Patent Publication Number: US-2023138195-A1

Title: Nonvolatile resistive memory device using dynamic reference in dual domain and read method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2021-0146348, filed on Oct. 29, 2021, in the Korean Intellectual Property Office, the disclosures of which are incorporated by reference herein in their entireties. 
     BACKGROUND 
     Embodiments of the present disclosure described herein relate to a semiconductor memory device, and more particularly, relate to a nonvolatile resistive memory device using a dynamic reference in a dual domain including a voltage domain and a time domain, and a read method thereof. 
     A semiconductor memory device is roughly divided into a volatile memory device and a nonvolatile memory device. Read and write speeds of the volatile memory device are fast, but when a power supply is interrupted, data stored therein disappears. In contrast, the nonvolatile memory device retains data stored therein even though external power supply is discontinued. Therefore, the nonvolatile memory device is used to store contents to be preserved regardless of whether a power is supplied. 
     In recent years, a resistive memory has emerged as a global concern. The resistive memory device stores data corresponding to logic ‘0’ or ‘1’ based on a difference in resistance size. Unlike a conventional dynamic random access memory (DRAM) or a conventional static random access memory (SRAM), which loses data when a power supply is interrupted, the resistive memory memorizes and stores a series of data even when power is turned off. Based on these characteristics, when the resistive memory is used, leakage power may be reduced by shutting off a power in a standby state. Accordingly, the resistive memory is being used in various fields such as large-capacity memory, neuromorphic hardware, and Internet of Things (IoT) devices. 
     A read operation of the resistive memory applies a method of applying a current to a device to read data stored in the form of resistance, and sensing a voltage formed at this time. A reference cell having an average value of resistances corresponding to logic values of ‘0’ and ‘1’ and a bit line connected to a cell to be read are precharged with the same voltage. After that, when a current is discharged through each device, the bit line of the cell in which ‘1’ having a larger resistance value than the reference cell is stored is discharged more slowly. On the other hand, the bit line of the cell in which ‘0’ having a smaller resistance value than the reference cell is stored is discharged faster. A sense amplifier (hereinafter referred to as a SA) compares the voltage of the reference cell with the voltage of the cell to be read, and amplifies a voltage difference to a greater extent to complete the read operation. 
     In the conventional read operation, the read voltage difference formed due to a small resistance difference according to the storage of ‘0’ and ‘1’ data is not large. A read error may occur due to process variations, the influence of an offset voltage of the sense amplifier, or noise. Therefore, there is a need for a technology capable of improving read reliability by compensating for a small resistance difference caused by the storage of ‘0’ and ‘1’ data in the resistive memory. 
     SUMMARY 
     Embodiments of the present disclosure provide a resistive memory device capable of providing high read reliability by reducing read errors due to process variations or noise, and a read method thereof. 
     According to an embodiment of the present disclosure, a method of reading a nonvolatile resistive memory device including a data cell and a reference cell includes precharging a first bit line connected to the data cell and a second bit line connected to the reference cell, discharging a voltage precharged to the first bit line and the second bit line to a source node through the data cell and the reference cell, sampling a reference voltage developed to the second bit line at a first time when a voltage of the first bit line reaches a threshold voltage, sampling a data voltage developed to the first bit line at a second time when a voltage of the second bit line reaches the threshold voltage, and sensing and amplifying a difference value between the reference voltage and the data voltage and outputting the sensed and amplified difference value as output data. 
     According to an embodiment, the first time may precede or follow the second time depending on a logic value stored in the data cell. 
     According to an embodiment, when logic ‘0’ is stored in the data cell, the first time may precede the second time. 
     According to an embodiment, a first reference voltage sampled at the first time may be higher than a second reference voltage at the second time. 
     According to an embodiment, when logic ‘1’ is stored in the data cell, the second time may precede the first time. 
     According to an embodiment, a first reference voltage sampled at the first time may be lower than a second reference voltage at the second time. 
     According to an embodiment of the present disclosure, a nonvolatile resistive memory device includes a cell array including a data cell and a reference cell, and a read circuit that reads data stored in the data cell through a first bit line connected to the data cell and a second bit line connected to the reference cell, and the read circuit samples a voltage developed to the second bit line as a reference voltage at a first time when a voltage developed to the first bit line reaches a threshold voltage. 
     According to an embodiment, the read circuit may sample the voltage developed to the first bit line as a data voltage at a second time when the voltage developed to the second bit line reaches the threshold voltage. 
     According to an embodiment, the read circuit may compare the reference voltage sampled at the first time with the data voltage sampled at the second time to determine a logic value stored in the data cell. 
     According to an embodiment, the read circuit may include a time domain detecting circuit that senses levels of the voltages developed to the first bit line and the second bit line to generate a sampling control signal, a sample/hold circuit that samples and holds the reference voltage at the first time, and samples and holds the data voltage at the second time, in response to the sampling control signal, and a sense amplifier that compares the reference voltage with the data voltage and outputs a result of the comparison as output data. 
