Abstract:
Semiconductor memory device row decoder structures have reduced layout area. A structure for erasing memory cells coupled to a single bitline includes a single bias driver for these cells, and a plurality of local voltage level converters coupled to the bias driver. At least one word line driver is coupled to each local level converter, to erase at least one of the memory cells. A global word line is also coupled to the word line driver. A method for erasing these memory cells includes biasing the local level converter, for powering in turn a component of the word line driver. In addition, an existing global word line driver powers another component of the word line driver, thus resulting in reduced design requirements for the local level converter. Each local level converter of the invention has therefore a simpler structure than the prior art, and thus occupies less area. The savings are multiplied by a large number, which results in significant savings in the overall area of the chip.

Description:
[0001]    This application claims priority from Korean Priority Document No. 2001-28258, filed on May 23, 2001 with the Korean Industrial Property Office, which document is hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to semiconductor memory devices, and more particularly, to semiconductor memory device row decoder structures having reduced layout area, and methods of operating the same.  
           [0004]    2. Description of the Related Art  
           [0005]    Semiconductor memory device is generally classified into the volatile and nonvolatile categories.  
           [0006]    Volatile memories perform quick read and write operations, but have the disadvantage of being erased, if an external power supply is cut off. These are further classified into dynamic random access memories (DRAMs) and static random access memories (SRAMs).  
           [0007]    On the other hand, nonvolatile memories keep the data stored in the memory cells, even if they lose power. These are further classified into mask read only memories (MROMs), programmable read only memories (PROMs) and electrically erasable and programmable read only memory (EEPROMs).  
           [0008]    Nonvolatile memories are mainly used for data that should not be lost if the power supply is lost. A result is that, in case of MROMs, PROMs or EEPROMs, common users are not free to perform erase and write (or program) processes. In other words, it is not convenient to erase or re-program the contents programmed at the on-board state. On the contrary, it is possible for the EEPROM system to perform the electrically erase and write processes in itself. Therefore, the EEPROM has been continuously expanded in the applications to be used as a system program storage device, or an auxiliary memory device requiring continuous renewal of its contents.  
           [0009]    In other words, it is highly desirable to develop the EEPROM that can be electrically erasable or programmable at high speed, for a variety of electronic devices that can be controlled by computers or microprocessors. Furthermore, since a relatively large area is taken for a hard disk device having a rotary magnetic disk to be used as an auxiliary memory device in a battery powered computer system at the size of a portable computer or notebook computer, designers of such systems have been greatly interested in development of an EEPROM having both a compact size and a high speed operation.  
           [0010]    Accordingly, a NOR type flash EEPROM having a flash erase function, appeared along with the advancement of EEPROM design technology, has been welcomed by users who have demanded a high speed memory device including faster programming, write and read operations than those of NAND type or AND type EEPROM.  
           [0011]    Referring now to FIG. 1 and FIG. 2, a general structure and a general operation of a NOR-type flash memory device are described.  
           [0012]    [0012]FIG. 1 shows a vertical cross-section of a memory cell transistor constructing a memory cell unit of a general NOR type flash memory. A n-type source region  3  is formed in p-type substrate  2  (also known as bulk). A n-type drain region  4  (also known as bit line B/L) is formed in p-type substrate  2 . Regions  3  and  4  define a channel between them, which has a thin (less than 100 angstroms) insulating layer  5  on it. A floating gate electrode  6  is formed on insulating layer  5 , over the channel. An insulating layer  7  is formed over the floating gate electrode  6 , and a control gate  8  (which may be called a word line W/L) is formed on insulating layer  7 . A gate voltage Vg is applied to control gate  8 .  
           [0013]    [0013]FIG. 2 is a table that indicates levels of voltage to be applied to the device of FIG. 1. These effect different modes of operation, such as program, erase, and read operation modes.  
           [0014]    First, the program operation is performed by injection of hot electrons from the drain region  4  and its adjacent channel region to the floating gate electrode  6 . Any injected electrons will remain there, even if the device is turned off, which is why the device is nonvolatile.  
           [0015]    As shown in the Table 1, an injection of hot electrons is made by applying a high level of voltage, 10V for instance, to the control gate electrode  8  and an adequate level of voltage, 5-6V for instance, to the drain region  4  for generation of hot electrons while the source region  3  and the p-type substrate region  2  are grounded. When enough negative charges are thus accumulated at floating gate electrode  6 , the memory cell transistor has a higher level of threshold voltage than that prior to the program operation.  
