Abstract:
A non-volatile memory IGFET device has a gate dielectric stack that is di lectrically equivalent to a layer of silicon dioxide having a thickness of to 170 Å or less. Above the dielectric stack is a polycrystalline silicon gate that is doped in an opposite manner to that of the source and drain regions of the transistor. By using a gate doping that is opposite to that of the IGFET source and drain regions the poly depletion layer that can occur during programming in modern and advanced memory devices is eliminated according to this invention. The device of this invention forms an accumulation layer in the poly rather than a depletion layer. This difference not only greatly improves the program speed, but allows for selecting the gate doping at levels as low as 10 11 /cm 3 , or less, without significantly compromising the program speed. Further, since the majority of the applied voltage in a device according to this invention is dropped over the gate dielectric, rather than shared between the gate dielectric and a depletion layer in the gate poly, the device of this invention can be scaled in gate dielectric thickness without significantly compromising the program speed.

Description:
This application is a divisional of patent application Ser. No. 09/082,167 filed May 20, 1998, now U.S. Pat. No. 6,140,676. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to improvements in semiconductor non-volatile memory transistors, and more specifically improvements in the gate design and processing of non-volatile transistors used in electrically erasable, electrically programmable read-only memories. 
     2. Description of the Related Art 
     Until recently, the gate structure of non-volatile memory transistors has been designed in a similar manner to that of conventional CMOS insulted gate field effect transistors (IGFETs) or MOSFETs. The difference between CMOS IGFETs and non-volatile IGFETs is primarily that non-volatile IGFETs include an added charge storage layer embedded in the gate dielectric. The charge storage layer is either a conductive element, such as a polycrystalline silicon (poly) floating gate, or a non-conductive element such as a dielectric which is capable of trapping charge. Older types of CMOS transistors have typically used a heavily doped N-type gate material for both N- and P-channel transistors in order to simplify processing and to achieve low poly resistivity. With the advent of deep sub-micron CMOS technology, a greater emphasis has been placed on reduced temperature processing, deeply scaled transistor geometries, and silicided polycrystalline silicon gates. This emphasis has led to changes in the gate structure that affect the doping of the poly. 
     In older technologies, the poly was typically doped by furnace diffusion processes using POC1 3  or Phosphine gas to produce a heavily doped N-type material. In newer technologies with channel length geometries at 0.7 microns and below, the furnace diffusion doping processes have been replaced with ion-implantation or low temperature in-situ doping during the poly deposition. These newer doping methods, which allow for substantially reduced thermal processing while doping the poly gate, are necessary to produce deeply scaled transistor geometries. Further, these newer doping methods allow for better doping control in the poly which is useful in facilitating the formation of a metal-silicide layer on top of the poly. Also, in newer technologies, it has been advantageous to use both N- and P-type doped poly, rather than simply N-type poly. Using P-type poly allows deeply scaled P-type MOS transistors to operate more efficiently at lower channel lengths due to the elimination of a buried channel that is usually required with N-type poly. Thus, N-type poly gates are often used in today&#39;s N-channel MOSFETs and P-type poly gates are often used in today&#39;s advanced P-channel MOSFETs. In these modern devices, the gate doping type is matched to the source and drain junction doping type. 
     Until recently, there has been no advantage in using different criteria for choosing a gate doping type for non-volatile memory devices from those used to choose the doping type for conventional MOSFETS. The choice has been primarily motivated by a desire to save costs by being compatible with processes used to produce conventional MOSFET devices. As a result, more recently developed doping methods and doping types for conventional MOSFETs have been applied to the construction of non-volatile memory transistors. Specifically, advanced N-channel non-volatile memory transistors are constructed using an N-type poly gate, advanced P-channel transistors are constructed using a P-type poly gate, and doping levels in both are often lower than what was used in the past. 
     In FIG. 1 memory transistor  10  shows an N-channel non-volatile insulated gate field effect transistor which includes a charge storage layer  32  embedded in its gate dielectric, according to prior art. The charge storage layer  32  is typically surrounded by at least a top dielectric  31  and a bottom dielectric  33  and resides between the N-type gate  12  and the channel  15  of the transistor. Channel  15  resides in the P-type silicon bulk  11  between the N-type source  14  and N-type drain  16  regions. The charge storage layer  32  is either a “floating gate”, typically of doped polycrystalline silicon, or a dielectric material capable of trapping charge carriers such as silicon nitride, silicon oxynitride, silicon-rich silicon dioxide, or a ferroelectric material. The thickness of the gate dielectric, the composite of layers  31 ,  32  and  33 , is typically dielectrically equivalent to 150 Å to 200 Å of silicon dioxide, although thinner dielectrics are currently under investigation. Note that transistor  10  could optionally include a silicide layer on top of the N-type gate  12 . 
