Patent Publication Number: US-7907455-B2

Title: High VT state used as erase condition in trap based nor flash cell design

Description:
TECHNICAL FIELD 
     The following description relates generally to methods and systems for operating a flash device using a high voltage state as an erased state. 
     BACKGROUND 
     Electronic devices with the ability to store information (electronic devices) are an important part of society. Electronic devices influence almost every aspect of life, ranging from business transactions to interpersonal communications. Examples of electronic devices include cellular telephones, personal digital assistants, and personal computers. One important aspect of electronic devices is the ability to store information in digital memory. Digital memory can be provided, for example, by a flash device. Flash memory has the advantages of being readable, rewritable, and non-volatile (i.e., flash memory can retain information without a draw from a constant source of power). Additionally, a flash memory device is relatively inexpensive to mass-produce, making it a desirable choice for personal applications such as storing digital photographs and storing digital music files. Moreover, flash devices generally have an expected lifespan of about one million programming cycles. 
     SUMMARY 
     The following presents a simplified summary of the information disclosed in the specification in order to provide a basic understanding of some aspects of the disclosed information. This summary is not an extensive overview of the disclosed information, and is intended to neither identify key or critical elements of the disclosed information nor delineate the scope of the disclosed information. Its sole purpose is to present some concepts of the disclosed information in a simplified form as a prelude to the more detailed description that is presented later. 
     Conventional flash devices store information in memory cells that are connected to each other via bitlines and wordlines to form a memory array. Each memory cell in a memory array is capable of storing one or more bits of information. A memory cell is programmed or erased by supplying appropriate program or erase voltage levels, respectively, to a wordline and bitline in the memory array that is connected to the cell to be programmed or erased. Conventional flash technologies typically use a low voltage state (e.g., low VT state) to represent a binary value of “1” as an erased state of the memory device. Program changes memory cells to a high voltage state (e.g., high VT state) to represent a binary value of “0.” Conventional flash devices are typically erased in units of memory called sectors or blocks instead of being erased at a logical cell level, wherein all bits in a given sector or block are switched to a predetermined polarity (e.g., low voltage state) when the sector or block is erased. With this conventional scheme, an erase operation generally includes pre-program of all cells in a sector or block, erase of all cells, and clean up of any over-erased cell. 
     The disclosed innovation herein provides systems and methodologies for using a high voltage state as an erase condition in a flash device. Logical cell mapping is changed from using a single physical memory cell to using two adjacent physical cells as a single logical cell, thereby creating a single program and erase entity. Logical cell erase, program, and/or read can be accomplished by using two channel regions in union. This combination can allow for single logical cell erasure in a flash device and the use of a high voltage state as an erased state. A default erased state can be a high voltage state. As a result, program operations can be performed by changing a voltage state of the single program and erase entity to a low voltage state, and erase operations can be performed by changing a voltage state of the single program and erase entity to a high voltage state. 
     The following description and the annexed drawings set forth certain illustrative aspects of the specification. These aspects are indicative, however, of but a few of the various ways in which the principles of the specification may be employed. Other advantages and novel features of the specification will become apparent from the following detailed description of the disclosed information when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of an exemplary dual bit flash device in accordance with one aspect of the specification. 
         FIG. 2  is a schematic illustration of a portion of an exemplary memory core, such as can include at least part of one of the cores depicted in  FIG. 1  in a virtual ground type configuration in accordance with one aspect of the specification. 
         FIG. 3  is a top view of a portion of an exemplary memory core, such as can include at least part of one of the cores depicted in  FIG. 1  in accordance with one aspect of the specification. 
         FIG. 4  is a cross sectional view of a portion of an exemplary memory device containing a single program and erase entity, such as that taken along line A-A of  FIG. 3  in accordance with one aspect of the specification. 
         FIG. 5  is a cross-sectional view of an exemplary single program and erase entity wherein charges can be trapped in the single program and erase entity in accordance with one aspect of the specification. 
         FIG. 6  is a schematic diagram illustrating a method for trapping charges in a single program and erase entity in accordance with one aspect of the specification. 
         FIGS. 7 and 8  are cross-sectional views of an exemplary single program and erase entity wherein charges can be trapped in one physical memory cell of the single program and erase entity in accordance with one aspect of the specification. 
         FIG. 9  is a cross-sectional view of an exemplary single program and erase entity wherein charges can be neutralized in the single program and erase entity in accordance with one aspect of the specification. 
         FIG. 10  is a schematic diagram illustrating a method for neutralizing charges in a single program and erase entity in accordance with one aspect of the specification. 
         FIG. 11  is a schematic diagram of an exemplary system for operating a flash device that contains a plurality of single program and erase entities in accordance with one aspect of the specification. 
         FIG. 12  is a flow diagram of an exemplary methodology for operating a flash device using single program and erase entities in accordance with one aspect of the specification. 
         FIGS. 13-15  illustrate Vt distributions of a physical memory cell of a single program and erase entity in accordance with one aspect of the specification. 
         FIG. 16  is a flow diagram of an exemplary methodology for operating a flash device using single program and erase entities, and using memory cells containing four or more data states in accordance with one aspect of the specification. 
         FIG. 17  illustrates cross-sectional views of an exemplary single program and erase entity wherein charges can be trapped and neutralized in the single program and erase entity in accordance with one aspect of the specification. 
         FIG. 18  is a schematic diagram illustrating a method for trapping charges and neutralizing trapped charges performed on an exemplary array of single program and erase entities in accordance with one aspect of the specification. 
         FIGS. 19   a  and  19   b  illustrate tables depicting exemplary mapping of voltage levels to a one-bit binary value in accordance with one aspect of the specification. 
         FIG. 20  is a schematic illustration for an exemplary method of programming and erasing single program and erase entities using a low voltage state as an erased state in accordance with one aspect of the specification. 
         FIG. 21  is a schematic illustration for an exemplary method of programming and erasing single program and erase entities using a high voltage state as an erased state in accordance with one aspect of the specification. 
         FIG. 22   a  and  22   b  illustrate tables depicting exemplary mapping of voltage levels to a two-bit binary value in accordance with one aspect of the specification. 
         FIG. 23  is a schematic illustration for an exemplary method of programming and erasing single program and erase entities using a low voltage state as an erased state when the single program and erase entities have a two-bit binary value in accordance with one aspect of the specification. 
         FIG. 24  is a schematic illustration for an exemplary method of programming and erasing single program and erase entities using a high voltage state as an erased state when the single program and erase entities have a two-bit binary value in accordance with one aspect of the specification. 
         FIG. 25  is a schematic diagram of an exemplary flash device that contains a plurality of single program and erase entities using a high voltage state as an erased state in accordance with one aspect of the specification. 
         FIG. 26  is a flow diagram of an exemplary methodology for operating a flash device that contains a plurality of single program and erase entities using a high voltage state as an erased state in accordance with one aspect of the specification. 
         FIG. 27  is a table for depicting an exemplary mapping of an erase direction indicator bit to a one-bit binary value in accordance with one aspect of the specification. 
         FIG. 28  is a schematic illustration for an exemplary method of programming and erasing single program and erase entities using an erase direction indicator bit in accordance with one aspect of the specification. 
         FIG. 29  is a table for depicting an exemplary mapping of an erase direction indicator bit to a two-bit binary value in accordance with one aspect of the specification. 
         FIG. 30  is a schematic illustration for another exemplary method of programming and erasing single program and erase entities using an erase direction indicator bit in accordance with one aspect of the specification. 
         FIG. 31  is a schematic diagram of an exemplary flash device that contains a plurality of single program and erase entities and an erase direction indicator cell in accordance with one aspect of the specification. 
         FIG. 32  is a flow diagram of an exemplary methodology for operating a flash device that contains a plurality of single program and erase entities using an erase direction indicator in accordance with one aspect of the specification. 
         FIG. 33  is a flow diagram of another exemplary methodology for operating a flash device that contains a plurality of single program and erase entities using an erase direction indicator in accordance with one aspect of the specification. 
         FIG. 34  is a schematic diagram of an exemplary flash device that emulates byte alterability in the flash device using single program and erase entities in accordance with one aspect of the specification. 
         FIG. 35  is a schematic illustration for an exemplary method of emulating byte alterability in a flash device using single program and erase entities in accordance with one aspect of the specification. 
         FIG. 36  is a flow diagram of an exemplary methodology for emulating byte alterability in a flash device using single program and erase entities in accordance with one aspect of the specification. 
         FIG. 37  is a top view of another exemplary dual bit flash device in accordance with one aspect of the specification. 
         FIG. 38  is a schematic illustration a portion of a flash memory core containing multiple virtual ground decoding schemes in accordance with one aspect of the specification. 
         FIG. 39  is a flow diagram of an exemplary methodology for operating a flash memory containing multiple virtual ground decoding schemes in accordance with one aspect of the specification. 
     
    
    
     DETAILED DESCRIPTION 
     Flash memory is a type of semi conductor computer memory with many desirable characteristics. Like read only memory, ROM, it is non-volatile, meaning that the contents of the memory are stable and retained without applied electrical power. A major advantage of flash over ROM is that the memory contents of flash can be changed after the device is manufactured. However, flash memory generally can not be written to, or programmed, at rates comparable to random access memory, RAM. Further, flash generally must be erased, either in its entirety or in large segments called sector or blocks, prior to changing its contents since pairs of memory cells share a bitline. 
     Typically, a conventional flash memory cell is described as being either in a low voltage state or low VT state (e.g., erased state) or a high voltage state or high VT state (e.g., programmed state). The low voltage state (e.g., erased state) is typically assigned to a binary value “1” and the high voltage state (e.g., programmed state) is typically assigned to a binary value 0. 
     Conventional flash memories generally allow bit changes only in one direction. A binary value “1” stored in a flash memory cell can be changed to a binary value “0” by a programming operation. Changing a binary value “0” by a programming operation into a binary value “1,” however, is generally not possible. Changing programmed cells (binary value “0”) to the logic state “1” is only possible with an erase operation. An erase operation cannot be performed on single bits, but only on a larger amount of data. 
     A page, sector, block, or array of conventional flash memory is generally erased before new data are stored in that page, sector, block, or array. Erasing is performed as a blanket operation wherein a page, sector, block, or array of memory cells is simultaneously erased. This conventional erase process is referred to as a page erase, sector erase, block erase, or flash erase (hereinafter, collectively referred to as a “sector erase”). The sector erase is generally a long process, typically measured in hundreds of milliseconds. Conventional flash memories do not support single logical cell erasure. As a result, conventional flash memories do not support byte alterability. This is a disadvantage compared to RAM and hard drives, which can be written directly, without an interposing erasure. 
