Patent Publication Number: US-7718492-B2

Title: Non-volatile memory cell circuit with programming through band-to-band tunneling and impact ionization gate current

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Cross-reference is made to the following related patent applications which are commonly owned by the same assignee as the present application: 
     The present application is a continuation of U.S. patent application Ser. No. 11/601,305 filed Nov. 16, 2006 (now U.S. Pat. No. 7,508,719 (issued Mar. 24, 2009)), in the name of inventor Andrew E. Horch and entitled “NON VOLATILE MEMORY CELL CIRCUIT WITH PROGRAMMING THROUGH BAND-TO-BAND TUNNELING AND IMPACT IONIZATION GATE CURRENT” which, in turn, claims the benefit of priority from U.S. Provisional Patent Application No. 60/839,771 filed on Aug. 24, 2006 in the name of the same inventor. 
     U.S. patent application Ser. No. 11/601,474 filed Nov. 16, 2006 (now U.S. Pat. No. 7,474,568 (Issued Jan. 6, 2009)) in the name of Andrew E. Horch and entitled “NON VOLATILE MEMORY WITH PROGRAMMING THROUGH BAND-TO-BAND TUNNELING AND IMPACT IONIZATION GATE CURRENT” may be found pertinent to this disclosure. 
     U.S. patent application Ser. No. 12/080,127 filed Mar. 31, 2008 (now U.S. Pat. No. 7,652,921 (Issued Jan. 26, 2010)) in the name of Andrew E. Horch and Bin Wang and entitled “MULTI-LEVEL NON-VOLATILE MEMORY CELL WITH HIGH-VT ENHANCED BTBT DEVICE” may also be found pertinent to this disclosure. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present description is related to the field of non-volatile memory, and, more specifically, to a non-volatile memory with programming through band-to-band tunneling and impact ionization gate current. 
     2. Description of the Related Art 
       FIG. 1  shows a prior art circuit design for a non-volatile memory cell. According to the prior art circuit observed in  FIG. 1 , the amount of charge residing on a floating gate  101  determines whether the cell is storing a logical “1” or a logical “0”. The floating gate  101  is referred to as “floating” because it is not coupled to the typical output end of a transistor (e.g., a source or drain node). 
     In operation, in order to “program” the memory cell to a first logical state, after turning transistor Q 3  “on” from SELECT line  104 , high energy electrons are added to the floating gate  101  from transistor Q 1 . Here, the WELL/SOURCE voltage  102  is set to a voltage that causes a substantial V SD  voltage drop from the source to the drain of the Q 1  transistor. For instance, according to one approach, the voltage of the WELL/SOURCE node  102  is set to a large positive voltage 2VDD where VDD is a DC supply voltage creating a large voltage drop across transistor Q 1 . This large voltage drop corresponds to the establishment of a high intensity electric field running from the source of transistor Q 1  to the drain of transistor Q 1 . 
     Transistor Q 1  is also “on” in the sense that the charge level on floating gate  101  corresponds to a floating gate  101  voltage that is sufficiently below V S −V T  (where V S  is the source voltage of transistor Q 1 =2VDD and V T  is the threshold or “turn-on” voltage of transistor Q 1 ). When transistor Q 1  is sufficiently turned “on”, a conductive channel is established within the substrate portion of transistor Q 1  just beneath its gate dielectric. 
     In transporting the current within transistor Q 1 , holes within transistor Q 1 &#39;s conductive channel flow from Q 1 &#39;s source to drain and, in so doing, are accelerated to very high velocities by the high intensity electric field. These high energy holes collide with the semiconductor substrate lattice from which transistor Q 1  is constructed. The collisions with the lattice create high energy electrons that have enough energy to surmount transistor Q 1 &#39;s gate dielectric energy barrier and travel to the floating gate  101  where they are collected. The collection of electrons on the floating gate  101  lowers the charge on the floating gate beneath some threshold which corresponds to a first logical state (e.g., a logical 1 or 0). 
     In order to “erase” the memory cell to a second logical state, electrons are removed from the floating gate  101  so as to effectively increase the charge on the floating gate  101  beyond a second threshold which corresponds to a second logical state. Here, electrons are tunneled by a Fowler-Nordheim tunneling mechanism from the floating gate  101  into the ERASE node  103  (i.e., the semiconductor substrate portion of transistor Q 2 ). In the case of Fowler-Nordheim tunneling, a large positive voltage is placed on the ERASE node  103  so as to create a strong electric field that runs from the semiconductor substrate portion of transistor Q 2  to the floating gate  101 . This large positive voltage causes the energy band structure of the gate dielectric of transistor Q 2  to resemble a sharp spike which promotes the tunneling of electrons through the energy barrier that resides between the floating gate  101  and the semiconductor substrate portion of transistor Q 2 . 
