Abstract:
The present invention relates to a flash memory cell with only four terminals and decoder circuitry for operating an array of such flash memory cells. The invention allows for fewer terminals for each flash memory cell compared to the prior art, which results in a simplification of the decoder circuitry and overall die space required per flash memory cells. The invention also provides for the use of high voltages on one or more of the four terminals to allow for read, erase, and programming operations despite the lower number of terminals compared to prior art flash memory cells.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a flash memory cell with only four terminals and decoder circuitry for operating an array of such flash memory cells. The invention allows for fewer terminals for each flash memory cell compared to the prior art, which results in a simplification of the decoder circuitry and overall die space required per flash memory cells. The invention also provides for the use of high voltages on one or more of the four terminals to allow for read, erase, and programming operations despite the lower number of terminals compared to prior art flash memory cells. 
       BACKGROUND OF THE INVENTION 
       [0002]    Non-volatile memory cells are well known in the art. One prior art non-volatile split gate memory cell  10 , which contains five terminals, is shown in  FIG. 1 . Memory cell  10  comprises semiconductor substrate  12  of a first conductivity type, such as P type. Substrate  12  has a surface on which there is formed a first region  14  (also known as the source line SL) of a second conductivity type, such as N type. A second region  16  (also known as the drain line) also of N type is formed on the surface of substrate  12 . Between the first region  14  and the second region  16  is channel region  18 . Bit line BL  20  is connected to the second region  16 . Word line WL  22  is positioned above a first portion of the channel region  18  and is insulated therefrom. Word line  22  has little or no overlap with the second region  16 . Floating gate FG  24  is over another portion of channel region  18 . Floating gate  24  is insulated therefrom, and is adjacent to word line  22 . Floating gate  24  is also adjacent to the first region  14 . Floating gate  24  may overlap the first region  14  to provide coupling from the first region  14  into floating gate  24 . Coupling gate CG (also known as control gate)  26  is over floating gate  24  and is insulated therefrom. Erase gate EG 28 is over the first region  14  and is adjacent to floating gate  24  and coupling gate  26  and is insulated therefrom. The top corner of floating gate  24  may point toward the inside corner of the T-shaped erase gate  28  to enhance erase efficiency. Erase gate  28  is also insulated from the first region  14 . Memory cell  10  is more particularly described in U.S. Pat. No. 7,868,375, whose disclosure is incorporated herein by reference in its entirety. 
         [0003]    One exemplary operation for erase and program of prior art non-volatile memory cell  10  is as follows. Memory cell  10  is erased, through a Fowler-Nordheim tunneling mechanism, by applying a high voltage on erase gate  28  with other terminals equal to zero volt. Electrons tunnel from floating gate  24  into erase gate  28  causing floating gate  24  to be positively charged, turning on the cell  10  in a read condition. The resulting cell erased state is known as ‘1’ state. 
         [0004]    Memory cell  10  is programmed, through a source side hot electron programming mechanism, by applying a high voltage on coupling gate  26 , a high voltage on source line  14 , a medium voltage on erase gate  28 , and a programming current on bit line  20 . A portion of electrons flowing across the gap between word line  22  and floating gate  24  acquire enough energy to inject into floating gate  24  causing the floating gate  24  to be negatively charged, turning off the cell  10  in a read condition. The resulting cell programmed state is known as ‘0’ state. 
         [0005]    Memory cell  10  is read in a Current Sensing Mode as following: A bias voltage is applied on bit line  20 , a bias voltage is applied on word line  22 , a bias voltage is applied on coupling gate  26 , a bias or zero voltage is applied on erase gate  28 , and a ground is applied on source line  14 . There exists a cell current flowing from bit line  20  to source line  14  for an erased state and there is insignificant or zero cell current flow from the bit line  20  to the source line  14  for a programmed state. Alternatively, memory cell  10  can be read in a Reverse Current Sensing Mode, in which bit line  20  is grounded and a bias voltage is applied on source line  24 . In this mode the current reverses the direction from source line  14  to bitline  20 . 
         [0006]    Memory cell  10  alternatively can be read in a Voltage Sensing Mode as following: A bias current (to ground) is applied on bit line  20 , a bias voltage is applied on word line  22 , a bias voltage is applied on coupling gate  26 , a bias voltage is applied on erase gate  28 , and a bias voltage is applied on source line  14 . There exists a cell output voltage (significantly &gt;0V) on bit line  20  for an erased state and there is insignificant or close to zero output voltage on bit line  20  for a programmed state. Alternatively, memory cell  10  can be read in a Reverse Voltage Sensing Mode, in which bit line  20  is biased at a bias voltage and a bias current (to ground) is applied on source line  14 . In this mode, memory cell  10  output voltage is on the source line  14  instead of on the bit line  20 . 
         [0007]    In the prior art, various combinations of positive or zero voltages were applied to word line  22 , coupling gate  26 , and floating gate  24  to perform read, program, and erase operations 
         [0008]    In response to the read, erase or program command, the logic circuit  270  (in  FIG. 2 ) causes the various voltages to be supplied in a timely and least disturb manner to the various portions of both the selected memory cell  10  and the unselected memory cells  10 . 
         [0009]    For the selected and unselected memory cell  10 , the voltage and current applied are as follows. As used hereinafter, the following abbreviations are used: source line or first region  14  (SL), bit line  20  (BL), word line  22  (WL), and coupling gate  26  (CG). 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE No. 1 
               
