Resistive memory device and operation

A method of operating a resistive memory device including a plurality of memory cells comprises determining whether to perform a refresh operation on memory cells in a memory cell array; determining a resistance state of each of at least some of the memory cells; and performing a re-writing operation on a first memory cell having a resistance state from among a plurality of resistance states that is equal to or less than a critical resistance level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No. 10-2014-0092660, filed on Jul. 22, 2014, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The inventive concept relates to a memory device, and more particularly, to a resistive memory device and a method of operating the same.

Research into next-generation memory devices that are non-volatile and do not require a refresh operation is driven by a demand for high capacity and low power consumption memory devices. Next-generation memory devices should ideally have the high integrity characteristic of dynamic random access memory (DRAM), the non-volatility of flash memories, and the high speed of a static RAM (SRAM). Examples of next-generation memory devices include phase change RAM (PRAM), nano floating gate memory (NFGM), polymer RAM (PoRAM), magnetic RAM (MRAM), ferroelectric RAM (FeRAM), and resistive RAM (RRAM).

SUMMARY

According to an aspect of the inventive concept, a method of operating a resistive memory device comprises determining whether to perform a refresh operation on memory cells in a memory cell array, determining a respective resistance state of each of the memory cells, and performing a re-writing operation on a first memory cell of the memory cells that has a resistance state from among a plurality of resistance states that is equal to or less than a critical resistance level.

The performing of the re-writing operation may include performing a set writing operation on the first memory cell.

The performing of the re-writing operation may include applying a current pulse to the first memory cell.

The first memory cell may include a memory cell having a resistance state other than an uppermost resistance state from among the plurality of resistance states.

The method may further include performing a pre-reading operation on some of the memory cells.

The method may further include classifying the memory cells as the first memory cell and a second memory cell having a resistance state that is greater than the critical resistance level.

The first memory cell may include a memory cell to which data is written through a set writing operation, and the second memory cell may include a memory cell to which the data is written through a reset writing operation.

When the data is written in the memory cells, a bias voltage may be applied to the first memory cell so that a filament is generated in a variable resistance device in the first memory, and a bias voltage may be applied to the second memory cell so that a filament in a variable resistance device is cut.

The determining of whether to perform the refresh operation may include determining to perform the refresh operation when a first reference time that is set in advance has passed after writing the data in the memory cells.

The determining of whether to perform the refresh operation may include increasing a counting value representing a number of reading operations when receiving a read command, determining whether the counting value has reached a first reference value, and determining to perform the refresh operation if the counting value reaches the first reference value.

The determining of whether to perform the refresh operation may include: reading data written in the memory cells; performing error detection with respect to the read data; and determining whether a number of detected errors in the read data exceeds a second reference value that is set in advance, wherein if the number of detected errors exceeds the second reference value, it is determined to perform the refresh operation.

The memory cell array may include a plurality of cell regions, and the performing of the re-writing operation may be performed by a cell region unit.

According to another aspect of the inventive concept, a method is provided for operating a resistive memory device that includes a plurality of memory cells. Each of the memory cells has a respective resistance state that is one of a plurality of resistance states. These resistance states may include a first and second groups of one or more resistance states. The first group of resistance states includes resistance states of higher resistance level than the resistance levels of the second group of resistance states. The method comprises reading the resistance states of the plurality of memory cells and re-writing the read resistance states to the plurality of memory cells. The frequency of performing the re-writing on those of the memory cells having resistance states of the first group of resistance states is less than frequency of performing the re-writing on those of the memory cells having resistance states of the second group of resistance states.

The plurality of resistance states may include a first resistance state and a second resistance state having a resistance level that is lower than a resistance level of the first resistance state, and the re-writing may be performed on a memory cell having the second resistance state between the first and second resistance states.

The re-writing may be not performed on a memory cell having an uppermost resistance state of the plurality of resistance states.

In yet another aspect of the inventive concept, a memory system includes a resistive memory device comprising a memory cell array, a write/read circuit, and control logic circuitry. The memory system is configured to determine whether to perform a refresh operation on memory cells in the memory cell array, to determine a respective resistance state of each of the memory cells, and to perform a re-writing operation on a first memory cell of the memory cells that has a resistance state from among a plurality of resistance states that is equal to or less than a critical resistance level.

DETAILED DESCRIPTION OF EMBODIMENTS

The inventive concept will now be described more fully with reference to the accompanying drawings in which exemplary embodiments of the inventive concept are shown. The inventive concept may, however, be embodied in many different forms, and should not be construed as being limited to the exemplary embodiments set forth herein. Thus, the inventive concept may include all revisions, equivalents, or substitutions which are included in the concept and the technical scope related to the inventive concept. Like reference numerals in the drawings denote like elements. In the drawings, the dimension of structures may be exaggerated for clarity.

Furthermore, all examples and conditional language recited herein are to be construed as being not limited to such specifically recited examples and conditions. Throughout the specification, a singular form may include plural forms, unless there is a particular description contrary thereto. Also, terms such as “comprise” or “comprising” are used to specify existence of a recited form, a number, a process, an operation, a component, and/or groups thereof, not excluding the existence of one or more other recited forms, one or more other numbers, one or more other processes, one or more other operations, one or more other components and/or groups thereof.

While terms “first” and “second” are used to describe various components, it is obvious that the components are not limited to the terms “first” and “second”. The terms “first” and “second” are used only to distinguish between each component. For example, a first component may indicate a second component or a second component may indicate a first component without conflicting with the inventive concept.

Unless expressly described otherwise, all terms including descriptive or technical terms which are used herein should be construed as having meanings that are obvious to one of ordinary skill in the art. Also, terms that are defined in a general dictionary and that are used in the following description should be construed as having meanings that are equivalent to meanings used in the related description, and unless expressly described otherwise herein, the terms should not be construed as being ideal or excessively formal.

Referring toFIG. 1, the memory system10may include a resistive memory device100and a memory controller200. The memory device100may include a memory cell array110, a write/read circuit120, and control logic130, and may include circuits for performing writing/reading operations on the memory cell array110as controlled by the control logic130. Since the memory cell array110includes resistive memory cells, the memory system10may be called a resistive memory system.

In response to a write/read request from a host, the memory controller200may read data stored in the memory device100or may control the memory device100to write data to the memory device100. In more detail, the memory controller200may provide an address ADDR, a command CMD, and a control signal CTRL to the memory device100, and thus may control a program (or write) operation, a reading operation, and/or an erase operation with respect to the memory device100. Also, data DATA to be written and/or read may be exchanged between the memory controller200and the memory device100.

Although not illustrated, the memory controller200may include a Random Access Memory (RAM), a processing unit, a host interface, and/or a memory interface. The RAM may be used as an operation memory of the processing unit. The processing unit may control operations of the memory controller200. The host interface may include a protocol for exchanging data between the host and the memory controller200. For example, the memory controller200may communicate with an external source (i.e., the host) by using at least one of various interface protocols including Universal Serial Bus (USB), MultiMediaCard (MMC), Peripheral Component Interconnect Express (PCI-E), Advanced Technology Attachment (ATA), Serial-ATA, Parallel-ATA, Small Computer System Interface (SCSI), Earth Science Data Interface (ESDI), and/or Integrated Drive Electronics (IDE).

The memory cell array110may include a plurality of memory cells (not shown) that are disposed respectively in regions where a plurality of first signal lines and a plurality of second signal lines cross each other. In this exemplary embodiment the plurality of first and second signal lines may be bit lines and word lines, respectively, while in other embodiments they may be word lines and bit lines respectively.

