Abstract:
A byte select circuit of a memory cell array wherein each column of the memory cell array has two byte select lines. A first byte select line is coupled to the even numbered rows in the column and a second byte select line is coupled to the odd numbered rows in the column. The second byte select line is configured to be driven to a low voltage level when the first byte select line is driven to a high voltage level, thereby minimizing or eliminating any parasitic voltage coupling between adjacent rows of memory cells.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to a non-volatile memory architecture. More specifically, the present invention is an EEPROM non-volatile memory architecture providing multiple byte select lines to alternating word line rows.  
       BACKGROUND ART  
       [0002]     EEPROM arrays include floating-gate memory cells arranged in rows and columns. When a floating gate of a programmed memory cell is charged with electrons, a source-drain path under the charged floating gate is nonconductive when a wordline select voltage is applied to a control gate of the cell. A nonconductive state is read as a “1” bit. If the floating gate of a non-programmed cell is either positively charged, neutrally charged, or slightly negatively charged, the source-drain path under the non-programmed floating gate is conductive when the wordline select voltage is applied to the control gate. The conductive state is read as a “0” bit.  
         [0003]     In memories based on a Fowler-Nordheim tunneling mechanism, an oxide between a floating gate of a transistor and a drain of the transistor must be fabricated to be very thin, typically only a few nanometers thick. When a voltage is applied across the control gate (to which the floating gate is strongly capacitively coupled) and the drain, a strong electrical field is produced. Electrons can then tunnel from the drain region via the thin oxide to the floating gate. A tunneling current in the opposite direction can be obtained by reversing the field. Thus, it is possible to write and erase a cell.  
         [0004]     Each column and row of an EEPROM array may contain thousands of cells. Control gates of each cell in a row are connected to a wordline. Prior to first programming, the source-drain paths of the cells begin to conduct at a relatively uniform control-gate threshold voltage, V t , since the floating gates are neutrally charged (having neither an excess nor a deficiency of electrons). An initial uniform threshold voltage may be, for example, +2.5 volts between the control gate and the source terminal. The initial uniform threshold voltage may be adjusted by appropriately doping the channel regions of the cells during fabrication.  
         [0005]     After programming, source-drain paths of the programmed cells have control-gate threshold voltages distributed over a voltage range, typically between −3.5 volts to −0.5 volts. After electrical erasure of the cells, the threshold voltages of the erased cells may be distributed over a range from perhaps +0.5 to 3.5 volts with a majority of the cells having erased threshold voltages near 2.5 volts. The actual range of erased threshold voltages is dependent on factors such as localized variations in the tunnel oxide thickness, geometrical areas of tunneling regions, capacitive coupling ratios between wordlines and floating gates, and relative strengths of erasing pulses. Using a lower-strength erasing pulse, the erased threshold voltage range may be from perhaps +1.5 to +3.5 volts with a majority of the cells having erased threshold voltages near 2.5 volts. With a higher-strength erasing pulse applied, the distribution may range from perhaps +3.0 to +6 volts with a majority of cells having erased threshold voltages near +4.5 volt. An excess of positive charges on the floating gates causes channel regions under the gates of floating gate transistors to be enhanced with electrons.  
         [0006]     In general, an extent of channel doping, programming pulse strength, erasing pulse strength, and other factors are chosen such that the source-drain path of a cell will either be conductive or non-conductive when applying a wordline select voltage to the control gate.  
         [0007]     With reference to  FIG. 1 , a portion  100  of a prior art memory array includes a first byte select transistor  101  connected to a gate of a first floating gate transistor  103 . A first wordline, WL(n), is connected to gates of both the first byte select transistor  101  and a first bit select transistor  105 . An asserted high value (e.g., a logical “1”) on the wordline, WL(n) allows both the first byte select transistor  101  and the first bit select transistor  105  to conduct, thereby allowing the first floating gate transistor  103  to be selected for read, write, or programming operations through a bitline, BL(m). The asserted high value on the wordline, WL(n), allows a source-drain current to flow from a byte select (i) line to a control gate of the first floating gate transistor  103 . The byte select (i) line is also arranged with seven additional floating gate transistors (not shown) in parallel with the first floating gate transistor  103  to store a byte of data. The portion  100  of the prior art memory array also includes a second byte select transistor  107 , a second floating gate transistor  109 , and a second bit select transistor  111 . A gate of the second bit select transistor  111  is connected to a second wordline, WL(n+1), and seven other floating gate transistors (not shown) in parallel with the second floating gate transistor  109 . The second floating gate transistor  109  is connected to the bitline, BL(m). Note that the first  101  and the second  107  byte select transistors are both connected to the same byte select (i) line.  