     The subject innovation described herein provides systems for facilitating a single logical cell erasure in a flash memory device. The single logical cell erasure can be accomplished by employing a single program and erase entity as a single logical cell. The single program and erase entity is a combination of neighboring drain/source regions of two adjacent memory cells. By mapping two adjacent physical cells as a single logical cell (e.g., single program and erase entity), the flash memory device can be programmed and erased on a byte or variable length basis. 
     The flash memory device that can be employed in the subject innovation can contain dual bit memory cells or mirror bit memory cells (hereinafter, collectively referred to as a “dual bit memory cell”) that have a semiconductor substrate with implanted conductive bitlines. The dual bit memory cell contains a charge trapping dielectric layer that can contain one or more layers and can be formed over the semiconductor substrate. For example, the charge trapping dielectric layer contains three separate layers: a first insulating layer, a charge trapping dielectric layer, and a second insulating layer. Wordlines are formed over the charge trapping dielectric layer substantially perpendicular to the bitlines. Programming circuitry controls two bits per cell by applying a signal to the wordline, which acts as a control gate, and changing bitline connections such that one bit is stored by source and drain being connected in one arrangement and a complementary bit is stored by the source and drain being interchanged in another arrangement. The details of the structure and manufacture of the dual bit flash memory device are not critical to the practice of the subject innovation. The details of the structure and manufacture of the dual bit flash memory device can be found in, for example, commonly-assigned U.S. Pat. No. 7,176,113, issued Feb. 13, 2007, which is hereby incorporated by reference. 
     The claimed subject matter is now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the claimed subject matter. It may be evident, however, that the claimed subject matter may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the claimed subject matter. 
       FIG. 1  illustrates a top view of an exemplary dual bit flash device  100 . The flash device  100  generally includes a semiconductor substrate  102  in which one or more high-density core regions  104  and one or more lower-density peripheral portions are formed. The high-density core regions  104  include one or more M by N arrays of individually addressable, substantially identical single program and erase entities  106 . The single program and erase entities include two adjacent dual bit flash memory cells. 
     The lower-density peripheral portions on the other hand typically include input/output (I/O) circuitry  108  and programming circuitry for selectively addressing the individual single program and erase entities. The programming circuitry is represented in part by and includes one or more wordline decoders  110  and one or more bitline decoders  112  that cooperate with the I/O circuitry  108  for selectively connecting a source, gate, and/or drain of selected addressed single program and erase entities to predetermined voltages or impedances to effect designated operations on the respective single program and erase entities (e.g., programming, reading, and erasing, and deriving necessary voltages to effect such operations). In one embodiment, the dual bit flash device  100  is a trap based NOR flash device. 
       FIG. 2  is a schematic illustration of a portion  200  of an exemplary memory core such as can include at least part of one of the M by N array cores  104  depicted in  FIG. 1 . The circuit schematic shows a group of physical memory cells  201  through  204  in a virtual ground type implementation, for example. The respective physical memory cells  201  through  204  are connected to a wordline  206 , which serves as a control gate, and pairs of the physical memory cells share a bitline. For instance, in the example shown, the physical memory cell  201  has associated bitlines  208  and  209 ; the physical memory cell  202  has associated bitlines  209  and  210 ; the physical memory cell  203  has associated bitlines  210  and  211 ; and the physical memory cell  204  has associated bitlines  211  and  212 . As such, cells  201  and  202  share bitline  209 , cells  202  and  203  share bitline  210  and cells  203  and  204  share bitline  211 , respectively. In the subject innovation, two adjacent physical memory cells can be combined to provide a single program and erase entity  215 ,  216  as a single logical cell. Depending upon a signal on the wordline and the connection of the bitlines in a single program and erase entity to an electrical source or drain, the single program and erase entities are capable of writing, reading, and erasing bits at locations  217  through  220 . 
       FIG. 3  illustrates a top view of a portion  300  of an exemplary memory core, such as can include the M by N array cores  104  depicted in  FIG. 1 . The memory  300  can be formed upon a semiconductor substrate  302  and have a plurality of implanted bitlines  304  extending substantially parallel to one another, and further include a plurality of formed wordlines  306  extending substantially in parallel to one another and at substantially right angles to the plurality of implanted bitlines  304 . It will be appreciated that the wordlines  306  and bitlines  304  have contacts and interconnections (not shown) to programming circuitry such as can be represented, at least in part, by wordline decoders and bitline decoders. 
       FIG. 4  is a cross sectional view of a portion of an exemplary memory device  400  containing a single program and erase entity as indicated by a dashed line  402 , such as that taken along line A-A of  FIG. 3 . The single program and erase entity  402  can contain two adjacent physical memory cells  404 ,  406  that share a common bitline  408 . The single program and erase entity can contain other two bitlines  410  that are not shared by the two adjacent physical memory cells  404 ,  406  (hereinafter, referred to as “non-common bitlines”). 
     The physical memory cell  404 ,  406  can be a dual bit memory cell. The physical memory cell  404 ,  406  has dual bit locations  412 ,  414 ,  4144 ,  418  where a charge can be stored. As will be described in detail below, the single program and erase entity  402  can store varying degrees of charge at two locations  414 ,  416  close to the common bitline  408 . It will be appreciated that the physical memory cell  404 ,  406  can correspond to the physical memory cells  201  through  204  depicted in  FIG. 2 . 
     The physical memory cell  404 ,  406  typically includes a charge trapping dielectric layer  420  that contains a charge trapping layer  422  sandwiched between two dielectric layers  424 ,  426 . In one embodiment, the two charge storage nodes  412 ,  414  are physically separated by a central dielectric (not shown) in the charge trapping dielectric layer  420 . The configuration and/or constituent of the charge trapping dielectric layer  420  can vary and are not critical to the subject innovation. The charge trapping dielectric layer  420  can contain a first insulating layer  424  (e.g., a bottom dielectric layer or a tunneling dielectric layer), a charge trapping layer  420 , and a second insulating layer  426  (e.g., a top dielectric layer). The first insulating layer  424  and/or the second insulating layer  426  can contain silicon oxide (e.g., SiO 2 ), other standard-K material (e.g., having a relative permittivity below ten), or a high-K material (e.g., having a relative permittivity, in one embodiment, above about ten and, in another embodiment, above about twenty). 
     The charge trapping dielectric layer  420  can contain any suitable charge trapping layer  422 . Examples of charge trapping layers  422  include nitrides (e.g., silicon nitride, silicon oxynitride, and silicon rich nitride), oxides, silicates, a high-k dielectric, for example, having a dielectric constant higher than that of silicon dioxide (SiO 2 ), and the like. In one embodiment, the first and second dielectric layers  424 ,  426  contain oxide dielectrics such as silicon dioxide (SiO 2 ) and the charge trapping layer contains nitride dielectrics such as silicon nitride (Si x N y ). The oxide-nitride-oxide configuration can be referred to as an ONO layer. Especially, when the nitride layer contains silicon rich nitride, the oxide-nitride-oxide configuration can be referred to as an ORO tri-layer. The oxide-nitride-oxide tri-layer can be fabricated by forming a first silicon oxide layer, forming a silicon nitride layer on the first silicon oxide layer, and forming a second silicon oxide layer on the silicon nitride layer. In another embodiment, the charge trapping dielectric layer  420  contains five separate layers, for example, oxide-nitride-polysilicon-nitride-oxide. The oxide-nitride-polysilicon-nitride-oxide configuration can be referred to as an ORPRO layer when the nitride layer contains silicon rich nitride. 
     The charge trapping layer  420  can be formed over a substrate  428  that can contain silicon or some other semiconductor material. The substrate  428  can be selectively doped with a p-type dopant, such as boron, for example, to alter its electrical properties. In the example illustrated, the substrate  428  has buried bitlines or bitline diffusions  408 ,  410 . The bitline diffusions can be formed by an implanted n-type dopant such as arsenic, phosphorous, and antimony, and can correspond to bitlines  208  through  212  in  FIG. 2 . A channel  430  is defined within the substrate between the bitlines (e.g., S/D extensions, deep S/D regions). 
     Overlying the upper dielectric layer  426  of the charge trapping dielectric layer  420  is a gate  432 . This gate  432  can be formed from a polysilicon material and can be doped with an n-type impurity (e.g., phosphorus) to alter its electrical behavior. The gate  432  can correspond to the wordlines  206  in  FIG. 2 . The gate  432  enables a voltage to be applied to the single program and erase entity  402  such that respective charges can, among other things, be stored within the single program and erase entity at locations  414 ,  416 . As will be described in detail below, the physical memory cell  404 ,  406  of the single program and erase entity  402  can store different amounts of charge  434 ,  436 ,  438 . 
       FIGS. 5-10  illustrate exemplary methods for trapping and neutralizing charges in charge trapping layers of single program and erase entities. By trapping charges in charge trapping layers, single program and erase entities can achieve a high voltage state (e.g., high VT state). By neutralizing charges in charge trapping layers, single program and erase entities can achieve a low voltage state (e.g., low VT state).  FIGS. 5-10  illustrate the use of channel hot-electron-injection and hot-hole-injection to achieve the desired voltage state. It should be appreciated that the particular method and memory cell architecture illustrated by  FIGS. 5-10  are only some of many possible high-voltage and/or high-current operations and memory cell architectures that can utilize the embodiments disclosed herein, and that all such operations and architectures are intended to fall within the scope of the hereto appended claims. A low voltage state may be referred to as a “low VT (low threshold voltage) state,” and the low voltage state can be equivalent to the low VT state in some instances. A high voltage state may be referred to as a “high VT (high threshold voltage) state,” and the high voltage state can be equivalent to the high VT state in some instances. 
       FIG. 5  specifically illustrates a method for trapping charges in a single program and erase entity  500  containing two adjacent physical memory cells  502 ,  504 . To trap charges in charge trapping layers  506  of single program and erase entities, a gate (e.g., a wordline)  508  can be set to high voltages (hereinafter, referred to as “hot-electron-injection gate voltage”), a common bitline  510  can be set to a predetermined potential above the non-common bitlines (hereinafter, referred to as “hot-electron-injection bitline voltage”), and non-common bitlines  512  that are not shared by the two adjacent physical cells can be connected to ground, left to float, or biased to a different voltage level. In one embodiment, the voltage across the wordline  508  can be about 9 volts and the voltage across the common bitline  510  can be about 4 volts. The applied voltages generate a vertical electric field through the charge trapping layer  506 , and generate a lateral electric field across a length of the channel  514  from the non-common bitlines  512  to the common bitline  510 . At a given voltage, the channels  514  invert such that electrons are drawn off the non-common bitlines and begin accelerating towards the common bitline. 