Each of the plurality of memory cells may be a single level cell (SLC) that stores one bit data, or may be a multilevel cell (MLC) that may store at least two-bit data. Alternatively, the memory cell array110may include both SLC and MLC. When one bit data is written to one memory cell, the memory cells may have two resistance level distributions in accordance with the written data. Alternatively, when two-bit data is written to one memory cell, the memory cells may have four resistance level distributions in accordance with the written data. In another exemplary embodiments, if a memory cell is a triple level cell (TLC) that stores three-bit data, the memory cells may have eight resistance level distributions according to the written data. However, embodiments of the inventive concept are not limited thereto. For example, in other embodiments each of the memory cells may store at least four-bit data.

In some embodiments the memory cell array110may include memory cells with a two-dimensional (2D) horizontal structure. In other possible embodiments the memory cell array110may include memory cells with a three-dimensional (3D) vertical structure.

In an exemplary embodiment, the memory cell array110may include a plurality of cell regions that may be defined in various ways. For example, the cell region may be a page including a plurality of memory cells connected to the same word line. As another example, the cell region may include a plurality of memory cells connected to the word lines and the bit lines. As yet another example, the word lines may be connected to one row decoder (or a row selection block) and the bit lines may be connected to one column decoder (or a column selection block), and the above-described cell region may be defined as a tile.

The memory cell array110may include resistance-type memory cells or resistive memory cells that include a variable resistor device (not shown). For example, when resistance of the variable resistor device that is formed of a phase change material (e.g., Ge—Sb—Te) is changed with temperature, a resistive memory device may be a phase change RAM (PRAM). As another example, when the variable resistor device is formed of complex metal oxide including an upper electrode, a lower electrode, and transition metal oxide therebetween, the resistive memory device may be a resistive RAM (RRAM). As yet another example, when the variable resistor device is formed with an upper electrode of a magnetic material, a lower electrode of the magnetic material, and a dielectric therebetween, the resistive memory device may be a magnetic RAM (MRAM). For the sake of the following discussion, it will be assumed that the memory cell array110is an RRAM.

The write/read circuit120performs writing/reading operations on the memory cells. The write/read circuit120may be connected to the memory cells via the bit lines, and may include a write driver that writes data to the memory cells and a sense amplifier that amplifies data read from the memory cells.

The control logic130may generally control the operations of the memory device100. Also, in order to ensure the reliability of data stored in the memory cell, the control logic130may control a data refresh operation with respect to the memory cell array110.

When a predetermined time has elapsed or repeated stress has been applied to the memory cell after writing data in the memory cell, resistance level of the memory cell may be changed, so that an incorrect value may be read when reading data from the memory cell. The control logic130of the memory device100in this exemplary embodiment may re-write data in the memory cell to refresh the data in the memory cell, in order to prevent a data error caused by the changed resistance level of the memory cell. Here, the control logic130may determine the memory cell that is to be refreshed from among the plurality of memory cells included in the memory cell array110, and may control timing of performing the refresh in the memory cell.

According to one or more exemplary embodiments, the control logic130may determine data retention characteristics of the memory cells based on resistance state or write state of the memory cells in which the data is written, and may perform re-writing operations on memory cells having a poor data retention characteristic.

In addition, if the memory cell array110includes resistive memory cells, a reset writing operation or a set writing operation may be performed on the resistive memory cells. For example, in the resistive memory cell a set state has a relatively low resistance level and a reset state has a relatively high resistance level. When a writing operation is performed on the memory cell that increases the resistance level, the writing operation may be defined as a reset writing operation. Conversely, when a writing operation is performed on the memory cell that reduces the resistance level, the writing operation may be defined as a set writing operation. For example, a forward bias voltage may be applied to the memory cell in order to perform the set writing operation, and a backward bias voltage may be applied to the memory cell in order to perform the reset writing operation. Here, the memory cell on which the set writing operation is performed, for example the memory cell having the set state or the low resistance state, may have a poor retention characteristic due to properties of the variable resistive device. The control logic130may determine whether the memory cell is in the set state or in the reset state based on the data read from the memory cell, and may perform the re-writing operation on the memory cell in the set state. Otherwise, the control logic130may determine the resistance state of the memory cell, and may perform the re-writing operation on the memory cell having the low resistance state.

In some exemplary embodiments, the data may be refreshed by performing the re-writing operation only on the memory cell having the poor data retention state, for example, the memory cell in the set state or the low resistance state, thus efficiently improving data reliability and reducing power consumption caused by the refresh operation.

In addition, the memory controller200and the memory device100may be integrated to a semiconductor device and thus may configure a memory card, for example a PC card (a PCMCIA card), a compact flash card (CF card), a smart media card (SM/SMC), a memory stick, a multimedia card (MMC, RS-MMC, or MMCmicroO, a secure digital (SD) card (SD, miniSD, or microSD), or universal flash storage (UFS). As another example, the memory controller200and the memory device100may be integrated to a semiconductor device to configure a Solid State Disk/Drive (SSD).

Operation of the memory device100included in the resistive memory system10with the aforementioned structure will now be described.

Referring toFIG. 2, the memory device100may include the memory cell array110, the write/read circuit120, the control logic130, a voltage generator140, a row decoder150, and a column decoder160.

The memory cell array110may be connected to a plurality of first signal lines, a plurality of second signal lines, and a plurality of memory cells respectively arranged on respective regions where respective first signal lines intersect respective second signal lines. One or more exemplary embodiments will now be described assuming the first signal lines are bit lines BL and the second signal lines are word lines WL, as shown inFIG. 2.

An address ADDR for indicating an access-target memory cell may be received. The address ADDR may include a row address X_ADDR for selecting a word line of the memory cell array110, and a column address Y_ADDR for selecting a bit line of the memory cell array110. The row decoder150is connected to the memory cell array110via the word lines WL, and may select at least one of the word lines in response to the row address X_ADDR. The row decoder150receives a voltage from the voltage generator140, and may transfer the voltage to the word lines WL. For example, the row decoder150may provide a selected word line WL with a ground voltage Vss or a reset write voltage Vreset, and may provide non-selected word lines WL with a column inhibit voltage Vihbx for blocking leakage current.

The column decoder160is connected to the memory cell array110via the bit lines BL, and may select at least one of the bit lines BL in response to the column address Y_ADDR. The column decoder160may receive a voltage from the voltage generator140, and may provide the bit lines BL with the voltage. Also, the column decoder160may receive a write current pulse or a write voltage pulse from the write/read circuit120and may transfer the write current pulse or the write voltage pulse to the bit lines BL. For example, the column decoder160may provide a selected bit line BL with a set voltage Vset, a read voltage Vread, or a ground voltage Vss, and may provide a column inhibit voltage Vihby for preventing leakage current to non-selected bit lines BL.

The write/read circuit120is connected to the bit line BL via the column decoder160to write data in the memory cell or to read data from the memory cell. The write/read circuit120may write data DATA from outside into the memory cell array110or output data written in the memory cell array110, as controlled by the control logic130. Also, the write/read circuit120may provide the control logic130with a write/read result RST_WR. For example, the write/read circuit120performs a verification operation in order to detect the result of the writing operation, and may provide the control logic130with the verification result, for example pass or fail information.

The write/read circuit120may include a write circuit121and a read circuit122. The write circuit121is connected to the selected bit line BL via the column decoder160to provide the selected memory cell MC with a program pulse, and thus performs a programming operation (writing operation). Thus, the data DATA to be stored may be input to the memory cell array110. Here, the programming pulse may be referred to as a write pulse. In the exemplary embodiment, the programming pulse may be a current pulse. In another exemplary embodiments, the program pulse may be a voltage pulse.