               
                   
               
               
                 PEO (Positive Erase Operation) Table 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                   
                 WL- 
                   
                 BL- 
                   
                 CG-unsel 
                 CG- 
                   
                 EG- 
               
               
                   
                 WL 
                 unsel 
                 BL 
                 unsel 
                 CG 
                 same sector 
                 unsel 
                 EG 
                 unsel 
               
               
                   
                   
               
             
          
           
               
                 Read 
                 1.0-2 
                 V 
                 0 V 
                 0.6-2 
                 V 
                 0 V- 
                  0-2.6 V 
                 0-2.6 V 
                 0-2.6 V 
                 0-2.6 V 
                 0-2.6 V 
               
               
                   
                   
                   
                   
                   
                   
                 FLT 
               
               
                 Erase 
                 0 
                 V 
                 0 V 
                 0 
                 V 
                 0 V 
                    0 V 
                 0-2.6 V 
                 0-2.6 V 
                 11.5-12 V  
                 0-2.6 V 
               
               
                 Program 
                 1 
                 V 
                 0 V 
                 1 
                 uA 
                 Vinh 
                 10-11 V 
                 0-2.6 V 
                 0-2.6 V 
                 4.5-5 V 
                 0-2.6 V 
               
               
                   
               
             
          
           
               
                   
                 SL 
                 SL-unsel 
               
               
                   
                   
               
             
          
           
               
                   
                 Read 
                 0 
                 V 
                 0 
                 V-FLT 
               
               
                   
                 Erase 
                 0 
                 V 
                 0 
                 V 
               
               
                   
                 Program 
                 4.5-5 
                 V 
                 0-1 
                 V/FLT 
               
               
                   
                   
               
             
          
         
       
     
         [0010]    In a recent application by the applicant—U.S. patent application Ser. No. 14/602,262, filed on Jan. 21, 2015, which is incorporated by reference—the applicant disclosed an invention whereby negative voltages could be applied to word line  22  and/or coupling gate  26  during read, program, and/or erase operations. In this embodiment, the voltage and current applied to the selected and unselected memory cell  10 , are as follows. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE No. 2 
               
               
                   
               
               
                 PEO (Positive Erase Operation) Table 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                   
                 WL- 
                   
                 BL- 
                   
                 CG-unsel 
                 CG- 
                   
                 EG- 
               
               
                   
                 WL 
                 unsel 
                 BL 
                 unsel 
                 CG 
                 same sector 
                 unsel 
                 EG 
                 unsel 
               
               
                   
                   
               
             
          
           
               
                 Read 
                 1.0-2 
                 V 
                 −0.5 V/ 
                 0.6-2 
                 V 
                 0 V- 
                  0-2.6 V 
                 0-2.6 V 
                 0-2.6 V 
                 0-2.6 V 
                 0-2.6 V 
               
               
                   
                   
                   
                 0 V 
                   
                   
                 FLT 
               
               
                 Erase 
                 0 
                 V 
                 0 V 
                 0 
                 V 
                 0 V 
                    0 V 
                 0-2.6 V 
                 0-2.6 V 
                 11.5-12 V  
                 0-2.6 V 
               
               
                 Program 
                 1 
                 V 
                 −0.5 V/ 
                 1 
                 uA 
                 Vinh 
                 10-11 V 
                 0-2.6 V 
                 0-2.6 V 
                 4.5-5 V 
                 0-2.6 V 
               
               
                   
                   
                   
                 0 V 
               
               
                   
               
             
          
           
               
                   
                 SL 
                 SL-unsel 
               
               
                   
                   
               
             
          
           
               
                   
                 Read 
                 0 
                 V 
                 0 
                 V-FLT 
               
               
                   
                 Erase 
                 0 
                 V 
                 0 
                 V 
               
               
                   
                 Program 
                 4.5-5 
                 V 
                 0-1 
                 V/FLT 
               
               
                   
                   
               
             
          
         
       
