Abstract:
A method for programming and erasing an array of NMOS electrically erasable programmable read only memory (EEPROM) cells that minimizes bit disturbances and high voltage requirements for the memory array cells and supporting circuits. In addition, the array of N-channel memory cells may be separated into independently programmable memory segments by creating multiple, electrically isolated P-wells upon which the memory segments are fabricated. The multiple, electrically isolated P-wells may be created, for example, by p-n junction isolation or dielectric isolation.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure, according to specific example embodiments, relates to semiconductor devices, and more specifically to N-channel Electrically Erasable Programmable Read Only Memory (EEPROM) (hereinafter memory) devices that may be programmed and erased with minimum bit disturbances and voltage withstand requirements for the memory array cells and supporting circuits. 
       BACKGROUND 
       [0002]    A common practice in fabricating Electrically Erasable Programmable Read Only Memory (EEPROM) was to produce N-channel cells over a P-well substrate because of a simpler manufacturing process and lower programming voltages. The approach used by Caywood; as disclosed in U.S. Pat. No. 5,986,931, entitled “Low Voltage Single Supply CMOS Electrically Erasable Read-Only Memory” which is a continuation-in-part of U.S. Pat. No. 5,790,455. and U.S. Pat. No. 5,986,931 (Caywood  2 ) and U.S. Pat. No. 5,790,455 (Caywood  1 ), incorporated by reference herein for all purposes; produces precisely the opposite configuration, i.e., P-channel devices over an N-well, which itself resides in a P-type substrate. The novelty of the Caywood approach is the reduction in magnitude of the applied voltage required for erasing and writing to the device while maintaining a similar writing speed as found in the related technology prior to Caywood as well as the elimination of certain components functionally necessary in the related technology. 
         [0003]    Referring to  FIG. 1 , the N-channel memory device related technology is illustrated. Each memory transistor (MEM) required a row select transistor (SEL), which controlled the data received from the bit lines (BL). Also, if byte addressing was desired, then the device included a byte select transistor (BYTE) for every eight memory transistors. The problem solved by Caywood with the advent of a P-channel/N-well device was the elimination of the row select transistors. Even after Caywood, byte selection still required the presence of the byte select transistors. The elimination of the byte select transistors resulted in the undesirable effect that the entire row must be reprogrammed following an erase operation. 
         [0004]    Referring to  FIG. 2 , the Caywood approach is illustrated in general terms for a single memory transistor  1 . The N-well  3  is created within a P-type substrate  2 . The P-channel for the drain  4  and source  5  is created within the N-well  3 . Poly  1  or the floating gate  6  of the memory transistor  1  is created after the active region for the drain  4  and source  5 . Poly  2  or the control electrode  7  of the memory transistor is fabricated over the floating gate. Various non-conductive layers  8  insulate the P-channel  4  and  5 , the floating gate  6  and the control electrode  7  from each other. 
         [0005]      FIG. 3  illustrates a plurality of cell rows  100 , typically connected to gate electrodes of memory transistors and a plurality of columns  200  typically connected to source and drain electrodes of memory transistors in the array, with both cell rows and cell columns existing on a single N-well  300  substrate. The limitation to the Caywood P-channel memory arrays, as shown in  FIG. 3 , is that all memory cells in any particular row must be selected, thus written or erased, during a particular operation. 
         [0006]    Alternatively stated, as disclosed by Caywood, the cell rows are not segmented such that some memory cells in the cell row may be selected for writing while other memory cells in the row are not. Thus, in order to program the contents of a single memory cell, the entire cell row must then be programmed in order to change the data in one memory cell. 
         [0007]    In many applications it is desired to change the data in the memory array, one byte at a time. In the N-channel device prior art, this feature was accomplished by the inclusion of a byte select transistor (BYTE) for each eight memory transistors as shown in  FIG. 1 . The disadvantage of this approach is the increased demand for silicon area to accommodate the overhead of the byte select transistor (BYTE). For example, from solely a transistor perspective, a byte select transistor (BYTE) for every eight memory transistors requires an 11 percent overhead (i.e., 1/9). 
         [0008]    Moreover, the capability of changing one byte at a time would give an endurance advantage over row select memory arrays because only one byte of cells would need to undergo the electrical stress of the programming cycle as opposed to the entire row. It is well known to those skilled in the art of semiconductor memory fabrication that one cause of EEPROM failure is attributable to excessive erase/write operations. 
