Abstract:
A nonvolatile memory device includes memory cell blocks each configured to comprise memory cells erased by an erase voltage, supplied to a word line, and a bulk voltage supplied to a bulk, a bias voltage generator configured to generate a first erase voltage, having a first pulse width and a first amplitude, in order to perform the erase operation of the memory cells and a second erase voltage, having a second pulse width narrower than the first pulse width and a second amplitude lower than the first amplitude, in order to perform an additional erase operation if an unerased memory cell is detected after the erase operation is performed, and a bulk voltage generator configured to generate the bulk voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Priority to Korean patent application number 10-2010-0066506 filed on Jul. 9, 2010, the entire disclosure of which is incorporated by reference herein, is claimed. 
     BACKGROUND 
     Exemplary embodiments relate to a nonvolatile memory device and a method of operating the same and, more particularly, to a nonvolatile memory device and a method of operating the same, which can be applied to an erase operation of a memory cell. 
     A flash memory device is a representative nonvolatile memory device and is divided into a NOR type and a NAND type. The NOR type flash memory device is chiefly used to store a small amount of information at high speed. The NAND type flash memory device is chiefly used to store a large amount of information. Furthermore, the flash memory device performs a read operation for data output, a program operation for data input, and an erase operation for data erasure. In particular, the program operation and the erase operation of the NAND type flash memory device are executed by means of Fowler-Nordheim (FN) tunneling of electrons which are generated in a tunnel insulating layer between the P-well well and the floating gate of a memory cell. That is, electrons are injected from the P-well to the floating gate of the memory cell via the tunnel insulating layer by means of FN tunneling, so that the program operation of the flash memory device is performed. In this program operation, electrons are injected into only the floating gates of selected ones of a plurality of memory cells included in a memory cell block. Furthermore, electrons within the floating gate of the memory cell are discharged toward the P-well by means of FN tunneling, so that the erase operation of the flash memory device is performed. In this erase operation, data stored in all the memory cells included in a selected one of a plurality of memory cell blocks is erased at the same time. That is, the erase operation is performed per memory cell block. 
       FIG. 1  is a circuit diagram of memory cells and pass gates for illustrating the erase operation of a known flash memory device. 
     Referring to  FIG. 1 , during the erase operation, a bias voltage Vb of 0 V is supplied to a global word line GWL, and a bulk voltage VBK 1  of 20 V is supplied to the P-wells of memory cells CA 1  to CAn and CB 1  to CBn (n is an integer). The sources and drains of the memory cells CA 1  to CAn and CB 1  to CBn become a floating state. Furthermore, a block selection signal BKSEL 1  of a voltage (Vcc) level is inputted to the gate of an NMOS transistor NM 1  coupled between the local word line WL 1  and the global word line GWL of a selected (that is, to be erased) memory cell block A. Furthermore, a bulk voltage VBK 2  of 0 V is supplied to the substrate (not shown) of the NMOS transistor NM 1 . The NMOS transistor NM 1  is turned on in response to the block selection signal BKSEL 1  and configured to couple the local word line WL 1  to the global word line GWL. Consequently, the voltage of the local word line WL 1  becomes 0 V, and a voltage difference of 20 V is generated between each of the control gates (not shown) of the memory cells CA 1  to CAn, coupled to the local word line WL 1 , and each of the P-wells of the memory cells CA 1  to CAn. Accordingly, the electrons of the floating gates of the memory cells CA 1  to CAn are discharged toward the P-wells, so that the erase operation of the memory cell block A is performed. 
     Meanwhile, a block selection signal BKSEL 2  of 0 V is supplied to the gate of an NMOS transistor NM 2  coupled between the local word line WL 2  and the global word line GWL of an unselected (that is, not to be erased) memory cell block B. Furthermore, a bulk voltage VBK 2  of 0 V is supplied to the substrate of the NMOS transistor NM 2 . The NMOS transistor NM 2  is turned off in response to the block selection signal BKSEL 2  and configured to separate the local word line WL 2  from the global word line GWL. Consequently, the local word line WL 2  becomes a floating state. Next, the bulk voltage VBK 1  of 20 V supplied to the P-wells of the memory cells CB 1  to CBn is drained out to the local word line WL 2  owing to a capacitive coupling phenomenon, so that the voltage level of the local word line WL 2  is boosted up to about 19 V. Accordingly, a fine voltage difference of about 1 V is generated between the local word line WL 2  and the P-wells of the memory cells CB 1  to CBn, and thus electrons are not discharged from the floating gates of the memory cells CB 1  to CBn. Consequently, during the time for which the erase operation of the memory cell block A is performed, the erase operation of the memory cell block B is not performed. 
     Meanwhile, with an increase in the number of erase/program cycles, a fast program phenomenon in which a threshold voltage exceeds a target voltage during the program operation or a slow erase phenomenon in which a threshold voltage is not sufficiently lowered below a target voltage during the erase operation is generated. This phenomenon is described in more detail below with reference to  FIG. 2 . 
       FIG. 2  is a characteristic graph illustrating a slow erase characteristic and a fast program characteristic according to the number of erase operations in a known art. 
     Referring to  FIG. 2 , although a program or erase operation is performed under the same conditions, a threshold voltage gradually becomes higher than a target voltage with an increase in the number of erase cycles after the program or erase operation. This phenomenon becomes worse because the amount of electrons trapped in a tunnel insulating layer between the floating gate of a memory cell and a semiconductor substrate is increased with an increase in the number of program/erase cycles. This means that the program operation is quickly performed or the erase operation is slowly performed. 
     Meanwhile, in order to discharge electrons accumulated in the floating gate toward the substrate during the erase operation, a high voltage has to be supplied to the substrate. With an increase in the voltage difference between the word line and the bulk of the substrate, the fast program and slow erase phenomena become worse. 
       FIG. 3  is a characteristic graph illustrating a slow erase characteristic and a fast program characteristic according to the levels of an erase voltage in a known art. 
     From  FIG. 3 , it can be seen that if an erase operation is performed in a state in which the voltage difference between the word line and the bulk is high (that is, in a high potential erase state), the fast program phenomenon and the slow erase phenomenon become worse. It can also be seen that if an erase operation is performed in the state in which the voltage difference between the word line and the bulk is relatively low (that is, in a low potential erase state), the fast program phenomenon and the slow erase phenomenon are minimized. 