     Thus, the ability to accurately control the logical state being held by the memory cell corresponds to the ability to control the amount of high energy electrons that are injected onto the floating gate  101  from the substrate portion of transistor Q 1  during a PROGRAM phase and the ability to accurately control the amount of electrons that tunnel from the floating gate  101  into the substrate portion of transistor Q 2  during an ERASE phase. With the ever decreasing size of transistor dimensions (e.g., in terms of gate length and gate dielectric thickness), however, the ability to control these transport mechanisms is proving to be increasingly difficult. 
     For instance, according to one incorrect realm of operation, “too many” electrons are tunneled into the substrate portion of transistor Q 2  during an ERASE phase resulting in the relative charge level of the floating gate  101  rising “too high” (e.g., at or too near V S −V T ). In this case, transistor Q 1  is not sufficiently “on” and, as a consequence, an insufficient amount of high energy electrons are created and injected onto the floating gate  101 . The result is that the first logical state cannot be reached in time. 
       FIG. 2  depicts the problem graphically.  FIG. 2  depicts the flow of high energy electrons onto the floating gate  101  during a PROGRAM phase (referred to as “impact ionization induced injection current”) as a function of the charge that exists on the floating gate  101  (in terms of the difference between the charge level&#39;s corresponding floating gate voltage and the source node voltage of transistor Q 1  (“normalized |V GS |”)). When the charge level is sufficiently low during the PROGRAM phase, the floating gate  101  voltage is sufficiently below V S -V T  which corresponds to region  201  of  FIG. 2 . The result is a sufficiently large, impact ionization induced injection current onto the floating gate that can reduce the floating gate  101  voltage to the first logical state during the PROGRAM phase. 
     However, if the charge level on the floating gate rises because too many electrons are tunneled off the floating gate  101  during an ERASE phase, the region of operation for a following PROGRAM phase is to the right of region  201  where a roll-off  202  in the curve is observed. This roll-off  202  corresponds to a drop in the impact ionization induced current that is injected to the floating gate during a PROGRAM phase that results from little or no I DS  current flowing through transistor Q 1  because of the large floating gate voltage (said another way, transistor Q 1  is not sufficiently “on” under such conditions). Thus, if the charge level on the floating gate rises too high from the ERASE phase, region  202  will be reached which corresponds to an impact ionization induced injection current magnitude that is too small to drop the floating gate charge level to the first logical state during the PROGRAM phase. 
     BRIEF SUMMARY 
     The present description gives instances of electronic circuitry having a first transistor having a first gate dielectric located between an electrically floating gate and a semiconductor substrate. The first injection current flows through the first gate dielectric to establish a first amount of electrical charge on the gate electrode. The electronic circuitry also includes a second transistor having a second gate dielectric located between the gate electrode and the semiconductor substrate. A band-to-band tunneling current flows between valence and conduction bands of the second transistor to create a second injection current that flows through the second gate dielectric to establish the first amount of electrical charge on the gate electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of this description will become more readily apparent from the following Detailed Description, which proceeds with reference to the drawings, in which: 
         FIG. 1  shows a prior art non-volatile memory circuit; 
         FIG. 2  shows a graph of a transistor&#39;s gate current resulting from impact ionization within the transistor&#39;s conductive channel; 
         FIG. 3A  shows an improved floating gate design capable of using both impact ionization current and band-to-band tunneling current to cause the floating gate to reach an amount of charge; 
         FIG. 3B  shows one embodiment of a graph of the gate electrode current for the circuit of  FIG. 3A  that demonstrates the contribution to the gate electrode current from both an impact ionization current component and a band-to-band tunneling current component; 
         FIG. 4  shows energy band diagrams for the respective transistors Q 1  and Q 2  of  FIG. 3A ; 
         FIG. 5  shows doping profiles for transistors Q 1  and Q 2  of  FIG. 3A ; 
         FIG. 6  depicts a method of operation of the circuit of  FIG. 3A ; 
         FIG. 7  depicts another method of operation of the circuit of  FIG. 3A ; 
         FIG. 8A  depicts a first non-volatile memory cell circuit utilizing the design of  FIG. 3A ; 
         FIG. 8B  depicts a second non-volatile memory cell circuit utilizing the design of  FIG. 3A ; 
         FIG. 8C  depicts a third non-volatile memory cell circuit utilizing the design of  FIG. 3A ; 
         FIG. 9  depicts the circuit of  FIG. 3A  implemented with n type transistors; 
         FIG. 10  depicts a non-volatile memory cell circuit utilizing the design of  FIG. 9 ; 
         FIG. 11  depicts a differential version of the non-volatile memory cell approach described herein; and 
         FIG. 12  depicts a method that can be executed by any of the non-volatile memory cell circuits illustrated in  FIGS. 8A ,  8 B,  8 C and  10 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3A  shows a circuit diagram for an improved non-volatile memory cell that avoids the problems described above with respect to the prior art circuit of  FIG. 1 .  FIG. 3B  graphically depicts a possible design point for the circuit of  FIG. 3A  that is directly comparable to  FIG. 2 . 