     
         [0011]    In another embodiment of U.S. patent application Ser. No. 14/602,262, negative voltages can be applied to word line  22  when memory cell  10  is unselected during read, erase, and program operations, and negative voltages can be applied to coupling gate  26  during an erase operation, such that the following voltages are applied: 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE No. 3 
               
               
                   
               
               
                 PNEO (Positive Negative Erase Operation) Table 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                   
                 WL- 
                   
                 BL- 
                   
                 CG-unsel 
                 CG- 
                   
                 EG- 
               
               
                   
                 WL 
                 unsel 
                 BL 
                 unsel 
                 CG 
                 same sector 
                 unsel 
                 EG 
                 unsel 
               
               
                   
                   
               
             
          
           
               
                 Read 
                 1.0-2 
                 V 
                 −0.5 V/0 V 
                 0.6-2 
                 V 
                 0- 
                 0-2.6 
                 V 
                 0-2.6 V 
                 0-2.6 V 
                 0-2.6 V   
                 0-2.6 V 
               
               
                   
                   
                   
                   
                   
                   
                 FLT 
               
               
                 Erase 
                 0 
                 V 
                 −0.5 V/0 V 
                 0 
                 V 
                 0- 
                 −(5-9) 
                 V 
                 0-2.6 V 
                 0-2.6 V 
                 8-9 V 
                 0-2.6 V 
               
               
                   
                   
                   
                   
                   
                   
                 FLT 
               
               
                 Program 
                 1 
                 V 
                 −0.5 V/0 V 
                 1 
                 uA 
                 Vinh 
                 8-9 
                 V 
                 CGINTH (4-6 V) 
                 0-2.6 V 
                 8-9 V 
                 0-2.6 V 
               
               
                   
               
             
          
           
               
                   
                 SL 
                 SL-unsel 
               
               
                   
                   
               
             
          
           
               
                   
                 Read 
                 0 
                 V 
                 0-FLT 
               
               
                   
                 Erase 
                 0 
                 V 
                 0 V 
               
               
                   
                 Program 
                 4.5-5 
                 V 
                 0-1 V/ 
               
               
                   
                   
                   
                   
                 FLT 
               
               
                   
                   
               
             
          
         
       