     According to an embodiment, the time domain detecting circuit may include a first half schmitt trigger that generates a first reference sampling control signal that controls to sample the reference voltage when the level of the voltage developed to the first bit line reaches the threshold voltage, a first inverter that inverts the first reference sampling control signal to generate a second reference sampling control signal, a second half schmitt trigger that generates a first data sampling control signal that controls to sample the data voltage when the level of the voltage developed to the second bit line reaches the threshold voltage, and a second inverter that inverts the first data sampling control signal to generate a second data sampling control signal. 
     According to an embodiment, the threshold voltage may be set depending on sizes of transistors configuring the first half schmitt trigger and the second half schmitt trigger. 
     According to an embodiment, the sample/hold circuit may include a first transmission gate that transfers the voltage developed to the first bit line at the second time in response to the first to second data sampling control signals, a first holding capacitor that stores the voltage transferred by the first transmission gate as the data voltage, a second transmission gate that transfers the voltage developed to the second bit line at the first time in response to the first to second reference sampling control signals, and a second holding capacitor that stores the voltage transferred by the second transmission gate as the reference voltage. 
     According to an embodiment, the sense amplifier may be activated after sampling of the reference voltage and the data voltage is completed. 
     According to an embodiment, the read circuit may include first and second precharge switches that respectively charges the first bit line and the second bit line to a precharge voltage in response to a precharge control signal, and first and second discharge switches that discharges the precharge voltage charged to the first bit line through the data cell, and discharges the precharge voltage charged to the second bit line through the reference cell, in response to a discharge control signal. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The above and other objects and features of the present disclosure will become apparent by describing in detail embodiments thereof with reference to the accompanying drawings. 
         FIG.  1    is a block diagram illustrating a resistive memory device, according to an embodiment of the present disclosure. 
         FIG.  2    is a diagram illustrating an example of a resistive memory device constituting a memory cell of a resistive memory device. 
         FIG.  3    is a circuit diagram specifically illustrating a configuration of a read circuit, according to the present disclosure. 
         FIG.  4    is a circuit diagram illustrating a basic operation of a time domain detecting circuit illustrated in  FIG.  3   . 
         FIG.  5    is a circuit diagram illustrating a detailed configuration of a time domain detecting circuit of  FIG.  4   . 
         FIG.  6    is a diagram illustrating a detailed configuration and operation of a sense amplifier of  FIG.  3   . 
         FIG.  7    is a flowchart illustrating a method of reading a resistive memory cell, according to an embodiment of the present disclosure. 
         FIG.  8    is a graph illustrating development trends of a data bit line and a reference bit line during a read operation of logic ‘0’. 
         FIG.  9    is a graph illustrating development trends of a data bit line and a reference bit line during a read operation of logic ‘1’. 
         FIG.  10    is a timing diagram illustrating control signals and voltage waveforms of respective nodes of a read circuit for implementing a read method of the present disclosure. 
         FIG.  11    is a graph illustrating an effect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, some embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In adding reference numerals to components of each drawing, the identical components may have the identical reference numerals as much as possible even though they are displayed on other drawings. Further, in describing the embodiment of the present disclosure, the detailed description of the related known configuration or function will be omitted when it is determined that it interferes with the understanding of the embodiment of the present disclosure. 
       FIG.  1    is a block diagram illustrating a resistive memory device, according to an embodiment of the present disclosure. Referring to  FIG.  1   , a resistive memory device  100  includes a cell array  110 , a word line decoder  120 , a column selection circuit  130 , a read circuit  140 , a write circuit  150 , and a control circuit  160 . 
     The cell array  110  may include a plurality of resistive memory cells. The resistive memory cells receive a write voltage or a read voltage through a word line WLs and bit lines BLm and BLr. The cell array  110  includes a data cell region  110   a  and a reference cell region  110   b . The data cell region  110   a  is a region in which data input externally are actually written, and the reference cell region  110   b  is a reference region for providing a reference voltage for reading the data cell region  110   a.    
     Data provided from the outside of the resistive memory device  100  are stored in the data cell region  110   a . In addition, reference cells for providing the reference voltage are formed in the reference cell region  110   b . Whether the data stored in the data cell region  110   a  is a logic ‘1’ or a logic ‘0’ may be identified with reference to the reference voltage. When any one word line WL 0  is selected to output data, a memory cell of the data cell region  110   a  and a reference cell of the reference cell region  110   b  will be selected. 
     The word line decoder  120  selects one of a plurality of word lines of the cell array  110  by decoding a row address R_ADD. The word line decoder  120  may provide a read word line voltage RWL to a selected word line, and may provide a cutoff voltage to unselected word lines, during a read operation. The word line decoder  120  may further include driver circuits for generating the read word line voltage RWL or the cutoff voltage. 
     The column selection circuit  130  may select at least one of a plurality of bit lines BL 1  to BLm and BLr with reference to a column address C_ADD. In addition, the column selection circuit  130  may select at least one of a plurality of source lines SL 1  to SLm and SLr according to the column address C_ADD. The column selection circuit  130  may connect at least one bit line of the bit lines BL 1  to BLm of the data cell region  110   a  and the bit line BLr of the reference cell region  110   b  to the read circuit  140  or the write circuit  150 , in response to the column address C_ADD. 