         [0009]    As the memory cell and supporting circuitry transistor circuits are fabricated with smaller geometries, voltage breakdown becomes more of a problem. Since the erase and write operations require relatively high voltages in comparison to read operations, the smaller geometry circuit elements are more voltage stressed than were older technology memory cells and supporting circuitry that used larger geometry transistors. 
       SUMMARY 
       [0010]    What is needed is a technique for programming and erasing an array of NMOS EEPROM cells that minimizes bit disturbances during programming and erasing, and minimizes the voltage withstand requirements for the memory array and supporting circuitry by biasing the cell elements for lowest voltage stress. 
         [0011]    In addition, byte selectability may be advantageously used for N-channel/P-well EEPROM technology by providing independently programmable memory segments within the EEPROM array other than with byte select transistors. This may be accomplished by providing an N-channel/P-well electrically erasable programmable read only memory array that is divided into independently programmable memory segments within a memory array by fabricating a plurality of P-wells within a deep N-well of the array or by segmenting the P-well of the array into sub-P-wells in the deep N-well. The independently programmable memory segments are achieved without the necessity for byte select transistors. Creating a plurality of P-wells within a deep N-well may be done with p-n junction isolation. Segmenting a P-well of the memory array may done by dielectric isolation. 
         [0012]    According to this disclosure, a memory array may comprise a plurality of P-wells within a deep N-well that is in a P-type substrate and each of the plurality of P-wells comprises a plurality of independently programmable memory segments. Each independently programmable memory segment is comprised of M memory cell columns and N memory cell rows. Each independently programmable memory segment may reside within a unique and separate P-well. Thus, each P-well contains an independently programmable memory segment. 
         [0013]    A memory array may comprise a P-well within a deep N-well that is within a P-type substrate wherein the P-well is segmented into a plurality of electrically isolated sub P-wells, M memory transistor columns within each of the plurality of electrically isolated sub P-wells and N memory transistor rows within each of the plurality of electrically isolated sub P-wells. 
         [0014]    A combination of positive and negative biases on the memory cell and support circuitry transistor elements, P-wells and deep N-well are used during write and erase operations for reducing voltage potential withstand requirements between the elements. For example, for a fifteen (15) volt programming method, use of approximately a four (4) volt bit line bias during program and erase operations minimizes cell disturbance and reduces cell junction voltage requirements to approximately eleven (11) volts. Use of approximately a four (4) volt word line bias during a program operation minimizes cell disturbance. Use of separate source select transistor biasing for each P-well during program and erase operations, with the drain and gate of each source select transistor coupled to the P-well voltage potential minimizes voltage withstand requirements for source select transistors. Use of approximately a four (4) volt word line bias during an erase operation minimizes voltage withstand requirements for row drivers. 
         [0015]    Commonly owned U.S. Pat. Nos. 6,222,761 B1;6,236,595 B1;6,300,183 B1; and 6,504,191 B2; all by Gerber et al., disclose PMOS EEPROM having independently programmable memory segments, all of which are incorporated by reference herein for all purposes. 
         [0016]    According to a specific example embodiment of this disclosure, a method of erasing selected ones of a plurality of memory segments in a memory array comprising a P-well in a deep N-well within a P-type substrate may comprise: setting the deep N-well to a positive voltage; setting the P-well to the positive voltage; setting a one of a plurality of word lines to a negative voltage; setting other ones of the plurality of word lines to substantially zero volts; setting a plurality of bit lines to the positive voltage; setting a source select gate line to the positive voltage; and setting a source select drain line to the positive voltage, wherein the selected ones are erased of the plurality of memory segments within the P-well and coupled to the one of the plurality of word lines. 
         [0017]    According to another specific example embodiment of this disclosure, a method of erasing selected ones of a plurality of memory segments in a memory array comprising a plurality of P-wells in a deep N-well within a P-type substrate, wherein each of the plurality of memory segments resides within a respective one of the plurality of P-wells may comprise: setting the deep N-well to a positive voltage; setting a one of the plurality of P-wells to the positive voltage and setting other ones of the plurality of P-wells to a negative voltage; setting a one of a plurality of word lines to the negative voltage; setting other ones of the plurality of word lines to substantially zero volts; setting a first plurality of bit lines in the one of the plurality of P-wells to the positive voltage; setting a second plurality of bit lines in the other ones of the plurality of P-wells to substantially zero volts; setting a first source select gate line associated with the one of the plurality of P-wells to the positive voltage; setting a first source select drain line associated with the one of the plurality of P-wells to the positive voltage; setting a second source select gate line associated with the other ones of the plurality of P-wells to the negative voltage; setting a second source select drain line associated with the other ones of the plurality of P-wells to the negative voltage; wherein the selected ones are erased of the plurality of memory segments within the one of the plurality of P-wells and coupled to the one of the plurality of word lines. 