     As the electrons move along the length of the channel  514 , the electrons gain energy and, upon attaining enough energy, the electrons jump over the potential barrier of the first dielectric layer (e.g., bottom dielectric layer)  516  and into the charge trapping layer  518 , where the electrons become trapped. The probability of electrons jumping the potential barrier in this arrangement is a maximum at locations  520 ,  522  close to the common bitline  510 , where the electrons have gained the most energy. These accelerated electrons are termed hot electrons and, once injected into the charge trapping layer  518 , stay in about the general area indicated as charge trapping regions  520 ,  522 . The trapped electrons tend to remain generally localized due to the low conductivity of the charge trapping layer  518  and the low lateral electric field therein. 
     The presence or absence of a trapped charge in the charge trapping layer  506  of the single program and erase entity  500  can then correspond to a bit of data stored by the single program and erase entity  500 . The presence or absence of a trapped charge can be read by determining the existence of a trapped charge in the charge trapping layer  518  at that location. Although not shown in  FIG. 5 , for a read operation, in addition to voltages applied to the gate  508  (e.g., the wordline), the non-common bitlines  512  are connected to drains and the common bitline  510  is connected a source. The voltage applied to the gate  508  causes a current from the non-common bitlines  512  to the common bitline  510 . The resulting current is measured, by which a determination is made as to the value of the data stored in the single program and erase entity  500 . For example, if the current is above a certain threshold, the bit is deemed unprogrammed or a logical one, whereas if the current is below a certain threshold, the bit is deemed to be programmed or a logical zero. 
     In this example, by setting the common bitline  510  to high voltages while setting both non-common bitlines  512  to ground, charges are trapped in charge trapping layers  506  of the two adjacent physical memory cells  502 ,  504  at the same time. The dotted line  524  illustrates one possible path through which electrons can move as they pass from the non-common bitlines  512  to the common bitline  510  of the single program and erase entity  500  based on the voltages applied to the single program and erase entity  500 . It should be appreciated that other electron paths are possible and that the exact path through which the electrons move can depend on the profile of the bitline junctions, the combination of the voltage across the wordline and the common bitline, and other appropriate factors. 
       FIG. 6  is a schematic diagram illustrating a method for trapping charges performed on an exemplary array  600  of single program and erase entities  602  as indicated by a dashed line. The single program and erase entities contain two adjacent physical memory cells  604 . The two adjacent physical memory cells  604  share a common bitline  606 . The single program and erase entities  602  are connected by other two bitlines  608  at both ends of the single program and erase entities. The other two bitlines  608  are not shared by the two adjacent physical memory cells that constitute a single program and erase entity; thus the other two bitlines are non-common bitlines. The physical memory cells  604  in the single program and erase entities  602  are also connected by wordlines. Since charges can be trapped in each single program and erase entity  602  by applying a suitable voltage on common bitlines  606  of the single program and erase entity  602 , the method can be performed on any selected single program and erase entity. Also, charges can be trapped in any selected single program and erase entity without any substantial electrical effect on adjacent single program and erase entities. 
     Trapping charges in single program and erase entities  602  can be conducted by a suitable mechanism, such as hot-electron-injection. Trapping charges with hot-electron-injection involves applying a relatively high voltage to the control gate (e.g., wordline), connecting the non-common bitlines  608  to ground, and connecting the common bitline  606  to a predetermined potential above the non-common bitlines  608 . When a resulting electric field is high enough, electrons collect enough energy to be injected from the non-common bitlines  608  onto charge trapping layers in the single program and erase entity  602 . As a result of the trapped electrons in the charge tapping layers, the threshold voltage of the single program and erase entity  602  increases. This change in the threshold voltage (and thereby the channel conductance) of the single program and erase entity  602  created by the trapped electrons is what causes the single program and erase entity  602  to be programmed or erased. 
     The method for trapping charges is performed on single program and erase entities  602  that are disposed next to each other in rows. Since each single program and erase entity  602  contains two adjacent physical memory cells and the single program and erase entities  602  are disposed next to each other in rows, common bitlines  606  shared by the two adjacent physical memory cells are on every other line. In other words, the common bitlines  606  of single program and erase entities  602  exist alternately in rows. As a result, trapping charges involves applying a suitable voltage (e.g., hot-electron-injection voltage or programming voltage) to at most every other bitline in rows. The position of the bitline whose voltage state is changed can be sequentially shifted to every other bitline or shifted to any common bitline of selected single program and erase entities. In one embodiment, trapping charges involve applying a suitable voltage to only odd numbers of bitlines or only even numbers of bitlines. In another embodiment, trapping charges does not involve sequentially applying a suitable voltage (e.g., hot-electron-injection voltage or programming voltage) to every bitline or two or more consecutive bitlines in rows. 
     In  FIG. 6 , charges are trapped in a single program and erase entity (SPEE 1 ) by applying a program gate voltage (PGV 1 ) (e.g., a hot-electron-injection gate voltage (HEIGV 1 )) to a wordline  610 , applying a program bitline voltage (PBV 1 ) (e.g., a hot-electron-injection bitline voltage (HEIBV 1 )) to a common bitline  606  of SPEE 1 , and connecting non-common bitlines  608  of SPEE 1  to ground. When subsequently trapping charges in another single program and erase entity (SPEE 2 ), the charges can be trapped by applying a program gate voltage (PGV 2 ) (e.g., a hot-electron-injection gate voltage (HEIGV 2 )) to a wordline, applying a programming bitline voltage (PBV 2 ) (e.g., a hot-electron-injection bitline voltage (HEIBV 2 )) to a common bitline  606  of SPEE 2 , and connecting non-common bitlines  608  of SPEE 2  to ground. 
     In one embodiment, by applying the gate bias on one of the two adjacent physical memory cells of a single program and erase entity, one of the two physical memory cells can be programmed.  FIG. 7  illustrates a programming operation performed on one physical memory cell of a single program and erase entity  700 . Specifically,  FIG. 7  illustrates a programming operation performed on a left physical memory cell  702  of a single program and erase entity  700 . In this example, the wordline  704  and the common bitline  706  of single program and erase entity  700  can be set to high voltages and the non-common bitline  708  of the left physical cell  702  can be set to ground. As a result of the voltages applied to the left physical cell  702 , electrons can be made to flow from the left non-common bitline  708  to the common bitline  706  and toward the charge trapping layer  710  of the left physical cell  702 . A charge provided by the electrons that enter the charge trapping layer  710  of the left physical cell  702  can then become trapped in the charge trapping layer  710  of the left physical cell at a location  712  close to the common bitline  706 , thereby enabling the single program and erase entity  700  to store a first bit of data in the left physical cell  702 . 
     Similar to the programming operation illustrated in  FIG. 7 , the other physical cell can be programmed by applying a bias voltage on the other physical cell.  FIG. 8  illustrates a programming operation performed on a right physical memory cell  802  of a single program and erase entity  800 . In this example, the wordline  804  and the common bitline  806  of single program and erase entity  800  can be set to high voltages and the non-common bitline  808  of the right physical cell  802  can be set to ground. As a result of the voltages applied to the right physical cell  802 , electrons can be made to flow from the right non-common bitline  808  to the common bitline  806  and toward the charge trapping layer  810  of the right physical cell  802 . A charge provided by the electrons that enter the charge trapping layer  810  of the right physical cell  802  can then become trapped in the charge trapping layer  810  of the right physical cell  802  at a location  812  close to the common bitline  806 , thereby enabling the single program and erase entity  800  to store a second bit of data in the right physical cell  802 . 
       FIG. 9  illustrates a cross-sectional view of an exemplary single program and erase entity  900  wherein charges can be neutralized in the single program and erase entity. More particularly,  FIG. 9  illustrates a method for neutralizing trapped charges in single program and erase entities via band-to-band hot-hole-injection. By neutralizing trapped charges in physical memory cells of single program and erase entities, the single program and erase entities can achieve a low voltage state. As will be described in detail below, the low voltage state can be assigned to an erased state or a programmed state. 
     To neutralize the charges, holes can be injected into the charge trapping layers  904  by applying a negative high voltage (hereinafter, referred to as “hot-hole-injection gate voltage”) to a gate (e.g., wordline)  906  and a positive high voltage of substantially equal magnitude (hereinafter, referred to as “hot-hole-injection bitline voltage”) to the common bitline  908  while allowing the non-common bitlines  910  to float. In one embodiment, the voltage across the wordline  906  is about −6 volts and the voltage across the common bitline  908  is about 6 volts. As a result of the neutralizing voltages applied to the single program and erase entity  900 , the holes can be made to flow through the physical memory cells  902  in a path such as the path illustrated by dotted line  912 . Further, the holes can be made to enter the charge trapping layers  904  near the common bitline  908 , thereby releasing charges provided by electrons in the charge trapping layer  904  at charge trapping regions  914 ,  916 . The charges can then flow out of the charge trapping layers  904  as a current  918 . 
       FIG. 10  is a schematic diagram illustrating a method for neutralizing trapped charges performed on an exemplary array  1000  of single program and erase entities  1002  as indicated by a dashed line. Single program and erase entities  1002  contain two adjacent physical memory cells  1004  on a substrate (not shown). The two adjacent physical memory cells  1004  share a common bitline  1006 . The single program and erase entities  1002  are connected by other two bitlines  1008  at both ends of the single program and erase entities. The other two bitlines are not shared by the two adjacent physical memory cells that constitute a single program and erase entity; thus the other two bitlines are non-common bitlines. The physical memory cells  1004  in the single program and erase entities  1002  are also connected by wordlines  1010 . Since trapped charges can be neutralized in each single program and erase entity  1002  by applying a suitable voltage on common bitlines  1006  of the single program and erase entities, the method can be performed on any selected single program and erase entity  1002 . Each single program and erase entity  1002  also can be erased without any substantial electrical effect on adjacent single program and erase entities. 
