Abstract:
The invention provides a method of programming in a nonvolatile semiconductor memory device, having a plurality of memory cell strings connected to a plurality of bitlines and constructed of a plurality of memory cell transistors whose gates are coupled to a plurality of wordlines, and a plurality of registers corresponding to the bitlines. The method involves applying a first voltage to a first one of the bitlines and applying a second voltage to a second one of the bitline, the first bitline being adjacent to the second bitline, the first and second voltages being supplied from the registers; electrically isolating the first and second bitlines from their corresponding registers; charging the first bitline up to a third voltage higher than the first voltage and lower than the second voltage; and applying a fourth voltage to a wordline after cutting off current paths into the first and second bitlines.

Description:
This application is a divisional of U.S. patent application Ser. No. 10/659,634, filed Sep. 9, 2003, now U.S. Pat. No. 6,807,098, which is a continuation of U.S. patent application Ser. No. 10/021,639, filed Dec. 12, 2001, now issued, Patent No. 6,650,566, both of which are incorporated herein by reference in their entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to nonvolatile semiconductor memory devices, and more specifically to a NAND-type flash memory device in which the threshold voltages of parasitic transistors interposed between memory cell transistors belonging to the same rows are controllable for a programming operation. 
   BACKGROUND OF THE INVENTION 
   NAND-type flash memory devices are in great demand for their high storage capacity and high integration density, without a need for refreshing, and their straightforward electrical functions of erasing and programming. The facilities of data retention of the NAND flash memory device, even during a power-off, are very useful for portable electronic systems such as mobile computers, digital still cameras, or PDAs. 
   The NAND flash memory device has memory cells electrically erased and programmed, i.e., EEPROM cells, each being formed of a source and a drain spaced apart from each other in a bulk region (or a semiconductor substrate), a floating gate positioned over a channel region between the source and drain, and a control gate over the floating gate. Insulation films are interposed between the control and floating gates, and between the floating gate and the channel region. 
     FIG. 1  shows an arrangement of a memory cell array including the EEPROM cells. Memory cell transistors M 15 ˜MC 0  connected in series form a cell string CS 0  (or CS 1 ). The memory cell transistor MC 15  is connected to bitline BL 0  (or BL 1 ) through string selection transistor SST, and the memory cell transistor MC 0  is connected to common source line CSL through ground selection transistor GST. Control gates of the memory cell transistors M 15  arranged in a row are coupled to wordline WL 15 . In the same manner, control gates of other cell transistors in a row are coupled to their corresponding wordlines. 
   Threshold voltages of the cell transistors are set at about −3V by erasing, and then a programming operation is carried out for a selected memory cell transistor to raise its threshold voltage. This is done by applying a high voltage of 20V to a corresponding wordline coupled to the selected cell transistor. Threshold voltages of the other non-selected cell transistors do not change from their current values. However, typically a programming disturbance occurs whereby memory cell transistors coupled to a wordline coupled to a selected memory cell transistor are undesirably programmed by the high-leveled program voltage even though the cell transistors are not selected in a programming operation. 
   A program inhibit technique for preventing the non-selected memory cell transistors from being undesirably programmed has been proposed in U.S. Pat. No. 5,677,873 entitled “Method of programming flash EEPROM integrated circuit memory devices to prevent inadvertent programming of nondesignated NAND memory cells therein”, or U.S. Pat. No. 5,991,202 entitled “Method for reducing program disturb during self-boosting in a NAND flash memory”. In a method of program inhibition called self-boosting, a current path towards a ground is cut off by applying 0V to a gate of the ground selection transistor GST. 0V is applied to a bitline (e.g., BL 0 ) assigned to a selected memory cell transistor while a non-designated bitline (e.g., BL 1 ) sees a program inhibit voltage of 3.3V or 5V of a power supply voltage VCC. 