     The read circuit  140  senses data written in the selected cell array  110  under a control of the control circuit  160  and outputs the sensed data to the outside as output data D_out. In detail, the read circuit  140  of the present disclosure may vary a sampling time of the reference cell depending on a logic value stored in a data cell. That is, the read circuit  140  may vary a sampling time of the reference cell depending on a logic value stored in the data cell, instead of using the fixed reference voltage provided by the reference cell. In detail, the read circuit  140  of the present disclosure may increase a sensing margin through a dynamic reference technology that varies the sampling time of the reference voltage. To this end, the read circuit  140  may include a time domain detecting circuit and a sample/hold circuit. The operation of these components will be described in detail with reference to  FIG.  3    to be described later. 
     The write circuit  150  writes write data D_in input under a control of the control circuit  160  into the cell array  110 . The write circuit  150  may provide a write voltage or a write current to the selected bit line BL. 
     The control circuit  160  may control the read circuit  140 , the write circuit  150 , and the word line decoder  120  in response to a write command, a read command, or a control signal. The control circuit  160  controls the read circuit  140 , the write circuit  150 , and the word line decoder  120  to synchronize a voltage of the word line WLs with a voltage of the bit line BL of the selected memory cell. 
     As described above, the resistive memory device  100  of the present disclosure uses a reference voltage of which sampling is controlled in a dual domain including a voltage domain and a time domain during a read operation. In detail, a difference between the data voltage and the reference voltage may be amplified by varying the sampling time of the reference voltage provided by the reference cell depending on the logic value stored in the data cell. The reference voltage has a larger difference from the data voltage provided by the data cell according to the dynamic variation of the sampling time. The read circuit  140  of the present disclosure may utilize the dynamic reference voltage in the dual domain to increase a read margin and may increase the reliability of read data. 
       FIG.  2    is a diagram illustrating an example of a resistive memory device constituting a memory cell of a resistive memory device. Referring to  FIG.  2   , a memory cell includes a selection device (e.g., a transistor) connected to a word line and a variable resistance device Rv for storing data. 
     The variable resistance device Rv may include a pair of electrodes  111  and  113  and a data storage film  112  formed between the electrodes  111  and  113 . The data storage film  112  may be formed of a bipolar resistance storage material or a unipolar resistance storage material. The bipolar resistance storage material is programmed to a set (or logic 1) or reset (or logic 0) state by a polarity of a pulse. The unipolar resistance storage material may be programmed to a set or reset state by the same pulse polarity. The unipolar resistance storage material includes single transition metal oxide such as NiOx or TiOx. The bipolar resistance storage material includes perovskite-based materials. 
     In the above, a variable resistance device has been described as an example of the resistive memory device of the present disclosure. However, resistive memory devices are not limited to the disclosure herein. It will be well understood that the resistive memory device may be provided in the form of any one of an STT-MRAM, a PRAM, and an FRAM. 
       FIG.  3    is a circuit diagram specifically illustrating a configuration of a read circuit, according to the present disclosure. Referring to  FIG.  3   , a connection relationship between the components of the cell array  110 , the column selection circuit  130 , and the read circuit  140  is illustrated. 
     The cell array  110  includes data cells and reference cells. Each of the data cells includes one of selection transistors ST 1  to STm and a variable resistance device Rn, and each of the reference cells includes a selection transistor STr and a reference variable resistance device Rr. When a read selection voltage VRWL is provided to the word line (assumed to be WL 0 ) selected during a read operation, the selection transistors ST 1  to STm and STr are turned on, and the variable resistance devices R 1  to Rm and Rr of the selected row are connected to the source lines SL 1  to SLm and SLr, respectively. 
     The column selection circuit  130  selectively connects the plurality of bit lines BL 1  to BLm and BLr and the plurality of source lines SL 1  to SLm and SLr to the read circuit  140  in response to column selection signals Y1 to Ym and Yr generated from the column address C_ADD. 
     The read circuit  140  may sample the reference voltage of the precharged bit line and the precharged reference bit line using a dynamic reference sampling technique in the voltage domain and the time domain. According to application of dynamic reference sampling, a level difference between the read voltage provided from the selected memory cell and the reference voltage provided from the reference cell may be amplified and sampled. For the dynamic reference sampling, the read circuit  140  may include a time domain detecting circuit  142  and a sample/hold circuit  144 . A read operation of the read circuit  140  performing dynamic reference sampling is as follows. 
     The read circuit  140  precharges the selected bit line and the reference bit line during the read operation. For example, when column selection signals Ym and Yr are activated, the read circuit  140  provides a precharge signal /Pre being a low level to turn on precharge switches PM 1  and PM 2  to sense the data stored in the variable resistance device Rm. Then, the precharge switches PM 1  and PM 2  are turned on, and a precharge source voltage VDDR is transferred to the bit line BLm and the reference bit line BLr to be charged. The read circuit  140  develops and samples voltages of the precharged bit lines BLm and BLr to identify the data stored in the variable resistance device Rm. To this end, the read circuit  140  activates a discharge signal Dis to a high level to turn on discharge switches NM 1  and NM 2 . 