     Trapped charges can be neutralized by a suitable mechanism, such as a hot-hole-injection. Neutralizing charges with hot-hole-injection involves applying a negative high voltage to the wordline  1010  and a positive high voltage of substantially equal magnitude to the common bitline  1006  while allowing the non-common bitlines  1008  to float. These applied voltages cause the electrons that are trapped into the charge trapping layer to undergo either Fowler-Nordheim tunneling or hot hole neutralization through a first dielectric layer (e.g., thin tunnel oxide layer) to either a substrate (P-well) or a source/drain depending on the type of method being performed. 
     In  FIG. 10 , charges trapped in a single program and erase entity (SPEE 1 ) are neutralized by applying an erase gate voltage (e.g., a hot-hole-injection gate voltage (HHIGV  1 )) to a wordline  1010 , applying an erase bitline voltage (e.g., a hot-hole-injection bitline voltage (HHIBV 1 )) to a common bitline  1006 , and allowing non-common bitlines  1008  to float. When subsequently neutralizing charges in another single program and erase entity (SPEE 2 ), the charges can be neutralized by applying a hot-hole-injection gate voltage (HHIGV 2 ) to a wordline  1010 , applying a hot-hole-injection bitline voltage (HHIBV 2 ) to a common bitline  1006  of the single program and erase entity, and allowing non-common bitlines  1008  of the single program and erase entity to float. 
     Single program and erase entities  1002  are disposed next to each other in rows. Since each single program and erase entity  1002  contains two adjacent physical memory cells  1004 , common bitlines  1006  shared by the two adjacent physical memory cells  1004  of single program and erase entities are on every other line in rows. In other words, the common bitlines  1006  of single program and erase entities  1002  exist alternately in rows. As a result, a method for neutralizing charges involves applying a suitable neutralizing voltage (e.g., hot-hole-injection voltage) to at most every other bitline in rows. The position of the bitline whose voltage state is changed can be sequentially shifted to every other bitline or shifted to any bitline of selected single program and erase entities. In one embodiment, a method for neutralizing charges involves applying a suitable voltage to only odd numbers of bitlines or only even numbers of bitlines. In another embodiment, a method for neutralizing charges does not involve sequentially applying a hot-hole-injection bitline voltage to every bitline, or two or more consecutive bitlines in rows. In yet another embodiment, a method for neutralizing charges does not involve a sector erase. 
       FIG. 11  illustrate a flash device  1100  for operating a flash device using single program and erase entities. The flash device can contain a memory core  1102  containing a plurality of single program and erase entities  1104  and one or more decoders such as a bitline decoder  1106  (e.g., Y decoder or column decoder) and a wordline decoder  1108  (e.g., X decoder or row decoder). The bitline decoder and the wordline controller can decode I/Os during various operations that are performed on the single program and erase entities (e.g., programming, reading, erasing). 
     Two adjacent physical memory cells in any suitable portion of or an entire of the memory array  1102  can be mapped as a single program and erase entity. Various operations can be performed on the flash device  1100  according to the mapping. Single program and erase entities can be selected by decoders, and programming, reading, and/or erasing are performed on a basis of the single program and erase entity. 
     The decoders  1106 ,  1108  can facilitate operating a portion of or an entire of the memory array on the basis of the single program and erase entity. The bitline decoder can select one or more columns and the wordline decoder can select one or more rows. Any suitable decoder can be employed as long as the decoder can select one or more single program and erase entities for access. In one embodiment, while operating a flash memory, the decoders do not perform a sector erase in a portion of or an entire of a memory core. In other words, in one embodiment, a bitline decoder does not sequentially select two or more consecutive bitlines for applying an erase bitline voltage. In another embodiment, the decoders do not select bitlines between the single program and erase entities (e.g., non-common bitlines) for applying an erase bitline voltage. 
     The decoders  1106 ,  1108  can select one or more single program and erase entities in response to received addresses. The decoders can select one or more addresses by any suitable technique. In one embodiment, a table (e.g., a lookup table) is employed to implement decoders. The table can drive a register-transfer-level (RTL) description to implement decoders (e.g., bitline decoders). The decoders can be a table-driven decoder. 
       FIG. 12  is a flow diagram of an exemplary methodology for operating a flash device using single program and erase entities. At  1200 , a flash device is programmed on a basis of a single program and erase entity. The single program and erase entity includes two adjacent dual bit physical memory cells of the flash device as a single logical cell. At  1204 , the flash device is erased on the basis of the single program and erase entity. The flash device can be programmed and then erased on the basis of the single program and erase entity. In another embodiment, the flash device can be erased and then programmed on the basis of the single program and erase entity. 
     Although not shown in  FIG. 12 , while erasing a portion of or an entire of the flash device on the basis of the single program and erase, a sector erase is not performed in the portion of or the entire of the flash memory. In another embodiment, while erasing a portion of or an entire of the flash device on the basis of the single program and erase, an erase pulse is not sequentially applied to two or more consecutive bitlines in the portion of or the entire of the flash memory. 
     The various operations (e.g., programming, erasing, reading) are performed on a basis of a single program and erase entity. In one embodiment, programming the flash memory device on a basis of a single program and erase entity involves applying a program pulse to a portion of or an entire of the flash memory on the basis of the single program and erase entity. In another embodiment, programming the flash memory device on a basis of a single program and erase entity involves applying a program gate voltage to a gate, applying a program bitline voltage to a common bitline of the single program and erase entity that is shared by the two adjacent dual bit physical memory cells, and connecting at least one of non-common bitlines of the single program and erase entity that are not shared by the two adjacent dual bit physical memory cells to ground. The program gate voltage can include a hot-electron-injection gate voltage and the program bitline voltage can include a hot-electron-injection bitline voltage. 
     Erasing can be performed on a basis of a single program and erase entity. In one embodiment, erasing the flash device on the basis of the single program and erase entity involves applying an erase pulse to a portion of or an entire of the flash memory on the basis of the single program and erase entity. In another embodiment, erasing the flash device on the basis of the single program and erase entity involves applying a erase gate voltage to a gate, applying a erase bitline voltage to a common bitline of the single program and erase entity that is shared by the two adjacent dual bit physical memory cells, and allowing non-common bitlines of the single program and erase entity that are not shared by the two adjacent dual bit physical memory cells to float. The erase gate voltage includes a hot-hole-injection gate voltage and the erase bitline voltage includes a hot-hole-injection bitline voltage. 
     Each bit of physical memory cells of single program and erase entities can be programmed to multiple levels. When voltages utilized to program single program and erase entities are increased or sustained for longer periods of time, the number of electrons or amount of charge stored in the single program and erase entities can be increased or otherwise varied. This allows the single program and erase entities to be utilized for additional data storage and/or programming states. For example, different amounts of charge can correspond to different programmed states. This technique is also called multi-level cell technology, which is useful to increase density and reduce manufacturing costs. 
     In one embodiment, dual bit physical memory cells of single program and erase entities contain storage nodes having two different states or levels, for example, 1 and 2. Level 1 can correspond to a situation where the storage nodes are blank or un-programmed, and level 2 can correspond to programmed. By way of illustration, charge storage nodes  414  and/or  416  described in  FIG. 4  can have two different states or levels. 
       FIG. 13  is a graph  1300  illustrating an unsigned Vt distribution  1302 ,  1304  of a charge storage node. The Vt distribution represents a population of physical memory cell threshold voltages centered about two discrete target threshold voltages. Each target threshold voltage occupies a range of Vt values designated by levels L1 and L2, respectively. Each level can be centered between upper and lower Vt limits (e.g., Vt0, Vt1, and Vt2). The various levels can be arbitrarily assigned corresponding binary states (e.g., L1=0 and L2=1, or L1=1 and L2=0) as desired by the user. When two storage nodes adjacent to common bitlines of single program and erase entities are programmed at the same time as described in  FIG. 5 , the single program and erase entities can be programmed to two levels. When the two storage nodes adjacent to common bitlines of single program and erase entities are programmed independently from each other as described in  FIGS. 7 and 8 , the single program and erase entities can be programmed to multiple levels such as four. 
     In another embodiment, dual bit physical memory cells of single program and erase entities contain storage nodes having four different states or levels, namely 1, 2, 3, and 4. Level 1 can correspond to a situation where the storage nodes are blank or un-programmed, and levels 2, 3 and 4 correspond to increased amounts of stored charge, respectively. By way of illustration, a level 2 can correspond to a relatively small amount of stored charge  434  in  FIG. 4 , while levels 3 and 4 can correspond to increasingly larger amounts of stored charge  436  and  438  in  FIG. 4 , respectively. 
       FIG. 14  is a graph  1400  illustrating a four level of Vt distribution  1402 ,  1404 ,  1406 ,  1408 . Vt distribution  1400  represents a population of memory cell threshold voltages centered about four discrete target threshold voltages. Each target threshold voltage occupies a range of Vt values designated by levels L1, L2, L3, and L4, respectively. Each level can be centered between upper and lower Vt limits, for example, Vt0, Vt1, Vt2, Vt3, and Vt4. In one embodiment, target threshold voltages Vt2, Vt3, and Vt4 can, for example, have values such as Vt2=1.5V, Vt3=2.1V, and Vt4=2.7V. 
     The various levels can be arbitrarily assigned corresponding binary states (e.g., L1=11, L2=10, L3=01, and L4=00, or L1=00, L2=01, L3=10, and L4=11) as desired by the user. When two storage nodes adjacent to common bitlines of single program and erase entities are programmed at the same time as described in  FIG. 5 , the single program and erase entities can be programmed to four levels. When the two storage nodes adjacent to common bitlines of single program and erase entities are programmed independently from each other as described in  FIGS. 7 and 8 , the two storage nodes can be programmed to multiple levels such as sixteen different combinations of charge (e.g., L1-L1, L1-L2, L1-L3, L1-L4, L2-L1, L2-L2, L2-L3, L2-L4, L3-L1, L3-L2, L3-L3, L3-L4, L4-L1, L4-L2, L4-L3, and L4-L4) providing the equivalent of four-bits-per-entity. The sixteen voltage levels can corresponds to sixteen data states of a single program and erase entity. 
     In a similar way to the four level charge storage node as described in  FIG. 14 , charge storage nodes of single program and erase entities can have any suitable number of levels.  FIG. 15  illustrates an unsigned Vt distribution  1500  having n levels. When two storage nodes adjacent to common bitlines of single program and erase entities are programmed at the same time as described in  FIG. 5 , the single program and erase entities can be programmed to n levels. When the two storage nodes adjacent to common bitlines of single program and erase entities are programmed independently from each other as described in  FIGS. 7 and 8 , the single program and erase entities can be programmed to multiple levels such as n 2  providing the equivalent of high bit-per-entity. Single program and erase entities can contain physical memory cells having any suitable combination of both positive and negative Vt distributions. In  FIG. 15 , for example, Vt0, Vt4, or another such Vtn level can be used as a zero voltage potential or another reference potential of the memory cells. 