     Referring to  FIG. 3A , transistor Q 1  and the Fowler-Nordheim tunneling transistor Q 3  (noting that reference Q 2  was used for the tunneling transistor with reference to  FIG. 1 ) operate as described above with respect to  FIG. 1 . That is, during a PROGRAM phase, high energy electrons (created from lattice collisions with carriers accelerated in Q 1 &#39;s conductive channel) are supposed to be injected to the floating gate  301  from the substrate portion of transistor Q 1  thereby lowering the floating gate  301  charge level and corresponding voltage to a first logical state. Moreover, during an ERASE phase, electrons are removed from the floating gate  301  by Fowler-Nordheim tunneling through the gate dielectric of transistor Q 3  so as to raise the charge level and corresponding voltage on the floating gate  301  to a second logical state. 
     If too many electrons are removed from the floating gate  301  during the ERASE phase so as to raise the floating gate&#39;s charge and corresponding voltage to too high a level to permit transistor Q 1  to adequately supply high energy electrons to the floating gate  301  during a subsequent PROGRAM phase, the floating gate&#39;s charge and voltage will nevertheless be properly lowered during the subsequent PROGRAM phase because transistor Q 2  will supply electrons to the floating gate  301  even though it may be subject to the same gate, source and drain node conditions as transistor Q 1 . Note that transistor Q 2 , similar to transistor Q 1 , uses the floating gate  301  at its own gate node thereby preserving the floating gate node&#39;s status as “floating” because it is not driven by a node of a transistor that is typically used as an output. 
     Here, transistor Q 2  is specifically designed to be “different” than transistor Q 1  so that it will inject current into the floating gate  201  by way of a different physical mechanism than transistor Q 1 . As a consequence, transistor Q 2  is capable of injecting electrons onto the floating gate  201  even though transistor Q 1  is not (e.g., in the case where a previous ERASE phase raised the voltage on the floating gate too high to sufficiently turn on transistor Q 1 ). 
       FIG. 3B  demonstrates the approach. Essentially,  FIG. 3B  shows that the total floating gate node current (represented as a solid line) is the summation of gate injection currents contributed by transistor Q 1  (represented as a “dash-dot” line) and transistor Q 2  (represented as a dotted line). Here, because transistor Q 2  injects electrons onto the floating gate  301  by way of a different physical mechanism than transistor Q 1 , its gate current curve is fundamentally different than the gate current curve of transistor Q 1 . This effectively corresponds to  FIG. 3B  depicting the addition of a new and different curve relative to the curve originally discussed with respect to  FIG. 2 . 
     The curve for transistor Q 2  shows increasing gate current, rather than a roll-off, moving to the right along the horizontal axis. Thus, according to the approach depicted in  FIG. 3B , transistor Q 2  effectively compensates for transistor Q 1  in that it increasingly contributes electrons to the floating gate as the region of operation moves to the right beyond region  311  where the curve for transistor Q 1  is observed to roll off. Better said, according to the approach of  FIG. 3B , when transistor Q 1  can no longer supply sufficient numbers of electrons to the floating gate (because the floating gate voltage is too high), transistor Q 2  “steps in” and provides such electrons instead. 