In detail, the write circuit121may perform the set writing operation that programs the memory cell MC so that the resistance of the memory cell MC is reduced. Also, the write circuit121may perform the reset writing operation that programs the memory cell MC so that the resistance of the memory cell MC is increased.

The read circuit122is connected to the selected bit line BL via the column decoder160, and may read the data DATA by sensing the resistance level of the selected memory cell MC. Thus, the data DATA stored in the memory cell array110may be output.

In detail, the read circuit122may perform a general reading operation on the memory cell MC if a read command is transmitted from the memory controller200. Also, the read circuit122may perform the reading operation on the memory cell MC before performing the writing operation on the memory cell MC, that is, a pre-reading operation for reading an initial resistance state of the memory cell MC.

After performing the writing operation on the memory cell MC, the read circuit122may perform a verify reading operation for determining whether the writing in the memory cell MC is finished.

In a general reading operation, the read circuit122may provide the read data DATA to a component outside of the memory device100, for example, the memory controller200. Also, in the pre-reading operation or the verify reading operation, the read circuit122may provide a writing/reading operation result RST_WR signal to the control logic130based on the read data DATA or a pass/fail signal representing whether the reading or writing of the data to the memory device100has succeeded.

The voltage generator140may generate various voltages for performing the writing, reading, and erasing operations with respect to the memory cell array110based on a voltage control signal CTRL_vol. The voltage generator140may generate voltages for driving the plurality of word lines WL and bit lines BL, for example, the set write voltage Vset, the reset write voltage Vreset, the row inhibit voltage Vinhx, and/or the column inhibit voltage Vinhy.

The control logic130may output various control signals for writing data DATA in or reading data DATA from the memory cell array110based on the command CMD, the address ADDR, and the control signal CTRL transmitted from the memory controller200. The various control signals output from the control logic130may be provided to the write/read circuit120, the voltage generator140, the row decoder150, and the column decoder160. Accordingly, the control logic130may control overall operations in the memory device100.

The control logic130may provide the write/read circuit120with various operation control signals CTRL_op. Also, the control logic130may provide the row decoder150with a row address X_ADDR, and may provide the column decoder160with a column address Y_ADDR. Moreover, the control logic130may control various operations in the memory device100with reference to the writing/reading operation results RST_WR transmitted from the write/read circuit120.

In addition, the memory device100in this exemplary embodiment may perform a refresh operation on the memory cell array110in order to prevent the data DATA written in the memory cell array110from being lost or changed due to a time lapse or repeated electric stresses applied to the memory cell array110. The memory device100may determine the resistance state of the memory cells MC so as to determine whether the re-writing operation for refreshing data will be performed on the memory cell or determine a frequency of the re-writing operation, based on the resistance state of the memory cell.

In this exemplary embodiment, the re-writing operation for refreshing data may be selectively performed on those of the memory cells MC that have low resistance or set states. The re-writing operation may be not performed on those of the memory cells MC having high resistance or reset states. The low resistance state and the high resistance state may be distinguished based on a critical resistance level that is set in advance. A resistance state having a voltage level equal to or less than the critical resistance level may be referred to as being in a low resistance state, and a resistance state having a voltage level greater than the critical resistance level may be referred to as being in a high resistance state.

In another exemplary embodiment, frequency of performing the re-writing operation may be determined based on the resistance state of the memory cells. That is, the re-writing frequency with respect to a memory cell having a relatively higher resistance state may be lower than that with respect to a memory cell having a relatively lower resistance state.

The control logic130may control the memory device100to perform the above refresh operation. The control logic130determines data retention characteristics of the memory cells in which the data is written, and controls the memory device100to perform re-writing operations on memory cells having poor data retention characteristics. The control logic130may determine on which memory cells the re-writing operation will be performed, based on the resistance states of the memory cells read through the write/read circuit120.

In this exemplary embodiment the control logic130may classify the memory cells as first memory cells having low resistance states and second memory cells having high resistance states based on the resistance states of the memory cells, and may control the memory device100to perform the re-writing operation on the first memory cells. The set writing operation may be performed on the first memory cells having the set state, and the reset writing operation may be performed on the second memory cells having the reset state. In the exemplary embodiment the control logic130may designate the memory cells having the highest resistance states as second memory cells, and the memory cells having resistance states other than the highest resistance state as first memory cells. Then the control logic130may control the memory device100to perform the re-writing operation on the first memory cells.

In one or more exemplary embodiments, in the re-writing operation the control logic130may control a bias voltage for the set writing operation to be applied to the first memory cells. In other exemplary embodiments, in the re-writing operation the control logic130may control a current pulse to be applied to the first memory cells.

In another possible exemplary embodiment, the control logic130may control the re-writing frequency on the memory cell having the relatively high resistance state to be lower than that on the memory cell having the relatively low resistance state, based on the resistance states of the memory cells that are read. The control logic130may control the memory device100to not perform the re-writing operation on the memory cells having the highest resistance states.

When controlling the timing of the refresh operation, the control logic130may monitor memory operations of the memory device100, for example writing/reading operations, and may determine whether refresh operation of the memory cell array110is necessary. In this exemplary embodiment, the control logic130may control the memory device100to perform the refresh operation on the entire memory cell array110or some memory cells in it upon receiving a command CMD requesting the refresh operation from the memory controller200. In another exemplary embodiment, the control logic130may monitor the memory operations of the plurality of cell regions included in the memory cell array110in a cell region unit, and may determine the necessity of the refresh operation in the cell region unit.

Referring toFIG. 3, the memory cell array110may include a plurality of memory blocks, and may include memory cells of a horizontal structure. The memory cell array110may include a plurality of word lines WL1through WLn, a plurality of bit lines BL1through BLm, and a plurality of memory cells MCs. A group of memory cells MC selected by the same word line WL may be defined as a page PAGE. The number of word lines WL, the number of bit lines BL, and the number of memory cells MC may vary among different embodiments. InFIG. 3as in general in these figures, large solid “dots” indicate where electrical lines actually electronically connect with each other rather than crossing over each other.

In this exemplary embodiment each of the plurality of memory cells MC may include a variable resistive device R and a selection device D. The variable resistance device R may be referred to as a variable resistance material, and the selection device D may be referred to as a switching device.

In this embodiment the variable resistance device R is connected between one of the bit lines BL1through BLm and the selection device D, and the selection device D is connected between the variable resistance device R and one of the word lines WL1through WLn. However, embodiments are not limited thereto. For example, the selection device D may be connected between one of the plurality of bit lines BL1through BLm and the variable resistance device R, and the variable resistance device R may be connected between the selection device D and one of the plurality of word lines WL1through WLn.

In this embodiment the variable resistance device R may be switched to one of a plurality of resistance states by an electric pulse applied thereto. The variable resistance device R may include a phase-change material, a crystallization status of which varies depending on an amount of electric current. The phase-change material may vary, and may be, for example, GaSb, InSb, InSe, Sb2Te3, or GeTe that are compounds of two elements, GeSbTe, GaSeTe, InSbTe, SnSb2Te4, or InSbGe that are compounds of three elements, and AgInSbTe, (GeSn)SbTe, GeSb(SeTe), or Te81Ge15Sb2S2that are compounds of four elements.

The phase-change material may have an amorphous state having a relatively high resistance and a crystalline state having a relatively low resistance. The phase of the phase-change material may be changed by Joule's heat generated in accordance with the amount of electric current. In addition, the data may be written by using the phase change.

In addition, according to another exemplary embodiment of the inventive concept, the variable resistance device R may include perovskite compounds, transmission metal oxide, magnetic materials, ferromagnetic materials, or antiferromagnetic materials, instead of the phase-change material.