     Single program and erase entities containing multi-level physical cells can increase the effective logical cell density by increasing the number of possible logical states or data states, thereby allowing a single program and erase entity to store information corresponding to more than one data bit. This can be done by using multiple (three or more, in the context of cell levels and states) threshold voltage (Vt) levels, which correspond to multiple data states per entity. 
     The quantity of charge stored in each storage node can influence an amount of current that flows between a common bitline and non-common bitline during a read operation, as well as a threshold voltage (Vt) required to cause such current to flow. Thus, the level of stored bits in a storage node can be determined by examining the currents as well as corresponding applied threshold gate (wordline) voltages. In particular, low currents and high gate voltages can be indicative of higher bit levels. Thus, when physical memory cells contain four level charge storage nodes, measured currents and/or threshold voltages that fall within first, second, third or fourth ranges can be indicative of a level 1, level 2, level 3 or level 4, respectively for the stored bit. 
       FIG. 16  is a flow diagram of an exemplary methodology for operating a flash device using single program and erase entities and using memory cells containing four or more data states. At  1600 , a flash device is programmed on a basis of a single program and erase entity. The single program and erase entity includes two adjacent dual bit physical memory cells of the flash device as a single logical cell. The dual bit physical memory cells can contain four or more data states. In one embodiment, each of the two adjacent dual bit physical memory cells can be programmed independently from each other. Since the memory cells contain four data states and each of the two adjacent dual bit physical memory cells can be programmed independently, the single program and erase entities can store four bits. Although not shown in  FIG. 16 , programming a portion of or an entire of the flash memory on the basis of the single program and erase entity can be performed by applying a program gate voltage to a gate, applying a program bitline voltage to a common bitline of the single program and erase entity that is shared by the two adjacent dual bit physical memory cells, and connecting at least one of two non-common bitlines of the single program and erase entity that are not shared by the two adjacent dual bit physical memory cells to ground. 
     At  1602 , the flash device is erased on the basis of the single program and erase entity. The flash device can be programmed and then erased on the basis of the single program and erase entity. In another embodiment, the flash device can be erased and then programmed on the basis of the single program and erase entity. Although not shown in  FIG. 16 , in yet another embodiment, at least one of the two adjacent dual bit physical memory cells are selected by a decoder for programming the flash device and one or more single program and erase entities are selected by decoders for erasing the flash device. 
       FIG. 17  illustrates cross-sectional views of an exemplary single program and erase entity wherein charges can be trapped and neutralized in the single program and erase entity. As disclosed above, trapping charges and neutralizing trapped charges in storage nodes can be performed on any selected single program and erase entity independently. Trapping charges and neutralizing trapped charges in storage nodes can be performed on a selected single program and erase entity without any substantial electrical effect on adjacent single program and erase entities. As a result, the subject innovation can provide two types of program and erase operations (Types 1 and 2). 
     In one embodiment, single program and erase entities are programmed to a high voltage state and erased to a low voltage state (Type 1). In other words, a high voltage state is assigned to a programmed state and a low voltage state is assigned to an erased state. The high voltage state in  FIG. 17  can correspond to  FIG. 5 . The low voltage state in  FIG. 17  can correspond to  FIG. 9 . In another embodiment, single program and erase entities are programmed to a low voltage state and erased to a high voltage state (Type 2). In other words, a low voltage state is assigned to a programmed state and a high voltage state is assigned to an erased state. Since trapping charges and neutralizing trapped charges in storage nodes can be performed on a single program and erase entity without any substantial electrical effect on adjacent single program and erase entities, erasing and programming can be accomplished on any suitable number of single program and erase entities such as on a byte or variable length basis. 
       FIG. 18  is a schematic diagram illustrating a method for trapping charges and neutralizing trapped charges performed on an exemplary memory array  1800  of single program and erase entities  1802 . Charges can be trapped in the same manner as described for trapping charges in connection with  FIG. 6 . Trapped charges can be neutralized in the same manner as described for neutralizing trapped charges in connection with  FIG. 10 . Trapping charges and neutralizing trapped charges can be performed on a basis of the single program and erase entity (e.g., in units of single program and erase entities). In one embodiment, neutralizing trapped charges does not require a sector erase. As a result, each single program and erase entity can be changed its voltage state independently from a high voltage state to a low voltage state and from a low voltage state to a high voltage state. 
     Trapping charges and neutralizing trapped charges can be performed on any suitable single program and erase entity in an array. Charges can be trapped in a single program and erase entity (SPEE 1 ) by applying a hot-electron-injection gate voltage (HEIGV) to a wordline  1804 , applying a hot-electron-injection bitline voltage (HEIBV 1 ) to a common bitline  1806 , and connecting non-common bitlines  1808  to ground. Trapped charges in SPEE 1  can be neutralized by applying a hot-hole-injection gate voltage (HHIGV) to a wordline  1804 , applying a hot-hole-injection bitline voltage (HHBV 1 ) to a common bitline  1806 , and allowing non-common bitlines  1808  to float. Since a sector erase is not required for neutralizing trapped charges, trapping charges and neutralizing trapped charges can be performed on the single program and erase entities at any suitable time without performing a sector erase. As a result, a high voltage state can be assigned to a programmed state and a low voltage state can be assigned to an erased state, or a low voltage state can be assigned to a programmed state and a high voltage state can be assigned to an erased state. 
     An erase operation of conventional flash memories is generally accomplished in one direction (e.g., from a high voltage state or binary value “0” to a low voltage state or binary value “1”) on a sector-by-sector basis. As a result, a program operation of conventional flash memories is also generally accomplished in one direction (e.g., from a low voltage state or binary value “1” to a high voltage state or binary value “0”). The conventional sector erase is typically accomplished by a pre-program cycle, erase cycle, and soft program cycle. The pre-programming puts each memory cell in a high voltage state or a programmed state. This is accomplished by applying a program pulse to each memory cell to store a charge in the charge trapping layer. This is done to eliminate or reduce the chance of removing too many electrons from the memory cells during the erase process. Once the pre-programming has been completed, erasing can be performed by one or more applications of short erase pulses (e.g., an erase cycle). After each erase pulse, an erase verification or read is performed to determine if each cell in the array is now “erased” (blank), yet remains “un-erased,” or “under-erased” (e.g., whether the cell has a threshold voltage above a predetermined limit). If an under-erased cell is detected, an additional erase pulse is applied to the entire sector, block, or array until all cells are sufficiently erased. With such a conventional erase procedure, however, some cells can become “over-erased” before other cells are sufficiently erased. A memory cell having a threshold voltage erased below a predetermined limit can be commonly referred to as being over-erased. For several reasons, it is undesirable for a memory cell to remain in an over-erased condition. If a memory cell is over-erased, the whole column can become leaky. When such an over-erased cell is detected, a soft program pulse is applied to the over-erased memory cell to pull its threshold voltage back into the normal population of erased cells (e.g., a soft-program cycle). 
     In the subject innovation, erasing can be accomplished in two directions, and programming can be also accomplished in two directions. The erase directions can include a first direction from a high voltage state or binary value “0” to a low voltage state or binary value “1,” and a second direction from a low voltage state or binary value “1” to a high voltage state or binary value “0.” The program direction include a first direction from a low voltage state or binary value “1” to a high voltage state or binary value “0,” and a second direction from a high voltage state or binary value “0” to a low voltage state or binary value “1.” 
     When single program and erase entities use a low voltage state as an erased state, an erase operation can be performed by applying a hot-hole-injection voltage to one or more selected single program and erase entities or all of the single program and erase entities (e.g., an erase cycle) and then soft-programming over-erased single program and erase entities (e.g., a soft-program cycle). In one embodiment, the erase operation does not require a pre-program cycle. When single program and erase entities use a high voltage state as an erased state, an erase operation can be performed by applying a hot-electron-injection voltage to one or more single program and erase entities or all of the single program and erase entities (e.g., pre-program cycle). In one embodiment, the erase operation does not require an erase cycle and/or a soft-program cycle. In both instances, the erase operations do not require all of the three cycles (e.g., a pre-program cycle, erase cycle, and soft program cycle). As a result, the erasing operations can reduce an erase time and reduce wear on single program and erase entities due to the reduced number of cycles. In addition, when single program and erase entities use a high voltage state as an erased state, the single program and erase entities can prevent or mitigate over erase problems. 
       FIGS. 19   a  and  19   b  illustrate tables  1900 ,  1902  depicting exemplary mapping of voltage levels to a one-bit binary value. Single program and erase entities can have a one-bit binary value when single program and erase entities have two discrete target threshold voltages as described in  FIG. 13 . Q 1  represents bit values. In FIG.  19   a , an erased state can be assigned to a binary value “1” and a programmed state can be assigned to a binary value “0.” Arrows in  FIG. 19  illustrate state transitions of single program and erase entities. Transition  1904  represents storing a binary “0” into single program and erase entities. Referring to  FIG. 13 , it can be seen that the transition  1904  represents a change in the charge distribution of single program and erase entities from region  1302  to region  1304 , or an increase in voltage threshold, and transition  1906  represents a change from region  1304  to region  1302 , or a decrease in voltage threshold. 
     In another embodiment of  FIG. 19   b , an erases state can be assigned to a binary value “0” and a programmed state can be assigned to a binary value “1.” Referring to  FIG. 13 , it can be seen that transition  1908  represents a change in the charge distribution of single program and erase entities from region  1302  to region  1304 , or an increase in voltage threshold, and transition  1910  represents a change from region  1304  to region  1302 , or a decrease in voltage threshold. 
       FIG. 20  is a schematic illustration for an exemplary method of programming and erasing single program and erase entities using a low voltage state as an erased state. In this example, single program and erase entities use a low voltage state (e.g., binary value “1”) as an erased state and a high voltage state (e.g., binary value “0”) as a programmed state. All bits of single program and erase entities can be initially binary value “1” or an erased state (State A). Data can be programmed by changing a voltage state of selected single program and erase entities from a low voltage state to a high voltage state. In this example, a hot-electron-injection voltage is applied to SPEE 1 , SPEE 2 , and SPEE 5 , and the data are programmed to “0011 0111” (State B). Then, the programmed single program and erase entities can be erased by applying a hot-hole-injection voltage. In this example, a hot-hole-injection voltage is applied to all of the single program and erase entities, or SPEE 1 , SPEE 2 , and SPEE 5 , and the data are erased to “1111 1111.” 