     In FIG. 2 memory transistor  10 ′ shows a P-channel non-volatile insulated gate field effect transistor which includes a charge storage layer  32  embedded in its gate dielectric, according to prior art. The charge storage layer  32  is typically surrounded by at least a top dielectric  31  and a bottom dielectric  33  and resides between the P-type gate  12 ′ and the channel  15 ′ of the transistor. Channel  15 ′ resides in the N-type silicon bulk  11 ′ between the P-type source  14 ′ and P-type drain  16 ′ regions. The charge storage layer  32  is either a “floating gate”, typically of doped polycrystalline silicon, or a dielectric material capable of trapping charge carriers such as silicon nitride, silicon oxynitride, silicon-rich silicon dioxide, or a ferroelectric material. The thickness of the gate dielectric, the composite of layers  31 ,  32  and  33 , is typically dielectrically equivalent to 150 Å to 200 Å of silicon dioxide, although thinner dielectrics are currently under investigation. Note that transistor  10 ′ could optionally include a silicide layer on top of the P-type gate  12 ′. 
     The amount and polarity of charge residing in the charge storage layer  32  affect the conductivity of the non-volatile transistor. The words “programmed” and “erased” are used here to describe two possible conductivity states that non-volatile transistors can achieve under two different charge storage conditions. It is recognized that the designation of the words “programmed” and “erased” is purely arbitrary and that these terms can be selected to represent different meanings depending on the application. Here, however, the terms “erased” and “programmed” are used in reference to relative levels of conductance. The terms “erased” or “erase”, and “programmed” or “program” are used to describe the “on” and “off” states, respectively. The primary difference between these two states is the level of conductance in non-volatile transistor while under read biases. An “on” state results when the non-volatile transistor is conductive and an “off” state results when the non-volatile transistor is non-conductive, or at least less conductive than a predetermined range of conductance that represents the “on” state. Further, the term “write” is used to describe an operation that intentionally sets the threshold voltage of a non-volatile memory transistor, either to the erase state or to the program state. 
     Unfortunately, we have discovered that matching the doping type of the gate to that of the source and drain junctions is not necessarily the optimal choice for building modern non-volatile memory transistor. So effects of using opposite gate and junction doping in non-volatile memory transistors are now being explored. The problem is that the traditional choice can lead to slow program timing and can reduce the scalability of a non-volatile transistor. These problems have not been a factor in devices that have been in production to date. However, as non-volatile device channel length geometries scale to 0.7 micron and below where the effective gate dielectric thickness is 170 Å or less the effects of gate doping become critical to the operation of the non-volatile transistor, as discussed below. 
     Non-volatile memory transistors oftentimes are written by placing a relatively high voltage on the gate with respect to the transistor channel. For example, a large negative potential (−10 to −20 volts) is placed on gate  12  relative to the channel in order to erase transistor  10  by way of quantum mechanical tunneling. Likewise a large positive potential (+10 to +20 volts) is placed on the gate  12  relative to the channel in order to program transistor  10 , again using quantum mechanical tunneling. Similar voltage magnitudes, but opposite polarities, are applied to the gate  12 ′ to erase and program transistor  10 ′. The large magnitude of the applied voltage is needed in order to both shorten tunneling distances and to lower tunneling barriers. This bias method enables charging the charge storage layer in a reasonable amount of time, typically within hundreds of microseconds to seconds. 
     The tunneling charge transport in transistors  10  and  10 ′ is described by way of equations known in the industry as Fowler-Nordheim Tunneling, Modified Fowler-Nordheim Tunneling, Direct Tunneling and Trap-Assisted Tunneling. These equations accurately predict that the rate of tunneling charge transport, or tunneling current, into Charge Storage Layer  32  is an exponential function of the electric field across the dielectric through which it is tunneling. The tunnel current that primarily affects write speed is the tunnel current through dielectric  33 . So the time it takes to either erase or program a transistor  10  or  10 ′ is a very strong function of the electric field imposed on dielectric layer  33  during a write operation, either erase or program. 
     Additionally, the electric field created by the gate voltage terminates in the channel region of the bulk and in the poly. When the field terminates in the poly, it does so by either forming an accumulated layer of free charge carriers or by forming a depletion layer at the interface between the poly and the top of the gate dielectric. When the free carriers are accumulated, the poly acts nearly like a metallic electrode and very little voltage is dropped within the poly. However, when the free carriers in the poly are repelled from the interface to form a depletion layer, a significant amount of voltage can be dropped in the poly depletion layer. 
