Abstract:
Programmable memory devices include a memory cell having an associated bit line. A buffer circuit couples the bit line to a data line. The buffer circuit has a sense node coupled to the bit line and includes a latch circuit having a latch node coupled to the data line. A control circuit resets the latch node between a program operation of the memory cell and its corresponding program-verify operation. The memory devices may be NAND-type flash memory devices and the memory cell may be one of a string of memory cells connected in series between the bit line and a common source line. A transistor may couple the data line to the latch node and a transistor may couple the latch node to the sense node. Methods of operating the same are also provided.

Description:
RELATED APPLICATION 
     This application claims priority to Korean Patent Application 2002-0026259, filed on May 13, 2002, the contents of which are herein incorporated by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to semiconductor memory devices, and in particular to electrically programmable semiconductor memory devices. 
     A variety of different types of memory devices are known, including read only memory (ROM), read-write memory and erasable programmable ROM (EPROM). One particular type of EPROM that has proven desirable in various applications is electrically erasable PROM (EEPROM), which may be erased and programmed, but, unlike certain read-write memory, may be operated without needing to refresh data stored in the memory. There is also a trend toward enhancing the storage capacity and the density of integration in such memory devices. A NAND-type flash memory is one example of an EEPROM non-volatile semiconductor memory that may provide high capacity and integration density without the need for refreshing stored data. 
     FIG. 1 is a circuit diagram illustrating an exemplary memory cell array for a conventional NAND-type flash memory device. As shown in FIG. 1, a memory cell array  1  includes a plurality of memory cell strings  2  extending in a column direction of the memory cell array. Each of the strings  2  includes a string selection transistor SST, a ground selection transistor GST, and a plurality of memory cells, shown in FIG. 1 as sixteen (16) memory cells MC 0 -MC 15  connected in series between the selection transistors SST and GST. A drain of the string selection transistor SST in each column is connected to a corresponding bit line, and its gate is connected to a string selection line SSL. A source of the ground selection transistor GST is connected to a common source line CSL and its gate is connected to a ground selection line GSL. Each of the memory cells MC 0 -MC 15  is illustrated as a floating gate transistor that has a control gate, a floating gate, a source, and a drain. The control gates of the memory cells MC 0 -MC 15  in each string are coupled to respective word lines WL 0 -WL 15  defining the rows of the memory cell array. 
     NAND-type flash memory devices typically program memory cells commonly connected to a selected word line concurrently. However, with a flash memory device, it is generally not possible to successfully program selected memory cells within a single program cycle. A single program cycle is generally insufficient as memory cells may have different coupling ratios, for example, due to variability of process conditions or materials during manufacture of the memory device. For example, memory cells of a relatively larger coupling ratio are typically programmed more rapidly than those of a relatively smaller coupling ratio. Therefore, when memory cells of a relatively larger coupling ratio are programmed, remaining memory cells having a relatively smaller,coupling ratio may be maintained at an erased state. 
     For such memory devices, following a program operation, it is known to execute a verify operation to determine if the program operation was successful. However, due to the circuit configuration, the potential of the common source line CSL can be increased when the verify operation is carried out. For example, the potential of the common source line CSL may be raised due to the resistances R 0 -Rm of the common source line CSL and current ION 0 -IONm flowing through memory cell(s) of an erased state in each string. This effect may be referred to as “CSL noise.” As a result of the CSL noise, a memory cell having a threshold voltage programmed to a level lower than a verify or pass voltage may appear to meet the pass voltage level for its threshold during the verify operation. In Such a case, an insufficiently programmed memory cell could be seen as a sufficiently programmed memory cell during the verify operation. 
     By way of example, assume that a selected memory cell (e.g., MC 0 ) is programmed up to a threshold voltage of about 0.3V and that the potential of the common source line CSL is raised by about 0.7V owing to current flowing through some of the memory cells during the verify operation. Under such conditions, the threshold voltage of the selected memory cell may be detected as exceeding 0.7V based on the raised potential of the common source line CSL. Thus, as illustrated in the graphical illustration of FIG. 2, after the program operation, threshold voltages of some of the memory cells can be distributed at voltage levels below the verify reference voltage Vref (e.g., about 0.7V) that corresponds to an off-cell. Therefore, when the program operation is completed, since memory cells having a threshold voltage of about 0.3V are at an under-program state, they may bee judged to be an on-cell during a read operation, resulting in a device failure. 