   At the same time, the power supply voltage VCC is applied to a gate of the string selection transistor SST so that a source of the string selection transistor is charged up to VCC-Vth (Vth is a threshold voltage of the string selection transistor). The string selection transistor is substantially shut off. Next, a high program voltage Vpgm and a pass voltage Vpass are applied to a selected wordline and non-selected wordlines, respectively, so that channel voltages of the non-selected memory cell transistors are boosted up to levels that prevent programming. The boosted channel voltages prohibit generation of F-N tunneling between the floating gate and channel region, preventing any change of the non-selected memory cell transistors from their primary erased states. 
   Nevertheless, there is still a problem of programming disturbance, because the non-selected memory cell transistors adjacent to a selected memory cell transistor tend to be programmed due to leakage current flowing through parasitic MOS (meal-oxide-semiconductor) transistors  10  as shown in FIG.  1 . The parasitic transistors  10  are connected between active regions (sources or drains) of cell transistors coupled to the same wordlines. 
   Referring to  FIG. 2  which shows a sectional diagram taken along the line A-A′ of  FIG. 1 , channel region  2  of program cell transistor MC 14   p  (to be programmed) and channel region  3  of program-inhibit cell transistor MC 14   i  act as a source and a drain, respectively, of the parasitic transistor. And the wordline W 14  acts as a gate of the parasitic transistor. A substrate region under field oxide  14  between the channel regions  2  and  3  is assigned to a channel region of the parasitic transistor. As a result, if the program voltage Vpgm is higher than a threshold voltage of the parasitic transistor, the parasitic transistor will be turned on, inducing a generation of leakage current flowing into the channel region  2  of the program cell transistor MCI  4   p  from the channel region  3  of the program-inhibit cell transistor MC 14   i  through the conductive parasitic transistor. Thus, the self-boosted voltage at the channel region  2  of the program-inhibit cell transistor MC 14   i  becomes lower, resulting in an undesirable programming disturbance. 
   There is a way to overcome the aforementioned problem by increasing the threshold voltage of the parasitic MOS transistor, using an ion implantation into the substrate region acting as the channel region of the parasitic transistor. However, it is preferred to restrict such an ion implantation into the substrate region because of a decrease in breakdown voltage in the drain region, or because of a scaling-down of a topological size of the memory cell array. Increasing the threshold voltage of the parasitic transistor by biasing the substrate  1  with a negative voltage is also not desirable because a longer time for charging the substrate  1  increases overall program time. 
   SUMMARY OF THE INVENTION 
   Embodiments of the invention provide a nonvolatile semiconductor memory device capable of preventing program disturbance due to parasitic MOS transistors, and a method thereof. 
   Other embodiments of the invention provide a nonvolatile semiconductor memory device capable of increasing a threshold voltage of a parasitic MOS transistor interposed between adjacent memory cell transistors, without increasing the wordline voltage during a program operation. 
   Particular example embodiments of a nonvolatile semiconductor memory include: a memory cell array formed of a plurality of memory cell strings each connected to a plurality of bitlines; a plurality of page buffers each connected to the bitlines; a plurality of transistors connected between the bitlines and the page buffers; and a bitline voltage controller applying a bitline control voltage to gates of the transistors. The bitline control voltage is charged to a first voltage during a first bitline setup period and charged to a second voltage during a second bitline setup voltage, the second voltage being lower than the first voltage. 
   Another aspect of the invention is a method of programming in a nonvolatile semiconductor memory device, which has a plurality of memory cell strings connected to a plurality of bitlines and constructed of a plurality of memory cell transistors whose gates are coupled to a plurality of wordlines, and a plurality of registers corresponding to the bitlines, including the steps of: applying a first voltage to a first one of the bitlines and applying a second voltage to a second one of the bitlines, the first bitline being adjacent to the second bitline, the first and second voltages being supplied from the registers; electrically isolating the first and second bitlines from their corresponding registers; charging the first bitline up to a third voltage higher than the first voltage and lower than the second voltage; and applying a fourth voltage to a wordline after cutting off the current paths into the first and second bitlines. 
   The first, the second, the third, and the fourth voltages are a ground voltage, a power supply voltage, an inhibit voltage, and a program voltage, respectively. 