     The time domain detecting circuit  142  senses levels of the developed voltages of each of the bit lines BLm and BLr to generate sampling control signals S REF , /S REF , S DATA , and/S DATA  for controlling the sample/hold circuit  144 . The time domain detecting circuit  142  generates the sampling control signals S REF , /S REF , S DATA , and IS DATA such that the reference voltage is sampled and held when the bit line voltage of the memory cell reaches a threshold voltage V T  as the bit line is discharged. In general, a resistance of the memory cell in which data of logic ‘0’ are stored is less than a resistance of the memory cell in which data of logic ‘1’ are stored. Accordingly, the bit line discharge rate of the memory cell storing the data of logic ‘0’ is greater than the bit line discharge rate of the memory cell storing the data of the logic ‘1’. Accordingly, the bit line voltage of the memory cell in which data of logic ‘0’ are stored by the bit line development will first reach the threshold voltage V T . In this case, the bit line of the memory cell in which data are stored will be referred to as a data bit line, and the bit line of the reference cell will be referred to as a reference bit line. The time domain detecting circuit  142  generates the sampling control signals S REF , /S REF , S DATA , and/S DATA  for varying sampling and holding times with respect to the reference bit line depending on the discharge rate of the data bit line. 
     The sample/hold circuit  144  samples and holds voltages of the data bit line BL DATA  and the reference bit line BL REF  in response to the sampling control signals S REF , /S REF , S DATA , and/S DATA . A sampling time of the data bit line BL DATA  is determined depending on a voltage change of the reference bit line BL REF . Since the reference bit line BL REF  is always discharged at the same rate, the sampling time with respect to the data bit line BL DATA  may be considered to be fixed. In contrast, since the discharge rate of the data bit line BL DATA  varies depending on the logic value stored in the memory cell, the discharge rate of the data bit line BL DATA  varies depending on the data stored in the memory cell. As a result, the sample/hold circuit  144  samples a voltage of the data bit line BL DATA  at a fixed time and samples a voltage of the reference bit line BL REF  at a variable time. 
     In detail, the sample/hold circuit  144  samples a voltage of the data bit line BL DATA  in response to the sampling control signals S REF  and/S REF  that turn on a transmission gate TG 1  at a fixed time, when the develop operation starts. In addition, the sampled voltage is maintained by using a first holding capacitor Cs 1 . The data voltage SA DATA  may be held by sampling and holding the voltage of the data bit line BL DATA . In contrast, the sample/hold circuit  144  samples a voltage of the reference bit line BL REF  in response to the sampling control signals S DATA  and/S DATA  that turn on a transmission gate TG 2  at a variable time after the develop operation is started, and holds the sampled voltage using a second holding capacitor Cs 2 . A reference voltage SA REF  may be held by sampling and holding the voltage of the reference bit line BL REF . 
     A sense amplifier  146  compares the data voltage SA DATA  with the reference voltage SA REF  when the sampling and holding with respect to the data bit line BL DATA  and the reference bit line BL REF  is completed, and outputs the comparison result as output data. For example, when the data voltage SA DATA  is greater than the reference voltage SA REF , the sense amplifier  146  may output logic ‘1’ as the output data D_out. For example, when the data voltage SA DATA  is less than the reference voltage SA REF , the sense amplifier  146  may output logic ‘0’ as the output data D_out. 
     By the operation of the read circuit  140  described above, the sampling time of the reference cell may vary depending on the data value stored in the data cell. Accordingly, the difference between the data voltage SA DATA  and the reference voltage SA REF  may be further amplified and provided to the sense amplifier  146 . 
       FIG.  4    is a circuit diagram illustrating a basic operation of a time domain detecting circuit illustrated in  FIG.  3   . Referring to  FIG.  4   , the time domain detecting circuit  142  includes half schmitt triggers H 1  and H 2 , and inverters INV 1  and INV 2 . 
     The first half schmitt trigger H 1  and the first inverter INV 1  generate the sampling control signals S DATA  and/S DATA  for sampling and holding the voltage of the reference bit line BL REF . The first half schmitt trigger H 1  generates the sampling control signal S DATA  when the voltage of the data bit line BL DATA  reaches the threshold voltage V T  by the development operation. In addition, the first inverter INV 1  inverts the sampling control signal S DATA  to output the sampling control signal/S DATA . The first half schmitt trigger H 1  will generate the sampling control signal S DATA  having a high-level when the level of the data bit line BL DATA  becomes the same as the level of the threshold voltage V T . 
     The second half schmitt trigger H 2  and the second inverter INV 2  generate the sampling control signals S REF  and/S REF  for sampling and holding the voltage of the data bit line BL DATA  The second half schmitt trigger H 2  generates the sampling control signal S REF  when the voltage of the reference bit line BL REF  reaches the threshold voltage V T  by the development operation. In addition, the second inverter INV 2  inverts the sampling control signal S REF  to output the sampling control signal /S REF . The second half schmitt trigger H 2  will generate the sampling control signal S REF  having a high-level when the level of the reference bit line BL REF  becomes the same as the level of the threshold voltage V T . The second inverter INV 2  inverts the sampling control signal S REF . 