     SUMMARY OF THE INVENTION 
     In some embodiments of the present invention programmable memory devices include a memory cell having an associated bit line. A buffer circuit couples the bit line to a data line. The buffer circuit has a sense node coupled to the bit line and includes a latch circuit having a latch node coupled to the data line. A control circuit resets the latch node between a program operation of the memory cell and its corresponding program-verify operation. The memory devices may be NAND-type flash memory devices and the memory cell may be one of a string of memory cells connected in series between a bit line and a common source line. A transistor may couple the data line to the latch node and a transistor may couple the latch node to the sense node. 
     In other embodiments of the present invention, the latch circuit includes a first and a second latch circuit. The first latch circuit has a first latch node selectively coupled to a third latch node, which is the latch node of the buffer circuit coupled to the data line, and a second latch node selectively coupled to a first reference voltage. The second latch circuit is coupled to the third latch node and has a fourth latch node coupled to the first reference voltage by a select circuit. The select circuit is responsive to signals on the first latch node and the sense node. 
     In further embodiments of the present invention, the control circuit is configured to selectively couple the first latch node to the third latch node to latch data on the first latch node during a program operation. The second latch control circuit may be configured to latch the data to the third latch node during the program operation. The control circuit may be configured to maintain program-inhibit data at the third latch node when data on the first latch node or data on the sense node is program-inhibit data. In particular embodiments, the control circuit is configured to reset the first latch node during a read operation and/or to selectively discharge the first latch node responsive to data on the sense node during a read operation. The control circuit may be configured to match data on the third latch node to data on the first latch node after discharge of the sense node during the read operation. 
     In other embodiments of the present invention, methods for programming a programmable semiconductor memory device include programming a plurality of memory cells of the semiconductor memory device. Latched programming data presented to ones of the plurality of memory cells is reset. After resetting the latched program data, the programming of the plurality of memory cells is verified in a program-verify operation in a single program cycle. In particular embodiments of the present invention, the memory device is a NAND-type flash memory device and the plurality of memory cells include a plurality of strings of memory cells, each of the strings of memory cells being connected in series between a bit line of the memory device and a common source line of the memory device. Accordingly, various embodiments of the present invention may preclude or reduce the likelihood of memory cells being insufficiently programmed because of a raised potential on a common source line during program-verify operations. 
     In further embodiments of the present invention, semiconductor memory devices include a memory cell array having a plurality of bit lines, a plurality of word lines, and a plurality of memory cells arranged in a matrix of the word lines and the bit lines. A page buffer group has a plurality of page buffers and is connected to the bit lines of the memory cell array. A column-pass gate circuit is connected between the page buffer group and a data bus. Each of the page buffers may include a first transistor having a drain connected to a first node, a source, and a gate connected to receive a first control signal; a first latch having a first latch node connected to the source of the first transistor and a second latch node; a second transistor having a drain connected to a sensing node, a source connected to the first node, and a gate connected to receive a second control signal; a second latch having a third latch node connected to the source of the second transistor and the first node, and a fourth latch node; a third transistor having a drain connected to the fourth latch node, a source, and a gate connected to the sensing node; a fourth transistor having a drain connected to the fourth latch node, a source, and a gate connected to the first latch node; and a fifth transistor having a drain connected to the sources of the third and fourth transistors, a source grounded, and a gate connected to receive a third control signal. 
     The first control signal may be activated such that data to be programmed is latched at the first latch node when a program operation commences. The data to be programmed may be latched at the third latch node of the second latch. When a program-verify operation commences, the third and fourth latch nodes of the second latch may be reset. The third control signal may be activated such that program-inhibit data is maintained at the third latch node of the second latch when either one of data on the sensing node and data on the first latch node of the first latch is program-inhibit data. 