   As the inhibit voltage is applied to the first (i.e., selected) bitline so as to shut off the parasitic transistor interposed between memory cells coupled to the same wordline, the threshold voltage of the parasitic transistor is increased up to a level higher than the program voltage, preventing program disturbance by way of the parasitic transistor. 
   The present invention will be better understood from the following detailed description of the exemplary embodiment thereof taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention, and many of the attendant advantages thereof, will become readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
       FIG. 1  is a circuit diagram of a general memory cell array in a NAND-type flash memory device; 
       FIG. 2  is a sectional physical diagram of the cell array taken along with the line A-A′ of  FIG. 1 ; 
       FIG. 3  is a circuit diagram of a memory cell array and a peripheral circuit thereof to perform a programming operation, according to an embodiment of the invention; 
       FIG. 4  is a circuit diagram of the bitline level controller shown in  FIG. 3 ; 
       FIG. 5  is a timing diagram illustrating an operation of the bitline level controller of  FIG. 4 ; 
       FIG. 6  is a timing diagram illustrating the programming operation performed by the circuit of  FIG. 3 , according a first embodiment of the invention; and 
       FIG. 7  is a timing diagram illustrating a programming operation performed by the circuit of  FIG. 3 , according to a second embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   It should be understood that the description of this preferred embodiment is merely illustrative and that it should not be taken in a limiting sense. In the following detailed description, several specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to one skilled in the art that the present invention may be practiced without these specific details. 
   Referring first to  FIG. 3  showing a circuit construction for performing a program operation according to the first embodiment of the invention, including memory cell array  100 , bitline level controller  110 , row decoder  120 , page buffer (register) circuit  130 , and column gate circuit  140 . The memory cell array  100  is formed of plural cell strings CS 0 , CS 1 , etc. Cell string CS 0 , for example, is formed of string selection transistor SST 0 , EEPROM cell transistors MC 0 ˜MC 15 , and ground selection transistor GST 0 . The string selection transistor SST 0  is connected to bitline BL 0 , and the ground selection transistor GST 0  is connected to common source line CSL. The cell transistors MC 15 ˜MC 0  are connected between the string and ground selection transistors, SST 0  and GST 0 , in serial. String selection line SSL, wordlines WL 0 ˜WLl 5 , and ground selection line GSL extend from the row decoder  120 , and are coupled to gates of the string selection transistors SST 0 , SST 1 , etc., control gates of the cell transistors MC 0 ˜MCl 5 , and the gates of the ground selection transistors GST 0 , GST 1 , etc., respectively. The bitline BL 0  is connected to node NO of page buffer (register)  130   a  of the page buffer circuit  130  through high-voltage adaptable NMOS transistor M 0 , and the bitline BL  1  is connected to node N 1  of page buffer (register)  130   b  of the page buffer circuit  130  through high-voltage adaptable NMOS transistor M 1 . Gates of the transistors M 0  and M 1  are coupled to a bitline level control signal BLC generated from the bitline level controller  110 . 
   The page buffers (e.g.,  130   a  and  130   b ) arranged in the page buffer circuit  130  each correspond to the bitlines (e.g., BL 0  and BL 1 ). In the page buffer  130   a , between VCC and the node NO is connected PMOS transistor M 2  whose gate is coupled to load enable signal LDE. Between NO and a ground voltage (or a substrate voltage VSS) is connected NMOS transistor M 4  whose gate is coupled to bitline discharge signal BLD. The node NO is connected to latch node LN 0  of latch circuit LO through high-voltage adaptable NMOS transistor M 6  whose gate is coupled to bitline selection signal BLS. Between counter nodes LN 0 , LN 0 B of the latch circuit LO and VSS are connected NMOS transistors M 8  and M 10 . Gates of the NMOS transistors M 8  and M 10  are coupled to the node NO and latch enable signal LTH, respectively. The latch node LN 0  is connected to data line DL through NMOS transistor M 12 , whose gate is coupled to column selection signal YS 0  generated from a column decoder (not shown). 