     
         [0012]    The CGINH signal listed above is an inhibit signal that is applied to the coupling gate  26  of an unselected cell that shares an erase gate  28  with a selected cell. 
         [0013]      FIG. 2  depicts an embodiment recently developed by applicant of an architecture for a flash memory system comprising die  200 . Die  200  comprises: memory array  215  and memory array  220  for storing data, memory arrays  215  and  220  comprising rows and columns of memory cells of the type described previously as memory cell  10  in  FIG. 1 , pad  240  and pad  280  for enabling electrical communication between the other components of die  200  and, typically, wire bonds (not shown) that in turn connect to pins (not shown) or package bumps that are used to access the integrated circuit from outside of the packaged chip or macro interface pins (not shown) for interconnecting to other macros on a SOC (system on chip); high voltage circuit  275  used to provide positive and negative voltage supplies for the system; control logic  270  for providing various control functions, such as redundancy and built-in self-testing; analog circuit  265 ; sensing circuits  260  and  261  used to read data from memory array  215  and memory array  220 , respectively; row decoder circuit  245  and row decoder circuit  246  used to access the row in memory array  215  and memory array  220 , respectively, to be read from or written to; column decoder circuit  255  and column decoder circuit  256  used to access bytes in memory array  215  and memory array  220 , respectively, to be read from or written to; charge pump circuit  250  and charge pump circuit  251 , used to provide increased voltages for program and erase operations for memory array  215  and memory array  220 , respectively; negative voltage driver circuit  230  shared by memory array  215  and memory array  220  for read and write operations; high voltage driver circuit  225  used by memory array  215  during read and write operations and high voltage driver circuit  226  used by memory array  220  during read and write operations. 
         [0014]    With flash memory systems becoming ubiquitous in all manner of computing and electronic devices, it is increasingly important to create designs that reduce the amount of die space required per memory cell and to reduce the overall complexity of decoders use in flash memory systems. What is needed is flash memory cell design that utilizes fewer terminals than in the prior art and simplified circuitry for operating flash memory cells that follow that design. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention relates to a flash memory cell with only four terminals and decoder circuitry for operating an array of such flash memory cells. The invention allows for fewer terminals for each flash memory cell compared to the prior art, which results in a simplification of the decoder circuitry and overall die space required per flash memory cells. The invention also provides for the use of high voltages on one or more of the four terminals to allow for read, erase, and programming operations despite the lower number of terminals compared to prior art flash memory cells. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a cross-sectional view of a non-volatile memory cell of the prior art to which the method of the present invention can be applied. 
           [0017]      FIG. 2  is a block diagram of a non-volatile memory device using the non-volatile memory cell of the prior art shown in  FIG. 1 . 
           [0018]      FIG. 3  is a block diagram of an embodiment of a non-volatile memory cell. 
           [0019]      FIG. 4  is a schematic representation of the non-volatile memory cell of  FIG. 3 . 
           [0020]      FIG. 5  is a block diagram of a non-volatile memory device using the non-volatile memory cell of  FIG. 3 . 
           [0021]      FIG. 6  depicts an embodiment of a row decoder for use with the memory device of claim  5 . 
           [0022]      FIG. 7  is a block diagram of decoder circuitry for use with the memory device of claim  5 . 
           [0023]      FIG. 8  depicts an embodiment of an erase gate decoder for use with the memory device of claim  5 . 
           [0024]      FIG. 9  depicts an embodiment of an erase gate decoder for use with the memory device of claim  5 . 
           [0025]      FIG. 10  depicts an embodiment of an erase gate decoder for use with the memory device of claim  5 . 
           [0026]      FIG. 11  depicts an embodiment of a source line decoder for use with the memory device of claim  5 . 
           [0027]      FIG. 12  depicts an embodiment of a source line decoder for use with the memory device of claim  5 . 
           [0028]      FIG. 13  depicts an embodiment of a source line decoder for use with the memory device of claim  5 . 
           [0029]      FIG. 14  depicts an embodiment of a source line decoder for use with the memory device of claim  5 . 
           [0030]      FIG. 15  depicts an embodiment of a source line decoder with a dummy flash memory cell for selectively pulling down to a low voltage or ground a source line. 
           [0031]      FIG. 16  depicts an embodiment of a dummy flash memory cell for selectively pulling down to a low voltage or ground a source line coupled to a selected flash memory cell. 
           [0032]      FIG. 17  depicts an embodiment of a control gate decoder for use with a memory device using the memory cell of claim  1 . 
           [0033]      FIG. 18  depicts an embodiment of a control gate decoder for use with a memory device using the memory cell of claim  1 . 
           [0034]      FIG. 19  depicts an embodiment of a gate decoder for use with a memory device using the memory cell of claim  1 . 
           [0035]      FIG. 20  depicts an embodiment of a latch voltage level shifter for use with the memory device of claim  5 . 
           [0036]      FIG. 21  depicts an embodiment of a latch voltage level shifter for use with the memory device of claim  5 . 
           [0037]      FIG. 22  depicts an embodiment of a high voltage current limiter for use with the memory device of claim  5 . 
           [0038]      FIG. 23  depicts an embodiment of a latch voltage level shifter for use with the memory device of claim  5 . 
           [0039]      FIG. 24  depicts an embodiment of an array of flash memory cells with a column of dummy memory cells for selectively pulling down to a low voltage or ground a selected source line. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0040]      FIG. 3  depicts an embodiment of an improved flash memory cell  300 . As with prior art flash memory cell  10 , flash memory cell  300  comprises substrate  12 , first region (source line)  14 , second region  16 , channel region  18 , bit line  20 , word line  22 , floating gate  24 , and erase gate  28 . Unlike prior art flash memory cell  10 , flash memory cell  300  does not contain a coupling gate or control gate and only contains four terminals—bit line  20 , word line  22 , erase gate  28 , and source line  14 . This significantly reduces the complexity of the circuitry, such as decoder circuitry, required to operate an array of flash memory cells. 
         [0041]    The erase operation (erasing through erase gate) and read operation are similar to that of the  FIG. 1  except there is no control gate bias. The programming operation also is done without the control gate bias, hence the program voltage on the source line is higher to compensate for lack of control gate bias. 
         [0042]    Table No. 4 depicts typical voltage ranges that can be applied to the four terminals for performing read, erase, and program operations: 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE NO. 4 
               
             
             
               
                   
               
               
                 Four Terminal Flash Device Operation Table 
               
             
          
           
               
                   
                   
                 WL- 
                   
                 BL- 
                   
                   
                   
                   
               
               
                   
                 WL 
                 unsel 
                 BL 
                 unsel 
                 EG 
                 EG-unsel 
                 SL 
                 SL-unsel 
               
               
                   
                   
               
             
          
           
               
                 Read 
                 0.7-2.2 V 
                 −0.5 V/0 V  
                 0.6-2 
                 V 
                 0 V/FLT 
                 0-2.6 V 
                 0-2.6 V 
                 0 V 
                 0 
                 V/FLT/VB 
               
               
                 Erase 
                 −0.5 V/0 V 
                 −.5 V/0 V 
                 0 
                 V 
                 0 V 
                  11.5 V 
                 0-2.6 V 
                 0 V 
                 0 
                 V 
               