           [0016]    Second, the read operation is performed by applying a level of positive voltage, 1V for instance, to the drain region  4  and a predetermined level of voltage, 4.5V for instance, to the control gate electrode  8  while the source region  3  and the substrate region  2  are grounded.  
           [0017]    After the program operation, those of the memory cell transistors having a higher level of threshold voltage function as off-cells during the read operation, to prevent current from flowing from the drain region  4  to the source region  3 . In this case, the memory cell transistor is called “an off-cell”. At this time, the programmed memory cell transistors generally have voltage distribution at the range of 6 to 7V.  
           [0018]    In the NOR type flash memory cell transistor, the erase operation is performed by generating a Fowler-Nordheim tunneling phenomenon (hereinafter referred to as F-N tunneling) from substrate  2  to control gate electrode  8 . For creating the F-N tunneling, it is required that a high level of negative voltage, −10V for instance, be applied to the control gate electrode  8 , and an adequate level of voltage, 5V for instance, to the bulk region as shown in the table  1 . In this case, the drain region  4  is set at high impedance, so that the erase operation is performed effectively. The aforementioned conditions of the erase operation form a strong magnetic field between the control gate electrode  8  and the bulk region, which bring about the F-N tunneling. Accordingly, any negative charges stored in floating gate electrode  6  are discharged to source region  3 . The F-N tunneling is commonly known to happen when the magnetic field of 6 to 7 MV/cm is applied to the conductive layer between the insulating layers. Even in the aforementioned memory cell transistor, the gate insulating layer  7  is formed in the thickness of 100 angstroms to permit the F-N tunneling to happen. As a result of the erase operation, the level of threshold voltage becomes lower at the memory cell transistor, than that of the case that electric charges are accumulated at the floating gate electrode  6 .  
           [0019]    During the read operation, the memory cell having the level of threshold voltage lowered by the erase operation functions as an on-cell, because of a current path from a drain region to a source region along the channel. At this time, the memory cell transistor is called an on-cell. The threshold voltage of the erased memory cell transistors is in the range of approximately 1V to 3V.  
           [0020]    In the general flash memory, many cells are formed in the bulk region for high integration. Moreover, they are connected, so that they can be erased simultaneously, during the aforementioned erase operation. These cells are also divided in regions, each of which has an erase unit. For instance, a group of memory cells that can be electrically erased as a unit of 64K byte are together called a sector.  
           [0021]    [0021]FIG. 3 shows a memory cell array in the prior art. In fact, FIG. 3 is made from FIG. 3A and FIG. 3B, which are considered joined at their common numerals.  
           [0022]    In FIG. 3, each of the word lines WLi is commonly connected to gates of n memory cells. Each of the bit lines BLi is commonly connected to drains of m cells.  
           [0023]    A plurality of unit sectors of 64 K byte are formed, and a core block has a row decoder for selectively driving word lines of the unit sectors. If the unit sector in the memory cell array is of 64 K byte, the number of word lines W/L is generally 1024 and the number of bit lines is 512.  
           [0024]    The memory device of FIG. 3 includes a plurality of sectors  300 - 300   n  that form a memory cell array.  
           [0025]    For each such sector, the device of FIG. 3 includes sector selectors  10 - 10   n,  for inputting a select signal to select one out of the sectors; level shift drivers  21 - 21   n;  high voltage drivers  30 - 30   n;  and erase voltage drivers  40 - 40   n  for applying a voltage necessary in driving a plurality memory cell transistors in the sectors.  
           [0026]    The device of FIG. 3 further includes global row decoders  800 - 800   n  commonly connected to the sectors for row decoding, partial decoders  50 - 50   n  arranged in each of the sectors; and local row decoders  900 - 900   n,    910 - 910   n,  . . . ,  930 - 930   n  correspondingly connected to global row decoders  800 - 800   n  for selecting word lines.  