   In the page buffer  130   b , between VCC and the node NI, is connected a PMOS transistor M 3  whose gate is coupled to load enable signal LDE. Between NI and the ground voltage VSS is connected NMOS transistor M 5  whose gate is coupled to bitline discharge signal BLD. The node N 1  is connected to latch node LN 1  of latch circuit LI through high-voltage adaptable NMOS transistor M 7  whose gate is coupled to bitline selection signal BLS. Between counter nodes LN 1 , LN 1 B of the latch circuit L 1  and VSS are connected NMOS transistors M 9  and M 11 . Gates of the NMOS transistors M 9  and M 11  are coupled to the node N 1  and latch enable signal LTH, respectively. The latch node LN 1  is connected to data line DL through a NMOS transistor M 13  whose gate is coupled to column selection signal YS 1  generated from a column decoder (not shown). 
   The bitline level controller  110  in  FIG. 4  includes bitline control voltage generator  210 , level shifter  220 , and CMOS transmission gate  230 . 
   In the bitline control voltage generator  210 , reference voltage VREF is applied to the gate of NMOS transistor M 25  of differential amplifier  211 . Node N 5  between resistors R 1  and R 2  is coupled to the gate of NMOS transistor M 26  of the differential amplifier  212 . The differential amplifier  212  is constructed of PMOS transistors M 22 ˜M 24  and of NMOS transistors M 25 ˜M 27 . The PMOS and NMOS transistors, M 22  and M 27 , connect the amplifier  212  to VCC and Vss, respectively. The gate of the PMOS transistor M 22  is coupled to VSS while the gate of the NMOS transistor M 27  is coupled to bitline control enable signal BLCE 4 . BLCE 4  is also applied to the gate of PMOS transistor M 21  connected between VCC and node N 3 , gate of NMOS transistor M 34  connected between the resistor R 2  and VSS, and to the input of NAND gate ND 1  through inverter INV 1 . The node N 3  is connected to the output of the differential amplifier  212  and to the gate of PMOS transistor M 30  which is connected between VCC and node N 4  connected to the resistor R 1 . Node N 4  is connected to the node N 6  through NMOS transistor M 28  whose gate and drain are coupled in common. Between VCC and the node N 6  is a PMOS transistor M 29  whose gate is coupled to bitline control enable signal BLCE 3 . BLCE 3  is also applied to the input of the NAND gate ND 1  together with the output of the inverter INV 1 . The output of NAND gate ND 1  is applied through inverter INV 2  to the gate of NMOS transistor M 30  connected between the node N 6  and VSS. 
   The output node N 6  of the bitline control voltage generator  210 , labeled bitline control voltage Vb 1   c , is connected to output terminal N 7  generating bitline control signal BLC via a transmission gate  230 . An N-channel electrode of the transmission gate  230  is coupled to bitline control enable signal BLCE 2 , while a P-channel electrode of the transmission gate is coupled to BLCE 2  through an inverter INV 3 . BLCE 2  is also applied to an input of NOR gate NR 1  together with another bitline control enable signal BLCE 1 . The output of the NOR gate NR 1  is applied to a gate of high-voltage adaptable NMOS transistor M 33  connected between the output terminal N 7  and VSS. A level shifter  220  converts VCC into program pass voltage Vpass in response to BLCE 1 . 
     FIG. 5  shows voltage waveforms of the signals and voltages in the bitline level controller  110 , along a sequence of a program operation. A voltage level of the bitline control signal BLC is set to Vpass in response to BLCEI going high, and then returns to the ground level in response to rising BLCE 3 , during period A (the first bitline set-up) within a bitline set-up time. During period B (subsequent to A) of the bitline set-up time. BLC goes up to only voltage Vfi′ from the ground level before programming, which second bitline set-up voltage Vfi′ may be seen from the BLC trace in  FIG. 5  to be lower than first bitline set-up up voltage Vpass. 