The selection device D may be connected between one of the plurality of word lines WL1through WLn and the variable resistance material R, and may control supply of the electric current to the variable resistance device R in accordance with a voltage applied to the connected word line and the bit line. In this exemplary embodiment the selection device D may be a PN junction diode or a PIN junction diode. An anode of the selection device D may be connected to the variable resistance device R and a cathode of the diode may be connected to one of the plurality of word lines WL1through WLn. Here, if a voltage difference between the anode and the cathode of the diode becomes greater than a threshold voltage of the diode, the diode is turned on so as to supply the electric current to the variable resistance device R. InFIG. 3, the selection device D is shown as a diode; however, one or more exemplary embodiments of the inventive concept are not limited thereto, that is, the selection device D may be another device that may be switched.

Referring toFIG. 4, the variable resistance device R included in the memory cell MC ofFIG. 3includes first and second electrodes EL1and EL2and a data storage film DS disposed between the first and second electrodes EL1and EL2.

The data storage film DS may be a bipolar resistive memory material or a unipolar resistive memory material. The bipolar resistive memory material may be programmed as a set state or a reset state in accordance with polarity of the pulse, and the unipolar resistive memory material may be a perovskite-based material. In addition, the unipolar resistive memory material may be programmed as a set state or a reset state according to the pulses of the same polarity, and the unipolar resistive memory material may be a transition metal oxide such as NiOx or TiOx.

Referring toFIG. 5A, a memory cell MCa includes a variable resistance device Ra which may be connected between a bit line BL and a word line WL. The memory cell MCa may store data by using voltages applied to the bit line BL and the word line WL.

Referring toFIG. 5B, a memory cell MCb may include a variable resistance device Rb and a bidirectional diode Db. The variable resistance device Rb may include a resistor material for storing data. The bidirectional diode Db is connected between the variable resistance device Rb and the word line WL, and the variable resistance device Rb may be connected between the bit line BL and the bidirectional diode Db. Locations of the bidirectional diode Db and the variable resistance device Rb may be exchanged. A leakage current flowing in non-selected resistor cells may be blocked by the bidirectional diode Db.

Referring toFIG. 5C, a memory cell MCc may include a variable resistance device Rc and a transistor TR. The transistor TR may be a selection device for supplying or blocking the electric current to the variable resistance device Rc according to a voltage of the word line WL, that is, a switching device. The transistor TR is connected between the variable resistance device Rc and the source line SL, and the variable resistance device Rc may be connected between the bit line BL and the transistor TR. Locations of the transistor TR and the variable resistance device Rc may be exchanged. The memory cell MCc may or may not be selected by the turning on/turning off of the transistor TR that is driven by the word line WL.

FIG. 6Ashows a case where the memory cell MC is an SLC, andFIG. 6Bshows a case where the memory cell MC is an MLC. InFIGS. 6A and 6B, horizontal axes indicate resistance, and vertical axes indicate number of memory cells MC.

Referring toFIG. 6A, if the memory cell MC is a single level cell SLC programmed with one bit, the memory cell MC may have a low resistance state LRS or a high resistance state HRS. In this exemplary embodiment resistance states RS1through RS4may be classified as high resistance states HRS and low resistance states LRS based on a critical resistance level Rref. The low resistance state LRS may be referred to as the set state, and the high resistance state HRS may be referred to as the reset state.

The low resistance state LRS and the high resistance state HRS may respectively correspond to one of data ‘0’ and data ‘1’. In this exemplary embodiment the resistance level R increases from data ‘0’ to data ‘1’, with the low resistance state LRS corresponding to data ‘0’ and the high resistance state HRS corresponding to data ‘1’.

An operation of applying a write pulse to the memory cell MC to switch the memory cell MC from the high resistance state HRS to the low resistance state LRS may be referred to as a set operation or a set writing operation. Applying the write pulse to the memory cell MC to switch the memory cell MC from the low resistance state LRS to the high resistance state HRS may be referred to as a reset operation or a reset writing operation.

Referring toFIG. 6B, if the memory cell MC is the multilevel cell programmed with two bits, a resistance value of the memory cell MC may have one of a first resistance state RS1, a second resistance state RS2, a third resistance state RS3, and a fourth resistance state RS4.

However, one or more exemplary embodiments are not limited thereto, that is, in another exemplary embodiment, the plurality of memory cells MC may include triple level cells TLCs storing data of three bits. Accordingly, the memory cells may have one of eight resistance states according to the written data. In another exemplary embodiment, the plurality of memory cells MC may include memory cells storing data of four-bits or greater.

Since the multi-level cell MLC has narrower intervals between resistance distributions than those of the single level cell SLC, a read error may occur due to a small variation in the resistance. Therefore, the resistance states RS1, RS2, RS3, and RS4may have resistor ranges that do not overlap with each other in order to ensure a read margin.

In this exemplary embodiment the resistance states RS1through RS4may be classified as either the high resistance state HRS or the low resistance state LRS, depending on their value relative to the critical resistance level Rref. For example, as reflected inFIGS. 6A and 6B, since the critical resistance level Rref is located between the first resistance state RS1and the second resistance state RS2, therefore the second, third, and fourth resistance states RS2, RS3, RS4will be classified as having resistance states LRS since they have lower resistance than the critical resistance level Rref. Conversely, since the first resistance state RS1is greater than the critical resistance level Rref, it will be classified as having the high resistance state HRS. Additionally, the low resistance state LRS may be referred to as the set state, while the high resistance state HRS may be referred to as the reset state.

Each of the first, second, third, and fourth resistance states RS1, RS2, RS3, RS4may correspond to one of data ‘00’, data ‘01’, data ‘10’, and data ‘11’. In this exemplary embodiment the resistance level R may increase in order of the data ‘11’, the data ‘01’, the data ‘00’, and the data ‘10’. That is, the fourth resistance state RS4may correspond to the data ‘11’, the third resistance state RS3may correspond to the data ‘01’, the second resistance state RS2may correspond to the data ‘00’, and the first resistance state RS1may correspond to the data ‘10’.

In,FIG. 7, the horizontal axis denotes voltage V, and the vertical axis denotes current I. If the memory cell MC is a multi-level cell MLC, the memory cell MC may have one of the plurality of resistance states according to data stored therein. For example, as shown inFIG. 7, the memory cell MC may have one of the first through fourth resistance states RS1, RS2, RS3, RS4. Here, the resistance level is the largest when the memory cell MC is in the first resistance state RS1, and the resistance level is the smallest when the memory cell MC is in the fourth resistance state RS4.

As shown in right side of the graph ofFIG. 7, the resistance level of the memory cell MC may be reduced through the set writing operation. Conversely, as shown in left side of the graph, the resistance level of the memory cell MC may be increased through the reset writing operation.

In a case where the resistance level of the memory cell MC is the first resistance state RS1, which is the largest, if a voltage equal to or greater than a threshold voltage Vth is applied to the memory cell MC, current in the memory cell MC is rapidly increased. Thus, it is not easy to control the current for writing the resistance level according to the data to be programmed. Therefore, in this exemplary embodiment, when performing the set writing operation on the memory cell MC, a write current Iset or a write current pulse may be applied to the memory cell MC.

In this exemplary embodiment, the memory cell MC may be switched from the first resistance state RS1to one of the second through fourth resistance states RS2, RS3, RS4according to the change in a magnitude of the set write current Iset or the set write pulse. In detail, when a first write current I1is applied to the memory cell MC, the memory cell MC may be switched from the first resistance state RS1to the fourth resistance state RS4. In addition, when a second write current I2is applied to the memory cell MC, the memory cell MC may be switched from the first resistance state RS1to the third resistance state RS3. Moreover, when a third write current I3is applied to the memory cell MC, the memory cell MC may be switched from the first resistance state RS1to the second resistance RS2.