       FIG. 21  is a schematic illustration for an exemplary method of programming and erasing single program and erase entities using a high voltage state as an erased state. In this example, single program and erase entities use a high voltage state (e.g., binary value “0”) as an erased state and a low voltage state (e.g., logic 1) as a programmed state. All bits of single program and erase entities can be initially binary value “0” or an erased state (State A). Data can be programmed by changing a voltage state of selected single program and erase entities from a high voltage state to a low voltage state. In this example, a hot-hole-injection voltage is applied to SPEE 3 , SPEE 4 , SPEE 6 , SPEE 7 , and SPEE 8 , and the data are programmed to “0011 0111” (State B). Then, the programmed single program and erase entities can be erased by applying a hot-electron-injection voltage. In this example, a hot-electron-injection voltage is applied to all the single program and erase entities or SPEE 3 , SPEE 4 , SPEE 6 , SPEE 7 , and SPEE 8 , and the data are erased to “0000 0000.” 
       FIGS. 22   a  and  22   b  illustrate tables  2200 ,  2202  depicting mapping of voltage levels to a two-bit binary value. In  FIG. 22   a , single program and erase entities can have a two-bit binary value when single program and erase entities have four discrete target threshold voltages as described in  FIG. 14 . Q 1  and Q 2  represent bit values. In one embodiment, an erases state can be assigned to a binary value “11” and programmed states can be assigned to binary values “10” (e.g., Program 1 state), “01” (e.g., Program 2 state), and “00” (e.g., Program 3 state), which correspond to increasingly larger amounts of stored charge. Arrows in  FIG. 22   a  illustrate state transitions of single program and erase entities. Transition  2204  from level 1 to level 2 represents storing a binary value “10” into single program and erase entities. Referring to  FIG. 14 , it can be seen that transition  2204  represents a change in the charge distribution of single program and erase entities from region  1402  to region  1404 , or an increase in voltage threshold. Although not shown in  FIG. 22   a , state transitions can be performed from one level to any other level. For example, a state of single program and erase entities is changed from Program 3 state to any other level including Program 2 state, Program 1 state, and Erase state. 
     In  FIG. 22   b , an erased state can be assigned to a binary value “00” and programmed states can be assigned to binary values “01” (e.g., Program 1 state), “10” (e.g., Program 2 state), and “11” (e.g., Program 3 state), which correspond to decreasingly less amounts of stored charge. Arrows in  FIG. 22   b  illustrate state transitions of single program and erase entities. Transition  2206  from level 4 to level 3 represents storing a binary value “01” into single program and erase entities. Although not shown in  FIG. 22   b , state transitions can be performed from one level to any other level. For example, a state of single program and erase entities is changed from Program 3 state to any other level including Program 2 state, Program 1 state, or Erase state. 
       FIG. 23  is a schematic illustration for an exemplary method of programming and erasing single program and erase entities using a low voltage state as an erased state when the single program and erase entities have a two-bit binary value. Single program and erase entities can use a low voltage state (e.g., binary value “11”) as an erased state and high voltage states (e.g., binary value “10,” “01” and “00”) as a programmed state. All bits of single program and erase entities can be initially binary value “11” or an erased state (State A). Data can be programmed by changing a voltage state of selected single program and erase entities from a low voltage state to high voltage states. In this example, hot-electron-injection voltages are applied to SPEE 1 , SPEE 3 , and SPEE 7 , and the data are programmed to “0011 0111 1111 0111” (State B). Then, the programmed single program and erase entities can be erased by applying hot-hole-injection voltages. In this example, hot-hole-injection voltages are applied to all the single program and erase entities, or SPEE 1 , SPEE 3 , and SPEE 7 , and the data are erased to “1111 1111 1111 1111.” 
       FIG. 24  is a schematic illustration for an exemplary method of programming and erasing single program and erase entities using a high voltage state as an erased state when the single program and erase entities have a two-bit binary value. Single program and erase entities can use a high voltage state (e.g., binary value “00”) as an erased state and low voltage states (e.g., binary value “01,” “10,” and “11”) as a programmed state. All bits of single program and erase entities can be initially binary value “00” or an erased state (State A). Data can be programmed by changing a voltage state of selected single program and erase entities from a high voltage state to a low voltage state. In this example, hot-hole-injection voltages are applied to SPEE 2 , SPEE 3 , SPEE 4 , SPEE 5 , SPEE 6 , SPEE 7 , and SPEE 8 , and the data are programmed to “0011 0111 1111 0111” (State B). Then, the programmed single program and erase entities can be erased by applying hot-electron-injection voltages to all the single program and erase entities or selected single program and erase entities (e.g., SPEE 2 , SPEE 3 , SPEE 4 , SPEE 5 , SPEE 6 , SPEE 7 , and SPEE 8 ), and the data are erased to “0000 0000 0000 0000.” 
     Although not shown, similarly to  FIGS. 19-22 , single program and erase entities having more than four bit binary values can use a low voltage state as an erased state or a high voltage value as an erased state. Physical memory cells of such single program and erase entities can have n levels of Vt distribution of a charge storage node as described for the Vt distribution  1500  in connection with  FIG. 15 . 
       FIG. 25  is a schematic diagram of an exemplary flash device  2500  that contains a plurality of single program and erase entities  2502  using a high voltage state as an erased state. The flash device can contain the plurality of single program and erase entities  2502  in a memory core  2504  and contain one or more decoders  2506 ,  2508  such as a bitline decoder (e.g., Y decoder or column decoder) and a wordline decoder (e.g., X decoder or row decoder) in the same manner as described for the flash device in connection with  FIG. 11 . 
     The decoders  2506 ,  2508  can select one or more single program and erase entities for erasing a portion of or an entire of the core of the flash device on a basis of the single program and erase entity by changing a voltage state of the single program and erase entity to a high voltage state. In one embodiment, the decoders include one or more bitline decoders that facilitate erasing a portion of or an entire of the flash device on the basis of the single program and erase entity with the proviso that a sector erase is not performed in the portion of or the entire of the core of the flash memory. In another embodiment, the decoders include one or more bitline decoders that facilitate erasing a portion of or an entire of the core of the flash device on the basis of the single program and erase entity with the proviso that an erase pulse is not sequentially applied to two or more consecutive bitlines in the portion of or the entire of the core of the flash memory. In yet another embodiment, the decoders select one or more single program and erase entities with the proviso that the decoders do not select bitlines between the single program and erase entities for applying an erase bitline voltage. In still yet another embodiment, for erasing a portion of or an entire of the core of the flash device on a basis of the single program and erase entity, the decoders select one or more single program and erase entities for applying a hot-electron-injection gate voltage to a gate, applying a hot-electron-injection bitline voltage to a common bitline of the single program and erase entity that is shared by the two adjacent dual bit physical memory cells, and connecting at least one of non-common bitlines of the single program and erase entity that are not shared by the two adjacent dual bit physical memory cells to ground. The memory cells in the memory core can include four or more data states, and the single program and erase entity can contain sixteen or more data states. 
       FIG. 26  is a flow diagram of an exemplary methodology for operating a flash device that contains a plurality of single program and erase entities using a high voltage state as an erased state. At  2600 , a portion of or an entire of a memory core of a flash device is erased by changing a voltage state of a single program and erase entity to a high voltage state. At  2602 , the portion of or the entire of the core of the flash device is programmed by changing a voltage state of the single program and erase entity to a low voltage state. The flash device can be erased and then programmed. In another embodiment, the flash device can be programmed and then erased. 
     Flash devices, systems, and methods disclosed herein can include an indicator bit that indicates an erase direction of a low voltage state or a high voltage state. When an indicator cell indicates a low voltage state such as an indicator bit “1” or “11,” an erase operation can be performed by applying a hot-hole-injection voltage to single program and erase entities or an erase cycle. When there are over-erased single program and erase entities, a soft program cycle can be performed after the erase cycle. In another embodiment, when the indicator cell indicates a high voltage state such as an indicator bit “0” or “00,” an erase operation can be performed by applying a hot-electron-injection voltage to single program and erase entities or a pre-program cycle. After a flash device is programmed, the flash device can be erased to either a high voltage state or a low voltage state according to the erase direction indicator bit. As a result, the erase direction indicator bit can reduce erase time and/or reduce a number of cycles (e.g., pre-program cycles, erase cycles, and soft-program cycles), thereby increasing system reliability, efficiency, and or durability. 
     The erase direction (e.g., a state of an indicator bit) can be changed at any suitable time. When an indicator bit is a low voltage state such as logical “1,” single program and erase entities can be programmed by changing their states from a low voltage state (e.g., binary value “1”) to a high voltage state (e.g., binary value “0”). Before erasing the data, the indicator bit can be changed from the low voltage state (e.g., binary value “1”) to a high voltage state (e.g., binary value “0”). Since the erase indicator bit is now the high voltage state, the single program and erase entities can be erased by changing their states to the high voltage state. In another embodiment, when an indicator bit is a high voltage state (e.g., binary value “0”), single program and erase entities can be programmed by changing their states from a high voltage state (e.g., binary value “0”) to a low voltage state (e.g., binary value “1”). Before erasing the data, the indicator bit can be changed from the high voltage state to a low voltage state. Since the erase indicator bit is now the low voltage state, the single program and erase entities can be erased by changing their states to the low voltage state. 
     Single program and erase entities can be read as erased or programmed by comparing binary values of the single program and erase entities with the indicator bit. When the indicator cell indicates a low voltage state such as an indicator bit “1” or “11,” single program and erase entities having the low voltage states can be read as erased and single program and erase entities not having the low voltage state can be read as programmed. When the indicator cell indicates a high voltage state such as an indicator bit “0” or “00,” single program and erase entities having the high voltage state can be read as erased and single program and erase entities not having the high voltage state can be read as programmed. 
       FIG. 27  illustrates a table  2700  for depicting a mapping of an indicator bit to a one-bit binary value. Single program and erase entities can have a one-bit binary value when single program and erase entities have two discrete target threshold voltages as described in  FIG. 13 . When the indicator cell indicates a low voltage state such as an indicator bit “1,” a low voltage state (e.g., binary value “1”) is read as erased and a high voltage state (e.g., binary value “0”) is read as programmed. When the indicator cell indicates a high voltage state such as an indicator bit “0,” a high voltage state (e.g., binary value “0”) is read as erased and a low voltage state (e.g., binary value “1”) is read as programmed. 