   The voltage level Vfi′ is a sum of a threshold voltage of a bitline level control transistor (i.e., M 0  or M 1 ) and a minimum source-to-bulk voltage that is required for turning the parasitic MOS transistor on. Thus, a level Vfi is provided by the bitline level controller  110  in order to inhibit programming. The inhibit voltage Vfi, higher than the ground voltage, is supplied to a bitline assigned to the program cell. The inhibit voltage Vfi should be established with consideration for a characteristic threshold voltage of a MOS transistor, the threshold voltage of the MOS transistor (i.e., the parasitic MOS transistor) being summarized in the following equation.
 
 Vt=Vto +γ(√{square root over (2φ f+Vsb )}−√ {square root over (2 φf )})   (1) 
 
wherein Vto is a threshold voltage when Vsb is 0V, wherein g is a process parameter and of is a physical parameter, as is known.
 
   Because Vt is affected from source-to-bulk voltage Vsb, the inhibit voltage Vfi should be established to shut off a leakage current between adjacent memory cell transistors, i.e., to make the threshold voltage of the parasitic MOS transistor (or a field voltage) be higher than a program wordline voltage, without increasing the wordline voltage during programming. Of course, Vfi is adjusted by the resistance values of the resistors R 1  and R 2  (See FIG.  4 ). To bias a bitline of the program cell on Vfi, the voltage level of the bitline level control signal BLC, i.e., Vfi′, should be set to Vfi+Vth 1  (Vth 1 : a threshold voltage of the NMOS transistor M 0  or M 1 ). 
   Assume that BL 0  is a bitline to be programmed and BL 1  is a bitline to be program-inhibited. When all of the bitline control enable signals BLCE 1 ˜BLCE 4  are held at low levels (e.g., ground levels), the NMOS transistor M 33  is turned on and thereby the bitline level control signal BLC is established at the ground level (or VSS). Next, when BLCE 1  goes to high level (or VCC) while BLCE 2 ˜BLCE 4  retain low levels, M 33  is turned off and thereby BLC is converted to the pass voltage Vpass by the level shifter  220 . The transmission gate  230  is turned off, and Vblc is set to VCC by PMOS transistor M 29  turned on by BLCE 3 . The voltage level of the bitline level control signal BLC, i.e. Vpass, is applied to the gate of the NMOS transistor M 0  or M 1 , so that a data bit “ 0 ” to be programmed is supplied to BL 0  or BL 1  to be programmed through the NMOS transistor M 0  or M 1 . When BLCE 1  falls to a low level and BLCE 2  and BLCE 3  rise up to high levels, the transmission gate  230  and the NMOS transistor M 30  are turned on. The output of the level shifter  220  is at ground level. Thus, BLC goes to ground level from Vpass through the discharging path of the transmission gate  230  and the NMOS transistor M 30 . 
   After the first bitline set-up period A, the second bitline set-up period B starts with a high transition of BLCE 4  while BLCE 1  remains low and BLCE 2  and BLCE 3  maintain high. As the NMOS transistors M 27  and M 34  are turned on, the differential amplifier  212  is conductive to compare the reference voltage VREF with a voltage at the node N 5  divided by the resistors R 1  and R 2 . If a voltage at the node N 4  is lower than Vfi′+Vth 28  (Vth 28  being a threshold voltage of the NMOS transistor M 28 ), i.e., the voltage at N 5  is lower than VREF, the voltage at N 4  is increased by current supplied through the PMOS transistor M 21 . When the voltage at N 4  reaches Vfi′+Vth 28 , Vblc becomes Vfi′ and thereby BLC is maintained at Vfi′ for the programming time. The Vfi′ is applied to the gate of the NMOS transistor M 1  or M 0 , so that a data bit “I” for the program inhibition is supplied to BL 1  or BL 0  to be program-inhibited through the NMOS transistor M 1  or M 0 . 
   After the programming time, a recovery operation is performed for which BLCE 1 , BLCE 2 , BLEC 3  and BLCE 4  respectively are low, high, low, and low, and Vbls and BLC are set on VCC. 