           [0027]    In the aforementioned structure, 10 row address signals are needed in selecting one W/L out of 1024 W/Ls at the selected selector during reading or programming operation. The ten address signals are divided into two groups, one with 7 signals and one with 3 signals. The seven signals serve to drive one of the 128 global row decoders  800 - 800   n,  to select one global word line (GWLi; i=0 to 127). The three address signals serve to drive one of the partial decoders  50 - 50   n  assigned to each of the sectors to thereby select one partial word line (PWLi; i=0 to 7).  
           [0028]    The global word lines (GWL 0  to GWL 127 ) are correspondingly connected to the local row decoders  900 - 900   n,    910 - 910   n,  . . . ,  930 - 930   n.  The local row decoder of the selected sector receives a signal of the enabled one of the eight partial word lines (PWLi; i=0 to 7) by the selected partial decoder. Therefore, one word line W/L connected to control gates of memory cells is selected.  
           [0029]    As shown in FIG. 3, each of the plurality of local row decoders  900 - 900   n,    910 - 910   n,  . . . ,  930 - 930   n  has a respective level shifter LSpq, where p, q take appropriate values. These are driven by the level shift drivers  21 - 21   n.    
           [0030]    The level shifters LSij are for switching from a low voltage to a high voltage. That is because the plurality of local row decoders  900 - 900   n,    910 - 910   n,  . . . ,  930 - 930   n  should receive a relatively low level of signal of the global word line GWLi, and output to the selected word line W/L a high voltage that is larger than power supply voltage.  
           [0031]    Level shifter LS 1  is comprised of a NAND gate N 1  for NAND-gating a signal of global word line GWL 0  and a sector select signal; first and second PMOS transistors P 1 , P 2  that are cross-coupled; a first input NMOS transistor N 3  connected to a gate of the second PMOS transistor P 2  at its drain, and connected to an output of the of the NAND gate at its source; and a second input NMOS transistor N 2  connected to a gate of the first PMOS transistor P 1  at its drain, and connected to an output of the of the NAND gate N 1  at its gate.  
           [0032]    According to such a structure, the signal of the global word line GWL 0  and the select signal SS 0  of sector selector  10  are NAND-gated, and the result is input to a gate of second input NMOS transistor N 2 . On the other hand, the signal of level shift driver  21  is input to a gate of first input NMOS transistor N 3 . The differential amplification signals that are amplified by difference in a voltage level of gate inputted to the first and second input NMOS transistors N 3 , N 2  are generated as an output signal of the level shifter, through the drain terminals of first and second PMOS transistors P 1 , P 2 . The level-shifted output of the level shifter is applied to the word line driver DR 1 , which is connected to the corresponding word line to thereby boost the selected word line to a high voltage.  
           [0033]    At this time, since the level shifters arranged in the local row decoders  900  to  900   n,    910  to  910   n,  . . . ,  930  to  930   n  should have NAND gates formed with a plurality of transistors, there has been a problem that the area occupied by the local row decoders must be increased. Furthermore, since all the internal transistors other than the NAND gates are made by devices for a high voltage, accordingly large transistors should be fabricated, that have a relatively large channel size and forming separate wells. With design requirements like these, however, it is difficult to reduce a chip size in view of the process aspect and layout aspect.  
           [0034]    As describe above, there has been a problem in the prior art in that the layout area is large, which in turn hinders higher integration of semiconductor memory devices.  
         SUMMARY OF THE INVENTION  
         [0035]    The invention improves on the limitations of the prior art. The invention provides semiconductor memory device row decoder structures having reduced layout area, and methods of operating the same.  
           [0036]    A method according to an embodiment of the invention is for erasing memory cells coupled to a single bitline. A local level converter is biased, for powering in turn a component of a word line driver. In addition, a global word line driver powers another component of the word line driver.  
           [0037]    An advantage of the method of the invention is that an existing component (i.e. the global word line driver) is used to power a portion of the word line driver. This way the local level converter has fewer design requirements, than a level shifter of the prior art.  
           [0038]    A structure according to an embodiment of the invention is for erasing memory cells coupled to a single bitline. The structure includes a single bias driver for these cells, and a plurality of local voltage level converters coupled to the bias driver. At least one word line driver is coupled to each local level converter, to erase at least one of the memory cells.  