     In other embodiments of the present invention, each of the page buffers includes a sixth transistor having a drain connected to the second latch node of the first latch, a source grounded, and a gate connected to receive a fourth control signal. The page buffers further include a seventh transistor having a drain connected to the first latch node of the first latch, a source, and a gate connected to the sensing node and an eighth transistor having a drain connected to the source of the seventh transistor, a source grounded, and a gate connected to receive a fifth control signal. 
     The fourth control signal may be activated to reset the first latch node of the first latch when a normal read operation commences. The fifth control signal may be activated to selectively discharge the first latch node of the first latch, based on a logic level of the sensing node, during a normal read operation. The sensing node may be discharged into a ground voltage after the first latch node of the first latch is set according to a logic level of the sensing node during the normal read operation. Furthermore, the third control signal may be activated to establish the third latch node of the second latch into data on the first latch node of the first latch, after the sensing node is discharged, during the normal read operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram illustrating a memory cell array of a conventional NAND-type flash memory device; 
     FIG. 2 is a graph illustrating a threshold voltage distribution of on-cells and off-cells for a conventional program-verify operation of the memory cell array of FIG. 1; 
     FIG. 3 is a circuit diagram of a NAND-type flash memory device according to some embodiments of the present invention; 
     FIG. 4 is a timing diagram illustrating operations for controlling the circuit of FIG. 3 for a program operation according to some embodiments of the present invention; 
     FIG. 5 is a timing diagram illustrating operations for controlling the circuit of FIG. 3 for a program-verify operation according to some embodiments of the present invention; 
     FIG. 6 is a flowchart diagram illustrating program and program-verify operations according to some embodiments of the present invention; 
     FIG. 7 is a graph illustrating a threshold voltage distribution of on-cells and off-cells that may result from program-verify operations in accordance with some embodiments of the present invention; and 
     FIG. 8 is a timing diagram illustrating operations for controlling the circuit of FIG. 3 for a read operation according to some embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. It will be understood that when an element such as a layer, region or substrate is referred to as being “on” or “connected to” or “coupled to” another element, it can be directly on or connected or coupled to the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
     Each embodiment described and illustrated herein includes its complementary conductivity type embodiment as well. References to source and drain of transistors herein are interchangeable and intended to encompass complementary conductivity type transistors or alternate technology type transistors except where a specific transistor type is referenced. 
     The present invention will now be further described with reference to FIGS. 3-8. FIG. 3 is a circuit diagram illustrating a NAND-type flash memory device according to some embodiments of the present invention. As shown in FIG. 3, the memory device includes a memory cell array  10 , a row decoder  20 , a page buffer group  30 , a column-pass gate circuit  40  and a control circuit  102 . The memory cell array  10  includes a plurality of memory cell strings  12 , each of which extends in the column direction. Each of the strings  12  includes a string selection transistor SST, a ground selection transistor GST, and a plurality of serially connected memory cell transistors. More particularly, FIG. 3 illustrates each string  12  as including sixteen (16) memory cells MC 0 -MC 15  connected in series between the selection transistors SST and GST. A drain of the string selection transistor SST in each column is connected to a corresponding bit line and a gate of the string selection transistor SST is connected to a string selection line SSL. A source of the ground selection transistor GST is connected to a common source line CSL and a gate of the ground selection transistor GST is connected to a ground selection line GSL. 
     Each of the memory cells MC 0 -MC 15  is illustrated as a floating gate transistor that has a control gate, a floating gate, a source, and a drain. The control gates of memory cells MC 0 -MC 15  in each string are coupled with corresponding word lines WL 0 -WL 15 . The word lines WL 0 -WL 15  are connected to the row decoder  20  (noted as “X-DEC” in FIG.  3 ). 