         [0018]    According to yet another specific example embodiment of this disclosure, a method of erasing selected ones of a plurality of memory segments in a memory array comprising a P-well in a deep N-well within a P-type substrate may comprise: setting the deep N-well to a first voltage; setting the P-well to a second voltage; setting a one of a plurality of word lines to a third voltage; setting other ones of the plurality of word lines to a fourth voltage; setting a plurality of bit lines to a fifth voltage; setting a source select gate line to a sixth voltage; and setting a source select drain line to seventh voltage, wherein the selected ones are erased of the plurality of memory segments within the P-well and coupled to the one of the plurality of word lines. 
         [0019]    According to still another specific example embodiment of this disclosure, a method of erasing selected ones of a plurality of memory segments in a memory array comprising a plurality of P-wells in a deep N-well within a P-type substrate, wherein each of the plurality of memory segments resides within a respective one of the plurality of P-wells may comprise: setting the deep N-well to a first voltage; setting a one of the plurality of P-wells to a second voltage; setting other ones of the plurality of P-wells to a third voltage; setting a one of a plurality of word lines to a fourth voltage; setting other ones of the plurality of word lines to a fifth voltage; setting a first plurality of bit lines in the one of the plurality of P-wells to a sixth voltage; setting a second plurality of bit lines in the other ones of the plurality of P-wells to a seventh voltage; setting a first source select gate line associated with the one of the plurality of P-wells to an eighth voltage; setting a first source select drain line associated with the one of the plurality of P-wells to a ninth voltage; setting a second source select gate line associated with the other ones of the plurality of P-wells to a tenth voltage; setting a second source select drain line associated with the other ones of the plurality of P-wells to an eleventh voltage; wherein the selected ones are erased of the plurality of memory segments within the one of the plurality of P-wells and coupled to the one of the plurality of word lines. 
         [0020]    According to another specific example embodiment of this disclosure, a method of programming selected ones of a plurality of memory segments in a memory array comprising a plurality of P-wells in a deep N-well within a P-type substrate, wherein each of the plurality of memory segments resides within a respective one of the plurality of P-wells may comprise: setting the deep N-well to a positive voltage; setting the plurality of P-wells to a first negative voltage; setting a one of a plurality of word lines to the positive voltage; setting other ones of the plurality of word lines to a second negative voltage; setting at least one of a plurality of bit lines to the first negative voltage; setting other ones of the plurality of bit lines to substantially zero volts; setting a source select gate line to the first negative voltage; and setting a source select drain line to the first negative voltage, wherein the selected ones are programmed of the plurality of memory segments coupled to the one of the plurality of word lines and the at least one of the plurality of bit lines. 