           [0039]    Each level converter of the invention has a simpler structure than the level shifter of the prior art, and therefore occupies less area. Furthermore, it may be made with smaller transistors, further conserving on the layout area. The savings are then multiplied by the large number of the local level converters, which results in significant savings in the overall area of the chip. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]    The advantages of the present invention will become apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:  
         [0041]    [0041]FIG. 1 is a cross sectional view showing the section structure of a NOR type memory cell transistor in the prior art.  
         [0042]    [0042]FIG. 2 is a table showing operational modes of the device of FIG. 1.  
         [0043]    [0043]FIG. 3 (made from FIG. 3A and FIG. 3B) is a circuit diagram showing a sector and row decoder structure of a general NOR type flash memory in the prior art.  
         [0044]    [0044]FIG. 4 (made from FIG. 4A and FIG. 4B) is a circuit diagram showing a sector and row decoder structure of a NOR type flash memory according to an embodiment of the present invention;  
         [0045]    [0045]FIG. 5 is a table showing operational modes of the device of FIG. 4.  
         [0046]    [0046]FIG. 6 is a circuit diagram of the Vpbias driver shown in FIG. 4;  
         [0047]    [0047]FIG. 7 is a circuit diagram of the sector selector as shown in FIG. 4;  
         [0048]    [0048]FIG. 8 is a circuit diagram of the Vpx driver as shown in FIG. 4;  
         [0049]    [0049]FIG. 9 is a circuit diagram of the Vex driver as shown in FIG. 4;  
         [0050]    [0050]FIG. 10 a circuit diagram of the partial decoder shown in FIG. 4; and  
         [0051]    [0051]FIG. 11 is a flowchart for illustrating a method according to the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0052]    Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. It should be noted that like reference numerals are used through the accompanying drawings for designation of like or equivalent parts or portion for simplicity of illustration and explanation. Also, in the following description, specifications will be made to provide a thorough understanding about the present invention. It is apparent to one skilled in the art that the present invention can be achieved without the specifications. There will be omission of detailed description about well known functions and structures, so that the description of key points of the present invention will not be obscured.  
         [0053]    [0053]FIG. 4 shows a sector and row decoder connection structure of a NOR type flash memory according to an embodiment of the present invention. In fact, FIG. 4 is made from FIG. 4A and FIG. 4B, which are considered joined at their common numerals.  
         [0054]    In FIG. 4, only a portion of the core block including the row decoder is shown; it should be understood that various functional blocks for a memory operation are omitted, so as not to obscure the description of the invention.  
         [0055]    The structure of FIG. 4 includes a plurality of sectors  300 - 300   n.  These include memory cell transistors that are connected to corresponding word lines at their gates, and to corresponding bit lines at their drains.  
         [0056]    The structure of FIG. 4 also includes global row decoders  100 ,  110 , . . . ,  100   n.  Global row decoders  100 ,  110 , . . . ,  100   n  decode a portion of address signals out of external address signals, to select global word lines.  
         [0057]    The structure of FIG. 4 further includes partial decoders  50 - 50   n  arranged in each of the sectors. Partial decoders  50 - 50   n  decode the remaining address signals out of the external address signals to thereby select partial word lines.  
         [0058]    The structure of FIG. 4 moreover includes local row decoders  200 - 200   n,    210 - 210   n,  . . . ,  230 - 230   n  connected to the global word lines of the global row decoders  100 ,  110 , . . . ,  100   n.  Local row decoders  200 - 200   n,    210 - 210   n,  . . . ,  230 - 230   n  receive a drive voltage of word line through the partial word lines, to thereby enable the word lines of the selected sectors.  
         [0059]    The structure of FIG. 4 additionally includes high voltage Vpx drivers  30 - 30   n.  High voltage Vpx drivers  30 - 30   n  generate a high voltage Vpx, to input it to local row decoders  200 - 200   n,    210 - 210   n,  . . . ,  230 - 230   n  in response to the sector address signal.  
         [0060]    The structure of FIG. 4 also includes erase voltage Vex drivers  40 - 40   n.  Erase voltage Vex drivers  40 - 40   n  generate an erase voltage Vex, to input it to the local row decoders  200 - 200   n,    210 - 210   n,  . . . ,  230 - 230   n  in response to the sector address signal.  
         [0061]    The structure of FIG. 4 further includes sector selectors  10 - 10   n.  Sector selectors  10 - 10   n  generate a sector select signal, to input it to the local row decoders  200 - 200   n,    210   210   n,  . . . ,  230 - 230   n  in response to the sector address signal.  