     As shown in FIG. 3, a positive bias is applied to the poly gate  12  relative to the channel  15  to program transistor  10 . In this example, ten volts is applied to gate  12  while the source, drain and bulk are held at ground. The voltage difference between the gate  12  and the bulk  11  creates an electric field that passes through layers  31 ,  32  and  33  and creates a depletion layer  20  in the gate poly and a depletion layer  21  to form the channel  15  in the bulk. The applied gate-to-bulk voltage creates depletion layer  20  because the electric field attracts the free electrons in the N-type poly gate  12  toward the electrode and away from the interface between the gate  12  and the top dielectric  31 . Likewise, the electric field created by the gate-to-bulk bias repels the free holes from interface between the P-type bulk and the bottom dielectric  33 , forming bulk depletion  21 . 
     The voltage difference between the gate electrode and the bulk electrode is called Vpp. Vpp is nearly equal to the sum of the voltages dropped across  20 ,  31 ,  32 ,  33 , and  21 ; namely 
     
       
           Vpp ≈Delta —   V _Poly+Delta —   V _Top —   Ox +Delta —   V _Storage_Layer+Delta —   V _Bottom —   Ox +Delta —   V _Bulk. 
       
     
     Unfortunately, the voltage drop in the poly, Delta_V_Poly, provides no value in forming the conditions required to tunnel charge through the dielectric layer  33 . In fact, when the depletion layer is present, the tunnel characteristics are much like what would be expected if the device were formed with a metallic gate electrode and the write voltage was lower by the amount dropped in the poly depletion. Since the tunnel current is an exponential function of the electric field across the dielectric, the write speed can be significantly degraded by the poly depletion voltage loss. 
     The circumstances are quite different when transistor  10  is being erased as shown in FIG.  4 . When a negative bias is applied to the N-type poly gate  12  relative to the channel  15  of the N-channel transistor  10 , the electric field serves to accumulate the free carriers  24  (electrons) in the poly  12  at the interface between the poly  12  and the top dielectric  31 . In this case, there is typically negligible electric field lost in the poly and the erase speed is not degraded by voltage lost in the poly. Further, independent of the gate structure, the erase bias serves to accumulate holes in the channel at the interface between the bulk and the bottom dielectric  33 , forming accumulation  23 . As a result, negligible voltage is typically dropped in the bulk and so the erase condition creates the ideal result of the Vpp being dropped only over layers  31 ,  32  and  33 ; namely 
     
       
           Vpp ≈Delta —   V _Top —   Ox +Delta —   V _Storage_Layer+Delta —   V _Bottom —   Ox.   
       
     
     Under good program conditions, as best seen in FIG. 3, little or no voltage is dropped in the N-type poly gate  12 . Preferably the voltage drop in the gate, Delta_V_Poly, will be much less than the write voltage, Vpp, applied to the gate. This was readily achieved in older technologies when the doping in the poly gate  12  was high, typically ≧10 20 /cm 3 , and the layers  31 ,  32  and  33  were relatively thick, equivalently ≧170 Å of SiO 2 . However, in more modern technologies which use a moderately or lightly doped poly gate  12  and which have relatively thin layers  31 ,  32  and  33 , the voltage drop in the poly depletion layer  20  can be quite substantial, as shown in FIG.  5 . In this case, there can be a significant amount of the applied voltage lost in the poly and the time required to program transistor  10  can be greatly increased. 
     The voltage lost in the poly depletion layer during a program operation has been calculated as shown in FIG. 6 for an applied voltage of 10 volts. In this plot, the percent of the applied voltage dropped in the poly is indicated on the vertical axis. The horizontal axis on the right hand side marks the doping concentration in the poly. The horizontal axis on the left hand side indicates the thickness of the gate dielectric in values equivalent to a thickness of SiO 2 . The shaded bands in the contour shows domains where the percentage of applied voltage dropped in the poly lies within a 5% range. Six bands of applied voltage drop in the poly are shown; specifically 0 to 5%, 5% to 10%, 10% to 15%, 15% to 20%, 20% to 25% and 25% to 30%. 
     As shown in FIG. 6, the percentage of applied voltage that is dropped in the poly increases rapidly as the doping concentration falls below 10 20 /cm 3 . Once the concentration reaches 10 18 /cm 3 , the percentage of applied voltage that is dropped in the poly achieves about 20%. With further reductions in doping concentration below 10 18 /cm 3 , the percentage of applied voltage that is dropped in the poly increases only gradually. This lack of sensitivity occurs because of the formation of an inversion layer in the poly in the lower concentrations. 