         [0008]     In operation, when the first floating gate transistor  103  is erased, a voltage (e.g., a high voltage above V cc  is preferred) is applied to the gate of the first byte select transistor  101  and a high voltage (e.g., 12-14 volts) is applied to the byte select (i) line, allowing electrons to transfer from a floating gate of the first floating gate transistor  103  through the mechanism of Fowler-Nordheim tunneling. However, due to the close proximity between adjacent byte select transistors  101 ,  107 , a source-drain current on the first byte select transistor  101  may induce a source-drain current on the second byte select transistor  107  thereby causing the second floating gate transistor  109  to be inadvertently erased. Even though the length of the line from the source of, for example, the second byte select transistor  107  is very short, due to an extremely high packing density of transistors in integrated circuits, the lines are close enough that parasitic effects, such as capacitive coupling, may readily occur.  
         [0009]     A block diagram of  FIG. 2  typifies an addressable portion of a prior art memory array and exemplifies how the capacitive coupling occurs between adjacent rows of memory cells contained within the same column. For example, the first byte select transistor  101  and the first floating gate transistor  103  of  FIG. 1  can be conceptualized as being located at a point b 0  (“bit  0 ”) of row x=1 and column y=1 of  FIG. 2 . Assuming wordlines for each of the rows (x=1 through x=4 is at logic high,) byte select ( 1 ) would, when asserted, activate the first byte select transistor  101  and select each of a plurality of data bytes (e.g., bits b 0 - b 7 ) within column y=1. Thus, rows x=1, x=2, and so on are all selected by byte select ( 1 ). Each column is thus controlled by a single byte select line (e.g., column y=0 is controlled by byte select line ( 0 ), column y=2 is controlled by byte select line ( 2 ) and so on.)  
         [0010]     Further, adjacent byte select lines, when activated, tend to parasitically couple to a mirrored prior column. In  FIG. 2 , column y=0 mirrors column y=1, column y=2 mirrors column y=3, and so on. Thus, when byte select ( 1 ) in column y=1 is activated, byte select ( 0 ) may also have a voltage coupled as well.  
         [0011]     Therefore, for robust and proper memory operation, it is desirable to eliminate or minimize any potential for parasitic cross coupling between memory cells in adjacent rows.  
       SUMMARY  
       [0012]     A byte select circuit of a memory cell array wherein each column of the memory cell array has two byte select lines. A first byte select line is coupled to the even numbered rows in the column and a second byte select line is coupled to the odd numbered rows in the column. The second byte select line is configured to be driven to a low voltage level when the first byte select line is driven to a high voltage level, thereby minimizing or eliminating any parasitic voltage coupling between adjacent rows of memory cells. In an exemplary embodiment, the present invention is a byte select circuit of an EEPROM memory cell array; the byte select circuit is arranged such that each column within the EEPROM array has two byte select lines. A first byte select line is coupled to a first plurality of memory cells in a first column of the memory cell array, the first plurality of memory cells being considered to be in an even row and selectable by a first wordline. A second byte select line is coupled to a second plurality of memory cells in the first column of the memory cell array. The second plurality of memory cells being considered to be in an odd row and selectable by a second wordline.  
         [0013]     Any parasitic coupling is minimized or completely eliminated by this arrangement of dual byte select lines coupled with voltages placed on the wordlines. For example, the second byte select line is configured to be driven to a low voltage level when the first byte select line is driven to a high voltage level. Concurrently, any wordlines in the array that are not immediately adjacent to the first wordline are allowed to float when a voltage is asserted on the first wordline. Any wordlines immediately adjacent to the first wordline are driven to approximately zero volts when a voltage is asserted on the first wordline. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a circuit diagram of a localized area of a prior art memory cell.  
         [0015]      FIG. 2  is a block layout of a section of a prior art memory cell.  
         [0016]      FIG. 3  is a circuit diagram of a localized area of an exemplary embodiment of a memory cell of the present invention.  
         [0017]      FIG. 4  is a portion of an exemplary embodiment of an arrangement of wordline and byte select lines of the present invention.  
