltage of 5V may be applied to the source terminal S of the cell transistor. 
         [0079]      FIG. 15  is a conceptual diagram for explaining a method of compensating resistance distribution of the resistive device included in the memory cell array  1400  of  FIG. 2 . Referring again to  FIGS. 2 and 3 , distances from each of the resistive devices included in the memory cell array  1400  to the sense amplifier  1540  may not be the same, and resistive distribution of the resistive devices themselves may not be the same. Therefore, the sensing margin of the sense amplifier  1540  for sensing output data may be decreased. The memory cell array  1400  of  FIG. 2  may decrease the resistance distribution of each of the resistive memory cells included in the memory cell array  1400  sensed in an input terminal of the sense amplifier  1540  by adjusting interface states of the cell transistors. 
         [0080]    In  FIG. 15 , R_D 0  denotes resistance distribution of the resistor, and R_OFFSET denotes resistance distribution of the compensation resistor for cancelling resistance deviation between the resistors. Further, R_EFF denotes an effective resistance value measured at the input terminal of the sense amplifier  1540  after the resistance deviation between the resistors is cancelled. In  FIG. 15 , it may be noted that the distribution of resistance R_P 1  is cancelled by resistance CR_P 1  of a cell transistor, the distribution of resistance R_P 2  is cancelled by resistance CR_P 2  of a cell transistor, and the distribution of a resistor R_P 3  is cancelled by resistance CR_P 3  of a cell transistor. Referring to  FIG. 15 , distribution of effective resistance of the resistive devices included in the memory cell array  1400  of the resistive memory device  1000  is near “0”. 
         [0081]      FIG. 16  is a diagram illustrating a sensing margin (SM) of a resistive device included in the memory cell array  1400  of  FIG. 2  before and after resistance distribution is compensated. Referring to  FIG. 16 , it may be noted that the sensing margin, that is a resistance value between a resistance value R_D 0  of data “0” and a resistance value R_D 1  of data “1”, is increased to an effective sensing margin SM_EFF when the compensation is done using resistance R_OFFSET of a cell transistor. 
         [0082]      FIG. 17  is a diagram illustrating range of a magnitude of a write current for programming a resistive device to compensate distribution of the resistive device included in the memory cell array of  FIG. 2 . In  FIG. 17 , I_WD denotes a write current for programming a resistive device in a normal operating mode, and I_WO denotes a write current for providing stress to the cell transistor, which causes interface state generation (INTERFACE STATE GENERATION). Referring to  FIG. 17 , the lower limit of a magnitude of the write current for providing the stress to the cell transistor may be determined as a current value that is not affected by a write current of the resistive device. The upper limit I_UL of a magnitude of the write current for providing the stress to the cell transistor may be determined as a current value that does not break down the resistive device, but higher values of write current can cause data storage hard failures (DSHF). 
         [0083]      FIG. 18  is a flowchart illustrating a method of compensating resistance distribution of a resistive memory device, in accordance with an embodiment of the inventive concept. The resistive memory device may include a plurality of resistive devices and a plurality of cell transistors serially connected to each of the resistive devices. Referring to  FIG. 18 , the method of compensating resistance distribution of a resistive device may include the following operations: 
         [0084]    (1) writing data of “0” or “1” with respect to all the memory cells included in a memory cell array (S 1 ); 
         [0085]    (2) selecting a memory cell from which data is read out (S 2 ); 
         [0086]    (3) reading out data from the selected memory cell (S 3 ); 
         [0087]    (4) determining whether the read data equals to a target value (S 4 ); and 
         [0088]    (5) ending a process when the data is equal to the target value, adjusting interface states of cell transistors when the data is not equal to the target value, and executing the operation  3  (S 3 ) (S 5 ). The operation of adjusting interface states of the cell transistors may be expressed as an offset write operation in this specification. 
         [0089]      FIG. 19  is a flowchart illustrating a method of adjusting the interface states of cell transistors in the method of compensating resistance distribution of the resistive memory device of  FIG. 18 . Referring to  FIG. 19 , the method of adjusting the interface states of cell transistors may include the following operations: 
         [0090]    (1) driving a program driver (S 51 ); 
         [0091]    (2) selecting a memory cell for which interface state is to be adjusted (S 52 ); 
         [0092]    (3) giving stress with respect to a cell transistor of the selected memory cell (S 53 ). 
         [0093]      FIG. 20  is a diagram illustrating a memory module  2100  including resistive memory devices according to embodiments of the inventive concept. Referring to  FIG. 20 , the memory module  2100  may include a printed circuit board (PCB)  2110 , a plurality of MRAM chips  2120 , and a connector  2130 . The plurality of MRAM chips  2120  may be bonded to top and bottom surfaces of the PCB  2110 . The connector  2130  may be electrically connected to the plurality of MRAM chips  2120  through conductive lines (not shown). Also, the connector  2130  may be connected to a slot of an external host. 
         [0094]      FIG. 21  is a simplified perspective view of a semiconductor device having a stack structure including a resistive memory device according to embodiments of the inventive concept. In the memory module  2100  of  FIG. 20 , each of the memory chips may include a plurality of semiconductor layers LA 1  to LAn shown in  FIG. 21 . In the stack semiconductor device  2400 , the plurality of stacked semiconductor layers LA 1  to LAn may be connected to one another through through-silicon vias (TSVs)  2420 . Each of the semiconductor layers LA 1  to LAn may include cell arrays  2410  including STT-MRAM cells. 