         [0062]    The structure of FIG. 4 additionally includes bias drivers  20 - 20   n.  Bias drivers  20 - 20   n  generate a bias voltage, to apply it to the local row decoders  200 - 200   n,    210 - 210   n,  . . . ,  230 - 230   n  in response to the sector address signal.  
         [0063]    Each of global row decoders  100 ,  110 , . . . ,  100   n  includes a logic gate  101  for gating the address signals using the external power supply voltage as a drive voltage, and an inverter  102  for inverting an output from the logic gate. Inverter  102  thus outputs a global word line drive voltage.  
         [0064]    Inverter  102  is supplied with the first voltage Vcx or second voltage Vematx as a drive voltage for operation. The first voltage Vcx is 0V during an erase operation, and a level of an external power voltage during the other modes. Meanwhile, the second voltage Vematx is a negative high-voltage during an erase operation, and a level of 0 V during the other modes.  
         [0065]    Each of local row decoders  200 - 200   n,    210 - 210   n,  . . . ,  230 - 230   n  receives a high voltage Vpx and an erase voltage Vex, as driving power voltages. These voltages VPex, Vex are supplied from Vpx drivers  30 - 30   n  and Vex drivers  40 - 40   n,  respectively.  
         [0066]    Each of the local row decoders  200 ,  210 , . . . ,  230  includes a local level converter L 1 , L 2 , . . . Ln. Similarly, each co the remaining local row decoders includes local level converters (unnumbered in FIG. 4). They also includes word line drivers DR 1  connected to their respective local level converter.  
         [0067]    As shown in FIG. 4, local level converter L 1  includes a PMOS transistor P 1 . Transistor P 1  of FIG. 4 receives a high voltage at its source, and a bias voltage Vpbiasi at its gate.  
         [0068]    Local level converter L 1  also includes an NMOS transistor N 1  having a drain coupled with a drain of PMOS transistor P 1 . Transistor N 1  receives a sector select signal SSi at its gate.  
         [0069]    Local level converter L 1  further includes a second NMOS transistor N 2  having a drain coupled with a source of the first NMOS transistor N 1 . Transistor N 2  has a gate coupled with the global word line.  
         [0070]    The converted output level of local level converter L 1  is applied to the word line drivers DR 1 , DR 2 , DR 3 . As will be seen, word line driver DR 1  is coupled with a corresponding word lines, so that the selected word line may be boosted to a high voltage.  
         [0071]    In particular, word line driver DR 1  includes a PMOS transistor P 3 . Transistor P 3  has a source coupled with partial word line PWLi, a gate receiving an output of the local level converter L 1 , and a drain coupled with word line WLi.  
         [0072]    Word line driver DR 1  also includes an NMOS transistor N 3 . Transistor N 3  has a drain coupled with the drain of the PMOS transistor P 3 , a grounded source, and a gate receiving an output of the local level converter L 1 .  
         [0073]    Word line driver DR 1  further includes an NMOS transistor N 4 . Transistor N 4  has a drain coupled with the partial word line, a gate coupled with global word line GWLi, and a source coupled with word line WLi.  
         [0074]    A contrast of FIG. 4 with the prior art of FIG. 3 is useful at this juncture. Local level converter L 1  of the invention has a simpler structure. That is because, since the wordline driver receives an input also from global word line GWLi, local level converter L 1  does not need to perform a differential amplification operation.  
         [0075]    Local level converter L 1  of the invention can perform a level converting operation, without even needing the NAND gates of level shifter LSpq shown in FIG. 3. Accordingly, level converter L 1  of the invention requires substantially less area from level shifter LSpq of the prior art.  
         [0076]    Moreover, due to the reduced design requirements, local level converter L 1  of the invention may be made with smaller transistors than those of level shifter LSpq. This further reduces the layout area for each of the local row decoders  200 - 200   n,    210 - 210   n,  . . . ,  230 - 230   n.    
         [0077]    A massive savings in area therefore results from the invention. The savings in area described above for one local row decoder, are to be multiplied by the large number of the local row decoders  200 - 200   n,    210 - 210   n,  . . . ,  230 - 230   n.  For example, if a sector is set at a capacity of 64 K byte, made out of 1024 W/L and 512 B/L, the multiplication is by the resulting numbers of both rows and columns.  