         [0021]    According to another specific example embodiment of this disclosure, a method of programming selected ones of a plurality of memory segments in a memory array comprising a plurality of P-wells in a deep N-well within a P-type substrate, wherein each of the plurality of memory segments resides within a respective one of the plurality of P-wells, may comprise: setting the deep N-well to a first voltage; setting a one of the plurality of P-wells to a second voltage; setting other ones of the plurality of P-wells to a third voltage; setting the third voltage to the second voltage; setting a one of a plurality of word lines to a fourth voltage; setting other ones of the plurality of word lines to a fifth voltage; setting at least one of a plurality of bit lines to a sixth voltage; setting other ones of the plurality of bit lines to a seventh voltage; setting source select gate lines associated with the plurality of P-wells to an eighth voltage; setting source select drain lines associated with the plurality of P-wells to a ninth voltage; wherein the selected ones are programmed of the plurality of memory segments coupled to the one of the plurality of word lines and the at least one of the plurality of bit lines. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    A more complete understanding of the present disclosure thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
           [0023]      FIG. 1  is a schematic diagram of byte selectable N-channel memory cells in a related technology incorporating byte select transistors and row select transistors; 
           [0024]      FIG. 2  is a cross section of a related technology P-channel memory transistor; 
           [0025]      FIG. 3  is an illustration of a related technology in which the matrix of P-channel memory transistors resides in a single N-well; 
           [0026]      FIG. 4  is an illustration of an N-channel memory array comprising two P-wells within a deep N-well and each P-well having an independently programmable memory segment, according to a specific example embodiment of the present disclosure; 
           [0027]      FIG. 5  is a schematic cross section elevational view of the specific example embodiment for a plurality of P-wells within a deep N-well as illustrated in  FIG. 4 ; 
           [0028]      FIG. 6  is a schematic cross section elevational view of a specific example embodiment of P-well segmentation trenching of N-wells illustrated in  FIG. 4 ; 
           [0029]      FIG. 7  is a schematic circuit diagram of the N-channel memory array illustrated in  FIG. 4 , according to a specific example embodiment of the present disclosure; 
           [0030]      FIG. 8  is a voltage matrix chart for a byte erase operation of the N-channel memory array circuit as illustrated in  FIG. 7 ; and 
           [0031]      FIG. 9  is a voltage matrix chart for a bit program operation of the N-channel memory array circuit as illustrated in  FIG. 7 . 
           [0032]    While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Referring now to the drawings, the details of example embodiments are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
         [0034]    Referring to  FIG. 4 , an N-channel memory array  10  comprising a plurality of P-wells (e.g.,  301  and  302 ) within a deep N-well  304  (e.g., see  FIGS. 5 and 6 ) and a plurality of independently programmable memory segments are shown. Each independently programmable memory segment is comprised of a matrix of memory cell transistors shown as cell rows  100  and cell columns  200 . The embodiment of  FIG. 4  segments the 16 cell columns  200  and the plurality of cell rows  100  of the memory array  10  into two independently programmable memory segments residing within the P-wells  301  and  302 , respectively, and shown in dashed lines. P-wells  301  and  302  are electrically separated from each other. 
         [0035]    In the specific example embodiment of the present disclosure, there are eight memory transistor columns within each P-well segment, thereby comprising byte (8 bit) segments. There are a common number of cell rows  100  within each P-well and the total number of rows  100  is determined by the desired size of the memory array  10 . In  FIG. 4 , N rows of memory transistors are illustrated. Not shown in  FIG. 4 , but discussed below and shown in subsequent diagrams, are source select transistors (see source select transistors  501 - 516  in  FIG. 7 ) at the bottom of each column  200  of the array  10 . 
         [0036]    In the embodiment of  FIG. 4  only two P-wells and two independently programmable memory segments are shown in byte format, e.g., 8 cell columns per memory segment, or a total of 16 cell columns. However, those skilled in the art will recognize that additional P-well segmentations are possible thus yielding additional independently programmable memory segments in byte format. Thus, for a byte format memory array  10 , the number of independently programmable memory segments multiplied by eight, e.g., the number of cell columns  200  per memory segment, equals the total number of cell columns  200  in the array  10 . 
         [0037]    Furthermore, each of the independently programmable memory segments which may be comprised of M cell columns, where M is either smaller or larger than a byte. The number M of cell columns  200 , alternative to the byte format, include, but are not limited to: 2, 4, 16, 32, 64, etc., cell columns  200  for each independently programmable memory segment. These various memory array  10  geometries are easily implemented according to the specific example embodiments of this disclosure. 
         [0038]    Each independently programmable memory segment may be comprised of a plurality of independently programmable memory units. An independently programmable memory unit is defined as those cell columns  200  which are common to a given cell row  100  and within a single independently programmable memory segment. The intersection of a cell column  200  and a cell row  100  defines a memory cell which may be a single memory transistor. Thus, for the specific example embodiment geometry illustrated in  FIG. 4 , each independently programmable memory unit is comprised of eight memory cells. Furthermore, the total number of independently programmable memory units for a given independently programmable memory segment is equal to the total number (N) of cell rows  100 . 
         [0039]    The functional relevance of the independently programmable memory unit may be as follows. A single independently programmable memory unit defines the smallest or most narrow portion of the memory array  10  that may be addressed by the write and erase memory operations described below. Additionally, all independently programmable memory units within a common cell row  100  may be simultaneously addressed by the read, write and erase memory operations. 