     In order to prevent the fast program phenomenon and the slow erase phenomenon, the erase operation has to be performed in a state in which the voltage difference between the word line and the bulk is reduced. In this case, however, the time that it takes to perform the erase operation time is lengthened, and the erase operation may not be normally performed. After the erase operation, an erase verification operation is performed. In the case where the erase operation is not normally performed, a corresponding block is treated as an invalid block and is not used. In this case, there are problems in that the number of available blocks is reduced and the data storage capacity is reduced. 
     BRIEF SUMMARY 
     According to exemplary embodiments of this disclosure, an increment of the operation time can be minimized and the generation of a fast program phenomenon and a slow erase phenomenon can be prevented by controlling the level of voltage supplied to a global word line and a substrate (or a bulk or P-well) during the erase operation of a selected memory block. 
     A nonvolatile memory device according to an aspect of this disclosure includes memory cell blocks each configured to include memory cells erased by an erase voltage, supplied to a word line, and a bulk voltage supplied to a bulk; a bias voltage generator configured to generate a first erase voltage, having a first pulse width and a first amplitude, in order to perform the erase operation of the memory cells and a second erase voltage, having a second pulse width narrower than the first pulse width and a second amplitude lower than the first amplitude, in order to perform an additional erase operation if an unerased memory cell is detected after the erase operation is performed; and a bulk voltage generator configured to generate the bulk voltage. 
     The bias voltage generator may be configured to lower the level of the second erase voltage whenever the additional erase operation is performed. The bias voltage generator may be configured to lower the level of the second erase voltage by a certain value whenever the additional erase operation is performed. The bias voltage generator may be configured to increase a decrement of the second erase voltage whenever the additional erase operation is performed. The bias voltage generator may be configured to decrease a decrement of the second erase voltage whenever the additional erase operation is performed. 
     A nonvolatile memory device according to another aspect of this disclosure includes memory cell blocks each configured to include memory cells erased by an erase voltage, supplied to a word line, and a bulk voltage supplied to a bulk; a bias voltage generator configured to supply the erase voltage to the word line in order to perform an erase operation of the memory cells; and a bulk voltage generator configured to supply a bulk voltage to a bulk in order to perform the erase operation and another bulk voltage, having a narrowed pulse width and a raised potential, to the bulk in order to perform an additional erase operation if an unerased memory cell is detected after the erase operation. 
     The bulk voltage generator may be configured to raise the level of the bulk voltage by a certain value whenever the additional erase operation is performed. The bulk voltage generator may be configured to increase an increment of the bulk voltage whenever the additional erase operation is performed. The bulk voltage generator may be configured to decrease an increment of the bulk voltage whenever the additional erase operation is performed. 
     The bulk voltage generator may be configured to narrow the pulse width of the bulk voltage whenever the additional erase operation is performed. 
     A method of operating a nonvolatile memory device according to another aspect of this disclosure includes performing an erase operation by supplying an erase voltage of a positive potential to the word lines of a selected memory block and supplying a bulk voltage to the bulk of memory cells coupled to the word lines, performing an erase verification operation for detecting an unerased memory cell from among the memory cells, and performing an additional erase operation by increasing the difference between the erase voltage and the bulk voltage and reducing the time taken for the erase voltage to be supplied to the word lines or the time taken for the bulk voltage to be supplied to the bulk, if the unerased memory cell is detected. 
     The level of the erase voltage may be lowered in order to increase the difference between the erase voltage and the bulk voltage whenever the additional erase operation may be performed. The level of the erase voltage may be lowered by a certain value in order to increase the difference between the erase voltage and the bulk voltage whenever the additional erase operation may be performed. A decrement of the erase voltage may be increased in order to increase the difference between the erase voltage and the bulk voltage whenever the additional erase operation is performed. A decrement of the erase voltage may be decreased in order to increase the difference between the erase voltage and the bulk voltage whenever the additional erase operation is performed. 
     The level of the bulk voltage may be raised in order to increase the difference between the erase voltage and the bulk voltage whenever the additional erase operation is performed. The level of the bulk voltage may be raised by a certain value in order to increase the difference between the erase voltage and the bulk voltage whenever the additional erase operation is performed. An increment of the bulk voltage may be increased in order to increase the difference between the erase voltage and the bulk voltage whenever the additional erase operation is performed. An increment of the bulk voltage may be decreased in order to increase the difference between the erase voltage and the bulk voltage whenever the additional erase operation is performed. The level of the erase voltage may be lowered and the level of the bulk voltage may be raised in order to increase the difference between the erase voltage and the bulk voltage whenever the additional erase operation is performed. 
     The level of the erase voltage may be lowered by a certain value and the level of the bulk voltage may be raised by a certain value, in order to increase the difference between the erase voltage and the bulk voltage whenever the additional erase operation is performed. A decrement of the erase voltage may be increased and an increment of the bulk voltage may be increased, in order to increase the difference between the erase voltage and the bulk voltage whenever the additional erase operation is performed. A decrement of the erase voltage may be decreased and an increment of the bulk voltage may be decreased, in order to increase the difference between the erase voltage and the bulk voltage whenever the additional erase operation is performed. 
     Whenever the additional erase operation is performed, the pulse widths of the erase voltage and the bulk voltage may be narrowed in order to reduce the time taken for the erase voltage to be supplied to the word lines and the time taken for the bulk voltage to be supplied to the bulk. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of memory cells and pass gates for illustrating the erase operation of a known flash memory device; 
         FIG. 2  is a characteristic graph illustrating a slow erase characteristic and a fast program characteristic according to the number of erase operations in a known art; 
         FIG. 3  is a characteristic graph illustrating a slow erase characteristic and a fast program characteristic according to the levels of an erase voltage in a known art; 
         FIG. 4  is a block diagram of a flash memory device according to an exemplary embodiment of this disclosure; 
         FIG. 5  is a detailed diagram of a memory cell array, a block selector, a second bias voltage generator, a bulk voltage generator, and an X-decoder shown in  FIG. 4 ; 
         FIG. 6  is a detailed diagram of memory cells, switching elements, the bulk voltage generator, and the bias voltage selection unit shown in  FIG. 5 ; 
         FIG. 7  is a flowchart illustrating a method of controlling an erase operation of the flash memory device according to an exemplary embodiment of this disclosure; 
         FIG. 8A  is a diagram showing an example of the switching element shown in  FIG. 6 ; 
         FIG. 8B  is a diagram showing a change in the energy potential according to a change in the bias voltage of a word line in the switching element of  FIG. 6 ; 
         FIGS. 9A to 9C  are waveforms illustrating a first exemplary embodiment in which voltage is supplied to a global word line and a P-well during the erase operation in  FIG. 5 ; 
         FIGS. 10A to 10C  are waveforms illustrating a second exemplary embodiment in which voltage is supplied to the global word line and the P-well during the erase operation in  FIG. 5 ; 
         FIG. 11  is a characteristic graph illustrating a shift in the threshold voltage of an unselected block during the erase operation; and 
         FIG. 12  is a characteristic graph illustrating a slow erase characteristic and a fast program characteristic according to the number of erase operations according to this disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, some exemplary embodiments of this disclosure will be described in detail with reference to the accompanying drawings. The figures are provided to allow those having ordinary skill in the art to understand the scope of the embodiments of the disclosure. 