               
                 Program 
                   1-1.5 V 
                 −.5 V/0 V 
                 1-3 
                 μA 
                 Vinh 
                   4.5 V 
                 0-2.6 V 
                 7-9 V   
                 0-1 
                 V/FLT 
               
               
                   
                   
                   
                   
                   
                 (~1.8 V) 
               
               
                   
               
             
          
         
       
     
         [0043]      FIG. 4  depicts a symbolic representation  400  of flash memory cell  300 . Symbolic representation  400  comprises symbols for the four terminals of flash memory cell  300 , namely, bit line  20 , word line  22 , erase gate  28 , and source line  14 . 
         [0044]      FIG. 5  depicts an embodiment of an architecture for a flash memory system comprising die  500 . Die  500  comprises memory arrays  501 ,  511 ,  521 , and  531 , for storing data, each of memory arrays  501 ,  511 ,  521 , and  531  comprising rows and columns of memory cells of the type described previously as flash memory cell  300  in  FIG. 3 . Die  500  further comprises sensing circuit  543  used to read data from memory arrays  501 ,  511 ,  521 , and  531 ; row decoder circuit  541  used to access the selected row in memory arrays  501  and  511  and row decoder circuit  542  used to access the selected row in memory arrays  521  and to be read from or written to; column decoder circuits  503 ,  513 ,  523 , and  533  used to access bytes in memory arrays  501 ,  511 ,  521 , and  531 , respectively, to be read from or written to; high voltage row decoder WSHDR  502 ,  512 ,  522 , and  532  used to provide high voltage to one or more terminals of the selected memory cell within memory arrays  501 ,  511 ,  521 , and  531 , respectively, depending on the operation being performed. 
         [0045]    Die  500  further comprises the following functional structures and sub-systems: macro interface pins ITFC pin  548  for interconnecting to other macros on a SOC (system on chip); low voltage generation (including a low voltage charge pump circuit) circuits  547  and high voltage generation (including a high voltage charge pump circuit) circuit  546  used to provide increased voltages for program and erase operations for memory arrays  501 ,  511 ,  521 , and  531 ; analog circuit  544  used by analog circuitry on die  500 ; digital logic circuit  545  used by digital circuitry on die  500 . 
         [0046]      FIG. 6  depicts row decoder  600  for 8 word lines in a sector within a memory array (such as memory array  501 ,  511 ,  521 , and  531 ). Row decoder  600  can be part of row decoder circuits  541  and  542  in die  500 . Row decoder  600  comprises NAND gate  601 , which receives pre-decoded address signals, here shown as lines XPA, XPB, XPC, and XPD, which select a sector within a memory array. When XPA, XPB XPC, and XPD are all “high,” then the output of NAND gate  601  will be “low” and this particular sector will be selected. 
         [0047]    Row decoder  600  further comprises inverter  602 , decoder circuit  610  to generate word line WL 0 , decoder circuit  620  to generate WL 7 , as well as additional decoder circuits (not shown) to generate word lines WL 1 , WL 2 , WL 3 , WL 4 , WL 5 , and WL 6 . 
         [0048]    Decoder circuit  610  comprises PMOS transistors  611 ,  612 , and  614  and NMOS transistors  613  and  615 , configured as shown. Decoder circuit  610  receives the output of NAND gate  601 , the output of inverter  602 , and pre-decoded address signal XPZB 0 . When this particular sector is selected and XPZB 0  is “low,” then WL 0  will be asserted. When XPZB 0  is “high,” then WL 0  will not be asserted. 
         [0049]    Similarly, decoder circuit  620  comprises PMOS transistors  621 ,  622 , and  624  and NMOS transistors  623  and  625 , configured as shown. Decoder circuit  620  receives the output of NAND gate  601 , the output of inverter  602 , and pre-decoded address signal XPZ 70 . When this particular sector is selected and XPZB 7  is “low,” then WL 7  will be asserted. When XPZB 7  is “high,” then WL 7  will not be asserted. 
         [0050]    It is to understood that the decoder circuits (now shown) for WL 1 , WL 2 , and WL 3 , WL 4 , WL 5 , and WL 6  will follow the same design as decoder circuits  610  and  620  except that they will receive the inputs XPZB 1 , XPZB 2 , XPZB 3 , XPZB 4 , XPZB 5 , and XPZB 6 , respectively, instead of XPZB 0  or XPZB 7 . 