Also, in an exemplary embodiment, the memory cell MC may be switched from the second resistance state RS2to the fourth or third resistance state RS4or RS3, according to the change in the magnitude of the set write current Iset or the set write current pulse, and the memory cell MC may be switched from the third resistance state RS3to the fourth resistance state RS4.

If the memory cell MC is in one of the second through fourth resistance states RS2, RS3, RS4, the current applied to the memory cell MC must be adjusted to increase to a peak (denoted as a dotted box inFIG. 7), and then decrease, in order to perform the reset writing operation. Thus, it is not easy to perform the reset writing operation on the memory cell MC by using a general square pulse. Therefore, in this exemplary embodiment a write voltage pulse Vreset may be applied to the memory cell MC when performing the reset writing operation on the memory cell MC.

In this embodiment the memory cell MC may be switched from the fourth resistance state RS4to one of the first through third resistance states RS1, RS2, RS3according to a change in the reset write voltage Vreset or a reset write voltage pulse. In detail, when a first write voltage V1is applied to the memory cell MC, the memory cell MC may be switched from the fourth resistance state RS4to the third resistance state RS3. Also, when a second write voltage V2is applied to the memory cell MC, the memory cell MC may be switched from the fourth resistance state RS4to the second resistance state RS2. In addition, when a third write voltage V3is applied to the memory cell MC, the memory cell MC may be switched from the fourth resistance state RS4to the first resistance state RS1.

In this exemplary embodiment, when the magnitude of the reset write voltage Vreset or the reset write voltage pulse is changed, the memory cell MC may be switched from the third resistance state RS3to the second or first resistance state RS2or RS1. In addition, the memory cell MC may be switched from the second resistance state RS2to the first resistance state RS1.

FIG. 8Ashows an operation in the variable resistance device when performing the set writing operation on the memory cell MC ofFIG. 3, andFIG. 8Bshows an operation in the variable resistance device when performing the reset writing operation on the memory cell MC ofFIG. 3.

Referring toFIG. 8A, during the set writing operation on the memory cell MC, the voltage of the first electrode EL1may be greater than that of the second electrode EL2, based on application of the write current pulse. Due to the voltage difference between the first and second electrodes EL1and EL2, oxygen ions and void Vo may be separated from each other in the data storage film DS, which is a metal oxide layer. In addition, ion migration may occur, so that oxygen ions O2− move toward the first electrode EL1.

Due to the ion migration, filaments including the void Vo may be generated in the data storage film DS, and accordingly resistance level of the memory cell MC may be reduced. Thickness of the filament and a density of the filament may vary depending on amplitude and pulse width of the write current pulse applied to the memory cell MC, so that resistance level R of the memory cell MC is changed.

Referring toFIG. 8B, during the reset writing operation on the memory cell MC, for example, the voltage of the second electrode EL2may be greater than that of the first electrode EL1, based on application of the write voltage pulse. Thus, ion migration occurs, so that oxygen ions O2− stored in the first electrode EL1move toward the data storage film DS, and accordingly oxygen ions O2− are recombined with the void Vo in the data storage film DS so that the filament is cut and the resistance level of the memory cell MC increases.

FIG. 9shows the resistance states of the memory cells MCs in a case where the memory cells MCs are multi-level cells MLC programmed with 2 bits. Referring toFIG. 9, each of the memory cells MC may have one of the first through fourth resistance states RS1, RS2, RS3, RS4. When the data is written in the memory cell MC through the set write or the reset writing operation, the memory cells MC may have resistance distributions as denoted in the figure by solid lines.

After that, the resistance distribution of the memory cell MC may be widened in a direction in which the resistance level increases, as denoted in the figure by dotted lines. After writing data in the memory cell MC, the resistance level of the memory cell MC may increase according to a time lapse or continuous stress applied thereon. In the variable resistance device of the memory cell MC, oxygen ions O2− are likely to return to stabilized states through oxidation reaction, that is, recombining with void Vo, and thus the filament is cut due to the oxidation and the resistance level of the memory cell MC is increased.

If the reset writing is performed on the memory cell MC, for example, if the memory cell MC is in the reset state RESET or the high resistance state HRS, amount of the filament is less than that for the set writing operation so that, for example, the memory cell MC is in the set state or the low resistance state. Thus, in this case the memory cell MC is less affected by the oxidation. Therefore, as shown inFIG. 9, in the high resistance state HRS variation in the resistance distribution of the memory cell MC may be less than that in the low resistance state LRS. Also, even if the resistance level of the memory cell MC increases in the high resistance state HRS, for example the first resistance state RS1, its resistance in the low resistance state LRS, for example the second resistance state RS2, is not affected. Therefore, the data retention property of the memory cells MC in the high resistance state HRS may be superior to that of the memory cells MC in the low resistance state LRS. Thus, according to one or more exemplary embodiments, re-writing is performed with respect to the memory cells MC in the low resistance state LRS or the set state SET having relatively poor data retention property in order to return the resistance levels of the memory cells MC to initial resistance levels, and thus, the data retention property of the memory cells MC in the low resistance state LRS is recovered and the data reliability of the memory device100(FIG. 2) may be improved.

FIGS. 10A and 10Bare graphs illustrating a method of operating a memory device according to an exemplary embodiment.FIG. 10Ashows a case where the memory cell is a single level cell SLC, andFIG. 10Bshows a case where the memory cell is a multi level cell MLC. InFIGS. 10A and 10B, horizontal axes denote time, and vertical axes denote resistance levels of the memory cells.

Referring toFIG. 10A, if the memory cells are single level cells SLC, when a writing operation is performed on the memory cells at t1, the memory cells become high resistance states HRS or low resistance states LRS. The high resistance state HRS and the low resistance state LRS may be classified according to a set critical resistance level Rref. As time has elapsed, the resistance level of the low resistance state LRS may increase. Accordingly, the memory device100determines to perform a refresh operation at t2, and then may perform a re-writing operation on the memory cells in the low resistance state LRS to recover the resistance level of the low resistance state LRS. Here, since the resistance level of the low resistance state LRS has to be reduced, a set writing operation may be performed on the memory cells of the low resistance state LRS.

In addition, since the resistance level of the high resistance state HRS does not change much between t1and t2, the re-writing operation may be not performed on the memory cells of the high resistance state HRS.

Referring toFIG. 10B, if the memory cells are multi level cells MLC, when a writing operation is performed on the memory cells at t1, the memory cells may have one of the plurality of resistance states RS1through RS4. Differences between the resistance levels of the resistance states RS1through RS4are not limited to the examples shown inFIG. 10B. The difference between the resistance levels of the first resistance state RS1and the second resistance state RS2may be greater than the differences between the resistance levels of the other resistance states RS2through RS4, for example.

The plurality of resistance states RS1through RS4may be classified as the high resistance state HRS or the low resistance state LRS based on comparison with the critical resistance level Rref. As shown inFIG. 10B, the first resistance state RS1, which is the highest resistance state, may be classified as the high resistance state HRS, and the other resistance states, for example, the second through fourth resistance states RS2through RS4, may be classified as the low resistance state LRS. After writing data on the memory cells, the resistance level of the low resistance state LRS may increase with time. Here, the resistance level of the lowest resistance state, in this case the fourth resistance state RS4, may increase the most, and the resistance level of the high resistance state HRS may increase only a little. Accordingly, the memory device100may determine to perform a refresh operation at t2, and may perform the re-writing operation on the memory cells of the low resistance state LRS to recover the resistance level. As described above, since the resistance level of the low resistance state LRS has to be reduced, the set writing operation may be performed on the memory cells in the low resistance state LRS. In an exemplary embodiment, the set writing operation may be performed by applying current pulses to the memory cells in the low resistance state LRS.