       FIG. 4  is a block diagram of a flash memory device according to an exemplary embodiment of this disclosure. 
     Referring to  FIG. 4 , the flash memory device  100  includes a memory cell array  110 , an input buffer  120 , a control logic circuit  130 , a high voltage generator  140 , an X-decoder  150 , a block selector  160 , a page buffer  170 , a Y-decoder  180 , a data I/O buffer  190 , and a pass/fail check circuit  200 . 
     The memory cell array  110  includes memory cell blocks MB 1  to MBK (K is a positive integer), each including a plurality of memory cells (not shown). The input buffer  120  receives a command signal CMD or an address signal ADD and outputs it to the control logic circuit  130 . The control logic circuit  130  receives the command signal CMD or the address signal ADD in response to external control signals /WE, /RE, ALE, and CLE. The control logic circuit  130  generates one of a read command READ, a program command PGM, and an erase command ERS in response to the command signal CMD. Furthermore, in response to the address signal ADD, the control logic circuit  130  generates a row address signal RADD and a column address signal CADD. 
     The high voltage generator  140  includes a bulk voltage generator  40 , a first bias voltage generator  50 , and a second bias voltage generator  60 . The bulk voltage generator  40  generates a bulk voltage VCB in response to one of the read command READ, the program command PGM, and the erase command ERS and supplies the bulk voltage VCB to the bulk (for example, a P-well) of memory cells. More particularly, the bulk voltage generator  40  generates the bulk voltage VCB of a low voltage (for example, 0 V) level in response to the read command READ or the program command PGM. Furthermore, the bulk voltage generator  40  generates the bulk voltage VCB of a high voltage (for example, 16 V to 20 V) level in response to the erase command ERS. Meanwhile, if, after an erase verification operation, the pass/fail check circuit  200  detects data outputted from the Y-decoder  180  and a memory cell on which an erase operation has not been normally performed is determined to exist as a result of the detection, the control logic circuit  130  controls the bulk voltage generator  140  so that the bulk voltage generator  140  controls the level of the bulk voltage VCB. For example, if an erase operation has not been normally performed, the level of the bulk voltage VCB may be raised in unit of 0.5 V or 1 V, and the increment may be changed according to the design. 
     The first bias voltage generator  50  generates a drain bias voltage VGD and a source bias voltage VGS in response to one of the read command READ, the program command PGM, and the erase command ERS, supplies the drain bias voltage VGD to a global drain selection line GDSL, and supplies the source bias voltage VGS to a global source selection line GSSL. More particularly, the first bias voltage generator  50  generates the drain bias voltage VGD and the source bias voltage VGS of a high voltage (for example, 4.5 V) level in response to the read command READ. The first bias voltage generator  50  generates the drain bias voltage VGD of an internal voltage (VCC, not shown) level and the source bias voltage VGS of a low voltage level in response to the program command PGM. Furthermore, the first bias voltage generator  50  generates the drain bias voltage VGD and the source bias voltage VGS of a low voltage level in response to the erase command ERS. 
     The second bias voltage generator  60  generates word line bias voltages VWF 1  to VWFJ (J is a positive integer), word line bias voltages VWS 1  to VWSJ (J is a positive integer), or word line bias voltages VWT 1  to VWTJ (J is a positive integer) in response to a decoding signal DEC and one of the read command READ, the program command PGM, and the erase command ERS and supplies them to global word lines GWL 1  to GWLJ (J is a positive integer). More particularly, the second bias voltage generator  60  generates the word line bias voltages VWF 1  to VWFJ in response to the read command READ. The second bias voltage generator  60  generates the word line bias voltages VWS 1  to VWSJ in response to the program command PGM. Furthermore, the second bias voltage generator  60  generates the word line bias voltages VWT 1  to VWTJ in response to the erase command ERS. Here, the second bias voltage generator  60  generates a positive voltage higher than 0 V in response to the erase command ERS. After an erase verification operation is performed, the pass/fail check circuit  200  detects data outputted from the Y-decoder  180 . If, as a result of the detection, there is a memory cell on which an erase operation has not been normally performed, the control logic circuit  130  controls the second bias voltage generator  60  so that the levels of the word line bias voltages VWT 1  to VWTJ are controlled. For example, if an erase operation has not been normally performed, the levels of the word line bias voltages VWT 1  to VWTJ are lowered in unit of 0.1 V to 0.5 V, and the decrement may be changed according to the design. 
     If an erase operation has not been normally performed, the bulk voltage generator  40  and the second bias voltage generator  60  control the levels of the output voltages. This is for the purpose of performing the erase operation again by increasing the voltage difference between the word line and the bulk. In this case, only one of the bulk voltage generator  40  and the second bias voltage generator  60  can control the levels of the output voltages so that the voltage difference between the word line and the bulk can be increased. Alternatively, both the bulk voltage generator  40  and the second bias voltage generator  60  may control the levels of the output voltages. This is described in detail later. 
     The X-decoder  150  generates the decoding signal DEC by decoding the row address signal RADD. The block selector  160  selects one or some of the memory cell blocks MB 1  to MBK in response to the decoding signal DEC and couples the local word lines (refer to WL 11  to WL 1 J of  FIG. 5 ) of a selected memory cell block (or selected memory cell blocks) and the respective global word lines GWL 1  to GWLJ. Furthermore, the block selector  160  couples the drain selection line (refer to one of DSL 1  to DSLK of  FIG. 5 ) of the selected memory cell block and the global drain selection line GDSL and couples the source selection line (refer to one of SSL 1  to SSLK of  FIG. 5 ) of the selected memory cell block and a global source selection line GSSL. The configurations and operations of the page buffer  170 , the Y-decoder  180 , and the data I/O buffer  190  will be understood by those having ordinary skill in the art to which this disclosure pertains, and a detailed description thereof is omitted. 
       FIG. 5  is a detailed diagram of the memory cell array, the block selector, the second bias voltage generator, the bulk voltage generator, and the X-decoder shown in  FIG. 4 . 