     The illustrated page buffer group  30  includes a plurality of page buffers (buffer circuits)  100 . For the embodiments shown in FIG. 3, one page buffer  100  is associated with two adjacent strings  12   e  and  12   o.  For ease of explanation, a single page buffer  100  is detailed in FIG. 3 but it is to be understood that the plurality of page buffers may each be similarly configured. As shown in FIG. 3, the page buffer  100  includes an NMOS transistor M 5 , a PMOS transistor M 6 , the first latch circuit  110 , the second latch circuit  120 , and a bit line selecting and biasing circuit  130 . The bit line selecting and biasing circuit  130  selects one of the corresponding bit lines BLe and BLo and connects the selected bit line to the first and second latch circuits  110  and  120  at the sensing (sense) node N 5 . The NMOS transistor M 5  has a gate connected to receive a control signal BLSLT, a drain connected to a node N 3  and a source connected to the node N 5 . The PMOS transistor M 6  has a gate connected to receive a control signal PLOAD, a source connected to a power supply voltage VDD and a drain connected to the node N 5 . The PMOS transistor M 6  supplies current to a selected bit line through the bit line selecting and biasing circuit  130  based on to a voltage level of the control signal PLOAD. 
     The first latch circuit  110  has a first latch node N 1  and a second latch node N 2 . The second latch circuit  120  has a third latch node N 3  and a fourth latch node N 4 . The third latch node N 3  is coupled to a data line DL through the column pass gate circuit  40  and to the sense node N 5  through the transistor M 5 . 
     The first latch circuit  110  receives data transferred through the column-pass gate circuit  40  and latches the received data until program and program-verify operations are completed. As illustrated in FIG. 3, the first latch circuit  110  includes four NMOS transistors M 1 , M 11 , M 12 , and M 13  and a first latch  112  including two inverters IN 1  and IN 2 . The NMOS transistor M 10  is connected between the first latch node N 1  and the third latch node N 3  (and to the column-pass gate circuit  40 ), and transfers data to be programmed from the column-pass gate circuit  40  to the latch  112  responsive to a control signal DIN. The first latch node N 1  is connected to an output of the inverter IN 1  and an input of the inverter IN 2 . 
     The NMOS transistor M 11  is connected between the second latch node N 2  (interconnection of an input of the inverter IN 1  and an output of the inverter IN 2 ) and a ground voltage VSS, and discharges the node N 2  responsive to a control signal RESET. Thus, the NMOS transistor M 11  may be used to allow charging of the node N 1  up to the power supply voltage through the inverter IN 1 . The NMOS transistors M 12  and M 13  are connected in series between the first latch node N 1  and the ground voltage VSS. A gate of the NMOS transistor M 12  is connected to the sense node N 5  and a gate of the NMOS transistor M 13  is connected to receive a control signal LCH 2 . 
     The second latch circuit  120  receives data transferred through the column-pass gate circuit  40  and stores the received data until a program operation is completed. The second latch circuit  120  latches data on the third latch node N 3  based on latched data in the first latch circuit  110  (first latch node N 1 ) and data on the sense node N 5  during a program-verify operation. As shown in FIG. 3, the second latch circuit  120  includes three NMOS transistors M 7 , M 8 , and M 9  and a latch  122  that includes two inverters IN 3  and IN 4 . An output of the inverter IN 3  and an input of the inverter IN 4  are connected to the third latch node N 3  (and to a data line DL through the column-pass gate circuit  40 ). An input of the inverter IN 3  and an output of the inverter IN 4  are connected to the fourth latch node N 4 . The NMOS transistor M 7  has a drain connected to the fourth latch node N 4  and a gate connected to the sense node N 5 . The NMOS transistor MS has a drain connected to the fourth latch node N 4  and a gate connected to the first latch node N 1  in the first latch circuit  110 . The NMOS transistor M 9  has a gate connected to a control signal LCH 1 , a drain connected to sources of the transistors M 7  and MS and a source connected to the ground voltage VSS. 