     The exponential dependence of tunneling current on linear changes in electric field causes the program time to increase by about an order of magnitude for every 10% decrease in voltage across the gate dielectric. The percentage of applied voltage that is dropped in the poly is less than 10% for thicknesses as low as 57 Å of equivalent gate dielectric when the poly doping concentration is 10 20 /cm 3  or higher. However, the percentage voltage drop exceeds 10% for a poly doping concentration of 10 19 /cm 3  when the gate dielectric thickness falls below only 170 Å. This result greatly limits the range of either the poly doping or the equivalent gate dielectric thickness if program speeds cannot be compromised, which is often the case. 
     Likewise, when a positive bias is applied to the P-type poly gate  12 ′ of P-channel transistor  10 ′ relative to channel  15 ′, the electric field serves to accumulate the free carriers (holes) in the poly  12 ′ at the interface between the poly  12 ′ and the top dielectric  31 . In this case, which is an erase condition, there is very little electric field lost in the poly and the erase speed is not degraded by voltage dropped in the poly. However, when a negative bias is applied to the poly gate  12 ′ relative to the channel  15 ′, the electric field serves to repel the free carriers in the poly  12 ′ from the interface and a depletion layer forms in the poly  12 ′ above the top dielectric  31 . In this case, which is a program condition, there can be a significant amount of the applied voltage lost in the poly in modern structures and the time required to program transistor  10 ′ can be greatly increased. 
     While writing a non-volatile memory transistor, it is desirable to achieve the fastest possible program time without compromising product yield and reliability. The program time directly affects the rate at which data can be stored in a non-volatile memory product. Erase speed is not so critical because data is being erased, not stored. Sections of the memory product can be erased in a background manner, long before those sections are selected to store data. Unfortunately, prior art embodiments are constructed to favor fast erase speeds and not fast program speeds as effective gate dielectrics scale to thicknesses of 170 Å and below. 
     Further, voltage drops in the poly significantly reduce the sensitivity of the write speed to variations in the thicknesses of dielectric layers  31 ,  32  and  33 . This is advantageous in establishing insensitivity to manufacturing induced thickness variations. However, this lack of sensitivity also makes it difficult to scale the program voltage of transistors  10  and  10 ′. As the layers  31 ,  32 , and  33  are reduced in thickness, the fraction of the applied voltage that is dropped in the poly depletion increases. Eventually, reductions in the thickness of layers  31 ,  32  and  33  has a diminishing impact on reducing the program voltage. This can occur once the layers  31 ,  32  and  33  produce a gate dielectric that is dielectrically equivalent in thickness to 170 Å or less of silicon dioxide. Thus, the non-volatile transistor becomes difficult to scale to take advantage of lower program voltages for deeply scaled technologies. Also, the lack of sensitivity to variation in the thickness of layers  31 ,  32  and  33  is replaced by a sensitivity to variations in doping in the poly, which is traditionally less controllable. 
     So without further innovation, deeply scaled non-volatile transistors for 0.7 micron technologies and below that have effective gate dielectrics thicknesses of 170 Å and below will provide faster erase speed, rather than faster program speed. Further, the scalability of program voltages by using conventional dielectric scaling methods is limited, making it difficult to integrate into a low-voltage CMOS process flow as device geometries reduce to below 0.7 microns. 
     SUMMARY OF THE INVENTION 
     In light of the above, therefore, it is an object of the invention to provide an improved non-volatile semiconductor memory device that provides better program write speed performance compared to prior art devices. 
     Another object of the invention is to provide an improved non-volatile semiconductor memory device that can be scaled to program using lower voltages without loss in program speed. 
     Yet another object of the invention is to provide an improved non-volatile semiconductor memory device that exhibits program speeds that are insensitive to variations in the doping level in the gate. 
     Still another object of the invention is to provide an improved non-volatile semiconductor memory device that scales to technology geometries of 0.7 microns and below without compromising program speeds. 
     A further object of the invention is to provide an improved non-volatile semiconductor memory device that utilizes a gate dielectric thickness that is dielectrically equivalent to 170 Å of SiO 2  or less without loss in program speed. 
     Yet a further object of the invention is to provide an improved non-volatile semiconductor memory device that can be constructed with a wide range of gate doping concentrations without significant loss in program speed. 
     The above and further objects, details and advantages of the invention will become apparent from the following detailed description, when read in conjunction with the accompanying drawings and appended claims. 