     Referring to  FIG. 5 , the memory cell block MB 1  of the memory cell array  110  includes memory cells M 111  to M 1 JT (J and T are positive integers), drain selection transistors DST 1 , and source selection transistors SST 1 . The memory cells M 111  to M 1 JT share bit lines BL 1  to BLT (T is a positive integer), local word lines WL 11  to WL 1 J (J is a positive integer), and a common source line CSL 1 . That is, the memory cells M 111  to M 11 T are coupled to the respective bit lines BL 1  to BLT through the drain selection transistors DST 1 , and the memory cells M 1 J 1  to M 1 JT are coupled to the common source line CSL 1  through the respective source selection transistors SST 1 . Furthermore, the gates of the memory cells M 111  to M 1 JT are coupled to the local word lines WL 11  to WL 1 J. Meanwhile, the gates of the drain selection transistors DST 1  are coupled to a local drain selection line DSL 1 . The gates of the source selection transistors SST 1  are coupled to the local source selection line SSL 1 . 
     Each of the memory cell blocks MB 2  to MBK of the memory cell array  110  has a similar configuration to the memory cell block MB 1 , and a detailed description thereof is omitted. The block selector  160  includes a block switching unit  161  and a plurality of switching units PG 1  to PGK (K is a positive integer). The block switching unit  161  generates block selection signals BSEL 1  to BSELK (K is a positive integer) in response to the decoding signal DEC received from the X-decoder  150 . The plurality of switching units PG 1  to PGK is disposed to correspond to the respective memory cell blocks MB 1  to MBK and is enabled or disabled in response to the respective block selection signals BSEL 1  to BSELK. 
     Each of the plurality of switching units PG 1  to PGK includes a plurality of switching elements. For example, the switching unit PG 1  includes switching elements GD 1 , G 11  to G 1 J, and GS 1 . Each of the switching units PG 2  to PGK has a similar configuration and operation to the switching unit PG 1 , and thus only the switching unit PG 1  is chiefly described. Preferably, the switching elements GD 1 , G 11  to G 1 J, and GS 1  may be implemented using NMOS transistors. Hereinafter, the switching elements GD 1 , G 11  to G 1 J, and GS 1  are referred to as NMOS transistors. The block selection signal BSEL 1  is inputted to the gates of the NMOS transistors GD 1 , G 11  to G 1 J, and GS 1 . The source of the NMOS transistor GD 1  is coupled to the global drain selection line GDSL, and the drain thereof is coupled to the local drain selection line DSL 1 . The sources of the NMOS transistors G 11  to G 1 J are coupled to the respective global word lines GWL 1  to GWLJ, and the drains thereof are coupled to the respective local word lines WL 11  to WL 1 J. The source of the NMOS transistor GS 1  is coupled to the global source selection line GSSL, and the drain thereof is coupled to the local source selection line SSL 1 . The NMOS transistors GD 1 , G 11  to G 1 J, and GS 1  are turned on or off at the same time in response to the block selection signal BSEL 1 . More particularly, when the block selection signal BSEL 1  is enabled, the NMOS transistors GD 1 , G 11  to G 1 J, and GS 1  are turned on. When the block selection signal BSEL 1  is disabled, the NMOS transistors GD 1 , G 11  to G 1 J, and GS 1  are turned off. When the NMOS transistors GD 1 , G 11  to G 1 J, and GS 1  are turned on, the global drain selection line GDSL is coupled to the local drain selection line DSL 1 , the global source selection line GSSL is coupled to the local source selection line SSL 1 , and the global word lines GWL 1  to GWLJ are coupled to the respective local word lines WL 11  to WL 1 J. 
     The second bias voltage generator  60  includes first to third pump circuits  61  to  63  and a bias voltage selection unit  64 . The pump circuit  61  generates read voltages VRD 1  and VRD 2  in response to the read command READ. Preferably, the read voltage VRD 1  has a high voltage (for example, 4.5 V) level, and the read voltage VRD 2  has a low voltage (for example, 0 V) level. During the read operation of the memory cell array  110 , the read voltage VRD 1  is supplied to a local word line coupled to the gates of unselected memory cells (that is, memory cells not to be read), and the read voltage VRD 2  is supplied to a local word line coupled to the gates of selected memory cells (that is, memory cells to be read). 
     The second pump circuit  62  generates program voltages VPG, VPS in response to the program command PGM. Preferably, the program voltages VPG, VPS have respective high voltage levels (for example, VPG=18 V and VPS=10 V). During the program operation of the memory cell array  110 , the program voltage VPG is supplied to a local word line coupled to the gates of memory cells to be programmed, and the program (or pass) voltage VPS is supplied to a local word line coupled to the gates of memory cells not to be programmed. 
     Furthermore, the third pump circuit  63  generates an erase voltage VERS of a positive level higher than 0 V in response to the erase command ERS. That is, during the erase operation, the third pump circuit  63  generates the erase voltage VERS so that voltage of a level higher than 0 V is supplied to the word lines of a selected memory block. At this time, in a memory block on which the erase operation is performed, the voltage difference between the word line and the bulk is lowered by the erase voltage VERS of a positive potential. The erase voltage VERS is preferably generated with a level to such an extent that the voltage difference between the word line and the bulk becomes 15 V to 20 V in the block on which the erase operation is performed. 
     Meanwhile, in the operation of the pass/fail check circuit  200  determining whether the erase operation has been normally performed, if data of a non-erase state (for example, 0) is detected (that is, the erase operation failed) from among the data outputted from the Y-decoder (refer to  180  of  FIG. 4 ), the third pump circuit  63  lowers the level of the erase voltage VERS in unit of 0.1 V to 0.5 V in response to the control signal of the control logic circuit  130 . In this case, the decrement may be changed according to the design. For example, a decrement of the erase voltage VERS may be fixed to a specific value. In some embodiments, whenever the erase operation is repeatedly performed, a decrement of the erase voltage VERS may be gradually increased or decreased. That is, the erase voltage VERS may be lowered in a primary or quadratic function way or may be lowered in an exponential function way. Accordingly, the voltage difference between the word line and the bulk is increased, and the erase operation is performed again according to the increased voltage difference. 