     Level  303  of  FIG. 3B  simply corresponds to a preferred level of total electron flow into the floating gate needed to properly lower the voltage of the floating gate node to the first logical state. Note that the solid curve representing the contribution of both transistors Q 1  and Q 2  is always at or above this level  303  for all operating regions of the memory cell. Here, note in particular the dramatic increase in electron flow at high floating gate node voltages for the circuit of  FIG. 3A  as compared to the circuit of  FIG. 1  as seen by comparing region  312  of  FIG. 3B  with region  212  of  FIG. 2 . 
     With an understanding that sufficient electron flow into the floating gate over a wider span of operating regions can be achieved through the introduction of a transistor Q 2  to the floating gate node that injects electrons to the floating gate by way of a different physical mechanism than transistor Q 1 , some discussion of the physical mechanism is appropriate. 
     Referring to  FIG. 3A  note that transistor Q 2  is tied to the WELL/SOURCE node  302  in the same manner as transistor Q 1 . Also recall from the discussion of  FIG. 1  that during the PROGRAM phase the WELL/SOURCE node  302  is set to a substantially high positive voltage (e.g., 2VDD) in order to establish a very strong electric field that runs from the source node of transistor Q 1  to the drain node of transistor Q 1 . According to the operation of the circuit of  FIG. 3A , the substantial positive voltage applied at the WELL/SOURCE node  302  during a PROGRAM phase results in a high intensity electric field that runs from the floating gate node  301  and terminates in the semiconductor substrate of transistor Q 2  (at least for very high floating gate voltages). 
     In alternate implementations in order to increase the strength of this electric field, the well (and source) nodes of transistor Q 2  may be set to a voltage that is less than 2VDD (e.g., to ground or any voltage between ground and 2VDD). In this case, the well and source nodes of transistors Q 1  and Q 2  are respectively driven to different voltages (e.g., 2VDD for the source and well nodes for transistor Q 1  and ground for the well and source nodes of transistor Q 2 ) and therefore require different DC drive circuits to drive them to their respective voltages. Also, like transistor Q 1 , transistor Q 2  may be technically “off” because the voltage of the floating gate node  301  is too large with respect to the source so as to prevent the formation of a conductive channel from the source node. 
     Referring now to  FIG. 4 , which shows the energy band diagram of transistor Q 2 , the high intensity electric field results in severe energy band bending in the N-Well of transistor Q 2  that results in “band-to-band” tunneling in the diode of drain and N-Well. According to the band-to-band tunneling mechanism, because of the influence of the high intensity electric field, electrons in the valence band tunnel into the conduction band and the holes left behind travel to the drain. The electrons are accelerated by the lateral electric field toward the channel region and may gain enough energy to travel across the Si0 2  barrier to the floating gate. 
     The high intensity electric field results in severe energy band bending in the semiconductor substrate of transistor Q 2  that results in “band-to-band” tunneling in the semiconductor substrate of transistor Q 2 . According to this band-to-band tunneling mechanism, because of the influence of the high intensity electric field, electrons in the valence band tunnel toward the floating gate and into the conduction band. The tunneling electrons create holes in the valence band which, also under the influence of the high intensity electric field, accelerate rapidly away from the gate dielectric of transistor Q 2  and deeper into the semiconductor substrate. These high energy holes collide with the semiconductor lattice and create high energy electrons that have sufficient energy to cross over the energy barrier between the semiconductor substrate and the floating gate node and therefore flow into the floating gate node where they are collected. Thus, in this manner, transistor Q 2  injects high energy electrons onto the floating gate. 
     According to a further design strategy, transistor Q 2  is purposely designed to have a higher intensity electric field beneath and near the edges of its gate than transistor Q 1  owing to the presence, for example, of more acceptor dopant atoms at least near the drain regions of transistor Q 2  as compared to transistor Q 1 . The presence of more acceptor atoms in the drain regions of transistor Q 2  results in the flux lines of the high intensity electric field flux in transistor Q 2  being terminated closer to the surface of the substrate beneath the gate dielectric in transistor Q 2  than in the drain regions of transistor Q 1  (which essentially means the gate-drain electric field is stronger in transistor Q 2  than in transistor Q 1 ). 
     The result is more severe energy band bending in the drain regions of transistor Q 2  toward the gate than in transistor Q 1 ; which, in turn, corresponds to the presence of substantial band-to-band tunneling within the drain regions of transistor Q 2  but not within transistor Q 1 . Again, note that like transistor Q 1 , transistor Q 2  may still be “off” in the sense that no conductive channel is created in the source of transistor Q 2  because the voltage on the floating gate  301  is too high with respect to the voltage of the source of transistor Q 2 . The carriers produced by band-to-band tunneling that are collected by the drain are sometimes called GIDL (Gate Induced Drain Leakage). 