     For some embodiments of the present invention, as illustrated in FIG. 3, the bit line selecting and biasing circuit  130  includes four NMOS transistors M 1 , M 2 , M 3 , and M 4 . The NMOS transistor M 1  is connected between a signal line VIRPWR and a bit line BLe and is turned on/off by a control signal VBLe. The NMOS transistor M 2  is connected between the signal line VIRPWR and a bit line BLo and is turned on/off by a control signal VBLo. The NMOS transistor M 3  is connected between the bit line BLe and the sense node N 5  and is turned on/off by a control signal BLSHFe. The NMOS transistor M 4  is connected between the bit line BLo and the sense node N 5  and is turned on/off by a control signal BLSHFo. 
     The control circuit  102  is coupled to the page buffer group  30  to provide control signals  104 . The control circuit  102  is configured to generate various of the control signals described with reference to FIG. 3 as will now be further described with reference to the timing diagrams of FIGS. 4,  5  and  8  and the flowchart of FIG.  6 . 
     FIG. 4 is a timing diagram illustrating operations for controlling the circuit of FIG. 3 for a program operation according to some embodiments of the present invention. FIG. 5 is a timing diagram similarly illustrating program-verify operations. FIG. 6 is a flowchart illustrating operations for program and program-verify operations as shown in the timing diagrams of FIGS. 4-5 according some embodiments of the present invention. 
     Referring first to FIG. 6 (and FIG.  3 ), at block S 100 , data to be programmed to memory cells of a selected page is loaded to the first latch  112  and the second latch  122 . The load procedure includes resetting of nodes N 1  and N 3  to a logic ‘1’ as illustrated between times t 1  and t 2  of FIG.  4  and programming of data loaded to nodes N 1  and N 3  through a column-pass gate circuit  40  between times t 2  and t 3  of FIG.  4 . Responsive to the control signal PLOAD being activated low and the control signal LCH 1  being activated high, NMOS transistors M 7  and M 9  are turned on. The third latch node N 3  is established at a logic high level (logic ‘1’) through the inverter IN 3  of the second latch  122 . When the control signal RESET is activated high, the NMOS transistor M 11  is turned on. The first latch node N 1  is established at a logic high level through the inverter IN 1  of the first latch  112 . Subsequently, at time t 2  (FIG.  4 ), the control signal PLOAD is inactivated high and the control signals LCH 1  and RESET are inactivated low. Afterwards, when the control signal DIN is activated high, data to be programmed is transferred to the nodes N 1  and N 3  from the column-pass gate circuit  40 . For example, if data ‘0’ is received through the column-pass gate circuit  40 , the nodes N 1  and N 3  have a logic low level. If data ‘1’ is received through the column-pass gate circuit  40 , the nodes N 1  and N 3  have a logic high level. 
     While a memory cell corresponding to a page buffer where data ‘0’ is to be loaded is programmed, a memory cell corresponding to a page buffer where data ‘1’ is loaded is program-inhibited. In other words, the threshold voltage for each data state must be set based on charging or not charging of the floating gate of the floating gate transistors of the respective memory cells. 
     At block S 110 , a program operation for selected memory cells is carried out based on states of the loaded data. The program operation is illustrated in FIG. 4 as being carried out between times t 3  and t 4 . During the program operation, as illustrated in FIG. 4, the control signals VBLo and BLSHFe have a logic high level of VDD+α, and the control signals VBLe and BLSHFo have a logic low level of VSS. Under these conditions, the bit line BLe is connected to the sense node N 5  through the NMOS transistor M 3  and the bit line BLo is connected to the line VIRPWR through the NMOS transistor M 2 . Subsequently, selected memory cells of a page or word line are programmed or program-inhibited based on the loaded data states. 
     At block S 120 , the second latch circuit  120  is reset. As illustrated in FIG. 5, between time t 5  and t 6  the control signals VBLe, VBLo, BLSHFe, BLSHFo, BLSLT, and PLOAD have a logic high level (VDD). The control signal VIRPWR has a logic low level (VSS or 0V). Thus, between times t 5  and t 6 , the third latch node N 3  is reset to a logic ‘0.’ 