     The bias voltage selection unit  64  selects the read voltages VRD 1  and VRD 2  and supplies them to the global word lines GWL 1  to GWLJ as the respective word line bias voltages VWF 1  to VWFJ, selects the program voltages VPG, VPS and supplies them to the global word lines GWL 1  to GWLJ as the respective word line bias voltages VWS 1  to VWSJ (J is a positive integer), or selects the erase voltage VERS and supplies it to the global word lines GWL 1  to GWLJ as the respective word line bias voltages VWT 1  to VWTJ, in response to the decoding signal DEC of the X-decoder  150 . 
     The bulk voltage generator  40  generates the bulk voltage VCB of a high voltage to be supplied to a bulk (for example, a P-well) in which the memory cells M 111  to M 1 JT (J and T are positive integers) are formed during an erase operation, in response to the erase command ERS. In this case, the bulk voltage VCB is generated with a level to such an extent that the voltage difference between the word line and the bulk becomes 15 V to 20 V in a block on which the erase operation is performed. 
     Meanwhile, in the operation of the pass/fail check circuit  200  determining whether the erase operation has been normally performed, if data of a non-erase state (for example, 0) is detected (that is, the erase operation failed) from among the data outputted from the Y-decoder (refer to  180  of  FIG. 4 ), the bulk voltage generator  40  raises the level of the bulk voltage VCB in unit of 0.5 V to 1 V in response to the control signal of the control logic circuit  130 . In this case, the increment may be changed according to the design. For example, an increment of the bulk voltage VCB may be fixed to a specific value. In some embodiments, whenever the erase operation is repeatedly performed, an increment of the bulk voltage VCB may be gradually increased or decreased. That is, the bulk voltage VCB may be raised in a primary or quadratic function way or may be raised in an exponential function way. Accordingly, the voltage difference between the word line and the bulk is increased, and the erase operation is performed again according to the increased voltage difference. 
     If the erase operation is not normally performed after the erase operation is performed in the state in which a positive voltage is supplied to the global word line as described above, the erase operation is performed again by controlling the output voltage of any one of or both the third pump circuit  63  and the bulk voltage generator  40  so that the voltage difference between the word line and the bulk is increased. Here, the output voltage of the third pump circuit  63  or the bulk voltage generator  40  preferably is controlled so that the voltage difference between the word line and the bulk is 15 V or higher. 
       FIG. 6  is a detailed diagram of the memory cells, the switching elements, the bulk voltage generator, and the bias voltage selection unit shown in  FIG. 5 . 
     Referring to  FIG. 6 , the bias voltage selection unit  64  includes a selection signal generator  65  and selection circuits S 1  to SJ (J is a positive integer). The selection signal generator  65  generates selection signals SL 1  to SLJ on the basis of the decoding signal DEC. The selection circuits S 1  to SJ includes switches SW 11  to SW 15 , . . . , SWJ 1  to SWJ 5  coupled to the respective global word lines GWL 1  to GWLJ. Each of the selection circuits S 1  to SJ receives the read voltages VRD 1  and VRD 2 , the program voltages VPG and VPS, and the erase voltage VERS. The selection circuits S 1  to SJ output the respective word line bias voltages VWF 1  to VWFJ, VWS 1  to VWSJ, or VWT 1  to VWTJ to the global word lines GWL 1  to GWLJ in response to the respective selection signals SL 1  to SLJ. More particularly, for example, the switches SW 11  to SW 15  of the selection circuit S 1  are coupled between the global word line GWL 1  and the read voltages VRD 1  and VRD 2 , the program voltages VPG and VPS, and the erase voltage VERS. The switches SW 11  to SW 15  are turned on or off according to the respective logic values of bits B 1  to B 5  of the selection signal SL 1 . In the case where the switches SW 11  to SW 15  are formed of NMOS transistors, when the logic values of the bits B 1  to B 5  are 1, the switches SW to SW 15  are turned on. Furthermore, when the logic values of the bits B 1  to B 5  are 0, the switches SW 11  to SW 15  are turned off. 
     For example, when one of the switches SW 11  or SW 12  is turned on, one of the read voltages VRD 1  or VRD 2  is supplied to the global word line GWL 1  as the word line bias voltage VWF 1 . When one of the switches SW 13  or SW 14  is turned on, one of the program voltages VPG, VPS is supplied to the global word line GWL 1  as the word line bias voltage VWS 1 . Furthermore, when the switch SW 15  is turned on, the erase voltage VERS is inputted to the global word line GWL 1  as the word line bias voltage VWT 1 . At this time, since the selection signal generator  65  generates the logic value of one of the bits B 1  to B 5  as 1 and the logic values of the remaining bits as 0, one of the switches SW 11  to SW 15  is turned on and the remaining switches are turned off. Consequently, one of the read voltages VRD 1  or VRD 2 , the program voltages VPG or VPS, and the erase voltage VERS is supplied to the global word line GWL 1 . Each of the selection circuits S 2  to SJ has a similar configuration and operation to the selection circuit S 1 , and a detailed description thereof is omitted. 
     In  FIG. 6 , each of the selection circuits S 1  to SJ is illustrated to include the five switches SW 11  to SW 15 . However, in the case where the selection circuits S 1  to SJ output the respective word line bias voltages VWF 1  to VWFJ, VWS 1  to VWSJ, or VWT 1  to VWTJ, the configurations of the selection circuits S 1  to SJ may be modified in various ways. 
     In  FIG. 6 , only the NMOS transistors G 11 , GK 1 , G 1 J, and GKJ, the local word lines WL 11 , WL 1 J, WLK 1 , and WLKJ, and the memory cells M 111 , M 11 T, M 1 J 1 , M 1 JT, MK 11 , MK 1 T, MKJ 1 , and MKJT coupled to the global word lines GWL 1 , GWLJ are illustrated, for the simplification of the figure. The gates of the memory cells M 111  to M 11 T are coupled to the local word line WL 11 , and the gates of the memory cells M 1 J 1  to M 1 JT are coupled to the local word line WL 1 J. Furthermore, the gates of the memory cells MK 11  to MK 1 T are coupled to the local word line WLK 1 , and the gates of the memory cells MKJ 1  to MKJT are coupled to the local word line WLKJ. The source and drain of the NMOS transistor G 11  are coupled to the global word line GWL 1  and the local word line WL 11 , respectively. The source and drain of the NMOS transistor GK 1  are coupled to the global word line GWL 1  and the local word line WLK 1 , respectively. Furthermore, the source and drain of the NMOS transistor G 1 J are coupled to the global word line GWLJ and the local word line WL 1 J, respectively, and the source and drain of the NMOS transistor GKJ are coupled to the global word line GWLJ and the local word line WLKJ, respectively. 
       FIG. 7  is a flowchart illustrating a method of controlling the erase operation of the flash memory device according to an exemplary embodiment of this disclosure. 