     According to the present invention, there is provided a non-volatile memory IGFET device that uses a gate dielectric stack that is dielectrically equivalent in thickness to 170 Å or less of silicon dioxide. Above the dielectric stack is a polycrystalline silicon gate that is doped in an opposite manner to that of the source and drain junctions of the transistor. By using a gate doping that is opposite to that of the IGFET source and drain junctions, the poly depletion layer that can occur during programming in modern and advanced memory devices is eliminated according to this invention. The device of this invention forms an accumulation layer in the poly rather than a depletion layer. This difference not only greatly improves the program speed, but allows for selecting the gate doping at levels as low as 10 11 /cm 3 , or less, without significantly compromising the program speed. Further, since the majority of the applied voltage in a device according to this invention is dropped over the gate dielectric, rather than shared between the gate dielectric and a depletion layer in the gate poly, the device of this invention can be scaled in gate dielectric thickness without significantly compromising the program speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the detailed description of preferred embodiments of the invention presented below and in the description of prior art above, reference is made to the accompanying drawings of which: 
     FIG. 1 shows a cross sectional view of an N-channel non-volatile memory transistor according to prior art that utilizes an N-type polycrystalline silicon gate; 
     FIG. 2 shows a cross sectional view of a P-channel non-volatile memory transistor according to prior art that utilizes a P-type polycrystalline silicon gate; 
     FIG. 3 shows a cross sectional view of an N-channel non-volatile memory transistor under program bias conditions according to prior art that utilizes an N-type polycrystalline silicon gate that is heavily doped to produce a small voltage drop in the gate; 
     FIG. 4 shows a cross sectional view of an N-channel non-volatile memory transistor under erase bias conditions according to prior art; 
     FIG. 5 shows a cross sectional view of an N-channel non-volatile memory transistor according to prior art that utilizes a moderately or lightly doped N-type polycrystalline silicon gate, resulting in a large voltage drop in the gate; 
     FIG. 6 shows the calculated percent voltage drop in the poly gate according to prior art during a program operation as a function of the doping concentration in the poly and the SiO 2  equivalent thickness of the gate dielectric; 
     FIG. 7 shows a cross sectional view of an N-channel non-volatile memory transistor according a preferred embodiment of the invention that utilizes a P-type polycrystalline silicon gate; 
     FIG. 8 shows a cross sectional view of a P-channel non-volatile memory transistor according a preferred embodiment of the invention that utilizes an N-type polycrystalline silicon gate; 
     FIG. 9 shows a cross sectional view of an N-channel non-volatile memory transistor according a preferred embodiment of the invention that utilizes a P-type polycrystalline silicon gate under program bias conditions; 
     FIG. 10 shows the calculated percent voltage drop in the poly gate according to a preferred embodiment of the invention during a program operation as a function of the doping concentration in the poly and the SiO 2  equivalent thickness of the gate dielectric; 
     FIG. 11 shows a cross sectional view of an N-channel non-volatile memory transistor according a preferred embodiment of the invention that utilizes a P-type polycrystalline silicon gate under erase bias conditions; 
     FIG. 12 shows a cross sectional view of a P-channel non-volatile memory transistor according a preferred embodiment of the invention that utilizes an N-type polycrystalline silicon gate under program bias conditions; 
     FIG. 13 shows a cross sectional view of a P-channel non-volatile memory transistor according a preferred embodiment of the invention that utilizes an N-type polycrystalline silicon gate under erase bias conditions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to a preferred embodiment of the invention, a non-volatile memory transistor device is provided that uses a gate dielectric stack that is dielectrically equivalent in thickness to 170 Å or less of silicon dioxide. Above the dielectric stack is a polycrystalline silicon gate that is doped to a type that is different from that of the source and drain junctions of an N-channel transistor. As seen in FIG. 7, a P-type poly gate  12 ″ is used in an N-channel non-volatile transistor device  10 ″. The P-type poly gate  12 ″ is doped with electrically active atoms of boron at a concentration of as little as 10 11 /cm 3  or less. Memory transistor  10 ″ comprises a non-volatile insulated gate field effect transistor which includes a charge storage layer  32  embedded in its gate dielectric. The charge storage layer  32  is typically separated from the bulk (or substrate)  11  by at least a bottom dielectric  33 . Memory transistor  10 ″ optionally includes a top dielectric  31  between the gate  12 ″ and charge storage layer  32 . The stack of layers  31 ,  32  and  33 , which comprise the gate dielectric of transistor  10 ″, resides between the P-type gate  12 ″ and the channel  15  of the  1 l transistor. Channel  15  resides in the P-type silicon bulk (or substrate)  11  between the N-type source  14  and N-type drain  16  regions. 
     The charge storage layer  32  is either a “floating gate”, typically of polycrystalline silicon, or a dielectric material capable of trapping charge carriers or charge polarization such as silicon nitride, silicon oxynitride, silicon-rich silicon dioxide, or a ferroelectric material. Bottom dielectric layer  33  is typically a thermally grown layer of silicon dioxide, but can be formed using any suitable materials that exhibit dielectric properties. Optional top dielectric layer  31  is typically a grown or deposited layer of silicon dioxide, however, it could be constructed by providing multiple layers of dielectric material, such as a three layer stack of silicon dioxide, silicon nitride and silicon dioxide, or other suitable technique. 