       FIG. 5  depicts an example of how transistor Q 2  may be made to have more dopant atoms than transistor Q 1 . Here, transistor Q 2  has more features in its dopant profile than transistor Q 1  as depicted by the presence of source/drain extensions  504 _ 2  and halos  505 _ 2  in transistor Q 2  but not in transistor Q 1 . According to one embodiment, the non-volatile memory cell is constructed with “I/O” transistors, which are different than the core logic transistors of the semiconductor die in which the non-volatile memory cell is constructed. Here, the I/O transistors have a thicker gate dielectric than the core logic transistors because the I/O transistors are expected to handle larger voltages associated with driving/receiving signals off/on the semiconductor die. 
     The non-volatile memory cell is constructed from these same I/O transistors because of the large voltages used at the WELL/SOURCE  302  and TUNNEL  303  nodes to induce the respective high energy electron injection and Fowler-Nordheim tunneling mechanisms. According to known prior art manufacturing processes, however, elaborate dopant profiles such as source/drain extensions, lightly doped drains and halos are not performed in the manufacture of I/O transistors, but rather, are used only in the manufacture of core logic transistors. 
     As is known in the art, halo implants  505 _ 2  have been used to prevent “punch-through” of the gate dielectric  502 _ 2  thereby permitting shorter channel regions in the core logic transistors and source/drain extensions  504 _ 2  have been used to reduce resistance in the substrate region below the gate spacer. Due to the lower operating voltages of the core logic transistors, these devices can tolerate a more highly doped channel and source/drain extension. Both source/drain (S/D) extension and lightly doped drain (LDD) implants are below the spacer. LDD is called LDD because the dopant concentration is significantly lower that of the S/D, while a S/D extension is approximately the same (e.g., LDD dopant concentration ˜1 e9 atoms/cm 3 ; S/D extension dopant concentration=source drain region dopant concentration ˜1 e20 atoms/cm 3 ). 
     Thus, one approach is to construct transistor Q 2 , which may be an I/O transistor in perhaps all other respects, except that it has one or more of the complex doping features traditionally implemented only into core logic transistors (such as source/drain extensions, lightly doped drains and halos). Alternatively or in combination, the density of dopant acceptor atoms (e.g., in the source/drain regions) may simply be made higher in transistor Q 2  than in transistor Q 1  (e.g., with just a higher core logic transistor threshold voltage implant). 
       FIGS. 6 and 7  illustrate methodologies of operation consistent with the discussion provided above. First, according to the methodology of  FIG. 6 , electrical charge may be written to a floating gate by injecting both a first injection current between a floating gate and a first transistor&#39;s semiconductor substrate region and a second injection current between the floating gate a second transistor&#39;s semiconductor substrate where the carriers of the second injection current are created by carriers created from a band-to-band tunneling current within the second transistor&#39;s semiconductor substrate  601 . Note that the methodology of  FIG. 6  covers those situations where both types of injection currents are simultaneously acting together to add electrons to the floating gate during a same PROGRAM phase (e.g., the inflection point in  FIG. 3  at level  303  where the two curves from the two different injection mechanisms overlap) as well as those situations where one type of injection current is used during one PROGRAM phase and the other type of injection current is used during another PROGRAM phase. 
       FIG. 7  elaborates on an example of the latter where injection during a first PROGRAM phase results from the injection of high energy electrons that were given enough energy to cross the gate dielectric energy barrier of a first transistor from collisions between the first transistor&#39;s substrate lattice and carriers accelerated in the first transistor&#39;s conductive channel (e.g., as discussed at length above with respect to transistor Q 1 )  701 . Then, during a subsequent ERASE phase too many electrons are removed from the floating gate resulting in a floating gate node voltage that is too high for the type of injection that occurred during the first write phase to occur  702 . Thus, during a second PROGRAM phase that follows the ERASE phase, high energy electrons are injected onto the floating gate that were given enough energy to surmount the gate dielectric energy barrier of a second transistor by high energy carriers created by a band-to-band tunneling current within the second transistor  703 . 