   Now, referring to  FIG. 6 , a programming operation according to the first embodiment of the invention will be described in detail. It is assumed that MC 14   p  is a memory cell transistor to be programmed, which means that BL 0  is selected while BL 1  is non-selected. Thus, the page buffer  130   a  assigned to BL 0  holds data bit “ 0 ” while the page buffer  130   b  assigned to BL 1  stores data bit “ 1 ”. Also, WL 14  coupled to the gate of MCl 4   p  is a selected wordline. The timing operation of the bitline level controller  110 , shown in  FIG. 5 , is illustrated in FIG.  6 . 
   At the beginning of the bitline set-up period, SSL goes to high level (hereinafter, referred to as VCC), BLS and BLC go to Vpass, and GSL, CSL, BLD, and LTH are low levels (hereinafter, referred to as GND). The NMOS transistors M 0  and M 1  are turned on by BLC of Vpass, and the string selection transistors SST 0  and SST 1  are turned on by SSL of VCC. The NMOS transistor M 6  is turned on by BLS of Vpass. As a result, during the first bitline set-up period A, BL 0  and BL 1  are established GND and VCC, respectively. Before starting the second bitline set-up period B after the bitlines BL 0  and BL 1  are sufficiently set up each to GND and Vpass, BLC and BLS fall to GND from Vpass, thereby electrically isolating the bitlines BL 0  and BL 1  from their corresponding page buffers (registers)  130   a  and  130   b.    
   Starting the second bitline set-up period B, BLC is charged to Vfi′ (=Vfi+Vth 1 ) as aforementioned in FIG.  5 . And LDE goes to GND from VCC. Thereby, the current paths through the PMOS transistors M 2 /M 3  and the NMOS transistors M 0 /M 1  are connected to BL 0 /BL 1 . Since BLC is at Vfi′, the selected bitline BL 0  is charged to Vfi while the non-selected bitline BL 1  remains at VCC that has been set in the first set-up period A. As the string selection transistors SST 0  and SST 1  are substantially in a shut-off state (there is no current flow), the cell strings CS 0  and CS 1  corresponding to BL 0  and BL 1  are in a floating state. 
   Consequently, in the programming period, the program voltage Vpgm is applied to the selected wordline WL 14 , and Vpass is applied to the non-selected wordlines WL 0 ˜WL 13  and WL 15 . Since the cell string CSI corresponding to the non-selected bitline BL 1  is in the floating state, a channel voltage of the program-inhibit memory cell transistor MC 14   i  rises to a level sufficient to prevent a F-N tunneling by way of a self-boosting mechanism induced from Vpgm. The boosted channel voltage of MC 14   i  prohibits migration of electrons from its channel region to the floating gate because there is no discharge path due to the VCC-charged BL 1 . Meanwhile, a channel voltage of the program cell transistor MV 14   p  is discharged to Vfi from a boosted level through BL 0  even though it raises the boosted level in response to Vpgm that performs the self-boosting. Therefore, the channel voltage of the selected memory cell transistor MC 14   p  is finally established at Vfi. 
   After programming, BL 0  and BL 1  are discharged to GND, and the page buffers (registers)  130   a  and  130   b  are reset. 
   At this point, the threshold voltage of the parasitic MOS transistor  10 , as shown in equation (1), is established at a level higher than Vpgm, the actual levels being proportional to the source-to-bulk voltage Vsb, which is identical to the channel voltage of the program cell transistor (i.e., MC  14 ), Vfi. Thus, the parasitic MOS transistor is turned on while Vpgm is applied to WL 14 , causing the leakage current flowing between MC  14   p  and MC 14   i  (See  FIG. 2 ) through the parasitic transistor to be cut off. As a result, the program disturbance due to the parasitic transistor is eliminated. 
     FIG. 7  shows another case of programming according to the second embodiment of the invention. Like  FIG. 6 ,  FIG. 7  assumes that MC 14   p  is a memory cell transistor to be programmed, which means that BL 0  is selected while BL 1  is non-selected. Thus, the page buffer (register)  130   a  assigned to BL 0  holds data bit “ 0 ” while the page buffer (register)  130   b  assigned to BL 1  stores data bit “ 1 ”. Also, WL  14  coupled to the gates of MC  14   p  and MC  14   i  is a selected wordline. The timing operation of the bitline level controller  110 , shown in  FIG. 5 , is also illustrated in FIG.  6 . 