       FIG.  5    is a circuit diagram illustrating a detailed configuration of a time domain detecting circuit of  FIG.  4   . Referring to  FIG.  5   , the time domain detecting circuit  142  includes the half schmitt triggers H 1  and H 2  and inverters INV 1  and INV 2  for setting the appropriate threshold voltage V T . 
     The first half schmitt trigger H 1  is triggered when the voltage V BL_DATA  of the data bit line BL DATA  becomes lower than the threshold voltage V T . In the bit line develop operation, the data bit line voltage V BL_DATA  is lowered by discharging from the precharged state. The first half schmitt trigger H 1  is provided as a half schmitt trigger triggered only when the level of the data bit line voltage V BL_DATA  becomes lower than the threshold voltage V T . The level of the threshold voltage V T  may be adjusted by adjusting the sizes of PMOS transistors P 1  and P 2  and an NMOS transistor N 1  receiving the data bit line voltage V BL_DATA . 
     The PMOS transistors P 1  and P 2  switch a power supply voltage VDD in response to the data bit line voltage V BL_DATA . In detail, the PMOS transistors P 1  and P 2  are turned off when the data bit line voltage V BL_DATA  is greater than the threshold voltage V T , and are turned on when the data bit line voltage V BL_DATA  is lower than the threshold voltage V T . The PMOS transistor P 3  stably maintains a level of the sampling control signal S DATA , which is the level of an output terminal of the first half schmitt trigger H 1 . In contrast to the PMOS transistors P 1  and P 2 , the NMOS transistor N 1  pulls down the output terminal of the first half schmitt trigger H 1  in response to the data bit line voltage V BL_DATA . The first inverter INV 1  inverts the sampling control signal S DATA  to output the sampling control signal /S DATA . 
     The second half schmitt trigger H 2  and the second inverter INV 2  are actually the same as the first Half Schmitt trigger H 1  and the first inverter INV 1 , except that a voltage to be sensed is a reference bit line voltage V BL_REF . The threshold voltage V T  at the same level as that of the first half schmitt trigger H 1  should be set by adjusting the sizes of PMOS transistors P 4  and P 5  and an NMOS transistor N 2  also in the second half schmitt trigger H 2 . 
       FIG.  6    is a diagram illustrating a detailed configuration and operation of a sense amplifier of  FIG.  3   . Referring to  FIG.  6   , a sensing enable signal SAE of the sense amplifier  146  is activated when sampling and holding of the data voltage SA DATA  and the reference voltage SA REF  are completed. 
     The sense amplifier  146  is provided to sense a voltage difference between the data bit line BL DATA  and the reference bit line BL REF , and may include PMOS transistors P 31 , P 32 , P 41 , and P 42 , NMOS transistors N 32 , N 32 , N 41 , N 42 , and N 50 , and a NOR gate. In particular, the sensing enable signal SAE corresponding to an output of the NOR gate is activated after sampling of the data voltage SA DATA  and the reference voltage SA REF  is completed, as may be seen from a truth table illustrated below of the corresponding drawing. Referring to the time domain sensing circuit  142  of  FIG.  4   , when the sampling control signal S REF  transitions to logic ‘1’, the data voltage SA DATA  is sampled. In addition, when the sampling control signal S DATA  transitions to logic ‘1’, the reference voltage SA REF  is sampled. Accordingly, after all of the sampling control signals S DATA  and S REF  are transitioned to logic ‘1’, the sensing enable signal SAE, which is the output of the NOR gate, may be activated as logic ‘1’. In contrast, when any one of the sampling control signals S DATA  and S REF  is logic ‘0’, the sensing enable signal SAE, which is the output of the NOR gate, will maintain logic ‘0’. 
       FIG.  7    is a flowchart illustrating a method of reading a resistive memory cell, according to an embodiment of the present disclosure. Referring to  FIGS.  3  and  7   , the dynamic reference voltage of the dual domain in which the sampling time and the level of the reference cell are varied depending on a data value stored in the data cell may be provided by the time domain detecting circuit  142  and the sample/hold circuit  144  of the present disclosure. 
     In operation S 110 , a bit line precharge operation by the read circuit  140  is performed for a read operation with respect to the selected memory cells. First, bit lines of a data cell and a reference cell selected by the read circuit  140  are precharged. When the variable resistance device Rm is sensed, the column selection signals Ym and Yr are activated. In addition, the read circuit  140  provides the precharge signal /Pre having a low level to turn on the precharge switches PM 1  and PM 2 . Then, the precharge switches PM 1  and PM 2  are turned on, and the data bit line BLm and the reference bit line BLr are respectively charged to a level of a precharge voltage V READ  by the precharge source voltage VDDR. 
     In operation S 120 , the read circuit  140  develops each of the data bit line BLm and the reference bit line BLr. That is, the read circuit  140  connects the precharged bit lines BLm and BLr to the source lines to identify data stored in the variable resistance device Rm, and will sense a voltage change. To this end, the read circuit  140  activates the discharge signal Dis to a high level to turn on the discharge switches NM 1  and NM 2 . Then, the source lines SLm and SLr are connected to a ground. In addition, as the read voltage is provided to the word line WL 0  by the word line decoder  120  (refer to  FIG.  1   ), the selection transistors STm and STr are turned on, and the variable resistance devices Rm and Rr are connected to the ground. Accordingly, discharging of the precharge voltage V READ  charged in each of the data bit line BLm and the reference bit line BLr may start. 