         [0095]      FIG. 22  is a block diagram of an example of a memory system  2500  including a resistive memory device according to embodiments of the inventive concept. Referring to  FIG. 22 , the memory system  2500  may include a memory controller  2510  and a resistive memory device  2520 . The memory controller  2510  may generate an address signal ADD and a command CMD, and provide the address signal ADD and the command CMD to the resistive memory device  2520  through buses. Data DQ may be transmitted from the memory controller  2510  to the resistive memory device  252 Q through the buses, or transmitted from the resistive memory device  2520  to the memory controller  2510  through the buses. The resistive memory device  2520  may be a resistive memory device  1000  according to an embodiment of the inventive concept, and may compensate for resistance distribution by adjusting interface states of the cell transistors. 
         [0096]      FIG. 23  is a block diagram of an example of a memory system  2600  including a resistive memory device and an optical link according to an embodiment of the inventive concept. Referring to  FIG. 23 , the memory system  2600  may include a controller  2620 , a resistive memory device  2630 , and a plurality of optical links  2610   a  and  2610   b  configured to interconnect the controller  2620  and the resistive memory device  2630 . The controller  2620  may include a control unit  2621 , a first control transmitter (CTx)  2622 , and a first control receiver (CRx)  2623 . The control unit  2621  may transmit a control signal SN 1  to the first transmitter  2622 . The first transmitter  2622  may include a first optical modulator  2622 _ 1 , which performs an electrical-to-optical (E/O) signal conversion by converting the control signal SN 1 , which is an electric signal, into a first optical transmission signal OPT 1 , and transmit the first optical transmission signal OPT 1  to the optical link  2610   a . The first control receiver (CRx)  2623  may include a first optical demodulator  2623 _ 1 , which performs an optical-to-electrical (O/E) signal conversion by converting a second optical receiving signal OPT 2 ′ received from the optical link  2610   b  into a data signal SN 2 , which is an electric signal, and transmit the data signal SN 2  to the control unit  2621 . 
         [0097]    The resistive memory device  2630  may include a second control receiver (CRx)  2631 , a resistive memory cell array  2632 , and a second control transmitter (CTx)  2633 . The second control receiver  2631  may include a second optical modulator  2631 _ 1 , which performs an optical-to-electrical (O/E) signal conversion by converting a first optical receiving signal OPT 1 ′ received from the optical link  2610   a  into the control signal SN 1 , which is the electric signal, and transmit the control signal SN 1  to the resistive memory cell array  2632 . In the resistive memory cell array  2632 , data may be written under the control of the control signal SN 1 , or the data signal SN 2  output by the resistive memory cell array  2632  may be transmitted to the second transmitter  2633 . The second control transmitter  2633  may include a second optical modulator  2633 _ 1 , which performs an electrical-to-optical (O/E) signal conversion by converting the data signal SN 2 , which is the electric signal received from the resistive memory cell array  2632 , into a second optical data signal QPT 2 , and transmit the second optical data signal QPT 2  to the optical link  2610   b.    
         [0098]      FIG. 24  is a block diagram of an example of an information processing system  2700  including a resistive memory device according to an embodiment of the inventive concept. Referring to  FIG. 24 , a resistive memory device  2711  may be mounted in a computer system. The information processing system  2700  may include a memory system  2710 , a modem  2720 , a central processing unit (CPU)  275 Q, a RAM  2740 , and a user interface  2730 , which may be electrically connected to a system bus  276 Q. 
         [0099]    The memory system  2710  may include the resistive memory device  2711  and a memory controller  2712 . Data processed by the CPU  2750  or externally input data may be stored in the resistive memory device  2711 . A semiconductor memory device including STT-MRAM cells may be applied to at least one of the resistive memory device  2711  and the RAM  2740 . That is, the semiconductor memory device including STT-MRAM cells may be applied to the resistive memory device  2711  configured to store a large capacity of data required for the information processing system  2700 , or the RAM  2740  configured to store data (i.e., system data) that needs to be rapidly accessed. Although not shown in  FIG. 24 , it would be apparent to those of ordinary skill in the art that an application chipset, a camera image processor (CIP), and an input/output (I/O) device may be further provided in the information processing system  2700 . 
         [0100]    Among resistive memory devices, an STT-MRAM may be an advanced memory that may satisfy low-cost large-capacity characteristics of a dynamic RAM (DRAM), operation speed of a static RAM (SRAM), and non-volatile characteristics of a flash memory. Accordingly, while a conventional system requiring an additional cache memory with high processing speed and an additional mass storage unit (e.g., a RAM) configured to store a large capacity of data, the cache memory and the mass storage unit may be replaced by an MRAM device according to an embodiment of the inventive concept. That is, since a memory device including an MRAM may rapidly store a large capacity of data, an information processing system may be configured more simply than in the conventional art. 
         [0101]    The resistive memory device according to embodiments of the inventive concepts is able to compensate for resistance distribution of resistive devices and parasitic resistors of lines by adjusting interface states of cell transistors included in the memory cell array and increase sensing margin of data. 
         [0102]    The inventive concept may be applied to a semiconductor device, particularly, a resistive memory device and a memory system including the same. 
         [0103]    The foregoing is illustrative of embodiments and is not to be construed as limiting thereof. Although a few embodiments have been described, those skilled in the art will readily appreciate that many modifications are possible without materially departing from the novel teachings and advantages. Accordingly, all such modifications are intended to be included within the scope of this inventive concept as defined in the claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function, and not only structural equivalents but also equivalent structures.