         [0051]    In the situation where this sector is selected and it is desired for WL 0  to be asserted, the output of NAND gate  601  will be “low,” and the output of inverter will be “high.” PMOS transistor  611  will be turned on, and the node between PMOS transistor  612  and NMOS transistor  613  will receive the value of XPZB 0 , which will be “low” when word line WL 0  is to be asserted. This will turn on PMOS transistor  614 , which will pull WL 0  “high” to ZVDD which indicates an asserted state. In this instance, XPZB 7  is “high,” signifying that WL 7  is to be not asserted, which will pull the node between PMOS transistor  622  and NMOS transistor  623  to the value of XPZB 7  (which is “high”), which will turn on NMOS transistor  624  and cause WL to be “low,” which indicates a non-asserted state. In this manner, one of the word lines WL 0  . . . WL 7  can be selected when this sector is selected. 
         [0052]      FIG. 7  depicts high voltage row decoder  700 . It will be recalled that in the embodiments of this invention, high voltage signals (e.g., 7-9V for the source line during a programming operation) are required to compensate for the lack of a coupling gate in the flash memory cells. High voltage decoder  700  comprises high voltage level shift enable circuit  710 , erase gate decoder  720 , and source line decoder  730 . 
         [0053]    High voltage level shift enable circuit  710  comprises high voltage level shift circuit  711  and low voltage latch  712 . Low voltage latch  712  receives word line (WL), enable (EN), and reset (RST) as input signals and outputs sector enable signal (SECEN) and sector enable signal bar (SECEN_N). Sector enable signal (SECEN) is provided as an input to high voltage level shift circuit  711 , which outputs sector enable signal high voltage (SECEN_HV 0  . . . SECEN_HVN for N sectors) and sector enable signal high voltage bar (SECEN_HV 0 _N . . . SECEN_HVN_N for N sectors). 
         [0054]    Erase gate decoder  720  comprises erase gate decoder  721  for row 0 in the sector, and similar erase gate decoders (not shown) for rows 1, . . . , N in the sector. Here, erase gate decoder  721  receives the sector enable signal high voltage (SECEN_HV 0 ) from high voltage level shift circuit  711 , its complement (SECEN_HV 0 _N), a voltage erase gate supply (VEGSUP), a low voltage erase gate supply (VEGSUP_LOW),sector enable signal (SECEN), and its complement (SECEN_N). Thus, the output EG 0  of erase gate decoder  721  can be at one of three different voltage levels: SECEN_HV 0  (high voltage), VEGSUP (normal voltage), or VEGSUP_LOW (low voltage). 
         [0055]    Similarly, source line decoder  730  comprises source line decoder  721  for row 0 in the sector, and similar source line decoders (not shown) for rows 1, . . . , N in the sector. Here, source line decoder  731  receives sector enable signal high voltage (SECEN_HV 0 ) from high voltage level shift circuit  711 , its complement (SECEN_HV 0 _N), a voltage source line supply (VSLSUP), a low voltage source line supply (VSLSUP_LOW), sector enable signal (SECEN), and its complement (SECEN_N). Thus, the output SL 0  of source line decoder  730  can be at one of three different voltage levels: SECEN_HV 0  (high voltage), VSLSUP (normal voltage), or VSLSUP_LOW (low voltage). 
         [0056]      FIG. 8  shows erase gate decoder  800 , which is an embodiment of erase gate decoder  720 . Erase gate decoder  800  comprises NMOS transistor  801  and PMOS transistors  802  and  803 , configured as shown. PMOS transistor  803  is a current limiter with EGHV_BIAS as a current mirror bias level. When this erase gate signal (EG) is to be asserted, EN_HV_N will be low (e.g., 0V or 1.2V or 2.5V), which will turn on PMOS transistor  802  and turn off NMOS transistor  801 , which will cause erase gate (EG) to be high (i.e. =VEGSUP, for example 11.5V). When this erase gate signal (EG) is to be not asserted, EN_HV_N will be high, which will turn off PMOS transistor  802  and turn on NMOS transistor  801 , which will cause erase gate (EG) to be low (i.e., =VEGSUP_LOW level, for example 0 v or 1.2V or 2.5V). 
         [0057]      FIG. 9  shows erase gate decoder  900 , which another embodiment of erase gate decoder  720 . Erase gate decoder  900  comprises NMOS transistor  901  and PMOS transistor  902 . Erase gate decoder  900  in this example does not contain a current limiter. When this erase gate signal (EG) is to be asserted, EN_HV_N will be low (e.g., 0V or 1.2V), which will turn on PMOS transistor  902  and turn off NMOS transistor  901 , which will cause erase gate (EG) to be high. When this erase gate signal (EG) is to be not asserted, EN_HV_N will be high, which will turn off PMOS transistor  902  and turn on NMOS transistor  901 , which will cause erase gate (EG) to be low (e.g., 0V or 1.2V or 2.5V). 