       FIGS. 8A through 8C  show different implementations of memory cell circuits that employ p type transistors as observed in  FIG. 3A . In the memory cell designs of  FIGS. 8A through 8C , transistors Q 1 , Q 2  and Q 3  operate as described above. Also, in each of  FIGS. 8A through 8C , the ERASE node  803  is driven by circuitry that creates a large positive voltage to attract electrons through Fowler-Nordheim tunneling through the gate dielectric of transistor Q 3 . This circuitry may include a charge pump circuit to raise the voltage higher than the supply voltage(s) supplied to the semiconductor chip; or, more simply, the semiconductor chip may receive the large voltage from an off-chip source and the circuitry simply routes this large voltage to the ERASE node (e.g., through a switch having a control input coupled to a logic circuit output to control the switch&#39;s state). 
     The WELL node  802 _A is driven by circuitry that also creates a large positive voltage (e.g., by any of the types of circuits described just above) to: 1) create a large I SD  current in transistor Q 1 ; and, 2) create a strong gate-to-drain electric field that causes band-to-band tunneling within the substrate of transistor Q 2 . As discussed above with respect to  FIG. 3A , the WELL/SOURCE nodes of transistors Q 1  and Q 2  may in fact be driven to different voltage levels (e.g., where the well and source of Q 2  is driven to a lower voltage (e.g., ground) than the well and source of transistor Q 1 ) by different driving circuits during the same PROGRAM phase in order to ensure the presence of a high intensity electric field running from the floating gate to the Q 2  substrate needed to induce band-to-band tunneling within transistor Q 2 . 
     With respect to  FIG. 8A , according to one embodiment, during the PROGRAM phase, the WELL/SOURCE node  802  is set to a large positive voltage (e.g., 2VDD), the ERASE node  803 _A is set to ground (so as to diminish the attraction of electrons to the substrate of the tunnel transistor Q 3 ), the SELECT lines  805 _A and  806 _A are set to ground to turn on their respective transistors Q 4  and Q 5  and permit any current flowing from Q 1  and Q 2  to flow into the BIT LINE  807 _A. Both of the SELECT lines  805 _A and  806 _A (as well as the other SELECT lines in the following figures) are driven in part by addressing logic circuitry that identifies the particular cell to be programmed. The BIT LINE  807 _A is set to ground to provide an electric potential “sink” for any current flowing from transistors Q 1  and Q 2 . 
     During the ERASE phase, the ERASE node  803 _A is set to a large positive voltage to induce Fowler-Nordheim tunneling of electrons into the substrate region of transistor Q 3 . The WELL/SOURCE node  802 _A is set to a low voltage (or low voltage for transistor Q 1  and high voltage for transistor Q 2 ) to ensure that these transistors do not add electrons to the floating gate during the ERASE phase. 
     During a READ phase, the WELL/SOURCE node  802 _A is set to a positive voltage (e.g., VDD) between the floating node voltage associated with the first and second logical states, SELECT line  805 _A is low to turn on Q 4  but select line  806 _A is high to turn Q 5  off. If the floating gate  801 _A is in the first logical state (low), transistors Q 1  and Q 2  are “on” but current can only flow from transistor Q 1  (and not from transistor Q 2 ) into the drain node  807 _A because transistor Q 5  is off. Here, turning Q 5  off to prevent any current flow from transistor Q 2  effectively prevents overloading of any standard prior art sense amplifier circuitry that is coupled to the BIT LINE  807 _A to sense the current flowing from the BIT LINE  807 _A. If the floating gate  801 _A is in the second logical state (high), transistors Q 1  and Q 2  are “off” and no current flows from the BIT LINE  807 _A. 
     The circuit of  FIG. 8B  operates similarly to that of the circuit of  FIG. 8A  except that the band-to-band-tunneling device Q 2  is configured to be incapable of transporting any currents other than the band-to-band-tunneling induced current injected to the floating gate node because its drain and source nodes are configured to be at the same electrical potential. Because the Q 2  device can not provide or sink any source-to-drain or drain-to-source current, its only activity is to inject band-to-band tunneling induced current (i.e., it cannot inject the type of gate current injected by transistor Q 1 ). It therefore has a separate PROGRAM line  808  that, in one embodiment, is set to the same potential as the bit line/drain node  807 _B during the PROGRAM phase and is set to the same potential as the source node  802 _B during a READ phase. Here, the PROGRAM line  808  may be driven by switch circuitry having a first input coupled to the output of the logic circuitry driving node  807 _B, a second input coupled to an output of the high voltage circuitry that drives node  802 _B and a control input driven by logic circuitry that indicates whether the cell is in the PROGRAM phase or the READ phase. SELECT line  806 _B is set to ground to turn on Q 5  only during the PROGRAM phase as well. 