FIG. 11is a diagram illustrating an example of the operation of the memory device according to the exemplary embodiment of the inventive concept. An example of the refresh operation of the memory device will now be described with reference toFIG. 11.

Referring toFIG. 11, the control logic130may include a refresh control unit131. The refresh control unit131is a circuit for controlling the refresh operation of the memory device100(FIG. 2), and may determine timing of performing the refresh, a cell region that is to be refreshed, and memory cells on which the re-writing will be performed.

In this exemplary embodiment, the refresh control unit131may receive a refresh command CMD from outside, for example from the memory controller200, and may determine the timing of performing the refresh operation based on the refresh command CMD. For example, the refresh control unit131may determine the timing of the refresh operation so that the refresh may be performed upon receiving the refresh command CMD, or a while after the receipt of the refresh command CMD. For example, if the memory device100is performing a writing or reading operation, the timing of the refresh operation may be determined so that the refresh may be performed after finishing the writing or reading operation.

In one or more exemplary embodiments, the memory cell array110may include a plurality of memory cell regions. In addition, the refresh control unit131may receive the refresh command CMD and an address ADDR indicating the cell region from the memory controller200, and then may determine the timing of the refresh operation by the cell region unit. In another exemplary embodiment, the refresh control unit131may determine the cell region that is to be refreshed from among the plurality of cell regions in the memory cell array110, and may determine to perform the refresh operation on the above determined cell region upon receiving the refresh command CMD from outside.

According to another exemplary embodiment, the refresh control unit131may increase a counting value representing the number of reading operations whenever receiving a read command CMD. Then the refresh control unit131may determine to perform the refresh operation when the counting value reaches a reference value. Here, the refresh control unit131may determine whether to perform the refresh operation by the cell region unit, by counting the read command CMD with respect to the cell region.

In another exemplary embodiment, the refresh control unit131may perform error detection from the data DATA read from the memory cell array110, and may determine to perform the refresh operation if the number of errors that have occurred is equal to or greater than a predetermined error reference value. The refresh control unit131may perform the error detection by the cell region unit, and then determine whether to perform the refresh operation.

In another exemplary embodiment, the refresh control unit131may include a timer (not shown) therein, and may determine to perform the refresh operation with a predetermined period in consideration of the characteristics of the memory device100. Another possibility would be to determine to perform the refresh operation on the memory cell in which the data is written when a predetermined time has passed after the data is written in the memory cell.

As described above, the refresh control unit131determines to perform the refresh operation, and then may determine resistance states of the memory cells.

In this exemplary embodiment, the refresh control unit131reads data from the memory cells, and may determine the resistance state of each of the memory cells based on the read data. The refresh control unit131may transmit a pre-read signal CTRL_pre to the write/read circuit120. In response to the pre-read signal CTRL_pre, the read circuit122may pre-read the resistance states of at least some of the memory cells in the memory cell array110. In this exemplary embodiment the read circuit122may read the resistance states of the memory cells by applying a voltage that is different from the read reference voltage during a general reading operation.

The read circuit122may provide the refresh control unit122with the resistance states of the memory cells as pre-read data Dpre in a bit-data format.

The refresh control unit131determines the resistance state of each of the memory cells based on the pre-read data Dpre, and may control the memory device100to perform the re-writing operation on memory cells having resistance states that are lower than the critical resistance level (such memory cells will be hereinafter referred to as first memory cells). The refresh control unit131determines the memory cells on which the re-writing operation is to be performed, and may provide the write/read circuit120with a re-writing signal RW. The control logic130may provide the row decoder150(FIG. 2) and the column decoder160(FIG. 2) with the address for selecting the memory cells on which the re-writing operation is to be performed. The write/read circuit120may perform the re-writing operation on the selected memory cells. In one exemplary embodiment, in order to recover the resistance levels of the first memory cells, the set writing operation may be performed for reducing the resistance level of the first memory cells when performing the re-writing operation. Also, as described above with reference toFIG. 7, the current pulses may be applied to the first memory cells when the set writing operation is performed.

In one or more exemplary embodiments, when it is determined to perform the refresh operation based on a count of reading operations or the error detection operation, the pre-reading operation may be omitted. For example, since the reading operation is performed in response to the read command CMD before determining the refresh operation, the read circuit122may store the read data in a buffer (not shown) included therein and may provide the refresh control unit131with the read data when it is determined that the refresh operation is to be performed. The refresh control unit131may determine the resistance state of each of the memory cells based on the read data, and may determine the memory cells on which the re-writing operation is to be performed.

In another exemplary embodiment, write information on the memory cells, for example address information of the memory cells written in the high resistance state or the memory cells written in the low resistance state, may be stored in a storage unit (not shown) of the memory device100(FIG. 1). When it is determined to perform the refresh operation, the refresh control unit131accesses the storage unit to determine the resistance states of the memory cells, and determines the memory cells on which the re-writing operation is to be performed. In this case, reading the memory cells in order to determine the resistance states of the memory cells may be omitted.

In another exemplary embodiment, the write information about the memory cells may be stored in the memory controller200(FIG. 1), which may provide the memory device100with the address ADDR indicating the cell regions in which the refresh operation is to be performed, and the memory cells on which the re-writing is to be performed, along with the refresh command CMD. The refresh control unit131determines the timing of the refresh operation based on the received refresh command CMD, and may control the re-writing operation to be performed on the memory cells corresponding to the address ADDR.

InFIG. 11the refresh control unit131is shown as included in the control logic130, but embodiments of the inventive concept are not limited thereto. The refresh control unit131may be realized as hardware or software on outer side of the control logic130for example.

FIG. 12is a block diagram showing an example of the refresh control unit131ofFIG. 11. Referring toFIG. 12, a refresh control unit131amay include a read cycle detector11, an error detector12, a refresh determination unit13, and a re-writing region selection unit14.

The read cycle detector11may detect the read cycle by using at least one of the command CMD and/or the address ADDR. The read cycle detector11may include a counter (not shown). If it is identified that the command CMD is the read command, the counter may increase a counting value. Also, if the refresh operation is performed by the cell region unit, the read cycle detector11may identify at least some of the bits in the address ADDR to detect the cell region that is necessary to be refreshed.

In addition, the error detector12performs error detection operation with respect to the read data Dread transmitted from the write/read circuit120, and detects the number of detected errors. Also, the error detector12may compare the number of detected errors with a predetermined reference value, and output the comparison result. The refresh determination unit13receives various detection results or comparison results from the read cycle detector11and the error detector12, determines whether to perform the refresh operation on at least some of the cell regions in the memory cell array110, and generates a determination result. The refresh control unit131amay control the refresh operation to be performed based on the determination result. The refresh control unit131amay transmit the pre-read signal CTRL_pre to the write/read circuit120(FIG. 1).

In addition, read data Dread (for example pre-read data Dpre may be transmitted from the write/read circuit120in response to the pre-read signal CTRL_pre. The pre-read data Dpre may include the read resistance states of the memory cells as bit data format. The re-writing region selection unit14determines the resistance state of each of the memory cells based on the pre-read data Dpre, and then determines the memory cells on which the re-writing operation is to be performed. As described above, the re-writing region selection unit14may determine the memory cells in the low resistance state as the memory cells on which the re-writing operation is to be performed.