     Optionally, transistor  10 ″ could include a refractory silicide layer on top of the P-type gate  12 ″. Further, transistor  10 ″ could optionally be constructed in a P-well formed in the bulk  11  or in a P-well nested inside an N-well, both formed in bulk  11 . In the cases, the bulk doping could be either N-type or P-type. 
     According to another preferred embodiment of the invention, a non-volatile memory transistor device is provided that uses a gate dielectric stack that is dielectrically equivalent in thickness to 170 Å or less of silicon dioxide. Above the dielectric stack is a polycrystalline silicon gate that is doped to a type that is different from that of the source and drain junctions of a P-channel transistor. As seen in FIG. 8, an N-type poly gate  12 ′″ is used in a P-channel non-volatile transistor device  10 ′″. The N-type poly gate  12 ′″ is doped with electrically active atoms of phosphorus, antimony or arsenic at a concentration of as little as 10 11 /cm 3  or less. Memory transistor  10 ′″ comprises a non-volatile insulated gate field effect transistor which includes a charge storage layer  32  embedded in its gate dielectric. The charge storage layer  32  is typically separated from the bulk (or substrate)  11 ′ by at least a bottom dielectric  33 . Memory transistor  10 ′″ optionally includes a top dielectric  31  between the gate  12 ′″ and charge storage layer  32 . The stack of layers  31 ,  32  and  33 , which comprise the gate dielectric of transistor  10 ′″, resides between the N-type gate  12 ′″ and the channel  15 ′ of the transistor. Channel  15 ′ resides in the N-type silicon bulk (or substrate)  11 ′ between the P-type source  14 ′ and P-type drain  16 ′ regions. 
     The charge storage layer  32  is either a “floating gate” of conductive material, typically of doped polycrystalline silicon, or a dielectric material capable of trapping charge carriers or charge polarization such as silicon nitride, silicon oxynitride, silicon-rich silicon dioxide, or a ferroelectric material. Bottom dielectric layer  33  is typically a thermally grown layer of silicon dioxide, but can be formed using any suitable materials that exhibit dielectric properties. Optional top dielectric layer  31  is typically a grown or deposited layer of silicon dioxide, however, it could be constructed by providing multiple layers of dielectric material, such as a three layer stack of silicon dioxide, silicon nitride and silicon dioxide. 
     Optionally, transistor  10 ′″ could include a refractory silicide layer on top of the N-type gate  12 ′″. Further, transistor  10 ′″ could optionally be constructed in an N-well formed in the bulk  11 ′ or in an N-well nested inside a P-well, both formed in bulk  11 ′. In these cases, the bulk doping could be either N-type or P-type. 
     As shown in FIG. 9, a positive bias is applied to the poly gate  12 ″ relative to the channel  15  to program transistor  10 ″. In this example, ten volts is applied to gate  12 ″ by way of electrode  50  while the source  14 , drain  16  and bulk  11  are held at ground by way of electrodes  51 ,  53  and  52 , respectively. Other bias conditions, such as different voltages or non-grounded bulk  11 , source  14  and drain  16  nodes, could be established for programming. The example in FIG. 9 more specifically shows that a voltage differential is created by applying a voltage to the gate  12 ″ which is different from the voltage at the channel surface. The voltage difference between the gate  12 ″ and the bulk  11  creates an electric field that passes through layers  31 ,  32  and  33  and creates a accumulation layer  22  of free holes in the gate poly and a depletion layer  21  to form the channel  15  in the bulk  11 . The applied gate-to-bulk voltage creates accumulation layer  22  because the electric field created by the gate-to-bulk bias attracts the free holes in the P-type poly gate  12 ″ to the interface between the gate  12 ″ and the top of the gate dielectric. Likewise, the electric field created by the gate-to-bulk bias repels the free holes in bulk  11  from interface between the P-type bulk  11  and the bottom dielectric  33 . 
     Since the voltage in the gate is dropped over an accumulation layer rather than a depletion layer, the voltage lost in the poly is much less for a given level of doping compared to prior art devices. This feature allows the poly to be doped to as low as 10 11 /cm 3 , or less, while achieving the same or better results as when the poly was doped at ≧10 20 /cm 3  in prior art devices. The voltage difference between the gate electrode  50  and the bulk electrode  52  is called Vpp. Vpp is nearly equal to the sum of the voltages dropped across  31 ,  32 ,  33 , and  21  since the voltage drop in accumulation layer  22  is typically negligible; namely 
     
       
           V pp≈Delta —   V _Top —   Ox +Delta —   V _Storage_Layer+Delta —   V _Bottom —   Ox +Delta —   V _Bulk. 