       FIG. 28  illustrates programming and erasing single program and erase entities using an erase direction indicator bit. In this example, a single program and erase entity  9  (SPEE 9 ) is an erase direction indicator and contain an indicator bit. Initially, an indicator bit of SPEE 9  can be a low voltage state (e.g., binary value “1”) and all other bits of single program and erase entities (e.g., SPEEs 1 - 8 ) can be initially binary value “1” (State A). Thus, SPEEs 1 - 8  are read as erased. Data can be programmed by changing a voltage state of selected single program and erase entities from a low voltage state (e.g., binary value “1”) to a high voltage state (e.g., binary value “0”). In this example, hot-electron-injection voltages are applied to SPEE 1 , SPEE 2 , and SPEE 5 , and the data are programmed to “0011 0111” (State B). By comparing the stored bits with the indicator bit “1,” binary value “0” of SPEE 1 , SPEE 2 , and SPEE 5  can be read as programmed. After programming and before erasing SPEEs 1 - 8 , the indicator bit can be changed from binary value “1” to binary value “0.” As a result, a binary value “0” is now an erased state. An erase operation on single program and erase entities therefore can be performed by changing their states to a high voltage state (e.g., binary value “0”) or by applying a hot-electron-injection voltage. In this example, SPEEs 1 - 8  are erased by applying a hot-electron-injection voltage, and the data are erased to “0000 0000” (State C). 
     Now the indicator bit of SPEE  9  is a binary value “0” and all other bits of single program and erase entities (e.g., SPEEs 1 - 8 ) are binary value “0” or erased (State C). Thus, data can be programmed by changing a voltage state of selected single program and erase entities from a high voltage state (e.g., binary value “0”) to a low voltage state (e.g., binary value “1”). In this example, a hot-hole-injection voltage is applied to SPEE 3 , SPEE 5 , SPEE 7 , and SPEE 8 , and the data are programmed to “0010 1011” (State D). By comparing the stored bits with the indicator bit “0,” binary value “1” of SPEE 3 , SPEE 5 , SPEE 7 , and SPEE 8  can be read as programmed. After programming and before erasing SPEEs 1 - 8 , the indicator bit can be changed from binary value “0” to binary value “1.” As a result, a binary value “1” then indicates an erased state. An erase operation on single program and erase entities therefore can be performed by changing their states to a low voltage state (e.g., binary value “1”) or by a applying hot-hole-injection voltage. In this example, SPEEs 1 - 8  are erased by applying a hot-hole-injection voltages to SPEE 1 , SPEE 2 , SPEE 4 , and SPEE 6 , and the data are erased to “1111 1111” (State A). 
     Similarly to  FIG. 27 , the subject innovation can contain an erase direction indicator bit when single program and erase entities contain two or more binary bit values. By way of example,  FIG. 29  illustrates a table  2900  for depicting a mapping of an indicator bit to a two-bit binary value. Single program and erase entities can have a two-bit binary value when single program and erase entities have four discrete target threshold voltages as described in  FIG. 14 . In this example, when the indicator cell indicates a low voltage state such as a binary value of “11,” binary values of “11” stored in single program and erase entities are read as erased and other binary values such as “10,” “01,” and “00” are read as programmed. When the indicator cell indicates a high voltage state such as a binary value of “00,” binary values of “00” stored in single program and erase entities are read as erased and other binary values such as “01,” “10,” and “11” are read as programmed. 
       FIG. 30  illustrates programming and erasing single program and erase entities using an erase direction indicator bit when single program and erase entities contain two binary bit values. In this example, a single program and erase entity  9  (SPEE 9 ) is an erase direction indicator and contains an indicator bit. Initially, an indicator bit of SPEE  9  can be binary value “11” and all other bits of single program and erase entities (e.g. SPEEs 1 - 8 ) can be initially binary value “11” (State A). Thus, SPEEs 1 - 8  are read as erased. Next, in this example, a hot-electron-injection voltage is applied to SPEE 1 , SPEE 3 , SPEE 5 , SPEE  6 , SPEE  7 , and SPEE  8 , and the data are programmed to “0011 0111 0001 0110” (State B). After programming and before erasing SPEEs 1 - 8 , the indicator bit can be changed from a binary value of “11” to a binary value “00.” As a result, a binary value “00” now indicates an erased state. In this example, with an erasing operation, SPEEs 1 - 8  are erased by applying a hot-electron-injection voltage to SPEE 2 , SPEE 3 , SPEE 4 , SPEE  6 , SPEE  7 , and SPEE  8 , and the data are erased to “0000 0000 0000 0000” (State C). 
     Now the indicator bit of SPEE  9  is binary value “00” and all other bits of single program and erase entities (e.g., SPEEs 1 - 8 ) are binary value “00” or erased (State C). On a next program operation, a hot-hole-injection voltage is applied to SPEE 2 , SPEE 3 , SPEE 6 , and SPEE 8 , and the data are programmed to “0010 1000 0011 0010” (State D). After programming and before erasing SPEEs 1 - 8 , the indicator bit can be changed from a binary value “00” to a binary value “11.” As a result, a binary value “11” is then an erased state. An erase operation on single program and erase entities therefore can be performed by changing their states to a binary value “11” or by applying hot-hole-injection voltages. In this example, a hot-hole-injection voltage is applied to SPEE 1 , SPEE 2 , SPEE 3 , SPEE 4 , SPEE 5 , SPEE 7  and SPEE 8 , and the data are erased to “1111 1111 1111 1111” (State A). At any suitable time between the program operations and the erase operations, the stored data can be read by comparing the stored bits with the indicator bit. 
       FIG. 31  is a schematic diagram of an exemplary flash device  3100  that contains a plurality of single program and erase entities  3102  and an indicator cell  3104  in a memory core  3106 . The flash device can contain the plurality of single program and erase entities and one or more decoders  3108 ,  3110  in the same manner as described for the flash memory  1100  in connection with  FIG. 11 . The indicator cell  3104  can contain an indicator bit as an erase direction of a low voltage state or a high voltage state. The indicator cell can be a single program and erase entity. The indicator cell can be programmed to indicate the low voltage state or the high voltage state as erase state. The indicator cell can exist at any suitable place. For example, the indicator cell is in the same memory core containing the single program and erase entities, as described in  FIG. 31 . When the indicator cell is in a row of single program and erase entities, the indicator cell can control an erase state of single program and erase entities in that row. Although not shown in  FIG. 31 , in another embodiment, the indicator cell exists in a core that is different from a core that contains a plurality of single program and erase entities. 
     The decoders  3108 ,  3110  can select one or more single program and erase entities for operating the flash device on a basis of the single program and erase entity by changing a voltage state of a single program and erase entity to a low voltage state or a high voltage state according to the erase direction. In one embodiment, the decoders select one or more single program and erase entities for erasing the flash device by applying a hot-hole-injection voltage to the single program and erase entity when the indicator cell indicates a low voltage state or by applying a hot-electron-injection voltage to the single program and erase entity when the indicator cell indicates a high voltage state. In another embodiment, the decoders select one or more single program and erase entities for programming the flash device by applying a hot election injection voltage to the single program and erase entity when the indicator cell indicates a low voltage state or by applying a hot-hole-injection voltage to the single program and erase entity when the indicator cell indicates a high voltage state. In yet another embodiment, after the indicator cell indicates a first erase direction and any operation of the flash device (e.g., erasing, programming, and/or reading) is performed with the first erase direction, the indicator cell indicates a second erase direction that is opposite to the first erase direction, and another operation of the flash device is performed with the second erase direction. 
       FIG. 32  is a flow diagram of an exemplary methodology for operating a flash device that contains a plurality of single program and erase entities using an erase direction. At  3200 , a first erase direction of a low voltage state or a high voltage state can be indicated. At  3202 , a flash memory device can be erased by changing a voltage state of a single program and erase entity in the first erase direction. At  3204 , the flash memory device can be programmed by changing a voltage state of the single program and erase entity in a second direction opposite to the first erase direction. In one embodiment, the flash memory device is read by comparing the voltage state of the single program and erase entity with the erase direction. An erase operation, a program operation, a read operation, or combinations thereof can be performed in any suitable order and any suitable number of times. For example, a first erase direction is indicated and an erase operation and a program operation are performed. And then, a second erase direction is indicated and another erase operation and program operation are performed. 
       FIG. 33  is a flow diagram of another exemplary methodology for operating a flash device that contains a plurality of single program and erase entities using an erase direction. At  3300 , a portion of or an entire of the flash memory is programmed on a basis of a single program and erase entity to a voltage state that is different from a first voltage state of an erase direction indicator cell. At  3302 , the first voltage state of the erase direction indicator cell is changed to a second voltage state. At  3304 , the portion of or the entire of the flash memory is erased on the basis of the single program and erase entity to a voltage state that is the same as the second voltage state of the erase direction indicator cell. Erasing the portion of or the entire of the flash memory on the basis of the single program and erase entity can involve neutralizing a charge in the single program and erase entity when the erase direction indicator cell indicates a low voltage state as an erased state, or trapping a charge in the single program and erase entity when the erase direction indicator cell indicates a high voltage state as an erased state. The method can further involve programming the portion of or the entire of the flash memory on the basis of the single program and erase entity to a voltage state that is different from a second voltage state of the erase direction indicator cell. 
       FIG. 34  illustrates another exemplary flash device  3400  that emulates an EEPROM features or functionalities (e.g., byte alterability) in a flash device using single program and erase entities  3402 . The flash device can contain a plurality of single program and erase entities  3402  in a core  3404  and one or more decoders such as a bitline decoder (e.g., Y decoder or column decoder)  3406  and a wordline decoder (e.g., X decoder or row decoder)  3408  in the same manner as described for the flash device  1100  in connection with  FIG. 11 . 
     For emulating EEPROM in a flash device, any suitable decoder (e.g., Y decoder, bitline decoder, or column decoder, and X decoder, wordline decoder, or row decoder) can be employed as long as the decoder can select one or more single program and erase entities for access. While emulating EEPROM in a flash memory, the decoders do not need to operate a sector erase. 
     The decoders can select one or more single program and erase entities in response to received addresses. The bitline decoder can select one or more columns and the wordline decoder can select one or more rows. The decoders can select one or more addresses by any suitable technique. In one embodiment, a table (e.g., a lookup table) is employed to implement decoders (e.g., bitline decoders). The table can drive a register-transfer-level (RTL) description to implement decoders (e.g., bitline decoders). The bitline decoders can include a table-driven decoder. 