     Referring to  FIGS. 5 and 7 , the levels of the erase voltage VWTJ and the bulk voltage VCB are set so that the erase voltage VWTJ has a positive potential and a difference between the erase voltage VWTJ and the bulk voltage VCB becomes 15 V at block S 701 . After the levels of the erase voltage VWTJ and the bulk voltage VCB are set, an erase operation is performed on all the flash memory cells of a memory block, selected in response to the block selection signal BLKWL, using the erase voltage VWTJ and the bulk voltage VCB at block S 702 . After the erase operation is finished, it is verified whether the erase operation has been normally performed at block S 703 . If, as a result of the verification, the erase operation has been normally performed on all the flash memory cells (that is, PASS), the erase operation is terminated. However, if, as a result of the verification, there is a flash memory cell on which the erase operation has not been normally performed (that is, FAIL), the levels of the erase voltage VWTJ and the bulk voltage VCB are set again and the erase operation is performed again. This is described in more detail below. 
     First, the number of erase operations is increased at block S 704 . It is then determined whether the number of erase operations is smaller than a set number at block S 705 . If, as a result of the determination, the number of erase operations is determined to be smaller than the set number, the level of the erase voltage VWTJ or the bulk voltage VCB is changed at block S 706 . Here, the levels of the erase voltage VWTJ and the bulk voltage VCB are changed so that the difference between the erase voltage VWTJ and the bulk voltage VCB gradually becomes greater than 15 V. A method of changing the levels of the erase voltage VWTJ and the bulk voltage VCB is described later. After the levels of the erase voltage VWTJ and the bulk voltage VCB are changed, the erase operation is performed again using the changed voltages at block S 702 . After the erase operation is performed, the steps S 703  to S 705  are performed again. 
     Meanwhile, if the erase operation is not normally performed until the number of erase operation becomes the set number, the corresponding block is treated as an invalid block at block S 707 . 
     The erase operation of the flash memory device  100  described with reference to  FIG. 7  is described in more detail with reference to  FIGS. 4 to 6 . First, the control logic circuit  130  generates the erase command ERS in response to the external control signals /WE, /RE, ALE, and CLE and the command signal CMD and generates the row address signal RADD in response to the address signal ADD. In response to the erase command ERS, the bulk voltage generator  40  of the high voltage generator  140  generates the bulk voltage VCB of a high voltage (for example, 17 V) level and supplies it to the bulk (or P-well) in which the memory cell blocks MB 1  to MBK are formed. Furthermore, in response to the erase command ERS, the first bias voltage generator  50  of the high voltage generator  140  generates the drain bias voltage VGD and the source bias voltage VGS of a low voltage (for example, 0 V) level. Accordingly, the drain bias voltage VGD is supplied to the global drain selection line GDSL, and the source bias voltage VGS is supplied to the global source selection line GSSL. 
     Meanwhile, the X-decoder  150  decodes the row address signal RADD and generates the decoding signal DEC. The second bias voltage generator  60  of the high voltage generator  140  generates the word line bias voltages VWT 1  to VWTJ in response to the erase command ERS and the decoding signal DEC and supplies them to the respective global word lines GWL 1  to GWLJ. More particularly, the third pump circuit  63  of the second bias voltage generator  60  generates the erase voltage VERS having a positive value in response to the erase command ERS. For example, the erase voltage VERS may have a value which is lower than the bulk voltage VCB, supplied to the P-well of memory cells during the erase operation, and is positive. Preferably, during the erase operation, the difference between the erase voltage VERS and the bulk voltage VCB supplied to the P-well of the memory cells may be set to be greater than or equal to 15 V. In response to the decoding signal DEC, the bias voltage selection unit  64  of the second bias voltage generator  60  selects the erase voltage VERS and outputs it as the word line bias voltages VWT 1  to VWTJ. More particularly, the selection signal generator  65  of the bias voltage selection unit  64  outputs all the values of the bits B 1  to B 5  of the selection signals SL 1  to SLJ as ‘00001’ in response to the decoding signal DEC. The switches SW 15  to SWJ 5  of the selection circuits S 1  to SJ of the bias voltage selection unit  64  are turned on and all the switches SW 11  to SWJ 1 , SW 12  to SWJ 2 , SW 13  to SWJ 3 , and SW 14  to SWJ 4  thereof are turned off, in response to the respective selection signals SL 1  to SLJ. Accordingly, the erase voltage VERS is supplied to the global word lines GWL 1  to GWLJ through the switches SW 15  to SWJ 5  as the word line bias voltages VWT 1  to VWTJ. 
     Furthermore, the block selector  160  selects one of the memory cell blocks MB 1  to MBK in response to the decoding signal DEC and couples the local word lines of the selected memory cell block to the respective global word lines GWL 1  to GWLJ. For example, if the memory cell block MB 1  is selected, the block switching unit  161  of the block selector  160  enables the block selection signal BSEL 1  and disables all the block selection signals BSEL 2  to BSELK in response to the decoding signal DEC. Consequently, only the switching unit PG 1  of the block selector  160  is enabled, and all the switching units PG 2  to PGK are disabled. More particularly, the switching elements GD 1 , G 11  to G 1 J, and GS 1  of the switching unit PG 1  are turned on at the same time, and all the switching elements GD 2  to GDK, G 21  to G 2 J, . . . , GK 1  to GKJ, and GS 2  to GSK of the switching units PG 2  to PGK are turned off. Accordingly, the drain selection line DSL 1  of the memory cell block MB 1  is coupled to the global drain selection line GDSL, and the source selection line SSL 1  thereof is coupled to the global source selection line GSSL. Consequently, the drain bias voltage VGD and the source bias voltage VGS of a low voltage level are supplied to the drain selection line DSL 1  and the source selection line SSL 1 , respectively, and thus the drain selection transistor DST 1  and the source selection transistor SST 1  are turned off. Accordingly, the drains and sources of the memory cells M 111  to M 1 JT of the memory cell block MB 1  become a floating state. 
     Furthermore, the local word lines WL 11  to WL 1 J of the memory cell block MB 1  are coupled to the respective global word lines GWL 1  to GWLJ. Consequently, the word line bias voltages VWT 1  to VWTJ of the global word lines GWL 1  to GWLJ are transferred to the respective local word lines WL 11  to WL 1 J. Accordingly, a voltage difference (for example, 15 V or higher) is generated between the bulk and each of the gates of the memory cells M 111  to M 1 JT of the memory cell block MB 1 , and electrons are discharged from the floating gates of the memory cells M 111  to M 1 JT by means of the voltage difference. Consequently, the erase operation of the memory cells M 111  to M 1 JT is performed. 