     In operation S 130 , the time domain detecting circuit  142  senses whether the level of the bit line voltage V BL_DATA  developed to the data bit line BLm reaches the threshold voltage V T . The half schmitt trigger H 1  of the time domain detecting circuit  142  activates the sampling control signals S DATA  and/S DATA  when the level of the bit line voltage V BL_DATA  falling by the discharge is the same as the preset threshold voltage V T . For example, when the level of the bit line voltage V BL_DATA  becomes the same as the preset threshold voltage V T , the sampling control signal S DATA  transitions from a low level to a high level. Then, the inverted sampling control signal /S DATA  will transition from the high level to the low level. 
     In operation S 135 , in synchronization with the transition of the sampling control signals S DATA  and /S DATA  reflecting the development state of the data bit line BLm, the sample/hold circuit  144  samples and holds the reference bit line voltage V BL_REF . The bit line discharge rate of the memory cell in which data of logic ‘0’ are stored in the variable resistance device Rm is faster than that of when logic ‘1’ is stored. Accordingly, in the read operation of the memory cell in which data of logic ‘0’ are stored, the sampling time of the reference bit line voltage V BL_REF  becomes earlier than that of the read operation of logic ‘1’. When the sampling time of the reference bit line discharged at a fixed rate is earlier, the relatively high reference voltage SA REF  is sampled and held. 
     In operation S 140 , the time domain detecting circuit  142  senses whether the level of the bit line voltage V BL_REF  developed to the reference bit line BLr reaches the threshold voltage V T . The half schmitt trigger H 2  of the time domain detecting circuit  142  activates the sampling control signals S REF  and/S REF  when the level of the bit line voltage V BL_REF  falling by the discharge is the same as the preset threshold voltage V T . For example, when the level of the bit line voltage V BL_REF  becomes the same as the preset threshold voltage V T , the sampling control signal S REF  transitions from a low level to a high level. Then, the inverted sampling control signal /S REF  will transition from the high level to the low level. 
     In operation S 145 , in synchronization with the transition of the sampling control signals S REF  and/S REF  reflecting the development state of the reference bit line BLr, the sample/hold circuit  144  samples and holds the data bit line voltage V BL_DATA . The variable resistance device Rr of the reference cell is fixed to a resistance value corresponding to the average value of each case in which the logic ‘0’ and the logic ‘1’ are stored. Accordingly, the discharge rate of the reference bit line BLr may be regarded as uniform based on the fixed resistance value. The sampling time of the data bit line, at which the sampling time is determined according to the discharge rate of the reference bit line BLr, may be fixed. Accordingly, the sampled data voltage SA DATA  will reflect the logic value ‘0’ or ‘1’ stored in the memory cell. 
     In operation S 150 , it is checked whether sampling and holding of the data voltage SA DATA  and the reference voltage SA REF  are completed through the sample/hold circuit  144 . Actually, the sampling and holding timings of the data bit line BLm and the reference bit line BLr are inevitably different due to the dynamic reference sampling technique. That is, during a read operation of a memory cell in which logic ‘0’ is stored, sampling and holding of the reference voltage SA REF  are completed earlier. In contrast, during the read operation of the memory cell in which the logic ‘1’ is stored, the sampling and holding of the reference voltage SA REF  will be later than the sampling and holding of the data voltage SA DATA . When the sampling and holding of the data voltage SA DATA  and the reference voltage SA REF  are completed (Yes direction), the procedure moves to operation S 160 . On the other hand, when it is determined that the sampling and holding of the data voltage SA DATA  and the reference voltage SA REF  are not completed (No direction), the subsequent procedure may wait until the sampling and holding of both the data voltage SA DATA  and the reference voltage SA REF  is completed. 
     In operation S 160 , when sampling and holding of both the data voltage SA DATA  and the reference voltage SA REF  are completed, an enable signal SAE for activating the sense amplifier  146  is provided. 
     In operation S 170 , the sense amplifier  146  compares the data voltage SA DATA  with the reference voltage SA REF  to determine whether the read data are the logic ‘0’ or the logic ‘1’. The sense amplifier  146  may provide the comparison result as the output data D_out. 
     According to the above-described resistive memory reading method, the sampling and holding time of the reference bit line may be varied by using the characteristic that the development speed of the bit line is different according to the logic value stored in the data cell. When the development speed of the data bit line is fast (e.g., in a logic ‘0’ read), the sampling time of the reference bit line is advanced, and the level of the sampled reference voltage SA REF  is relatively increased. In contrast, when the development rate of the data bit line is slow (e.g., in a logic ‘1’ read), the sampling time of the reference bit line is relatively delayed, and the level of the sampled reference voltage SA REF  is relatively decreased. As a result, the difference between the reference voltage and the data voltage in the voltage domain and the time domain may be amplified by varying the sampling time of the reference voltage SA REF  developed to the reference bit line with reference to the development speed of the data bit line. 