         [0040]    Referring to  FIG. 5 , depicted is a schematic cross section elevational view of the specific example embodiment for a plurality of P-wells within a deep N-well as illustrated in  FIG. 4 . P-well  301  and P-well  302  are formed in a deep N-well  304 . The deep N-well  304  is formed in a P-type substrate  308 . 
         [0041]    Referring to  FIG. 6 , depicted is a schematic cross section elevational view of a specific example embodiment of P-well segmentation trenching of N-wells as illustrated in  FIG. 4 . P-well  301   a  and P-well  302   a  are formed by dividing a single P-well with a trench  306  extending into the deep N-well  304  and filled with an insulating material. The deep N-well  304  is formed in a P-type substrate  308 . 
         [0042]    Referring to  FIG. 7 , depicted is a schematic circuit diagram of the N-channel memory array illustrated in  FIG. 4 , according to a specific example embodiment of the present disclosure. The memory array  10  is comprised of a plurality of N channel memory transistors  401 - 1  to  416 - n  which are laid out in a typical column/row matrix. Also shown are a row of N-channel source select transistors  501 - 516 . Only one source select transistor  501 - 516  is necessary for each bit line BL 1 -BL 16 . 
         [0043]    Two separate P-wells with accompanying independently programmable memory segments are shown in dashed lines drawn around a group of cells. Contained within P-well  301  are 8 memory transistor columns (only three are shown for clarity) and N memory transistor rows. P-well  302  is identical to P-well  301 , however, P-well  302  is electrically isolated from P-well  301 . Note that each independently programmable memory segment corresponds to a P-well and thus, the quantity of P-wells is equal to the quantity of independently programmable memory segments. The upper left independently programmable memory unit in P-well  301  is enclosed in a solid line box  702  to indicate that this is the target independently programmable memory unit (e.g., target byte) for the write, erase, and read operations described herein below. 
         [0044]    The control electrodes of the N-channel memory transistors  401 - 1  to  416 - n  for each row are connected to common word lines WL 1  to WLn, respectively. The drain electrodes of the memory transistors of any particular column are connected to a common bit lines BL 1 -BL 16 , respectively. The source electrodes of each memory transistor in a particular column are commonly connected to a respective one of the source select transistors  501 - 516 . The source select transistors for each P-well are controlled by two control lines SSG and SSD, connected to the gates and drains, respectively, of the source select transistors. Hence, source select transistors  501 - 508  in P-well  301  are controlled by the control lines SSG 1  and SSD 1 , and source select transistors  509 - 516  in P-well  302  are controlled by the control lines SSG 2  and SSD 2 . Voltage potentials at P-well  301  and P-well  302  may also be independently controlled, as represented by node  704  and  706 , respectively, for independent selection of an erase, program or read operation of only a memory segment contained within a certain P-well. However, it is contemplated and within the scope of the present disclosure that there may be only one P-well in a deep N-well of an NMOS EEPROM using the voltage combinations for selection of a byte erase, bit program or read operation. 
         [0045]    In this disclosure the IEEE standard 1005 will be followed for consistent nomenclature. Writing or programming a memory cell bit is defined as placing electrons onto the floating gate of the memory transistor. Erasing is defined as removing electrons from the floating gate of the memory transistor. The various writing, erasing and reading operations are performed by applying different combinations of voltages onto the word lines WLx, bit lines BLx, source select transistor gates SSGx, source select transistor drains SSDx, and P-wells, as described herein more fully below. 
         [0046]    Referring to  FIG. 8 , depicted is a voltage matrix chart for a byte erase operation of the N-channel memory array circuit illustrated in  FIG. 7 . For an erase operation, the word line WLx may be either at ground potential, e.g., zero (0) volts, or at some relatively high programming voltage, e.g., approximately minus eleven (−11) volts. For erasing the target independently programmable memory unit (e.g., target byte), the gates of the memory transistors  401 - 1  to  416 - 1  ( FIG. 7 ) are driven to approximately −11 volts over the WL 1  control line. The electric field resulting from a relatively high voltage potential, in relation to the P-well  301  biased to approximately 4 volts, causes electrons to tunnel from the floating gate across the dielectric layer and to the P-well of the transistors  401 - 1  to  408 - 1  ( FIG. 7 ), thus erasing the transistors  401 - 1  to  408 - 1  ( FIG. 7 ). 