         [0058]      FIG. 10  shows erase gate decoder  1000 , which is another embodiment of erase gate decoder  720  that uses only PMOS transistors. Erase gate decoder  1000  comprises PMOS transistors  1001  and  1002 , which share a common well. Erase gate decoder  1000  in this example does not contain a current limiter. When this erase gate signal (EG) is to be asserted, EN_HV_N will be low and EN_HV will be high, which will turn on PMOS transistor  1002  and turn off PMOS transistor  1001 , which will cause erase gate (EG) to be high. When this erase gate signal (EG) is to be not asserted, EN_HV_N will be low and EN_HV will be high, which will turn off PMOS transistor  1002  and turn on PMOS transistor  1001 , which will cause erase gate (EG) to be low (e.g., 0V or 1.2V or 2.5V). 
         [0059]      FIG. 11  shows source line decoder  1100 , which is an embodiment of source line decoder  730 . Source line decoder  1100  comprises NMOS transistors  1101 ,  1102 ,  1103 , and  1104 , configured as shown. NMOS transistor  1101  pulls the source line (SL) low during a read operation in response to the SLRD_EN signal. NMOS transistor  1102  pulls the source line (SL) low during a programming operation in response to the SLP_EN signal. NMOS transistor  1103  performs a monitoring function, through output VSLMON. NMOS transistor  1104  provides a voltage to source line (SL) in response to the EN_HV signal. 
         [0060]      FIG. 12  shows source line decoder  1200 , which is another embodiment of source line decoder  730 . Source line decoder  1200  comprises NMOS transistors  1201 ,  1202 , and  1203 , configured as shown. NMOS transistor  1201  pulls the source line (SL) low during a programming operation in response to the SLP_EN signal. NMOS transistor  1202  performs a monitoring function, through output VSLMON. NMOS transistor  1203  provides a voltage to source line (SL) in response to the EN_HV signal. 
         [0061]      FIG. 13  shows source line decoder  1300 , which is another embodiment of source line decoder  730 . Source line decoder  730  comprises NMOS transistors  1301  and  1302 , configured as shown. NMOS transistor  1301  pulls the source line (SL) low during a programming operation in response to the SLP_EN signal. NMOS transistor  1302  provides a voltage to source line (SL) in response to the EN_HV signal. 
         [0062]      FIG. 14  shows source line decoder  1400 , which is another embodiment of source line decoder  730  that uses only PMOS transistors. Source line decoder  1400  comprises PMOS transistors  1401 ,  1402 , and  1403 , configured as shown. PMOS transistor  1401  pulls the source line (SL) low during a programming operation in response to the EN_HV signal. PMOS transistor  1402  performs a monitoring function, through output VSLMON. PMOS transistor  1403  provides a voltage to source line (SL) in response to the EN_HV_N signal. 
         [0063]      FIG. 15  depicts source line decoder  1500 , which is another embodiment of source line decoder  730  that is a variation of source line decoder  1400  in  FIG. 14 . Source line decoder comprises source line decoder  1400 . The source line (SL) of source line decoder  1400  is connected to the source line  1620  of selected memory cell  1620  and source line  1520  of a dummy memory cell  1510  during read operations. Dummy memory cell  1510  follows the same construction as selected memory cell  1610 , which can be based on the design of memory cell  300 , except that dummy memory cell  1510  is not used to store data. 
         [0064]      FIG. 16  shows additional detail regarding selected memory cell  1620  and dummy memory cell  1520 . When selected memory cell  1620  is in read mode or erase mode, source line  1620  and source line  1520  are coupled to ground through dummy memory cell  1510  and dummy bitline  1526  which is coupled to ground. Dummy memory cell  1510  is required to be erased before read operation. This will pull source line  1520  and source line  1620  to ground. 
         [0065]    When selected memory cell  1610  is in program mode, bitline  1526  is coupled to an inhibit voltage such as VDD. This will place dummy memory cell  1510  in a program inhibit mode which will maintain dummy memory cell  1520  in am erased state. A plurality of the dummy cells, such as dummy memory cell  1510 , can be connected to memory cell  1610  through their source lines to strengthen the pull down of the source line  1620  to ground. 
         [0066]      FIG. 17  depicts control gate decoder  1700 , which is a control gate decoder that can be used with the prior art design of  FIGS. 1-2 , and which is not needed in the embodiments of  FIGS. 3-16 . Control gate decoder  1700  comprises NMOS transistor  1701  and PMOS transistor  1702 . NMOS transistor  1701  will pull down the control gate signal (CG) in response to the signal EN_HV_N. PMOS transistor  1702  will pull up the control gate signal (CG) in response to the signal EN_HV_N. 