     Meanwhile, the drain selection lines DSL 2  to DSLJ of the memory cell blocks MB 2  to MBK are separated from the global drain selection line GDSL, and the source selection lines SSL 2  to SSLJ thereof are also separated from the global source selection line GSSL. Furthermore, all the local word lines WL 21  to WL 2 J, . . . , WLK 1  to WLKJ of the memory cell blocks MB 2  to MBK are separated from the global word lines GWL 1  to GWLJ. Accordingly, the local word lines WL 21  to WL 2 J, . . . , WLK 1  to WLKJ are boosted by the bulk voltage VCB of a high voltage (for example, 20 V) level, supplied to the memory cells of the memory cell blocks MB 2  to MBK. Consequently, a boosting voltage VBST close to the bulk voltage VCB is generated in the local word lines WL 21  to WL 2 J, . . . , WLK 1  to WLKJ. In this case, the operations of the NMOS transistors G 21  to G 2 J, . . . , GK 1  to GKJ coupled between the global word lines GWL 1  to GWLJ and the local word lines WL 21  to WL 2 J, . . . , WLK 1  to WLKJ of the memory cell blocks MB 2  to MBK are described in more detail with reference to  FIGS. 8A and 8B .  FIGS. 8A and 8B  show a cross-sectional view and energy potential of the NMOS transistor GK 1 . Each of the operations of the NMOS transistors G 21  to G 2 J, . . . , GK 2  to GKJ is similar to that of the NMOS transistor GK 1 , and a detailed description thereof is omitted. 
       FIG. 8A  shows the cross-sectional view of the NMOS transistor GK 1  which is a switching element coupled to the local word line WLK 1  of the memory cell block MBK. The word line bias voltage VWT 1  having a positive value is supplied to the source  72  of the NMOS transistor GK 1 , and the block selection signal BSELK of a low (for example, 0 V) level is supplied to the gate  74  of the NMOS transistor GK 1 . Furthermore, the boosting voltage VBST is supplied to the drain  73  of the NMOS transistor GK 1 . Since the block selection signal BSELK is in a low level, the NMOS transistor GK 1  is turned off. Furthermore, since the word line bias voltage VWT 1  has a positive value, the energy potential of the source ( 72 ) region is decreased to Ev 2 , as shown in  FIG. 8B . Accordingly, the amount of electrons flowing from the source  72  to a substrate  71  is reduced, and thus the amount of electrons flowing into the local word line WLK 1  coupled to the drain  73  is reduced. Since the leakage current generated in the NMOS transistor GK is reduced, the local word line WLK 1  maintains the level of the boosting voltage VBST. Consequently, data of the memory cells coupled to the local word line WLK 1  is not erased. 
     Unlike the above, if the word line bias voltage VWT 1  of 0 V is supplied to the source  72 , the energy potential of the source ( 72 ) region is increased to Ev 1 , as shown in  FIG. 8B . Accordingly, the amount of electrons flowing from the source  72  to the substrate  71  is increased, and thus the amount of the leakage current generated in the NMOS transistor GK 1  is increased. In order to reduce the leakage current of the NMOS transistor GK 1 , the energy potential of the source ( 72 ) region needs to be reduced. 
     After an erase operation is performed under the above conditions, whether all the memory cells of a block on which the erase operation has been performed have been normally erased is determined. This may be determined based on data outputted through the Y-decoder  180  from the page buffer  170 . For example, after a read operation is performed for every string in the state in which 0 V is supplied all the word lines, if data outputted through the Y-decoder  180  is ‘1’, the erase operation may be determined to have been normally performed. If the data is ‘0’, it may be determined that there is a memory cell on which the erase operation has not been normally performed. In a known art, if a fail cell exists as in the latter case, a corresponding block is treated as an invalid block and is not used, thereby reducing the data storage capacity. In this disclosure, however, an erase operation is performed again by increasing the voltage difference between the word line and the bulk, thereby being capable of minimizing the generation of an invalid block. The process of performing the erase operation by controlling the voltage difference is described in more detail below. 
       FIGS. 9A to 9C  are waveforms illustrating a first exemplary embodiment in which voltage is supplied to the global word line and the P-well during the erase operation in  FIG. 5 .  FIGS. 10A to 10C  are waveforms illustrating a second exemplary embodiment in which voltage is supplied to the global word line and the P-well during the erase operation in  FIG. 5 . 
     Referring to  FIG. 9A , the erase voltage VWTJ of a specific level, having a positive value, is supplied to the global word line GWL. The bulk voltage VCB, which is 15 V or higher than the erase voltage VWTJ, is supplied to the bulk PWELL. In this state, an erase operation is performed. In a first erase operation, the erase voltage VWTJ and the bulk voltage VCB are supplied in the form of a pulse having a first pulse width W 1 . After the first erase operation is completed, the pass/fail check circuit  200  detects data outputted through the Y-decoder  180  in an erase verification operation and determines whether there is a memory cell on which the first erase operation has not been normally performed based on the detected data. 
     If, as a result of the determination, there is a memory cell on which the first erase operation has not been normally performed, the second bias voltage generator  60  lowers the level of the erase voltage VWTJ and supplies it to the global word line GWL. Here, the erase voltage VWTJ has a second pulse width W 2  narrower than the first pulse width W 1  of the erase voltage VWTJ supplied in the first erase operation. Consequently, the voltage difference between the global word line GWL and the bulk PWELL is increased. In the state in which the voltage difference has been increased, a second erase operation is performed again. 
     It is then determined whether there is a memory cell on which the second erase operation has not been normally performed. If, as a result of the determination, there is a memory cell on which the second erase operation has not been normally performed, the second bias voltage generator  60  lowers the level of the erase voltage VWTJ in unit of 0.1 V to 0.5 V in order to increase the voltage difference and supplies the lowered erase voltage VWTJ to the global word line GWL. Here, the erase voltage VWTJ may have a third pulse width W 3  narrower than the second pulse width W 2  of the erase voltage VWTJ supplied in the first erase operation. 
     The above erase method is called an Incremental Stepping Pulse Erase (ISPE) method. The erase operation is performed again by increasing the voltage difference according to the ISPE method. If all the memory cells are normally erased in the process of performing the erase operation again, the erase operation is stopped. However, if there is a fail memory cell even after the erase operation is performed a certain number of times, a corresponding block is treated as an invalid block. The number of erase operations performed again may be changed according to the design. For example, the number of erase operations performed again may be determined to such an extent that the erase operations are completed within a target time. 