       FIG. 35  is a schematic illustration for an exemplary method of emulating byte alterability in a flash device using single program and erase entities. As disclosed in connection with  FIGS. 17 and 18 , the subject innovation can allow changing a voltage state of a single program and erase entity on a basis of a single program and erase entity, not a sector basis. Since the high voltage state and low voltage state in a single program and erase entity can be changed alternately (e.g., programmed or erased) without a sector erase, the subject innovation can emulate byte alterability (e.g., EEPROM byte alterability) in a flash device. 
     By way of illustration, in  FIG. 35 , single program and erase entities use a high voltage state (e.g., binary value “0”) as an erased state and a low voltage state (e.g., binary value “1”) as a programmed state. A row of single program and erase entities (SPEEs 1 - 8 ) can contain data “0010 1011” (State A). The data can be re-written to “0011 0111” (State B). The data can be a user data. The data of State B can be re-programmed to “1111 0111” (State C), which can be in a program data phase. Then, the data of State C can be re-written to an erase data phase “1111 0111” (State D). Finally, the data of State D can be re-written to a final data such as a user data “0011 0111” (State E). 
     Although not shown in  FIG. 35 , charge storage nodes of single program and erase entities can have any suitable number of levels as described in connection with  FIGS. 13 ,  14 , and  15 . For example, the single program and erase entities can be programmed to four levels as described in connection with  FIG. 14 . This can facilitate emulating byte alterability using four-bits-per-entity. 
       FIG. 36  is a flow diagram of an exemplary methodology of emulating byte alterability using a flash device based on a single program and erase entity. The method can be used as EEPROM emulation. At  3600 , a voltage state in a single program and erase entity can be changed to a first voltage state (e.g., a high voltage state or a low voltage state) on a basis of a single program and erase entity. At  3602 , the voltage state in the single program and erase entity can be changed to a second voltage state that is different from the first voltage state. Although not shown in  FIG. 36 , the voltage state in the single program and erase entity can be change by applying a hot-electron-injection voltage or a hot-hole-injection voltage to a portion of or an entire of the flash memory on the basis of the single program and erase entity. 
     The single program and erase entity includes two adjacent dual bit physical memory cells of the flash device as a single logical cell. Thus, in one embodiment, the voltage state in the single program and erase entity can be changed to a high voltage state or a low voltage by applying a hot-electron-injection gate voltage to a gate, applying a hot-electron-injection bitline voltage to a common bitline of the single program and erase entity that is shared by the two adjacent dual bit physical memory cells, and connecting non-common bitlines of the single program and erase entity that are not shared by the two adjacent dual bit physical memory cells to ground, or applying a hot-hole-injection gate voltage to a gate, applying a hot-electron-injection bitline voltage to a common bitline of the single program and erase entity that is shared by the two adjacent dual bit physical memory cells, and allowing non-common bitlines of the single program and erase entity that are not shared by the two adjacent dual bit physical memory cells to float. In another embodiment, the methodology in  FIG. 36  further involves erasing the flash device by changing the voltage state to a predetermined voltage state on the basis of the single program and erase entity. For example, prior to programming a flash device and/or after programming a flash device, the flash device can be erased on the basis of the single program and erase entity to a predetermined voltage state. The flash device can be erased by applying an erase pulse to a portion of or an entire of the flash memory on the basis of the single program and erase entity. 
       FIG. 37  is a top view of another exemplary flash device  3700  containing sector configure resisters  3702 . The memory device  3700  includes one or more memory cores (e.g., memory cell arrays)  3704 , one or more sector configure resisters  3702 , one or more input/output (I/O) registers  3706 , one or more bitline decoders  3708 , and one or more wordline decoders  3710 . The memory cell array  3704  includes a plurality of memory cells arranged in an ordered array of rows and columns in the same manner as described for the memory core  104  in connection with  FIG. 1 . 
     A flash device can include a combination of two different storage schemes or methodologies using one or more sector configure registers. The combination can share the same support logic in a flash device. In one embodiment, a flash device contains a combination of a conventional dual bit decoding scheme with a single program and erase entity decoding scheme. The conventional dual bit decoding scheme can be employed in a dual bit memory device for high density storage. The single program and erase entity decoding scheme can be employed for emulating EEPROM functionality in a flash device, as described in connection with  FIGS. 34-36 . Thus, the combination of the dual bit decoding scheme and the single program and erase entity decoding scheme can provide both dual bit high density storage and EEPROM emulation in a single flash device. 
     The single program and erase entity decoding scheme employs a single program and erase entity as a single logical cell. In the single program and erase entity decoding scheme, the single program and erase entities are programmed and erased to a high voltage state or a low voltage state as a single logical cell. The dual bit decoding scheme is a conventional decoding scheme employed for storing data in a conventional flash device. Details of the conventional dual bit decoding scheme are not critical to the subject innovation. The details of the conventional dual bit decoding scheme can be found in, for example, commonly-assigned U.S. Pat. No. 7,130,210 issued Oct. 31, 2006, which is hereby incorporated by reference. In the dual bit decoding scheme, the dual bit memory cells are typically programmed to a high voltage state and are erased to a low voltage state on a sector basis. 
     The memory core  3704  of the flash device  3700  can contain two regions for the two decoding schemes. Each region can contain one or more sectors, independently. A first region of the memory core can be selected for the single program and erase entity decoding scheme (e.g., EEPROM emulation), and a second region of the memory core can be selected for the dual bit decoding scheme (e.g., conventional storage). Selection from the conventional storage and the EEPROM emulation in a flash memory can be achieved by using one or more sector configure resisters  3702 . The sector configure resister can store data related to selection of virtual ground mapping of the memory array. The configure registers form register means for storing selection information or data. For example, the sector configure registers store performance variable data for operating the first region on the basis of the single program and erase entity and/or performance variable data for operating the second region in the dual bit decoding scheme. 
     Selection information and data stored in the sector configure registers  3702  can be provided to the memory device  3700  using the I/O register  3706 . Contents of the sector configure register  3702  can serve as inputs to a state machine (not shown) which controls various operations (e.g., read, erase and programming) of the memory device  3700 . The state machine can perform embedded operations based on the input from the sector configure register  3702  to complete reading, erasing and programming automatically without user interaction. 
     In the first region performing EEPROM emulation functionality, various operations can be performed on a byte or variable length basis. As described above, single program and erase entities can be operated on a byte or variable length basis. For example, program and erase operations can be performed on a byte or variable length basis. In the second region performing dual bit storage functionality, erase operations and/or program operations are typically performed on a sector or block basis. A block can be any suitable size, such as 16 rows by 256 words per row. 
     Sector configure registers  3702  can include any suitable register, data storage circuit, or the like. In one embodiment, the sector configure registers are delay registers or D-type flip flops. In another embodiment, any suitable data storage circuit can be fashioned to provide the functionality of the sector configure registers. For example, a master-slave flip flop can be substituted, or the sector configure registers can be provided with other operational features such as hardware set. In yet another embodiment, the sector configure registers are combined with combinatorial logic, timing or clocking signals, or stored data to provide added flexibility. 
     The flash device  3700  can contain one or more first decoders for the first region and one or more second decoders for the second region. The first decoders can select one or more single program and erase entities in the first region for operating the single program and erase entities in accordance with the single program and erase entity decoding scheme. The second decoders can select one or more dual bit memory cells for performing, for example, a sector erase, in accordance with the dual bit decoding scheme. The first and second decoders can be any suitable decoder. For example, the first decoders are any of the decoders as described above in connection with  FIG. 25 . 
       FIG. 38  is a schematic illustration a portion of a memory core  3800  containing multiple virtual ground decoding schemes (e.g., First Sector Group  3802  and Second Sector Group  3804 ). In this embodiment, the memory core  3800  contains two ground decoding schemes of a single program and erase entity decoding scheme in the first sector group  3802  and a conventional dual bit decoding scheme in the second sector group  3804 . 
     In the first sector group  3802 , various operations can be performed on a basis of the single program and erase entity on a single bit or variable bit length basis. As disclosed above, in the single program and erase entity decoding scheme, the single program and erase entities can be operated (e.g., program, erase, and read) as a single logical cell. In the second sector group  3804 , physical memory cells  3808  are used for various operations as a single logical cell. Erase operations and/or program operations are typically performed on a sector or block basis in the second sector group. 
     Each of the two sector groups  3802 ,  3804  can contain one or more sectors. The memory core  3800  can contain any suitable number of sectors for the conventional dual bit decoding scheme and for the single program and erase entity decoding scheme. In one embodiment, about 1% of sectors or more and about 100% of sectors or less in a memory core are associated with a dual bit decoding scheme and about 0% of sectors or more and about 99% of sectors are associated with single program and erase entity decoding scheme. In another embodiment, about 2% of sectors or more and about 90% of sectors or less in a memory core are associated with a dual bit decoding scheme and about 10% of sectors or more and about 98% of sectors are associated with single program and erase entity decoding scheme. In yet another embodiment, about 5% of sectors or more and about 80% of sectors or less in a memory core are associated with a dual bit decoding scheme and about 20% of sectors or more and about 95% of sectors or less are associated with single program and erase entity decoding scheme. 
       FIG. 39  is a flow diagram of an exemplary methodology for operating a flash memory device containing multiple virtual ground decoding schemes. At  3900 , a decoding scheme is selected for a first region and a second region of the flash memory device. The first region and the second region can be selected by one or more sector configure registers. A single program and erase entity decoding scheme can be selected for the first region and a dual bit decoding scheme can be selected for the second region. At  3902 , the first region is operated on a basis of a single program and erase entity. In the single program and erase entity decoding scheme, the first region of the flash device can be programmed and erased on the basis of the single program and erase entity. At  3904 , the second region can be operated in a conventional dual bit decoding scheme. The second region can be programmed and/or erased on a sector basis. 
     What has been described above includes examples of the disclosed innovation. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the disclosed innovation, but one of ordinary skill in the art can recognize that many further combinations and permutations of the disclosed innovation are possible. Accordingly, the disclosed innovation is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term “contain,” “includes,” “has,” “involve,” or variants thereof is used in either the detailed description or the claims, such term can be inclusive in a manner similar to the term “comprising” as “comprising” is interpreted when employed as a transitional word in a claim. 
     INDUSTRIAL APPLICABILITY 
     The systems, structures, and methods described herein are useful in the field of flash device manufacturing and useful in operating flash devices.