     At block S 130 , a program-verify operation is performed. If insufficiently programmed memory cells are seen as sufficiently programmed memory cells (i.e., satisfying the verify threshold reference voltage) owing to the CSL noise, the third latch node N 3  is established at a logic ‘1’ between times t 7  and t 8  of FIG.  5 . Also, for a program-inhibited memory cell, the third latch node N 3  is set to a logic ‘1’ between times t 7  and t 8  of FIG.  5 . On the other hand, if a selected memory cell is not programmed sufficiently, the third latch node N 3  is established at a logic ‘0’ level. 
     The program-verify operation of block S 130  will now be further described below with reference to FIGS. 3 and 5. For purposes of this explanation, it is assumed that a potential of the common source line CSL is raised by about 0.7V owing to current flowing through un-programmed memory cells and that a threshold voltage of an insufficiently programmed memory cell is about 0.3V. Voltage levels of control signals used for the program-verify procedure S 130  are illustrated in FIG.  5 . More particularly, between times t 6  and t 7 , a read voltage VREAD is applied to the string and ground selection lines SSL and GSL, a voltage of about 0.8V is applied to a selected word line, and a voltage of about 4.5V is applied to unselected word lines. Concurrently, the ground voltage VSS is applied to the control signals VBLe and BLSLT and the control signal VBLo is retained at the power supply voltage VDD. Between times t 6  and t 7 , 1.5V, VSS, and 1.1V voltage levels are sequently applied to the control signal BLSHFe and the ground voltage is applied to the control signal PLOAD. Between times t 7  and t 8 , the control signal LCH 1  is activated high. Accordingly, data to be latched at the node N 3  is determined based on the logic level of the node N 1  and whether a selected memory cell is programmed. At this time, a pre-charged level, that is, a logic ‘1’ continues to be maintained at the sense node N 5  of a page buffer that corresponds to a sufficiently programmed cell and to a memory cell that is judged as a sufficiently programmed cell owing to the CSL noise even though it is not programmed sufficiently. Thus, the node N 3  is set to a logic ‘1’ because a current path is formed between the node N 4  and the ground voltage VSS through the NMOS transistors M 7  and M 9  in the second latch circuit  120 . 
     If a selected memory cell is not programmed, a pre-charged voltage on the sense node N 5  is discharged through a corresponding string  12 . In this case, as the NMOS transistor M 7  is turned off, the fourth latch node N 4  retains a logic ‘0’ state as a reset state. If data held in the latch  112  at the first latch node N 1  is a logic ‘1,’ when the control signal LCH 1  is activated between times t 7  and t 8 , a current path is formed between the node N 4  and the ground voltage VSS through the NMOS transistors M 8  and M 9 . The node N 4  then is established at a logic ‘0’ level. 
     At block S 140 , it is determined whether the third latch node N 3  has a logic ‘1’ level. If the third latch nodes N 3  of all page buffers  100  have a logic ‘1’ level, the program operation is completed. If at least one of the nodes N 3  has a logic ‘0’ level, the procedure returns to block S 110  to repeat program and program-verify operations. 
     In accordance with various embodiments of the present invention, when the next program cycle is carried out, the number of programmed memory cells may be increased, which may reduce the CSL noise. Accordingly, a memory cell insufficiently programmed in a previous program cycle may be correctly identified as an on-cell during the subsequent program-verify operation at block S 130 . The logic level of the node N 3  transitions from a logic high level (VDD) to a logic low level (VSS). That is, the node N 3  is reset to a logic ‘0’ level. 
     If resetting of the node N 3  as shown at block S 120  for various embodiments of the present invention is not, a selected memory cell may be judged as a successfully programmed cell even though it is not programmed sufficiently (threshold voltage not sufficient). As a result, the node N 3  may be set to a logic ‘1’ level. Such a memory cell may then continue to be program-inhibited during subsequent program cycles. Accordingly, various embodiments of the present invention provide for resetting the node N 3  to a logic ‘0’ level before the program-verify operation is performed at block S 130 . Thus, although an insufficiently programmed memory cell may be judged as a successfully programmed cell, due to the CSL noise, it can be reprogrammed during a subsequent program cycle when the node N 3  is reset to a logic ‘0’ level. As a result, as illustrated in the graph of FIG. 7, threshold voltages of programmed memory cells may be distributed at a higher region than the reference voltage Vref (=0.7V) that is used to judge whether a memory cell is normally programmed. 