     During the READ phase the program node  808  is held at the same potential as the source/n-well node  802 _B and the BTBT select node  806 _B is held low (to turn Q 5  “on”) to keep the drain, source and well voltages of transistor Q 2  about the same thereby preventing the Q 2  device from injecting any band-to-band tunneling current onto the floating gate  801 _B and disturbing the read operation. During an unselected PROGRAM (i.e., a different floating gate that shares the same PROGRAM line  808  is being programmed) the BTBT select node  806 _B is held high to turn device Q 5  off and shield device Q 2  from having a high electric field between its well and drain. As such, the BTBT select node  806 _B may be driven by logic circuitry that indicates whether the memory cell is selected and in READ mode or in some other state. 
     The circuit of  FIG. 8C  operates similarly to that of the circuit of  FIG. 8A  except that the BIT LINE  807 _C is capable of receiving current that flows from Q 2  during a READ phase because of the single SELECT line  805 _C and corresponding transistor Q 4 . 
       FIG. 9  demonstrates that, rather than using p-type devices (as illustrated in  FIG. 3A ), n-type devices may be used. In the case of an n-type approach, the sources of Q 1  and Q 2  are tied to a p type well node (rather than an n type WELL node). During a PROGRAM phase, a low voltage (e.g., ground) is applied to the WELL/SOURCE node  902 . If the floating gate voltage is large (e.g., because the tunneling device Q 3  removed too many electrons from the floating gate), transistor Q 1  will be “on” and will therefore inject high energy electrons created by collisions between Q 1 &#39;s substrate lattice and carriers that were accelerated in Q 1 &#39;s conductive channel. Likewise, if the floating gate voltage is significantly low (e.g., because the Q 3  device did not tunnel enough electrons) so as to fail to turn transistor Q 1  on, transistor Q 2  will inject electrons to the floating gate node  901  that were induced by band-to-band tunneling with Q 2 . Here, Q 2  may have its channel region doped with acceptor atoms (e.g., like a p type core logic transistor) so as to create a device that will exhibit a strong electric field running from the floating gate into the substrate so as to induce band-to-band tunneling. 
       FIG. 10  shows a memory cell constructed with n type Q 1  and Q 2  transistors that operates as described just above with respect to  FIG. 9 . The circuit operates similarly to the circuit of  FIG. 8A  in the sense that during a PROGRAM phase, transistors Q 4  and Q 5  are on and the WELL node is supplied with a substantial low voltage (with switch circuitry having an input coupled to the source of the low voltage and a control input coupled to logic circuitry that contemplates the operating state the memory cell is in). In this case, if either of transistors Q 1  and Q 2  have an I DS  current, the current will flow from BIT LINE  1007 . During an ERASE phase, a positive voltage capable of drawing electrons into the well of transistor Q 3  through Fowler-Nordheim tunneling is applied to ERASE node  1003 . Here, transistor Q 3  should have an isolated well so that it can be pulled to a high voltage. Note that transistor Q 3  can be p type while transistors Q 1  and Q 2  can be n type. During a READ phase, transistor Q 5  is turned off and transistor Q 4  is turned on (e.g., with logic circuitry that contemplates the memory cell&#39;s operating state) so as to only permit current being sinked by transistor Q 1  (and not Q 2 ) to flow through BIT LINE  1007 . 
     An alternate use of the circuit of  FIG. 10 , which simply reverses the logical states associated with the PROGRAM and ERASE phases from the embodiments discussed above, is to PROGRAM through transistor Q 3  (i.e., during the PROGRAM phase electrons are removed from the floating gate thereby raising its voltage to a first logical state) and to ERASE with transistors Q 1  and Q 2  (i.e., during the ERASE phase electrons are added to the floating gate thereby reducing its voltage to a second logical state). 