In another exemplary embodiment, if the write information about the memory cells is stored in the storage unit included in the memory device100(FIG. 1) or the memory controller200(FIG. 1), the re-writing region selection unit14accesses the storage unit of the memory device100to determine the resistance states of the memory cells and determine the memory cells on which the re-writing operation is to be performed.

FIG. 13is a flowchart illustrating an exemplary method of operating the memory device according to embodiments ofFIGS. 1 through 12.

Referring toFIG. 13, it is determined (S110) whether to perform the refresh operation with respect to at least some of the memory cells in the memory cell array. In one exemplary embodiment, when a refresh command is transmitted from outside, for example from the memory controller, it may be determined to perform the refresh operation. In another exemplary embodiment, when a predetermined reference time has passed after the writing is performed on the memory cells, it may be determined to perform the refresh operation on the memory cells. In another exemplary embodiment, the writing or reading operation of the memory device is monitored, and then a time point when the memory cells need to be refreshed may be determined through the above operation.

When it is determined to perform the refresh operation, the resistance state of each of the memory cells may be determined (S120). In one exemplary embodiment, the resistance states of the memory cells may be determined by reading data from the memory cells. The reading operation may be a pre-reading operation before performing the re-writing operation. In another exemplary embodiment, the resistance states of the memory cells may be determined based on the write information about the memory cells.

Each of the memory cells may have one of a plurality of resistance states that are classified according to resistance level. The memory cells may be classified as in the low resistance state or in the high resistance state based on their respective determined resistance states. As described above, the low resistance state and the high resistance state may be determined based on critical resistance level that is set in advance. In one exemplary embodiment, the uppermost resistance state having the highest resistance level from among the plurality of resistance states may be classified as the high resistance state, and the remaining resistance states may be classified as the low resistance states. Here, the high resistance state may be referred to as the reset state and the low resistance state may be referred to as the set state.

The re-writing operation is then performed (S130) on the memory cells in the low resistance state. The memory cells in the low resistance state are the memory cells in which the data is written in the set writing operation, and may have poor data retention characteristic since the resistance levels of the memory cells increase with time after writing the data in the memory cells. The set writing operation is performed again on the memory cells in the low resistance state to reduce the resistance levels, and thus the resistance levels of the memory cells in the low resistance state may be recovered. In this way the data retention characteristics of the memory cells in the low resistance state may be compensated for, and data reliability of the memory cells may be improved.

Referring toFIG. 14, the memory device may receive (S210) a refresh command from the outside, for example from the memory controller. In this exemplary embodiment the memory controller determines whether the refresh operation is performed by using the memory device, that is, the timing of performing the refresh, and the memory device may perform the refresh operation in response to the refresh command transmitted from the memory controller.

In order to perform the refresh operation, a pre-reading operation from the memory cells may be performed (S220). When resistance levels of the memory cells are sensed through the pre-reading operation, the resistance state of each of the memory cells may be determined (S230). As described above, the write/read circuit120(FIG. 2) provides the control logic130or the refresh control unit131with the pre-read data, and the control logic130or the refresh control unit131may determine the resistance state of each of the memory cells based on the pre-read data. Each of the memory cells may have one of the plurality of resistance states. In addition, the plurality of resistance states may be classified as the low resistance state or the high resistance state.

After that, the re-writing operation, for example, the set writing operation on the memory cells in the low resistance state is performed (S240) to recover the resistance level of the low resistance state.

Whether to perform the refresh operation in the memory device100ofFIG. 1may be determined according to a status of performing the writing/reading operation of the memory device100. Referring toFIG. 15, a read command is received (S310) from outside, for example from the memory controller (S310). In this exemplary embodiment the read command and an address indicating a cell region that is to be read may be received together.

A reading operation is performed (S320) on the memory cells indicated by the address. Read data may be output to the memory controller, or may be provided to the refresh control unit131.

In addition, a read cycle may be detected (S330) with performance of or after performing the reading operation (S320). Whenever the read command is received, a counting value representing the number of reading operations increases to detect the read cycle. As described above, the read cycle may denote the read cycle of the entire memory cell array or of some of the memory cell array.

According to a result of detecting the read cycle, it may be determined (S340) whether arefresh operation is necessary. For example, if the counting value does not reach a predetermined reference value, the memory device may determine that the refresh operation is not necessary and may finish (S350) the reading operation. If the counting value reaches the predetermined reference value, the memory device determines that the refresh operation is necessary, and then, may perform the refresh operation on the memory cell array. The refresh operation may be performed on the entire memory cell array or on some of the memory cell array.

The resistance states of at least some memory cells in the memory cell array, for example, the memory cells from which data is read according to the above address, may be determined (S360). In this exemplary embodiment, the resistance states of the above memory cells are pre-read, and then the resistance states may be determined. In another exemplary embodiment, the resistance states of the memory cells may be determined based on the data read (S320) as described above.

After that, the memory cells are classified as either memory cells in the low resistance state or memory cells in the high resistance state based on the resistance state, and the re-writing operation, for example, the set writing operation, may be performed (S370) on the memory cells in the low resistance state.

Whether to perform the refresh operation in the memory device100(FIG. 1) may be determined in the memory device100based on a status of performing the writing/reading operation of the memory device100.

Referring toFIG. 16, a read command may be received (S410) after being transmitted from outside. In one exemplary embodiment, the read command and an address indicating the memory cell region that is to be read may be transmitted together.

The reading operation is performed (S420) on the memory cells indicated by the address. The read data may be output to the memory controller, and may be provided to the refresh control unit131.

An error detection operation may be performed (S430) with respect to the read data in order to verify the read data. According to an error detection result, the number of errors may be determined (S440), for example by determining the number of fail bits.

It may be determined (S450), based on the number of errors detected for example, whether the refresh operation needs to be performed. If the number of errors is equal to or less than a reference value, the reading operation may be finished (S460) without performing an additional refresh operation.

However, if the refresh operation needs to be performed, the resistance state of each of the memory cells is determined (S470) to determine on which memory cells the re-writing operation is to be performed. In this exemplary embodiment, the resistance states of the memory cells are pre-read to determine the read resistance states. In another exemplary embodiment, the resistance states of the memory cells may be determined based on the data read (S420) as discussed above. The memory cells may be classified as being in the low resistance state or in the high resistance state based on the resistance states, and the re-writing operation, for example, the set writing operation, may be performed (S480) on the memory cells in the low resistance state.

FIG. 17illustrates the distribution of the resistance levels of the memory cell MC when the memory cell MC is a multi level cell programmed with two bits. As described above with reference toFIG. 6B, the memory cell MC may have one of the first through fourth resistance states RS1through RS4.

The resistance states may be classified as in the high resistance state HRS or the low resistance state LRS based on comparison with the critical resistance level Rref. As illustrated, the critical resistance level Rref may be located between the second resistance state RS2and the third resistance state RS3. Thus, the third resistance state RS3and the fourth resistance state RS4, which are lower than the critical resistance level Rref, may be referred to as low resistance states LRS. Conversely, the first resistance state RS1and the second resistance state RS2, which are greater than the critical resistance level Rref, may be referred to as high resistance state HRS. Also, the low resistance state LRS may be referred to as the set state, and the high resistance state HRS may be referred to as the reset state. The memory cells in the low resistance state LRS, that is, the third and fourth resistance states RS3and RS4, are the memory cells on which the set writing operation is performed, and the memory cells in the high resistance state HRS, that is, the first and second resistance states RS1and RS2, are the memory cells on which the reset writing operation is performed.

In this exemplary embodiment, when the memory device100(FIG. 1) is refreshed, the re-writing operation is performed on the memory cells having the third and fourth resistance states RS3and RS4, which are low resistance states LRS, and not on the memory cells having the first and second resistance states RS1and RS2, which are high resistance states HRS.