   At the beginning of the bitline set-up period, SSL goes to VCC, BLS and BLC go to Vpass. At the same time, GSL, CSL, BLD, and LTH maintain GND. The NMOS transistor M 0  and M 1  are turned on by BLC of Vpass, and the string selection transistors SST 0  and SST 1  are turned on by SSL of VCC. The NMOS transistor M 6  is turned on by BLS of Vpass. As a result, during the first bitline set-up period A, BL 0  and BL 1  are charged respectively to GND and VCC. 
   In the second bitline set-up period B, Vcsl is applied to the common source line CSL in order to prevent punch-through in the ground selection transistors GST 0  and GST 1 . And BLS falls to GND from VCC to electrically isolate the bitlines BL 0  and BL 1  from their corresponding page buffers  130   a  and  130   b . At the same time, LDE goes to Vload from VCC in order to supply load current Iload to BL 0  and BL 1  for a predetermined time tfi. 
   The time for charging the selected bitline BLn up to the inhibit voltage Vfi is associated with load current Iload in the following equation (2):
 
 tfi =( CBL×Vfi )/ Iload   (2) 
 
wherein CBL is capacitance of the bitline.
 
   BLC is charged to Vfi′ (=Vfi+Vth 1 ) as aforementioned with respect to FIG.  5 . And LDE goes to GND from VCC. Thereby, the current paths through the PMOS transistors M 2 /M 3  and the NMOS transistors M 0 /M 1  are connected to BL 0 /BL 1 . Since BLC is at Vfi′, the selected bitline BL 0  is charged to Vfi while the non-selected bitline BL 1  maintains VCC that has been set in the first set-up period A. At this time, as the string selection transistors SST 0  and SST 1  are substantially in a shut-off state (there is no current flow), the cell strings CS 0  and CS 1  corresponding to BL 0  and BL 1  are in a floating state, i.e. current into the first and second bitlines is substantially inhibited. 
   Consequently, during the programming period, the program voltage Vpgm is applied to the selected wordline WL  14 , and Vpass is applied to the non-selected wordlines WL 0 ˜WL 13 , and WL  15 . As aforementioned, since the cell string CS 1  corresponding to the non-selected bitline BL 1  is in the floating state, a channel voltage of the program-inhibit memory cell transistor MC 14   i  rises to a level sufficient to prevent a F-N tunneling by way of a self-boosting mechanism induced from Vpgm. The boosted channel voltage of MC 14   i  to prohibits migration of electrons from its channel region to the floating gate because there is no discharge path due to the VCC-charged BL 1 . Meanwhile, a channel voltage of the program cell transistor MV  14   p  is discharged to Vfi from a boosted level through BL 0  even though it raises the boosted level in response to Vpgm that performs the self-boosting. Therefore, the channel voltage of the selected memory cell transistor MCI  4   p  is finally established at Vfi. 
   After programming, BL 0  and BL 1  are discharged to GND, and the page buffers  130   a  and  130   b  are reset. 
   At this point, the threshold voltage of the parasitic MOS transistor  10 , as shown in the equation (1), is established at a level higher than Vpgm, being proportional to the source-to-bulk voltage Vsb that is identical the channel voltage of the program cell transistor (i.e., MC  14 ), Vfi. Thus, the parasitic MOS transistor is turned on while Vpgm is applied to WL 14 , causing the leakage current flowing between MC 14   p  and MC  14   i  (See  FIG. 2 ) through the parasitic transistor to be cut off. As a result, the program disturbance due to the parasitic transistor is eliminated. 
   As aforementioned, as the inhibit voltage is applied to the selected bitline in order to turn off the parasitic transistor interposed between memory cells coupled to the same wordline, the threshold voltage of the parasitic transistor is increased up to a level higher than the program voltage and thereby memory devices employing the present invention are able to be free from the program disturbance caused by the parasitic transistor. 
   Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as described in the accompanying claims.