         [0018]      FIG. 5  is a block layout of a section of an exemplary embodiment of a memory cell incorporating the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]     With reference to  FIG. 3 , a portion  300  of an exemplary embodiment of a memory array of the present invention includes a first byte select transistor  301  coupled to a gate of a first floating gate transistor  303 . A first wordline, WL(n) is coupled to gates of both the first byte select transistor  301  and a bit select transistor  305 . As with the prior art, an asserted high value (e.g., a logical “1”) on the wordline, WL(n) allows both the first byte select transistor  301  and the bit select transistor  305  to conduct, thereby allowing the first floating gate transistor  303  to be selected for read, write, or programming operations through a bitline, BL(m). The asserted high value on the wordline, WL(n), allows a source-drain current to flow from a byte select (i o ) line to a control gate of the first floating gate transistor  303 . The byte select (i o ) line is also arranged with seven additional floating gate transistors (not shown) in parallel with the first floating gate transistor  303  to store a byte of data. The drain of the first byte select transistor  301  is coupled to an odd byte select line.  
         [0020]     The portion  300  of the exemplary embodiment of the present invention also includes a second byte select transistor  307 , a second floating gate transistor  309 , and a second bit select transistor  311 . A gate of the second bit select transistor  311  is coupled to a second wordline, WL(n+1). The second floating gate transistor  309  is connected to the bitline, BL(m). The drain of the second byte select transistor  307  is coupled to an even byte select line (“Byte Select (i e )”). Seven other floating gate transistors (not shown) are in parallel with the second floating gate transistor  309 .  
         [0021]     Maintaining two byte select lines for each column of memory (i.e., an odd byte select line (“Byte Select (i o )”) and an even byte select line (“Byte Select (i e )”) prevents effects of capacitive coupling of the prior art.  
         [0022]     For example, with reference to  FIG. 4 , adjacent rows of byte select transistors are coupled alternately to either an odd byte select line (i o ) or an even byte select line (i e ). Byte select transistors in odd rows, i.e., a first, second, and third odd byte select transistor,  401   n -  401   n+4 , are coupled to the odd byte select line (i o ). Byte select transistors in even rows, i.e., a first, second, and third even byte select transistor,  402   n+1 - 402   n+5 , are coupled to the even byte select line (i e ). Also shown are sense lines, S(n)-S(n+5). If a high voltage is placed on, for example, odd byte select line (i o ) and wordline WL (n+2), only the second odd byte select transistor  401   n+2  will conduct. If any parasitic voltage is induced on wordlines or sense lines immediately proximate to wordline WL (n+2) and sense lines S(n+2), namely wordlines WL(n+1) and WL (n+3) and sense lines S(n+1) and S(n+3), no high voltage can be conducted on those wordlines since a voltage on the even byte select line (i e ) is driven low (e.g., to ground) while WL(n+2) is high. Sense lines located distally to sense line S(n+2) (e.g., sense lines S(n), S(n+4), S(n+5), etc.) can be allowed to float. Control circuitry or software to maintain appropriate voltage levels (i.e., high, low, or float) can be readily conceived by one of skill in the art.  
         [0023]     With reference to  FIG. 5 , an addressable portion of an exemplary embodiment of a memory array of the present invention is illustrative of how parasitic effects are minimized or eliminated. For example, the first byte select transistor  301  and the first floating gate transistor  303  of  FIG. 3  can be conceptualized as being located at a point b 0  (not shown explicitly) of row x=1 and column y=1 of  FIG. 5 . Assuming wordline x=1 is at logic high, byte select ( 1   o ) would, when asserted, activate the first byte select transistor  301  and select only a single data byte within column y=1. Thus, rows x=1 and x=3 in column y=1 are selected by byte select ( 1   o ). rows x=1 and x=3 in column y=2 are selected by byte select ( 2   o ) and so on. In contrast, rows x=2 and x=4 in column y=1 are selected by byte select ( 1   e ); rows x=2 and x=4 in column y=2 are selected by byte select ( 2   e ). Each column is thus controlled by two byte select lines depending on whether an even or odd row is chosen. Further, a skilled artisan will readily recognize that parasitic coupling between sense lines or wordlines on mirrored columns is effectively minimized or eliminated as well as parasitic coupling between adjacent rows. Thus, data may be reliably erased, programmed, and read without an effect on or from adjacent rows in an array of memory cells.  
         [0024]     In the foregoing specification, the present invention has been described with reference to specific embodiments thereof. It will, however, be evident to a skilled artisan that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. For example, skilled artisans will appreciate that additional byte select lines may be added to each column with, of course, a concomitant increase in complexity of control signals. Further, byte select lines may be laid out in various geometries (e.g., at a single side of each column as opposed to a mirrored arrangement) to minimize parasitic coupling between adjacent select lines as well. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.