         [0047]    Conversely, using WL 2  as an example, the control electrodes of memory transistors  401 - 2  to  408 - 2  are biased at approximately 0 volts, and the P-well  301  is biased at approximately 4 volts. Under these conditions, no tunneling occurs because of an absence of a sufficient electric field between these memory transistors  401 - 2  to  408 - 2  and the P-well  301 . Thus, memory transistors  401 - 2  to  408 - 2  are not erased. 
         [0048]    With respect to memory transistors  409 - 2  to  416 - 2 , the P-well  302  is at approximately −11 volts and the control electrodes at zero (0) volts potential results in a N-type inversion layer under the poly  2  layer of each of the memory transistors  409 - 2  to  416 - 2 . With BL 9 - 16  at approximately 0 volts and the drain electrodes of memory transistors  409 - 2  to  416 - 2  tied to the inversion layer, there is no voltage potential between the control electrode and the inversion layer at the surface of the P-well  302 . Thus, even with the P-well  302  biased to approximately −11 volts, no tunneling occurs thereby precluding an erase operation for memory transistors  409 - 2  to  416 - 2 . 
         [0049]    For the erase operation, the bit line to each of the columns BL 1 : 8  is set to approximately 4 volts, each of the columns BL 9 : 16  is set to approximately 0 volts, SSG and SSD for the select transistors  501 - 508  are set to approximately 4 volts, and the P-well  301  is biased to 4 volts. This permits a sufficient voltage potential between the floating gate of the memory transistors  401 - 1  to  408 - 1 , controlled by WL 1  at approximately −11 volts and the P-well  301  at approximately 4 volts (a difference in potential of approximately 15 volts). Electrons tunnel from the floating gate across the dielectric layer to the P-well  301 , thus positively charging the floating gate. Conversely, P-well  302  is biased to approximately −11 volts, thereby failing to create a sufficient voltage potential between the control electrodes of the memory transistors  409 - 1  to  416 - 1  that are within P-well  302  and the P-well  302 . Without a sufficient voltage differential, tunneling cannot occur and the erase cycle is not accomplished. Thus, by providing for separate and isolated P-wells, the N-channel memory transistors in any row may be organized in byte selectable segments where byte selection is effected, at least in part, by the application of, or biasing at, different voltage potentials, the plurality of the P-wells themselves. 
         [0050]    Referring to  FIG. 9 , depicted is a voltage matrix chart for a bit program operation of the N-channel memory array circuit illustrated in  FIG. 7 . In this example, word line WL 1  is biased to approximately 4 volts, the remainder of the word lines WL 2 :n are biased to approximately −7 volts; and the select lines SSG 1 , SSG 2 , SSD 1  and SSD 2 , and P-wells  301  and  302  are biased to approximately −11 volts. The floating gates of the memory transistors  401 - 1  through  416 - 1  are capacitively coupled to a voltage sufficient to turn these transistors on hard, thereby creating an inversion layer. Bit line BL 2  is biased to approximately −11 volts while bit lines BL 1  and BL 3 : 16  are biased to approximately 0 volts. This causes the inversion layer under the floating gate and pass gate of transistor  402 - 1  to be biased at −11 volts and the inversion layer under the floating gates and pass gates of transistors  401 - 1  and  403 - 1  through  416 - 1  to be biased at 0 volts. This creates a sufficient electric field to cause electron tunneling from the inversion region in the P-well  301  to the floating gate of the transistor  402 - 1  and thus charges the floating gate of the memory transistor  402 - 1 , but not the other floating gates (refer to  FIG. 7 ). 
         [0051]    Conversely, with WL 1  biased to approximately 4 volts and bit lines BL 1  and BL 3 : 16  biased to approximately 0 volts, there is insufficient electric field for electron tunneling from the inversion region to the floating gate for memory transistors  401 - 1  and  403 - 1  to  416 - 1 . Thus, the write operation is not accomplished for memory transistors  401 - 1  and  401 - 3  to  416 - 1 . In the target independently programmable memory unit (e.g. target byte  702 ) of  FIG. 7 , identified by the rectangle, a binary pattern may be entered into the memory cells  401 - 1  through  408 - 1  by setting the appropriate bit lines BL 1 :BL 8  to approximately −11 volts or 0 volts. Bit lines set to approximately −11 volts will write the memory cell. Bit lines set to substantially ground or approximately zero volts will remain in an unchanged state. 
         [0052]    While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.