       FIG. 11  shows a differential non-volatile memory cell utilizing the approach being described herein. A pair of “sub” non-volatile memory cells “A” and “B” that utilize both impact ionization induced injection current and band-to-band tunneling induced injection current are observed. Essentially, in order to achieve differential operation, the two sub cells A and B are kept at opposite logical states (i.e., when sub-cell A is storing a logical “1” sub-cell B is storing a logical “0”, and, when sub-cell B is storing a logical “0” sub-cell B is storing a logical “1”). Said another way, the two cells are set in opposing PROGRAM and ERASE phases such that when one sub-cell is in the PROGRAM phase the other sub-cell is set into the ERASE phase. The well/source nodes  1102 _A,  1102 _B and erase nodes  1103 _A,  1103 _B are therefore set to appropriate voltage levels, consistent with the teachings provided above, to effect this operation. During a read phase, one of the cells will supply current to the differential sense amplifier  1105  while the other will not. Which cell supplies the current determines the logical state “read” by the sense amplifier  1105 . 
       FIG. 12  shows a methodology that is consistent with at least one circuit described above. According to the methodology of  FIG. 12 , a select line is activated to permit a current to flow through a first transistor and a first injection current is injected between the first transistor&#39;s electrically floating gate and the first transistor&#39;s semiconductor substrate to place an amount of charge on the floating gate. The first injection current is caused by the current. A voltage is also applied to a second transistor&#39;s semiconductor substrate well region and a second injection current is injected between the gate electrode and the second transistor&#39;s semiconductor substrate in order to place the amount of charge on the floating gate. The second injection current is caused by a band-to-band tunneling current with the second transistor. The band-to-band tunneling current is caused by energy band bending between the gate electrode and the second transistor&#39;s semiconductor substrate. The energy band bending is caused at least in part by the voltage. 
     The electrical circuits described in this document can be manufactured in any number of ways, as will be appreciated by persons skilled in the art. One such way is as an integrated circuit, as described below. 
     Schematic-type inputs can be provided for the purpose of preparing one or more layouts. These inputs can include as little as a schematic of a circuit, to more including relative sizes of circuit components and the like, as will be appreciated by a person skilled in the art for such inputs. These inputs can be provided in any suitable way, such as merely in writing, or electronically, as computer files and the like. Some of these computer files can be prepared with the assistance of suitable design tools. Such tools often include instrumentalities for simulating circuit behaviors and the like. 
     These inputs can be provided to a person skilled in the art of preparing layouts. This, whether the person is within the same company, or another company, such as under a contract. 
     A layout can be prepared that embodies the schematic-type inputs by the person skilled in the art. The layout is itself preferably prepared as a computer file. It may be additionally checked for errors, modified as needed, and so on. 
     In the above, computer files can be made from portions of computer files. For example, suitable individual designs can be assembled for the electrical components and circuits indicated in the schematic-type inputs. The individual designs can be generated anew, or selected from existing libraries. In the layout phase, the assembled designs can be arranged to interoperate, so as to implement as integrated circuit(s) the electrical circuit(s) of the provided schematic-type inputs. These computer files can be stored in storage media, such as memories, whether portable or not, and the like. 
     Then a special type of computer file can be synthesized from the prepared layout, in a manner that incorporates the prepared layout, which has the embodied schematic-type inputs. Such files are known in the industry as IC chip design files or tapeout files, and express instructions for machinery as to how to process a semiconductor wafer, so as to generate an integrated circuit that is arranged as in the incorporated layout. 
     The synthesized tapeout file is then transferred to a semiconductor manufacturing plant, which is also known as a foundry, and so on. Transferring can be by any suitable means, such as over an electronic network. Or a tapeout file can be recorded in a storage medium, which in turn is physically shipped to the mask manufacturer. 
     The received tapeout file is then used by mask making machinery as instructions for processing a semiconductor wafer. The wafer, as thus processed, now has one or more integrated circuits, each made according to the layout incorporated in the tapeout file. If more than one, then the wafer can be diced to separate them, and so on. 
     In this description, numerous details have been set forth in order to provide a thorough understanding. In other instances, well-known features have not been described in detail in order to not obscure unnecessarily the description. 
     A person skilled in the art will be able to practice the present invention in view of this description, which is to be taken as a whole. The specific embodiments as disclosed and illustrated herein are not to be considered in a limiting sense. Indeed, it should be readily apparent to those skilled in the art that what is described herein may be modified in numerous ways. Such ways can include equivalents to what is described herein. 
     The following claims define certain combinations and subcombinations of elements, features, steps, and/or functions, which are regarded as novel and non-obvious. Additional claims for other combinations and subcombinations may be presented in this or a related document.