       
     
     The voltage lost in the poly accumulation layer during a program operation for the current invention has been calculated as shown in FIG. 10 for an applied voltage of 10 volts. In this plot, the percent of the applied voltage dropped in the poly is indicated on the vertical axis. The horizontal axis on the right hand side marks the doping concentration in the poly. The horizontal axis on the left hand side indicates the thickness of the gate dielectric in values equivalent to a thickness of SiO 2 . The shaded bands in the contour indicate domains where the percentage of applied voltage dropped in the poly lies within a 5% range. Two bands show specifically 0 to 5% and 5% to 10% applied voltage drop in the poly. 
     As shown in FIG. 6, the percentage of applied voltage that is dropped in the poly is less than 10% for thicknesses over all equivalent gate dielectric thicknesses and poly doping concentrations considered. These particular calculations assume a temperature of 150° C. For lower temperatures, doping levels as low as 10 10 /cm 3  would produce less than 10% voltage drop in the poly. Thus, manufacturers building devices according to the current invention can choose from a wide range of possible thicknesses and doping levels without significantly affecting program speeds. 
     In this embodiment, there is negligible voltage drop in the poly as long as the doping level in the poly is approximately ≧10 11 /cm 3 . The poly doping in this device can be set to a value that optimizes conditions unrelated to the program speed since a depletion layer does not form at the interface between the poly and the top of the gate dielectric under program bias conditions. The applied program voltage efficiently drops across layers  31 ,  32 ,  33  and deletion layer  21 . Since the tunnel current is an exponential function of the electric field across the dielectric, the write speed can be significantly improved by eliminating the poly depletion voltage loss as shown in this embodiment. 
     When transistor  10 ″ is being erased as shown in FIG. 11 a negative bias is applied to the P-type poly gate  12 ″ relative to the channel  15  of the N-channel transistor  10 ″, the electric field serves to form a depletion layer  20 ″ at the interface between the poly gate  12 ″ and the top it dielectric  31 . In this example, negative ten volts is applied to gate  12 ″ by way of electrode  50  while the source  14 , drain  16  and bulk  11  are held at ground by way of electrodes  51 ,  53  and  52 , respectively. Other bias conditions, such as different voltages or non-grounded bulk  11 , source  14  and drain  16  nodes, could be established for erasing. 
     The example in FIG. 11 more specifically shows that a voltage differential is created by applying a voltage to the gate  12 ″ which is different from the voltage at the channel surface. In this case, there can be an appreciable voltage lost in the poly depletion layer  20 ″ and the erase speed can be degraded by this lost voltage. However, as stated before, the erase speed is far less critical to system designs than the program speed, and as shown in FIG. 6, the maximum voltage drop in the poly is limited to ≦25% due to the inversion layer that forms in lightly doped poly. Further, independent of the gate structure, the erase bias serves to accumulate holes in the channel at the interface between the bulk  11  and the bottom dielectric  33 , forming free hole accumulation  23 . As a result, negligible voltage is dropped in the bulk  11 , so the erase condition results in the Vpp being dropped over layers  31 ,  32 ,  33  and depletion layer  20 ″; namely 
     
       
           Vpp ≈Delta —   V _Poly+Delta —   V _Top —   Ox +Delta —   V _Storage_Layer+Delta —   V _Bottom —   Ox.   
       
     
     Ideally, the voltage dropped in the poly depletion  20 ″ in this embodiment is small compared to Vpp. This will maximize the voltage dropped over layers  31 ,  32  and  33 . However, this condition is far less critical compared to prior art devices where the voltage dropped in the poly depletion affected the program speed. Since the erase speed can often be compromised without significantly affecting system level performance, it is preferable to accept any speed penalty in the erase speed, rather than the program speed. Further, unlike the program conditions of prior art devices, there is a negligible voltage lost in the bulk  11  during an erase since no depletion forms in the bulk during an erase. Thus, a write speed reduction caused by the voltage drop in the poly depletion  20 ″ is not further compounded by a voltage drop in the bulk as occurs in prior art devices under programming conditions. 
     This is a considerable improvement over prior art where depletion layers formed in both the poly and the bulk during a program operation. In this embodiment, both the program and the erase bias conditions create only one depletion layer and so the program and erase conditions and performance are more symmetric. Further, although the program condition includes a detrimental depletion layer, the depletion can be minimized by the use of channel doping techniques, such as either a buried channel or a depletion channel. 