       FIG.  8    is a graph illustrating development trends of a data bit line and a reference bit line during a read operation of logic ‘0’. Referring to  FIG.  8   , during a read operation of a memory cell in which data of logic ‘0’ are stored, the sampling of the reference bit line voltage V BL_REF  occurs faster than the sampling of the data bit line voltage V BL_DATA  at a sampling and holding time Trs 1  of the reference bit line BL REF . 
     During the develop operation, a change in the reference bit line voltage V BL_REF  developed to the reference bit line BL REF  is indicated by a dotted line. In addition, the data bit line voltage V BL_DATA  developed to the data bit line BL DATA  during the develop operation is indicated by a solid line. The data bit line BL DATA  and the reference bit line BL REF  are charged to the precharge voltage V READ  by the precharge operation, and are discharged by the develop operation. 
     During the develop operation, the discharge rate of the data bit line BL DATA  of the memory cell in which the logic ‘0’ is stored is faster than the discharge rate of the reference bit line BL REF . Accordingly, a point P 1  at which the data bit line voltage V BL_DATA  reaches the threshold voltage V T  corresponds to the first sampling time Trs 1 . Accordingly, the first sampling time Trs 1  at which the bit line voltage V BL_DATA  of the data bit line BL DATA  reaches the threshold voltage V T  becomes the reference sampling time for sampling and holding the reference bit line voltage V BL_REF . When a level (a point P 2 ) of the reference bit line voltage V BL_REF  is sampled at the first sampling time Trs 1 , the sampled voltage becomes the reference voltage SA REF  input to the sense amplifier  146 . 
     On the other hand, a point P 4  at which the reference bit line voltage V BL_REF  reaches the threshold voltage V T  corresponds to a data sampling time Td. In addition, the data sampling time Td becomes the reference sampling time for sampling and holding the data bit line voltage V BL_DATA . When a level (a point P 3 ) of the data bit line voltage V BL_DATA  is sampled at the data sampling time Td, the sampled voltage becomes the data voltage SA DATA  input to the sense amplifier  146 . 
     When the logic ‘0’ data is read, the sampling time of the reference bit line becomes earlier than the data sampling time, and consequently, a higher reference voltage SA REF  may be provided to the sense amplifier  146 . As a result, it may be seen that a sensing margin SM is significantly increased than a sensing margin SM′ when the reference bit line and the data bit line are simultaneously sampled. 
       FIG.  9    is a graph illustrating development trends of a data bit line and a reference bit line during a read operation of logic ‘1’. Referring to  FIG.  9   , during a read operation of a memory cell in which data of logic ‘1’ are stored, sampling of the reference bit line BL REF  occurs later than sampling of the data bit line voltage V BL_DATA . 
     During the develop operation, a change in the reference bit line voltage V BL_REF  developed to the reference bit line BL REF  is indicated by a dotted line. In addition, the data bit line voltage V BL_DATA  developed to the data bit line BL DATA  during the develop operation is indicated by a solid line. The data bit line BL DATA  and the reference bit line BL REF  are charged to the precharge voltage V READ  by the precharge operation, and are discharged by the develop operation. 
     During the develop operation, the discharge rate of the data bit line BL DATA  of the memory cell in which the logic ‘1’ is stored is slower than the discharge rate of the reference bit line BL REF . Accordingly, a point P 1 ′ at which the data bit line voltage V BL_DATA  reaches the threshold voltage V T  corresponds to a second sampling time Trs 2 . The second sampling time Trs 2  at which the data bit line voltage V BL_DATA  reaches the threshold voltage V T  becomes a reference sampling time for sampling and holding the reference bit line voltage V BL_REF . When a level (a point P 2 ′) of the reference bit line voltage V BL_REF  is sampled at the second sampling time Trs 2 , the sampled voltage becomes the reference voltage SA REF  input to the sense amplifier  146 . 
     On the other hand, a point P 4  at which the reference bit line voltage V BL_REF  reaches the threshold voltage V T  corresponds to a data sampling time Td. In addition, the data sampling time Td becomes the reference sampling time for sampling and holding the data bit line voltage V BL_DATA  When a level (a point P 3 ′) of the data bit line voltage V BL_DATA  is sampled at the data sampling time Td, the sampled voltage becomes the data voltage SA DATA  input to the sense amplifier  146 . 
     When the logic ‘1’ data is read, the sampling time of the reference bit line becomes slower than the data sampling time, and consequently, a lower reference voltage SA REF  may be provided to the sense amplifier  146 . As a result, it may be seen that the sensing margin SM is significantly increased than the sensing margin SM′ when the reference bit line and the data bit line are simultaneously sampled. 
       FIG.  10    is a timing diagram illustrating control signals and voltage waveforms of respective nodes of a read circuit for implementing a read method of the present disclosure. Referring to  FIG.  10   , a read operation when logic ‘1’ is stored in a data cell is illustrated. 
     At time T 0 , precharge for a read operation starts. For the precharge operation, the read circuit  140  provides the precharge signal Pre having a high level to turn on the precharge switches PM 1  and PM 2 . In this case, a selection voltage RWL of the word line may be maintained at a low level, and the enable signal SAE of the sense amplifier  146  may be deactivated. Then, the precharge switches PM 1  and PM 2  are turned on, and the data bit line BLm, the reference bit line BLr, and each of the sensing nodes BL DATA  and BL REF  are charged to the level of the precharge voltage V READ  by the precharge source voltage VDDR. 