         [0078]    Referring also to FIG. 5, the row decoding operation for the structure of FIG. 4 is explained below. FIG. 5 is a table that shows the applied voltages versus the operational modes. The structure of FIG. 4 is assumed to be made by 1024 wordlines (W/L) and 512 bitlines (B/L).  
         [0079]    Ten (10) row address signals are needed in selecting one W/L out of 1024 W/Ls at the selected sector during reading or programming operation. The ten address signals are divided into two groups, one of 7 signals and one of 3 signals.  
         [0080]    The seven signals serve to drive one of the 128 global row decoders  100 ,  110 , . . . ,  100   n.  This selects one of the global word line (GWLi; i=0 to 127).  
         [0081]    The three address signals serve to drive one of the partial decoders  50 - 50   n  assigned to each of the sectors. This way, one of the partial word lines (PWLi; i=0 to 7) is selected.  
         [0082]    The global word lines (GWL 0  to GWL 127 ) are correspondingly coupled with the local row decoders  200 - 200   n,    210 - 210   n,  . . . ,  230 - 230   n.  The local row decoder of the selected sector receives a signal of the enabled one (out of the eight) partial word line (PWLi; i=0 to 7) by the selected partial decoder. Accordingly, that one W/L connected to control gates of memory cells is selected during a read operation or during a program operation.  
         [0083]    Global row decoders  100 ,  110 , . . . ,  100   n  receive a second voltage Vematx as a drive power voltage during an erase operation. The second voltage Vematx is supplied as a negative high voltage, −10 V for instance, during an erase operation.  
         [0084]    Erase voltage Vex drivers  40 - 40   n  are activated in response to the sector address, thereby inputting an erase voltage Vexi (for instance 10V), to the local level converter of the selected sector. The SSi, Vpxi and PWLi are supplied with 0V, and the Vpbiasi is supplied with a negative high voltage (for instance, −10V).  
         [0085]    Accordingly, the output of the local level converter in the local row decoder of the selected sector becomes 0V. Therefore all the W/Ls of the selected sector are supplied with the negative high voltage, and the output of the local level converter in the local row decoder of the non-selected sector becomes a negative high voltage. This causes all the W/Ls of the non-selected sector to be supplied with 0V.  
         [0086]    [0086]FIG. 6 is a circuit diagram of the Vpbias driver  20  shown in FIG. 4. Driver  20  comprises NAND gate  22 , inverters  23 ,  24  for inverting the outputs of NAND gates  22 , an inverter  25  for inverting a program signal, a PMOS transistor  26  connected to an output of inverter  25  at its gate, a PMOS transistor  27  connected to a drain of PMOS transistor  26  at its source, an NMOS transistor  28  receiving a program signal at its gate and connected between the bias terminal and the drain of the transistor  27  by its drain-source channel, and an NMOS transistor  29  receiving the output of inverter  24  through its gate and in parallel connected to NMOS transistor  28 .  
         [0087]    In the structure shown in FIG. 6, a program enable signal is input to one side of NAND gate  22  as a logic “high” during a programming operation. In case that the corresponding sector is selected, the sector address SAi is applied as a logic “high” to other input terminals of NAND gates  22 . Accordingly, inverter  25  serves to apply a low level to a gate of PMOS transistor  26 , while NMOS transistor  28  is also turned on. As a result, the output signal Vpbiasi from the drain terminal of PMOS transistor  27  is determined as a voltage level of Vpw-Vtp. In this determination, Vpw is a voltage supplied to a word line during a program operation, and Vtp is a threshold voltage of PMOS transistors  26 ,  27 .  
         [0088]    On the other hand, in a read operation, a read enable signal becomes a logic “high”. In case that the corresponding sector is selected, the sector address SAi is applied as a logic “high”. Accordingly, the output of the inverter  24  becomes “high”, turning on NMOS transistor  29 . As a result, output signal Vpbiasi from the drain terminal of PMOS transistor  27  becomes 0V. In addition, in an erase operation, since the erase bias voltage Vebias is applied as a negative high voltage (e.g. −10V), the output voltage Vpbiasi becomes the same voltage as the erase bias voltage Vebias (e.g. −10V).  
         [0089]    [0089]FIG. 7 is a circuit diagram of one of the sector selectors  10 - 10   n  shown in FIG. 4. Circuit  10  comprises an inverter  11  for inverting an erase enable signal, a NAND gate  12  for receiving the output of the inverter  11  and the sector address SAi to thereby generate a NAND response, and an inverter  13  for inverting the output of the NAND gate  12 .  
         [0090]    The operation of the sector selector  10  is as follows. During an erase operation, the output signal SSi of inverter  13  becomes “low”, regardless of whether a sector is selected or not. If the sector had been selected at the previous time of a non-erase operation, the output SSi becomes “high”, but if the sector had not been selected, the output SSi becomes “low”.  
         [0091]    [0091]FIG. 8 is a circuit diagram of the Vpx driver  30  shown in FIG. 4. Driver  30  comprises NAND gates  31 ,  33 , inverters  32 ,  34  for inverting the outputs of NAND gates  31 ,  33 , inverters  35 ,  38  for inverting a program signal that is output from inverter  32 , PMOS transistors  36 ,  37  connected to outputs of inverters  35 ,  38  at its gates, inverters  35 - 1 ,  38 - 1  for inverting a read signal that is output from inverter  34 , and PMOS transistors  36 - 1 ,  37 - 1  connected to outputs of inverters  35 - 1 ,  38 - 1  at its gates.  
         [0092]    During a read operation, the read enable signal is applied to the driver circuit  30  as “high”. In the case of the selected sector, the read signal is input as “high”, and the voltage Vpr that is applied to the source of the PMOS transistor  36 - 1  is output through the output terminal Vpxi.  
         [0093]    During a program operation, since the program enable signal is input as “high”, the program signal becomes “high” in the case of the selected sector, and the voltage Vpw appears at the output terminal Vpxi.  
         [0094]    [0094]FIG. 9 is a circuit diagram of the Vex driver  40  shown in FIG. 4. Driver  40  comprises a NAND gate  41 , an inverter  42 , and an inverter comprised of a combination of P type MOS transistor  43  and N type MOS transistor  44 . In this case, the erase signal becomes “high”, and in case of the selected sector, the voltage that is applied to the terminal Vneg is transmitted to the terminal Vexi.  
         [0095]    [0095]FIG. 10 is a circuit diagram of the partial decoder  50  shown in FIG. 4. Partial decoder  50  includes a NAND gate  51  and an inverter  52  to enable the corresponding partial word line pWL&lt;i&gt;. When the selector is selected and the signal S&lt;i&gt; generated by the three address signals is “high”, the voltage level is transmitted to the output terminal pWL&lt;i&gt;.  
         [0096]    Referring now to FIG. 11, a flowchart  1100  is shown for describing a method according to an embodiment of the invention. The method of flowchart  1100  may also be practiced by the device of FIG. 4. It will be understood that, although many of the boxes are given sequentially, many of them may optionally be performed concurrently.  
         [0097]    The method of flowchart  1100  is for driving a word line of an electrically programmable and erasable semiconductor memory device, which has a local level and a word line driver coupled to the local level converter and to the word line.  
         [0098]    According to a box  1110 , the local level converter is biased.  
         [0099]    According to a box  1160 , a voltage from an output of the biassed local level converter is applied to the word line driver. In addition, a voltage from a global word line is concurrently applied to the word line driver to drive the word line.  
         [0100]    In the preferred embodiment, the local level converter is made of a PMOS transistor and first and second NMOS transistors, and the output of the local level converter is a drain of the PMOS transistor. Then, box  1110  is performed as follows.  
         [0101]    According to a box  1120 , a high voltage is applied to a source of a PMOS transistor.  
         [0102]    According to a box  1130 , a bias voltage is applied to a gate of the PMOS transistor.  
         [0103]    According to a box  1140 , a sector select signal is applied to a gate of a first NMOS transistor.  
         [0104]    According to a box  1150 , the global word line drive voltage is applied to a gate of a second NMOS transistor.  
         [0105]    While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the sprit and scope of the appended claims. For example, the local row decoder of the present invention may be applied to even a nonvolatile semiconductor memory having NAND or AND structure. In addition, the logic gates may be replaced with the corresponding equivalent circuit devices or other logic devices.