FIG. 18illustrates the method of operating the memory device when the memory cells have the resistance distribution shown inFIG. 17. Referring toFIG. 18, when a writing operation is performed on the memory cells at t1, the memory cells may have one of the plurality of resistance states RS1through RS4. Here, the memory cells in the high resistance states HRS, for example, the first and second resistance states RS1and RS2, are the memory cells on which the reset writing operation is performed, and thus may be in the reset states. The memory cells in the low resistance state, for example, the third and fourth resistance states RS3and RS4, are the memory cells on which the set writing operation is performed, and thus in the set states. Since the memory cells in the low resistance state LRS have poor data retention characteristics, their resistance levels may increase as time has passed. Thus, the memory device100(FIG. 1) may determine to perform the refresh operation at t2, and may perform the re-writing operation on the memory cells in the low resistance states LRS to recover the resistance levels. However, resistance levels of the memory cells in the high resistance state HRS do not increase much, and thus the re-writing operation may not be necessary. Accordingly, in this exemplary embodiment the re-writing operation may be selectively performed only on the memory cells in the low resistance state LRS, e.g., the third and fourth resistance states RS3and RS4.

FIG. 19is a graph illustrating a method of operating the memory device according to another exemplary embodiment of the inventive concept. The memory cells may have one of the plurality of resistance states RS1through RS4, and in this case the resistance states RS1through RS4may be classified as high resistance state HRS or low resistance state LRS depending on comparison with the critical resistance level Rref.

In addition, in this exemplary embodiment frequency of the refresh operation may be determined according to the resistance states. For example, in the case of the first resistance state RS1, the refresh operation may be performed at a first refresh period T1that is the smallest, and thus the frequency of performing the refresh operation may be highest in the first resistance state RS1. In the case of the second resistance state RS2, the refresh operation may be performed at a second refresh period T2that is longer than the first refresh period T1. In the case of the third resistance state RS3, the refresh operation may be performed at a third refresh period T3that is longer than the second refresh period T2. In case of the fourth resistance state RS4that is the high resistance state HRS, the refresh operation may be not performed. As the resistance level of the resistance states lowers, increase in the resistance level according to a time lapse may rise. According to this method of operating the memory device in this exemplary embodiment, the refresh cycle and the refresh frequency may vary depending on the resistance states of the memory cells so that the refresh operation may be performed effectively and the data may be stabilized.

InFIG. 20, a schematic block diagram of a memory system including a resistive memory device according to an exemplary embodiment, components denoted by the same reference numerals as those ofFIG. 1have the same functions as those ofFIG. 1, and thus detailed descriptions thereof are omitted.

Referring toFIG. 20, a memory controller200amay include an error detector210. The error detector210may be, for example, an error correction code (ECC) engine. The error detector210performs an error detection operation with respect to the data read from a memory device100ato determine whether the refresh operation is to be performed. The error detector210may transmit a command CMD for performing the refresh operation to the memory device100aif the number of detected errors is equal to or greater than a predetermined reference value. In addition, the error detector210may perform error detection by the cell region unit, and may then transmit to the memory device100athe command CMD for performing the refresh operation, and an address ADDR indicating the cell region in which the refresh operation is to be performed.

On receiving the command CMD for performing the refresh operation, the memory device100amay perform the refresh operation on all or some of the memory cells in the memory cell array110. Here, as described above, the memory device100amay perform a pre-reading operation of data from the memory cells to determine the resistance states of the memory cells, and then may perform the re-writing operation on the memory cells that are determined to have the low resistance state.

FIG. 21is a block diagram of a memory card system1000having a memory system applied thereto in accordance with exemplary embodiments including those ofFIGS. 1 through 20.

Referring toFIG. 21, the memory card system1000may include a host1100and a memory card1200. The host1100may include a host controller1110and a host connector1120. The memory card1200may include a card connector1210, a card controller1220, and a memory device1230. The memory card1200may be embodied by using the exemplary embodiments ofFIGS. 1 through 20.

The host1100may write data to the memory1200or may read data stored in the memory card1200. The host controller1110may transmit a command CMD, a clock signal CLK generated by a clock generator (not shown) in the host1100, and data DATA to the memory card1200via the host connector1120.

In response to the command CMD received via the card connector1210, the card controller1220may store the data DATA in the memory device1230in synchronization with a clock signal generated by a clock generator (not shown) in the card controller1220. The memory device1230may store the data DATA that is transmitted from the host1100.

The memory card1200may be embodied as a compact flash card (CFC), a microdrive, a smart media card (SMC), a multimedia card (MMC), a Security Digital Card (SDC), a memory stick, or a USB flash memory drive for example.

FIG. 22illustrates a resistive memory module2000according to an exemplary embodiment. Referring toFIG. 22, the resistive memory module2000may include memory devices2210through2240, and a control chip2100. Each of the memory devices2210through2240may be embodied by using the exemplary embodiments ofFIGS. 1 through 20. In response to various signals transmitted by an external memory controller, the control chip2100may control the memory devices2210through2240. For example, in response to various commands and addresses that are transmitted from an external source, the control chip2100may activate the memory devices2210through2240corresponding to the various commands and addresses and thus may control writing/reading operations. Also, the control chip2100may perform various post processing operations on read data output from each of the memory devices2210through2240, e.g., the control chip2100may perform error detection and correction operations on the read data.

In this exemplary embodiment, the memory devices2210through2240determine the resistance states of the memory cells and perform the re-writing operation selectively on the memory cells in the low resistance or reset state to perform the refresh operation effectively.

FIG. 23is a block diagram of a solid state disk (SSD) system3000including a memory system applied thereto, according to an exemplary embodiment.

Referring toFIG. 23, the SSD system3000may include a host3100and an SSD3200. The SSD3200exchanges signals with the host3100via a signal connector, and may receive a power input from a power connector. The SSD3200includes an SSD controller3210, an auxiliary power device3220, and a plurality of memory devices3230,3240,3250. The SSD3200may be embodied by using the exemplary embodiments ofFIGS. 1 through 20.

FIG. 24is a block diagram of a computing system4000including a memory system according to an exemplary embodiment.

Referring toFIG. 24, the computing system4000may include a memory system4100, a processor4200, a RAM4300, an input/output (I/O) device4400, and a power supply device4500. Although not illustrated inFIG. 24, the computing system4000may further include ports capable of communicating with a video card, a sound card, a memory card, or a USB device, or other electronic devices. The computing system4000may be embodied as a PC, a portable electronic device such as a notebook computer, a mobile phone, a personal digital assistant (PDA), or a camera.

The processor4200may perform particular calculations or tasks. In one or more exemplary embodiments, the processor4200may be a micro-processor or a Central Processing Unit (CPU). The processor4200may perform communication with the RAM4300, the I/O device4400, and the memory system4100via a bus4600such as an address bus, a control bus, or a data bus. Here, the memory system4100may be embodied by using the exemplary embodiments ofFIGS. 1 through 20.

In one or more exemplary embodiments, the processor4200may also be connected to an extended bus such as a Peripheral Component Interconnect (PCI) bus.

The RAM4300may store data for operations of the computing system4000. As described above, the memory device according to the one or more exemplary embodiments of the inventive concept may be applied to the RAM4300. Alternatively, a DRAM, a mobile DRAM, an SRAM, a PRAM, an FRAM, or an MRAM may be used as the RAM4300.

The I/O device4400may include an input unit such as a keyboard, a keypad, or a mouse, and an output unit such as a printer or a display. The power supply device4500may supply an operating voltage for the operations of the computing system4000.