     Similar benefits result in the embodiment disclosed in FIG. 8 for a P-channel non-volatile memory transistor. As shown in FIG. 12, a negative bias is applied to the poly gate  12 ′″ relative to the channel  15 ′ to program transistor  10 ′″. In this example, negative ten volts is applied to gate  12 ′″ by way of electrode  50 ′ while the source  14 ′, drain  16 ′ and bulk  11 ′ are held at ground by way of electrodes  51 ′,  53 ′ and  52 ′, respectively. Other bias conditions, such as different voltages or non-grounded bulk  11 ′, source  14 ′ and drain  16 ′ nodes, could be established for programming. The example in FIG. 12 more specifically shows that a voltage differential is created by applying a voltage to the gate  12 ′″ which is different from the voltage at the channel surface. The voltage difference between the gate  12 ′″ and the bulk  11 ′ creates an electric field that passes through layers  31 ,  32  and  33  and creates an accumulation layer  25  of free electrons in the gate poly  12 ′″ and a depletion layer  26  to form the channel  15 ′ in the bulk  11 ′. The applied gate-to-bulk  64  voltage creates accumulation layer  25  because the electric field created by the gate-to-bulk bias attracts the free electrons in the N-type poly gate  12 ′″ to the interface between the gate  12 ′″ and the top of the gate dielectric. Likewise, the electric field created by the gate-to-bulk bias repels the free electrons in bulk  11 ′ from interface between the N-type bulk  11 ′ and the bottom dielectric  33 . 
     Since the voltage in the gate is dropped over an accumulation layer rather than a depletion layer, the voltage lost in the poly is much less for a given level of doping compared to prior art devices. This feature allows the poly to be doped to as low as 10 11 /cm 3 , or less, while achieving the same or better results as when the poly was doped at ≧10 20 /cm 3  in prior art devices. The voltage difference between the gate electrode  50 ′ and the bulk electrode  52 ′ is called Vpp. Vpp is nearly equal to the sum of the voltages dropped across  31 ,  32 ,  33 , and  26 ; namely 
     
       
           Vpp ≈Delta —   V _Top —   Ox +Delta V_Storage_Layer +Delta —   V _Bottom —   Ox +Delta —   V _Bulk. 
       
     
     In this embodiment, there is negligible voltage drop in the poly gate  12 ′″, as long as the doping level in the poly is approximately ≧10 11 /cm 3 . The poly doping in this device can be set to a value that optimizes conditions unrelated to the program speed since a depletion layer does not form at the interface between the poly and the top of the gate dielectric under program bias conditions. The applied program voltage efficiently drops across layers  31 ,  32 ,  33  and depletion layer  26 . Since the tunnel current is an exponential function of the electric field across the dielectric, the write speed can be significantly improved by eliminating the poly depletion voltage loss as shown in this embodiment. 
     When transistor  10 ′″ is being erased as shown in FIG. 13 a positive bias is applied to the N-type poly gate  12 ′″ relative to the channel  15 ′ of the P-channel transistor  10 ′″, the electric field serves to form a depletion layer  27  at the interface between the poly gate  12 ′″ and the top of the gate dielectric. In this example, positive ten volts is applied to gate  12 ′″ by way of electrode  50 ′ while the source  14 ′, drain  16 ′ and bulk  11 ′ are held at ground by way of electrodes  51 ′,  53 ′ and  52 ′, respectively. Other bias conditions, such as different voltages or non-grounded bulk  11 ′, source  14 ′ and drain  16 ′ nodes, could be establish d for erasing. The example in FIG. 13 more specifically shows that a voltage differential is created by applying a voltage to the gate  12 ′″ which is different from the voltage at the channel surface. In this case, there is voltage lost in the poly depletion layer  27  and the erase speed can be degraded by this lost voltage. However, as stated before the erase speed is far less critical to system designs than the program speed. Further, independent of the gate structure, the erase bias serves to accumulate electrons in the channel at the interface between the bulk  11 ′ and the bottom dielectric  33 , forming free electron accumulation  28 . As a result, negligible voltage is dropped in the bulk and so the erase condition results in the Vpp being dropped primarily over layers  31 ,  32 ,  33  and  12  depletion layer  27 ; namely 
     
       
           Vpp ≈Delta —   V _Poly+Delta —   V _Top —   Ox +Delta   —   V _Storage_Layer+Delta —   V _Bottom —   Ox.   
       
     
     Again, this result is a considerable improvement over prior art where depletion layers formed in both the poly and the bulk during a program operation. In this embodiment, both the program and the erase bias conditions create only one depletion layer and so the program and erase conditions and performance are more symmetric. Further, although the program condition includes a detrimental depletion layer, the depletion can be minimized by the use of channel doping techniques, such as either a buried channel or a depletion channel. 
     Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed.