Patent Document

PRIORITY STATEMENT 
     This application is a divisional of U.S. application Ser. No. 11/653,866, filed Jan. 17, 2007, now U.S Pat. No. 7,733,695 which claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2006-0015821, filed on Feb. 17, 2006, in the Korean Intellectual Property Office (KIPO), the entire contents of each of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention relate to nonvolatile semiconductor memory devices and related methods of operation. 
     2. Discussion of Related Art 
     Generally, read and write (programming/erase) operations for memory cells in a nonvolatile semiconductor memory device are carried out by controlling bit line voltages corresponding to selected memory cells. In order to properly drive bit lines voltages during a read or programming operation, contemporary nonvolatile semiconductor memory devices provide one or more input/output circuits to temporarily store the data to be written into or read from the memory cells. 
       FIG. 1  is a diagram illustrating a conventional nonvolatile semiconductor memory device. As shown, the device includes a memory cell array  10 . The memory cell array  10  comprises a plurality of even and odd grouped bit lines (BLe&lt;n:1&gt; and BLo&lt;n:1&gt;), and corresponding strings St of memory cells for storing data received from bit lines (BLe&lt;n:1&gt; and BLo&lt;n:1&gt;) and outputting read data to the bit lines (BLe&lt;n:1&gt; and BLo&lt;n:1&gt;). Each pair of bit lines BLe and BLo is connected to an input/output circuit  20 . 
       FIG. 2  illustrates a portion of the memory cells in the memory cell array  10  in some additional detail. As shown, the memory cell array  10  comprises a plurality of cell strings (Ste&lt;n:1&gt; and STo&lt;n:1&gt;) each respectively connected to one of the bit lines (BLe&lt;n:1&gt; and BLo&lt;n:1&gt;). Each cell string in the illustrated example is formed from a string selection transistor (SST) connected to its corresponding bit line, a ground selection transistor (GST) connected to a common source line (CSL), and a plurality of memory cells (MC) connected in series between the string selection transistor (SST) and the ground selection transistor (GST). As shown in  FIG. 1 , more than one string may be connected to a bit line ( FIG. 2  shows only one string per bit line for clarity). 
     Each one of the memory cells (MC) comprises a floating-gate transistor having a source, a drain, a floating gate and a control gate. The memory cells (MC) may be programmed using the Channel Hot Electron (CHE) effect or the Fowler-Nordheim (F-N) tunneling effect. These techniques are both conventionally understood. 
     As shown in  FIG. 1 , two adjacent bit lines are configured to constitute a pair of bit lines. However, each bit line may be selected in relation to a unique column address. Therefore, in this disclosure the two bit lines, (i.e., an even bit line and an odd bit line) may be referred to individually or collectively as a “bit line” without further differentiation. 
       FIG. 1  further shows that the memory device includes a row selector  510  and control logic  500 . The control logic  500  receives command and address information (e.g., from a host system), and generates control signals to control the operation of the row selector  510  and the input/output circuits  20 . The commands may be at least one of a read command and a write command. The address information indicates an address of at least one memory cell in the memory cell array  10 . In particular, the control logic  500  decodes the address information into a row address and a column address. 
     Based on the command and the row address, the control logic  500  controls the row selector  510  to assert the appropriate word lines WLi, string selection transistor (SST) and a ground selection transistor (GST) to select a row of memory cells MC for a read or write operation. Based on the command and the column address, the control logic  500  generates the control signals supplied to the input/output circuits  20  as described in detail below. 
     As shown in  FIG. 1 , each input/output circuit  20  includes a bit line biasing and coupling circuit  110 , a bit line blocking circuit  120 , a page buffer  150  and a column gate  160 . Data to be written to a selected memory cell is supplied on a data input line  200 . The data is loaded and latched in the page buffer  150  via the associated column gate  160 . Data stored in the page buffer  150  is thus provided to bit line BLe or BLo through the bit line (BL) blocking circuit  120  and the BL bias and coupling circuit  110 . Thereafter, a programming operation may be performed relative to the selected memory cell. In similar fashion, data read from a selected memory cell is transferred via the BL bias and coupling circuit  110  and the BL blocking circuit  120 , and temporarily stored in the page buffer  150 . Data thus stored in page buffer  150  may be transferred to an output data line  300  in response to a column gate signal applied to the column gate  160  by the control logic  500 . 
       FIG. 3  illustrates the circuits comprising the input/output circuit  20  in greater detail. As shown, the bit line biasing and coupling circuit  110  includes first and second high voltage transistors  112  and  114  connected in series between the even and odd bit lines BLe and BL 0 . The node connecting the first and second high voltage transistors  112  and  114  receives a bit line bias BLPWR. As will be appreciated, the bit line bias BLPWR is, generally, either a reference ground voltage Vss or a power supply voltage Vdd. First and second control signals SHLDe and SHLDo are supplied to the gates of the first and second high voltage transistors  112  and  114 . 
     Because, in this example, the first and second high voltage transistors  112  and  114  are NMOS transistors, if the first and second control signals SHLDen and SHLDo are high, then the bit line bias BLPWR is supplied to the even and odd bit lines BLe and BLo to, for example, pre-charge the bitlines. If the first and second control signals SHLDe and SHLDo are low, then the then the bit line bias BLPWR is not supplied to the even and odd bit lines BLe and BLo. In view of the function of the first and second high voltage transistors  112  and  114 , these transistors are often referred to as pre-charge transistors because they are used to pre-charge the bit lines. 
     The bit line biasing and coupling circuit  110  also includes a third high voltage transistor  116  connected to the even bit line BLe and a fourth high voltage transistor  118  connected to the odd bit line BLo. The outputs of the third and fourth high voltage transistors  116  and  118  are connected together and to the bit line blocking circuit  120 . The third and fourth high voltage transistors  116  and  118  receive third and fourth control signals BLSLTe and BLSLTo at their gates, respectively. When the third and fourth control signals BLSLTe and BLSLTo are high, the third and fourth high voltage transistors  116  and  118  couple the even and odd bit lines BLe and BLo to the blocking circuit  120 . When the third and fourth control signals BLSLTe and BLSLTo are low, the third and fourth high voltage transistors  116  and  118  de-couple the even and odd bit lines BLe and BLo from the blocking circuit  120 . As a result, the third and fourth high voltage transistors  116  and  118  are also commonly referred to as de-coupling transistors. 
     As shown in  FIG. 3 , the blocking circuit  120  includes a single high voltage transistor connecting the bit line bias and coupling circuit  110  to the page buffer  150 . The high voltage blocking transistor  120  receives a fifth control signal SOBLK at its gate. When the fifth control signal SOBLK is high, the high voltage blocking transistor  120  connects the bit line bias and coupling circuit  110  with the page buffer  150 . When the fifth control signal SOBLK is low, the high voltage blocking transistor  120  blocks the connection between the bit line bias and coupling circuit  110  and the page buffer  150 . 
       FIG. 3  further shows the details of the page buffer  150 . Because the page buffer  150  shown in  FIG. 3  is so well known it will not be described in detail for the sake of brevity. It is sufficient to note that the page buffer  150  does include a latch  152  for temporarily storing input or output data with respect to the memory cell array  10 . 
     Also, as further shown in  FIG. 3  the column gate  160  includes a transistor connecting the page buffer  150  to the input data line  200  and the output data line  300 . The column gate transistor  160  receives a column gate signal YG at its gate. When the column gate signal is high, the input data line  200  and the output data line  300  are connected with the page buffer  150 . When the column gate signal YG is low, the input data line  200  and the output data line  300  are disconnected with the page buffer  150 . 
       FIG. 3  further provides an indication of the layout architecture of the memory cell array  10  and the input/output circuit  20 . As shown,  FIG. 3  indicates that the memory cell array  10  is formed in a cell array or pocket-Pwell region  600  of a semiconductor substrate while the input/output circuit  20  is formed over a high voltage transistor region  700  and a low voltage transistor region  800 . More specifically, the bit line bias and coupling circuit  110  and the bit line blocking circuit  120 , which include high voltage transistors are formed in a high voltage transistor region  700  of the semiconductor substrate. By contrast, the page buffer  150  and column gate  160  are formed in a low voltage transistor region  800  of the semiconductor substrate. 
       FIG. 4  illustrates a cross-sectional perspective view of the layout architecture of the memory cell array  10  and input/output circuit  20  shown in  FIG. 3 . It will be understood that the layout architecture shown in  FIG. 4  is not a true cross-section of the semiconductor substrate, but instead, is a side view of the semiconductor substrate. Furthermore it will be understood, that for the sake of clarity, many details to create an operational circuit layout have not been shown. Namely,  FIG. 4  is a graphical representation (e.g., the low voltage transistor region  800  including the page buffer  150  has been represented as the Nwell  804  and Pwell  802 ). Still further, the processing steps and techniques to produce the layout architecture shown in  FIG. 4  will not be described as these are well-known and readily understood from  FIG. 4 . As shown in  FIG. 4 , a p-type substrate  900  has an N-type well  602  formed therein. A pocket-Pwell  600  is formed in the Nwell  602 . The pocket-Pwell (P-Pwell)  600  defines the cell array or pocket-Pwell region  600 , and the memory cell transistors are formed in this pocket-Pwell region  600 . 
       FIG. 4  also shows that a Pwell  802  and Nwell  804  are formed in the substrate  900  disposed away from the Nwell region  602 . The Pwell region  802  and Nwell region  804  form the low voltage transistor region  800 . As graphically shown in  FIG. 4 , it is in this region that transistors are formed to create the page buffer  150 , column gate  160 , etc. 
     The p-type substrate  900  disposed between the Pwell  802  and the Nwell  602  is where the high voltage transistors of the bit line biasing and coupling circuit  110  and the blocking circuit  120  are formed. Namely, the portion of the p-type substrate  900  disposed between the Pwell  802  and the Nwell  602  forms the high voltage transistor region  700 .  FIG. 4  shows the second, fourth and blocking high voltage transistors  114 ,  118  and  120  in the high voltage region  700 . While  FIG. 4  has not been drawn to scale,  FIG. 4  does illustrate the size relationship between the transistors formed in the different regions. Namely,  FIG. 4  shows that the high voltage transistors, because of their need to transfer and block high voltages, are significantly greater in size then the transistors in the low voltage transistor region  800  or the transistors in the cell array region  600 . As a result, a significant portion of the substrate  900  is devoted towards the high voltage transistor region  700 . 
     As mentioned above, the high voltage transistors and the high voltage transistor region  700  are formed relatively large as a result of the high voltages they must withstand during operation.  FIG. 5  illustrates an example of an erase operation and the high voltages incurred by the high voltage transistors and the high voltage transistor region  700 . As shown, during an erase operation, an erase voltage of about 20 volts is induced on the bit lines BLe and BLo as a result of the pocket-Pwell  600  being biased to 20 volts. However, the gates of the high voltage bit line biasing transistors  112  and  114  are supplied with zero volts to prevent the 20 volt bias from being transferred out upon the conductor upon which the bias control signal BLPWR is received. As shown in  FIG. 5 , this creates great stress in the high voltage pre-charge transistors  112  and  114 , and necessitates their large size. 
     As further shown in  FIG. 5 , 20 volts is applied to the gates of the high voltage de-coupling transistors  116  and  118  such that these transistors transfer the 20 volts received along the bit lines BLe and BLo. As shown in  FIG. 5 , transferring such a high voltage also induces great stress in these transistors, and necessitates the large size of the high voltage de-coupling transistors  116  and  118 . The 20 volts transferred by the high voltage de-coupling transistors  116  and  118  is supplied to the high voltage blocking transistor  120 , which receives the power supply voltage VDD at its gate. The high voltage blocking transistor  120  throttles the voltage received, such that only a threshold difference of the power supply voltage reaches the low voltage transistor region  800 . 
     SUMMARY OF THE INVENTION 
     The principles of the present invention have application to various types of non-volatile memories, those currently existing and those contemplated for use in new technology. Implementations of the present invention, however, are described with respect to a flash electrically erasable and programmable read-only memory (EEPROM), wherein the storage elements are floating gates, as exemplary. 
     In one embodiment, the non-volatile memory device includes a well of a first conductivity type formed in a substrate, and a first plurality of memory cell transistors connected in series to a bit line formed in the well. A buffer is formed in the substrate outside the well and is connected to the bit line. At least one de-coupling transistor is configured to de-couple the buffer from the bit line, and the de-coupling transistor is formed in the well. 
     For example, the de-coupling transistor may be formed between the first plurality of memory cells and the buffer without an intervening plurality of memory cells. 
     One embodiment further includes a blocking transistor connected to the de-coupling transistor and formed outside the well. The blocking transistor is configured to selectively cut off the buffer from the bit line. 
     Another embodiment of the non-volatile memory device also includes a well of a first conductivity type formed in a substrate, and a plurality of memory cell transistors connected in series to a same bit line formed in the well. A buffer is formed in the substrate outside the well and is connected to the bit line. At least one pre-charge transistor may be configured to selectively apply a pre-charge voltage to the bit line, and the pre-charge transistor is formed in the well. 
     For example, the pre-charge transistor may be formed between the first plurality of memory cells and the buffer without an intervening plurality of memory cells. 
     One embodiment further includes a blocking transistor connected to the de-coupling transistor and formed outside the well. The blocking transistor is configured to selectively cut off the buffer from the bit line. 
     Yet another embodiment of the non-volatile memory device includes a well of a first conductivity type formed in a substrate, and a first plurality of memory cell transistors connected in series to a bit line formed in the well. A buffer is formed in the substrate outside the well and is connected to the bit line. At least one de-coupling transistor is configured to de-couple the buffer from the bit line, and the de-coupling transistor is formed in the well. At least one pre-charge transistor is configured to selectively apply a pre-charge voltage to the bit line, and the pre-charge transistor is formed in the well. 
     One embodiment further includes a blocking transistor connected to the de-coupling transistor and formed outside the well. The blocking transistor is configured to selectively cut off the buffer from the bit line. 
     In one embodiment, the de-coupling transistor and the pre-charge transistor may be formed between the first plurality of memory cells and the buffer without an intervening plurality of memory cells. 
     In another embodiment, a second plurality of memory cells are connected in series to the bit line and formed in the well. The de-coupling transistor may be disposed after the first and second pluralities of memory cells along a first direction, while the pre-charge transistor may be disposed between the first and second pluralities of memory cells in the first direction. 
     A further embodiment of the non-volatile memory device includes a first plurality of memory cell transistors connected in series to a bit line, a second plurality of memory cell transistor connected in series to the bit line, and a buffer connected to the bit line. At least one pre-charge transistor is configured to selectively apply a pre-charge voltage to the bit line, and the pre-charge transistor is formed between the first plurality of memory cells and the second plurality of memory cells along the first direction. 
     One embodiment further includes a de-coupling transistor configured to decouple the bit line from the buffer, and the de-coupling transistor is disposed after the first and second pluralities of memory cell transistors in the first direction. 
     A still further embodiment of the non-volatile memory device includes a first region of a substrate having a plurality of memory cell transistors connected in series to a bit line, and a second region of the substrate having a buffer connected to the bit line. At least one de-coupling transistor is configured to de-couple the buffer from the bit line, and the de-coupling transistor is formed in the first region. At least one well in the substrate defines the one of the first and second regions. 
     An additional embodiment of the non-volatile memory device includes a first region of a substrate having a plurality of memory cell transistors connected in series to a bit line, and a second region of the substrate having a buffer connected to the bit line. At least one pre-charge transistor is configured to selectively apply a pre-charge voltage to the bit line, and the pre-charge transistor is formed in the first region. At least one well in the substrate defines one of the first and second regions. 
     Another embodiment of the non-volatile memory device includes a first region of a substrate having a plurality of memory cell transistors connected in series to a bit line, and a second region of the substrate having a buffer connected to the bit line. At least one de-coupling transistor is configured to de-couple the buffer from the bit line, and the de-coupling transistor is formed in the first region. At least one pre-charge transistor is configured to selectively apply a pre-charge voltage to the bit line, and the pre-charge transistor is formed in the first region. At least one well in the substrate defines one of the first and second regions. 
     The present invention also relates to a method of erasing a portion of a non-volatile memory device. 
     In one embodiment of the method, an erase voltage is applied to a well formed in a substrate. The well includes a plurality of memory cell transistors connected in series to a bit line. The method also involves having a gate of at least one de-coupling transistor float. The de-coupling transistor is formed in the well and is configured to selectively de-couple the bit line from a buffer formed in the substrate outside the well. 
     One embodiment of the method further includes applying a turn off voltage to a gate of a blocking transistor to turn off the blocking transistor. The blocking transistor is connected to the de-coupling transistor and is formed outside the well. The blocking transistor is configured to selectively cut off the buffer from the bit line. 
     Another embodiment of the method includes applying an erase voltage to a well formed in a substrate. The well includes a plurality of memory cell transistors connected in series to a bit line. The method also involves having a gate of at least one pre-charge transistor float. The pre-charge transistor is formed in the well and is configured to selectively apply a pre-charge voltage to the bit line. 
     In one embodiment, the method further includes applying a turn off voltage to a gate of a blocking transistor to turn off the blocking transistor. The blocking transistor is formed outside the well, and the blocking transistor is configured to selectively cut off the buffer from the bit line. 
     Another embodiment of the method of erasing a portion of a non-volatile memory device includes applying an erase voltage to a well formed in a substrate. The well includes a plurality of memory cell transistors connected in series to a bit line. The method also involves having a gate of at least one de-coupling transistor and at least one pre-charge transistor float. The de-coupling transistor is formed in the well and is configured to selectively de-couple the bit line from a buffer formed in the substrate outside the well. The pre-charge transistor is formed in the well and is configured to selectively apply a pre-charge voltage to the bit line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given below and the accompanying drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus are not limiting on the present invention and wherein: 
         FIG. 1  is a diagram illustrating a prior art nonvolatile semiconductor memory device; 
         FIG. 2  is a diagram further illustrating the memory cell array shown in  FIG. 1 ; 
         FIG. 3  is a diagram illustrating the input/output circuit, in the prior art nonvolatile semiconductor memory device shown in  FIG. 1 , in greater detail; 
         FIG. 4  illustrates a cross-sectional perspective side view of the layout architecture of the memory cell array and input/output circuit shown in  FIG. 3 ; 
         FIG. 5  illustrates an example of an erase operation and the high voltages incurred by the high voltage transistors in the high voltage transistor region of  FIG. 3 ; 
         FIG. 6  illustrates a nonvolatile semiconductor memory device in accordance with an embodiment of the present invention; 
         FIG. 7  illustrates a cross-sectional perspective side view of the layout architecture shown in  FIG. 6 ; 
         FIG. 8  illustrates an erase operation performed according to an embodiment of the present invention with the layout architecture of  FIG. 6 ; and 
         FIG. 9  illustrates another embodiment of a layout architecture according to the present invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Example embodiments of the present invention will be described below in detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as being limited to only the embodiments set forth herein. 
       FIG. 6  illustrates a layout architecture of a non-volatile semiconductor memory device according to an embodiment of the present invention. As shown in  FIG. 6 , the layout architecture is similar to that of the prior art shown in  FIG. 3  except for some important differences. Namely, the high voltage transistor region  700 ′ in  FIG. 6  no longer includes the bit line biasing and coupling circuit  110 . Instead, as shown in  FIG. 6  a bit line biasing and coupling circuit  110 ′ has been formed in the cell array or pocket-Pwell region  600 ′. As shown, the bit line biasing and de-coupling circuit  110 ′ in  FIG. 6  has the same transistor architecture as that of the bit line biasing and coupling circuit  110  shown in  FIG. 3 , but the first, second, third and fourth transistors  112 ′,  114 ′,  116 ′ and  118 ′ forming the bit line biasing and coupling circuit  110 ′ have been labeled using new references numbers. The new references numbers have been used to demonstrate that the transistors in the bit line biasing and coupling circuit  110 ′ are not the same as in the bit line biasing and coupling circuit  110 . Namely, the first, second, third and fourth transistors  112 ′,  114 ′,  116 ′ and  118 ′ are not high voltage transistors. Furthermore, the bitline biasing and coupling circuit  110 ′ has been illustrated as including two circuits: a bitline biasing circuit  111  and a de-coupling circuit  113 . The bitline biasing circuit  111  includes the structure of the first and second transistors  112 ′ and  114 ′, and the de-coupling circuit  113  includes the structure of the third and fourth transistor  116 ′ and  118 ′. 
       FIG. 7  illustrates a cross-sectional perspective side view of the layout architecture shown in  FIG. 6 . It will be understood that the layout architecture shown in  FIG. 7  is not a true cross-section of the semiconductor substrate, but instead, is a side view of the semiconductor substrate. Furthermore it will be understood, that for the sake of clarity, many details to create an operational circuit layout have not been shown. Namely,  FIG. 7  is a graphical representation (e.g., the low voltage transistor region  800  including the page buffer  150  has been represented as the Nwell  804  and Pwell  802 ). Still further, the processing steps and techniques to produce the layout architecture shown in  FIG. 7  will not be described as these are well-known and readily understood from  FIG. 7 .  FIG. 7  shows the p-type substrate  900  having the Nwell  602  formed therein.  FIG. 7  further shows that the pocket-Pwell  600 ′ is formed in the Nwell  602  and that the transistors of the cell array  10  as well as the transistors of the bit line bias and coupling circuit  110 ′ are formed in the pocket-Pwell region  600 ′. A Pwell region  802  and Nwell region  804  are also formed in the p-type substrate  900 , and comprise the low voltage transistor region  800  in which the page buffer  150  is formed. Disposed between the low voltage transistor region and the Nwell  602  is the portion of the p-type substrate  900  serving as the high voltage transistor region  700 ′. As  FIG. 7  shows, the high voltage transistor region  700 ′ only includes the high voltage blocking transistor  120 . 
     While not drawn to scale,  FIG. 7  is intended to show the size relationship between the transistors formed in the different regions. In particular,  FIG. 7  demonstrates that the transistors forming the bit line biasing and coupling circuit  110 ′ are significantly smaller than the high voltage blocking transistor  120 . It is also intended, while not drawn to scale, that  FIG. 7  illustrate the size difference relationship between the transistors forming the bit line biasing and coupling circuit  110 ′ with respect to the transistors formed in the bit line biasing and coupling circuit  110  shown in  FIG. 4 . Again, comparison of  FIGS. 7 and 4  demonstrates that the transistors in the bit line biasing and coupling circuit  110 ′ are significantly smaller than their counterpart transistors in the prior art of  FIG. 4 . In particular, the transistors in the bit line biasing and coupling circuit  110 ′ are not high voltage transistors and are one-quarter the size of the high voltage transistors in the bit line biasing and coupling circuit  110 . 
     As will be appreciated, the layout architecture according to this embodiment of the present invention allows for a significantly smaller high voltage transistor region  700 ′ without a commensurate significant increase in the size of the pocket-Pwell region  600 ′. As such, a noticeable savings in precious semiconductor space is obtained through the layout architecture of the present invention. 
     As described in the Background of the Invention section, the transistors in the bit line biasing and coupling circuit  110  where designed as high voltage transistors because of the high voltage stresses experienced by these transistors. However, in the layout architecture of  FIG. 6 , these high voltage stresses have been eliminated through the new layout architecture, and this has enabled the use of non-high voltage transistors in the bit line bias and coupling circuit  110 ′.  FIG. 8  provides a representative example of the lack of high voltage stresses experienced by the transistors of the bit line biasing and coupling circuit  110 ′. 
     As a comparative example to the erase operation illustrated in  FIG. 5  with respect to the prior art,  FIG. 8  illustrates the erase operation performed with the layout architecture of  FIG. 6 . As shown, the same 20 volt erase voltage is applied to the P-Pwell  600 ′ in the well-known manner. This induces a substantially 20 volt erase voltage on the bit lines. Each of the pre-charge transistors  112 ′ and  114 ′ have their gates floating. Also, each of the de-coupling transistors  116 ′ and  118 ′ have their gates floating. As a result, stresses are not induced on these transistors, but the voltage does flow out the conductor providing the bit line biasing control signal BLPWR. As shown in  FIG. 8 , a switch  950  is used to supply either a reference ground voltage Vss or the power supply voltage Vdd as the bit line biasing control signal BLPWR. Furthermore, a high voltage protection transistor  952  is disposed between the switch  950  and the node connecting the pre-charge transistors  112 ′ and  114 ′. The high voltage protection transistor  952  may be disposed in the high voltage transistor region  700 ′. The power supply voltage Vdd is supplied to the gate of the high voltage protection transistor  952 , such that the high voltage protection transistor  952  shields the switch  950  from the high voltage generated during the erase operation. 
     As with the embodiment of  FIG. 5 , the high voltage blocking transistor  120  receives the power supply voltage Vdd at its gate. The high voltage blocking transistor throttles the voltage received, such that only a threshold difference of the power supply voltage reaches the low voltage transistor region  800 . 
       FIG. 9  illustrates a still further embodiment of the present invention. This embodiment is the same as  FIG. 6  except that the bit line biasing circuit  111  and the de-coupling circuit  113  have been separated. In the embodiment of  FIG. 6 , the bit line biasing circuit  111  and the de-coupling circuit  113  were formed at the end of the P-Pwell region  600 ′ such that no memory cells intervene between the bit line bias and coupling circuit  110 ′ and the page buffer  150 . However, in the embodiment of  FIG. 9 , the bit line biasing circuit  111  is formed within the memory cell array such that memory cells are disposed between the bit line biasing circuit  111  and the page buffer  150 . In the embodiment of  FIG. 9 , the bit line biasing circuit  111  is formed midway in the memory cell array such that an equal number of memory cells are disposed on either side of the bit line biasing circuit  111 . It will be understood, however, that other positions for the bit line biasing circuit  111  are possible, and the present invention is not limited to the embodiment shown in  FIG. 9 . 
     Although the present invention has been described in connection with several teaching embodiments, it is not limited thereto. It will be apparent to those skilled in the art that various substitutions, modifications and changes may be thereto without departing from the scope of the invention as defined by the following claims.

Technology Category: 3