     Thus with a page buffer structure according to some embodiments of the present invention, although an insufficiently programmed cell is detected as an off-cell due to the CSL noise, it is reprogrammed during repeated program cycle(s) so it may be provided a threshold voltage corresponding to an off-cell. Therefore, under-programming of memory cells, and resulting device failures, may be reduced or eliminated. 
     FIG. 8 is a timing diagram illustrating operations for controlling the circuit of FIG. 3 for a read operation according to embodiments of the present invention. Referring to FIG. 8 (and FIG.  3 ), between times t 11  and t 12 , a ground voltage VSS is applied to a string selection line SSL, a ground selection line GSL, and word lines WL 0 -WL 15 . Concurrently, the ground voltage VSS is applied to the BLSHFe and BLSHFo control lines and a-power supply voltage VDD is applied to the PLOAD control line. As a result, the bit lines BLe and BLo and the sense node N 5  are reset to the ground voltage VSS. Furthermore, as the control signal RESET is activated high, the node N 1  is established at a logic high level. 
     Between times t 12  and t 13 , the bit line BLe and the node N 5  are pre-charged as the control signal VBLe is set to the ground voltage VSS, the control signal BLSHFe has about a 1.5V level and the control signal PLOAD has the ground voltage VSS. Concurrently, the ground voltage VSS is applied to a selected word line and a read voltage VREAD is applied to unselected word lines. Under these conditions, if a selected memory is a programmed cell, the bit line BLe maintains a pre-charged voltage, so that the sense node N 5  continues to maintain a pre-charged voltage. On the other hand, if a selected memory cell is an erased cell, a pre-charged voltage of the bit line BLe is discharged into the ground voltage VSS through the selected memory cell and a voltage of the sense node N 5  is also discharged into the ground voltage VSS. 
     When the control signal LCH 2  is activated high between times t 13  and t 14 , the voltage level of the node N 1  is determined based on whether a selected memory cell is programmed. For example, when a selected memory cell is programmed, the node N 1  is discharged into the ground voltage VSS. On the other hand, when a selected memory cell has an erased state, or when the sense node N 5  has a logic low level, the node N 1  maintains a logic high level (VDD). 
     As the power supply voltage VDD is applied to the VBLe, BLSHFe, BLSHFo, and BLSLT at time t 14 , the node N 5  is discharged into the ground voltage VSS. When the control signal LCH 1  is activated high at time t 15 , the logic state of the node N 3  is established based on the logic level of the node N 3 . For example, if the node N 1  has a logic high level, the node N 3  is set to a logic high level. If the node N 1  has a logic low level, the node N 3  is established at a logic low level. The voltage (state) of the node N 3  is transferred to the data line DL through a column-pass gate circuit  40 . 
     As described above, for the illustrated NAND-type flash memory device, during a a normal read operation, data sensed through a bit line BLe and a sense node N 5  is latched at a node N 1  of the first latch  112 . The data at the node N 1  is also latched at a node N 3  of the second latch  122 . 
     In addition, flash memory devices according to various embodiments of the present invention provide a copy-back mode where data is sensed from memory cells of any page and then the sensed data is programmed in memory cells of a different page. In order to perform such a copy-back mode, a conventional NAND-type flash memory device typically uses an inverse read mode or check-bit cells. However, using a page buffer structure as described for various embodiments of the present invention, data read out from memory cells of any page is latched at nodes N 1  and N 3 , and then may be programmed in memory cells of a different page through the aforementioned program procedures. However, operations related to resetting the nodes N 1  and N 3  as described above are skipped when the copy-back mode is performed. Accordingly, the NAND-type flash memory device according to some embodiments of the present invention may perform the copy-back mode without the need for using an inverse read mode or check-bit cells. 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.