         [0067]      FIG. 18  depicts control gate decoder  1800  that uses only PMOS transistors, which is another embodiment of a control gate decoder that can be used with the prior art design of  FIGS. 1-2 , and which is not needed in the embodiments of  FIGS. 3-16 . Control gate decoder  1800  comprises PMOS transistors  1801  and  1802 . PMOS transistor  1801  will pull down the control gate signal (CG) in response to the signal EN_HV. PMOS transistor  1802  will pull up the control gate signal (CG) in response to the signal EN_HV_N. 
         [0068]      FIG. 19  depicts EG/CG/SL gate decoder  1900 , that can be used with the prior art design of  FIGS. 1-2 , and in the embodiments of  FIGS. 3-16 , thus showing the amount of space saved through the present invention. Gate decoder  1900  comprises PMOS transistors  1901 . PMOS transistor  1901  will pull low the gate signal (EG/CG/SL) high in response to the signal EN_HV_N. If EN_HV_N is not asserted, then the value of EG/CG/SL will float. The EG/CG/SL gate is pre-charged to a low bias level first before being enabled to a high voltage level. 
         [0069]      FIG. 20  depicts latch voltage level shifter  2000  with adaptive high voltage VH and low VL supplies. Latch voltage level shifter comprises a latch comprising inverters  2001  and  2002  and NMOS transistors  2003 ,  2004 ,  2005 ,  2006 , and  2007 , in the configuration shown. Latch voltage level shifter receives input  2012  to reset (input RST_SECDEC) and input  2010  to set, meaning enabling, (inputs WL 0  and SET_SECDEC) and produces output  2020  and  2022 . Latch voltage level shifter will adaptively change the magnitudes of a “high” voltage or a “low” voltage to minimize the voltage stress. The latch inverters  2001  and  2002  received power supply high VH and power supply low VL. Initially when enabling by the inputs  2010 / 2012 , VH is Vdd, e.g. 1.2V, and VL is gnd. Then VH starts to ramp up to an intermediate VH level, e.g. 5V. At this VH level, VL then ramps to an intermediate VL level, e.g., 2.5V. After VL reached the intermediate VL level, VH then ramps to final high voltage supply VHVSUP level, e.g., 11.5V. At this point, the voltage across the inverters is only 11.5V−2.5V=9V, hence reducing the voltage stress across them. 
         [0070]      FIG. 21  depicts latch voltage shifter  2100 . Latch voltage shifter  2100  comprises low voltage latch inverter  2109 , NMOS transistors  2103 ,  2104 ,  2107 , and  2108 , and PMOS transistors  2101 ,  2102 ,  2105 , and  2106 , in the configuration shown. Latch voltage shifter  2100  receives EN_SEC as an input and outputs EN_HV and EN_HV_N, which have a larger voltage swing than EN_SEC and ground. 
         [0071]      FIG. 22  depicts high voltage current limiter  2200 , which comprises a PMOS transistor that receives VEGSUP_LOC and outputs VEGSUP with a limited current (acting as a current bias) This circuit can be used with the circuits that do not have local current limiter such as in  FIG. 9,10,17,18,19  to limit current. 
         [0072]      FIG. 23  depicts latch voltage shifter  2300  with a current limiter for read operations. Latch voltage shifter  2300  comprises latch voltage shifter  2100  from  FIG. 21 . It also comprises current limiter  2310  comprising PMOS transistor  2301  and current source  2302 . Current limiter  2310  is connected to current limiter  2310  through switch  2303 . Latch voltage shifter  2100  also is connected to the signal HVSUP_GLB through switch  2304 . During a read operation, latch voltage level shifter  2100  will be connected to current limiter  2310  through switch  2303 . The outputs (e.g., approximately one Vt threshold voltage below Vdd2.5V) of the latch voltage level shifter  2100  control the gate of the EG and CG decoders as in  FIGS. 8,9,10,17,18,19 . When not in a read operation, latch voltage level shifter  2100  will be connected to HVSUP_GLB through switch  2304 . 
         [0073]      FIG. 24  depicts an array with source line pulldown  2400 , which utilizes the designs of  FIGS. 15 and 16 . Array with source line pulldown  2400  comprises a plurality of memory cells organized into rows (indicated by word lines WL 0 , . . . WL 7 ) and columns (indicated by bit lines BL 0 , . . . , BL 31 ). An exemplary memory cell pair is memory cell pair  2401 , which comprises one cell coupled to word line  2402  (WL 0 ) and another cell coupled to word line  2404  (WL 1 ). The two cells share erase gate  2403  (EG 0 ) and source line  2406  (SL 0 ). A column of dummy memory cells also is present, here shown attached to bit line BL_PWDN 1 . An exemplary dummy memory cell pair is dummy memory cell pair  2407 , which comprises one cell coupled to word line  2402  (WL 0 ) and another cell coupled to word line  2404  (WL 1 ). The two cells share erase gate  2403  (EG 0 ) and source line  2406  (SL 0 ). The selected memory cells and dummy memory cells can be configured during read operations as discussed previously for  FIGS. 15 and 16 .