     At time T 1 , the precharge ends and bit line development and sampling starts. For bit line development, the precharge signal Pre transitions to a low level, and the precharge switches PM 1  and PM 2  are turned off. In addition, the read circuit  140  activates the discharge signal Dis to a high level to turn on the discharge switches NM 1  and NM 2 . Then, the source lines SLm and SLr are connected to the ground. In addition, the word line read voltage RWL is provided to the word line WL 0  by the word line decoder  120  (refer to  FIG.  1   ) to turn on the selection transistors STm and STr. Then, one sides of the variable resistance devices Rm and Rr are connected to the ground, and the other sides of the variable resistance devices Rm and Rr are connected to the sensing nodes BL DATA  and BL REF , respectively. Accordingly, discharging of the precharge voltage V READ  charged in each of the data bit line BLm and the reference bit line BLr may start. 
     During the development process, as the voltages V BL_DATA  and V BL_REF  of the sensing nodes BL DATA  and BL REF  are discharged, the voltage level decreases. Since the data stored in the memory cell are logic ‘1’, the discharge rate of the data bit line voltage V BL_DATA  is slower than the discharge rate of the reference bit line voltage V BL_REF . Accordingly, at time T 2 , the reference bit line voltage V BL_REF  first reaches the threshold voltage V T . Subsequently, at time T 3 , the data bit line voltage V BL_DATA  will reach the threshold voltage V T . 
     At the time T 2  when the reference bit line voltage V BL_REF  reaches the threshold voltage V T , the sampling control signal S REF  is activated to a high level, and the sample/hold circuit  144  samples and hold the data bit line voltage V BL_DATA . The data voltage SA DATA  sampled and held by the sampling control signal S DATA  is maintained through the sample/hold circuit  144 . Subsequently, at the time T 3  when the data bit line voltage V BL_DATA  reaches the threshold voltage V T , the sampling control signal S DATA  is activated to a high level, and the sample/hold circuit  144  samples and holds the reference bit line voltage V BL_REF . The reference voltage SA REF  sampled and held by the sampling control signal S DATA  is maintained through the sample/hold circuit  144 . 
     At time T 4 , the enable signal SAE for activating the operation of the sense amplifier  146  is activated. Then, the sense amplifier  146  compares the data voltage SA DATA  with the reference voltage SA REF  to determine a logic value of the read data, and will provide the comparison result as the output data D_out. 
       FIG.  11    is a graph illustrating an effect of the present disclosure. Referring to  FIG.  11   , the distribution of the data voltage SA DATA  and the reference voltage SA REF  respectively input to the sense amplifier  146  (refer to  FIG.  3   ) during the read operation of data ‘0’ and the read operation of data ‘1’ is illustrated. 
     The distribution of the data voltage SA′ DATA  and the reference voltage SA′ REF in a general case in which the dynamic reference voltage of the present disclosure is not used during the read operation of data ‘0’ is also illustrated. The reference voltage SA′ REF  sampled simultaneously with the data voltage SA′ DATA  provided by the data cell in which the logic ‘0’ is stored is distributed in a relatively narrow voltage band, but the voltage difference or the sensing margin SM′ to be identified by the sense amplifier  146  is relatively small. In contrast, when the dynamic reference voltage of the present disclosure is used, the voltage band in which the data voltage SA DATA  and the reference voltage SA REF  are distributed is relatively wide. However, the voltage difference or the sensing margin SM to be identified by the sense amplifier  146  is much greater than the sensing margin SM′ when the dynamic reference voltage is not used. 
     The distribution of the reference voltage SA′ REF  and the data voltage SA′ DATA  in a general case in which the dynamic reference voltage of the present disclosure is not used during the read operation of data ‘1’ is illustrated. It may be seen that the sensing margin SM′, which is the difference between the data voltage SA′ DATA  provided by the data cell in which the logic ‘1’ is stored and the reference voltage SA′ REF  sampled at the same time, is less than the sensing margin SM when the technology of the present disclosure is applied. 
     In conclusion, it may be confirmed that the difference value (the sensing margin) between the data voltage SA DATA  and the reference voltage SA REF  provided to the sense amplifier  146  during the read operation using the dynamic reference voltage of the present disclosure may be dramatically increased. This feature may provide high read reliability even in environments such as process variations or noise. 
     According to an embodiment of the present disclosure, a resistive memory device that uses a dynamic reference voltage in a dual domain including a time domain and a voltage domain is provided. According to the present disclosure, a high read margin may be provided by using a reference voltage that dynamically changes depending on data stored in a target cell selected for a read operation. 
     The above description refers to embodiments for implementing the present disclosure. Embodiments in which a design is changed simply or which are easily changed may be included in the present disclosure as well as an embodiment described above. In addition, technologies that are easily changed and implemented by using the above embodiments may be included in the present disclosure. While the present disclosure has been described with reference to embodiments thereof, it will be apparent to those of ordinary skill in the art that various changes and modifications may be made thereto without departing from the spirit and scope of the present disclosure as set forth in the following claims.