     Meanwhile, when an erase operation is performed again, if the erase voltage VWTJ and the bulk voltage VCB, having a voltage difference greater than a voltage difference between the erase voltage VWTJ and the bulk voltage VCB supplied in a previous erase operation and also having a narrower pulse width than that of the previous erase operation, are supplied as described above, the voltage difference between the local word line and the bulk (that is, P-well) to which the erase voltage VWTJ is supplied from the global word line is increased, but the time that it takes to supply the erase voltage VWTJ and the bulk voltage VCB is decreased. That is, the time for which the high voltage difference is maintained between the local word line and the bulk is reduced. Consequently, the generation of a fast program phenomenon or a slow erase phenomenon can be prohibited by the high voltage difference between the local word line and the bulk. 
     In the above method, when the erase operation is performed again, the voltage difference between the word line and the bulk is increased by lowering the level of the erase voltage VWTJ supplied to the global word line GWL. However, as shown in  FIG. 9B , the bulk voltage generator  40  may raise the level of the bulk voltage VCB in unit of 0.5 V to 1 V so as to increase the voltage difference between the word line and the bulk. In some embodiments, as shown in  FIG. 9C , the second bias voltage generator  60  may lower the erase voltage VWTJ and, at the same time, the bulk voltage generator  40  may raise the bulk voltage VCB so that the voltage difference between the word line and the bulk is increased. In this case, when an erase operation is performed again, if the erase voltage VWTJ and the bulk voltage VCB, having a voltage difference greater than a voltage difference between the erase voltage VWTJ and the bulk voltage VCB supplied in a previous erase operation and also having a narrower pulse width than that of the previous erase operation, are supplied as described above, the time for which the high voltage difference is maintained is reduced, thereby being capable of prohibiting the generation of a fast program phenomenon or a slow erase phenomenon. 
     In the above methods, the difference between the erase voltage VWTJ and the bulk voltage VCB (that is, the voltage difference between the local word line and the bulk) is regularly increased. However, the difference between the erase voltage VWTJ and the bulk voltage VCB (that is, the voltage difference between the local word line and the bulk) may be controlled as shown in  FIGS. 10A to 10C . For example, whenever the erase operation is performed, a decrement of the erase voltage VWTJ may be increased as shown in  FIG. 10A . In some embodiments, whenever the erase operation is performed, an increment of the bulk voltage VCB may be increased as shown in  FIG. 10B . In some embodiments, whenever the erase operation is performed, a decrement of the erase voltage VWTJ and an increment of the bulk voltage VCB may be increased as shown in  FIG. 10C . 
     According to the present disclosure, the generation of an invalid block can be minimized, and a reduction in the threshold voltage due to a slow erase phenomenon in an unselected block on which an erase operation has not been normally performed or the generation of a fast program or slow erase phenomenon due to repeated erase operations can be prohibited. 
       FIG. 11  is a characteristic graph illustrating a shift in the threshold voltage of an unselected block during the erase operation. 
     Referring to  FIG. 11 , in a known art, the leakage current is generated in a switching element (for example, G 1 J (J is a positive integer) of  FIG. 5 ). For this reason, a slow erase phenomenon in which voltage flowing into a word line is gradually lowered and an erase operation is performed in order to prohibit the erase operation in an unselected block is generated. Consequently, there is a problem in that the threshold voltage of a memory cell in the unselected block is lowered. In the present disclosure, however, in order to prevent the generation of the leakage current in a switching element (for example, G 1 J of  FIG. 5  (J is a positive integer)), an erase operation is performed in the state in which the erase voltage of a positive potential has been supplied to the global word line. Accordingly, a slow erase phenomenon is rarely generated in an unselected block. Consequently, a shift in the threshold voltage can be minimized. 
       FIG. 12  is a characteristic graph illustrating a slow erase characteristic and a fast program characteristic according to the number of erase operations according to this disclosure. 
     Referring to  FIG. 12 , in a first erase operation, the voltage difference between the word line and the bulk is maintained to such an extent that the first erase operation can be normally performed. If the first erase operation is not normally performed, a second erase operation is performed again by gradually increasing the voltage difference. Accordingly, although several hundreds of thousands of the erase operations are performed, the fast program phenomenon and the slow erase phenomenon are generated within approximately 0.5 V. It can be seen that the fast program phenomenon or the slow erase phenomenon is rarely generated in the present disclosure, as compared with the known case where the fast program phenomenon and the slow erase phenomenon are generated at 2 V or higher in  FIG. 2 . 
     The embodiments of this disclosure can have the following advantages. 
     First, when an erase operation is performed, not 0 V, but voltage higher than 0 V is supplied to the global word line. Accordingly, the generation of the leakage current can be prevented in a switching element coupled between the global word line and the local word line. Consequently, voltage flowing into the word line of an unselected block on which an erase operation has not been normally performed can be prevented from decreasing, thereby preventing the generation of a slow erase phenomenon in the unselected block. 
     Second, in a known operation of verifying whether an erase operation has been normally performed after the erase operation, if there is a memory cell on which the erase operation has not been normally performed, a corresponding block is treated as an invalid block and not used, thereby reducing the data storage capacity. In this disclosure, however, if there is a memory cell on which the erase operation has not been normally performed, the erase operation is performed again by increasing the voltage difference between the word line and the bulk. Accordingly, the generation of an invalid block can be minimized, and thus a reduction in the data storage capacity can be minimized. 
     Third, if an erase operation is initially performed in the state in which the voltage difference between the word line and the bulk is high, the electrical properties of a memory cell may be deteriorated because electrons are trapped in the tunnel oxide layer or stress is applied thereto. In the present disclosure, however, an erase operation is performed using a minimum voltage difference for the erase operation. If there is a memory cell on which the erase operation has not been normally performed, the erase operation is performed again using a raised voltage difference. Accordingly, the amount of electrons trapped in the tunnel oxide layer or stress applied thereto can be minimized, thereby increasing the lifespan of the memory cell. 
     Fourth, in the present disclosure, an initial erase operation is performed using a minimum voltage difference. If there is a memory cell on which the erase operation has not been normally performed, the erase operation is performed again using an increased voltage difference. Although several hundreds of thousands of read and erase operations are performed, the generation of a fast program or slow erase phenomenon can be prevented or minimized. 
     Consequently, reliability of an erase operation can be improved, the generation of failure can be minimized, and the lifespan of a device can be increased.