Patent Publication Number: US-8987818-B1

Title: Integrated MOS power transistor with thin gate oxide and low gate charge

Description:
RELATED APPLICATIONS 
     This Patent application is a continuation in part of U.S. patent application Ser. No. 12/618,515, filed Nov. 13, 2009, and entitled, “IMPROVED MOS POWER TRANSISTOR.” This application incorporates U.S. patent application Ser. No. 12/618,515 in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of power transistors. More particularly, the present invention relates to the field of integrated MOS power transistors with reduced gate charge. 
     BACKGROUND OF THE INVENTION 
     A power supply is a device or system that supplies electrical or other types of energy to an output load or group of loads. The term power supply can refer to a main power distribution system and other primary or secondary sources of energy. A switched-mode power supply, switching-mode power supply or SMPS, is a power supply that incorporates a switching regulator. While a linear regulator uses a transistor biased in its active region to specify an output voltage, a SMPS actively switches a transistor between full saturation and full cutoff at a high rate. The resulting rectangular waveform is then passed through a low-pass filter, typically an inductor and capacitor (LC) circuit, to achieve an approximated output voltage. 
     SMPS is currently the dominant form of voltage conversion device because of its high power conversion efficiency, small size and weight, and low cost. SMPS takes input power from a source, such as a battery or wall socket, and converts the input power into short pulses according to the demand for power from the circuits coupled to the SMPS output. 
     MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are commonly used in SMPS. MOSFETs are commonly manufactured separately, as discrete transistors. Each MOSFET is then connected to other integrated circuits that are part of the SMPS. Using discrete devices in this manner increases cost and size of the overall SMPS. 
     High performing MOSFETs are significant to the conversion efficiency of SMPS because MOSFETs are some of the most power dissipating components in the SMPS. Also, the maximum possible switching frequency of the MOSFETs dictates the size, cost, and power losses in the inductors and capacitors included in the SMPS output filter circuits. Under normal SMPS operation, MOSFETs are turned on and off rapidly, so for efficient operation the MOSFETs should have low values of both resistance and gate capacitance. 
     A MOSFET has a gate, a drain, and a source terminal, as well as a fourth terminal called the body, base, bulk, or substrate. The substrate simply refers to the bulk of the semiconductor in which the gate, source, and drain lie. The fourth terminal functions to bias the transistor into operation. The gate terminal regulates electron flow through a channel region in the substrate, either enabling or blocking electron flow through the channel. Electrons flow through the channel from the source terminal towards the drain terminal when influenced by an applied voltage. 
     The channel of a MOSFET is doped to produce either an N-type semiconductor or a P-type semiconductor. The drain and source may be doped of opposite type to the channel, in the case of enhancement mode MOSFETs, or doped of similar type to the channel as in depletion mode MOSFETs. The MOSFET utilizes an insulator, such as silicon dioxide, between the gate and the substrate. This insulator is commonly referred to as the gate oxide. As such, the gate terminal is separated from the channel in the substrate by the gate oxide. 
     When a voltage is applied between the gate and source terminals, the electric field generated penetrates through the gate oxide and creates a so-called “inversion layer”, or channel, at the semiconductor-insulator interface. The inversion channel is of the same type, P-type or N-type, as the source and drain, so as to provide a channel through which current can pass. Varying the voltage between the gate and substrate modulates the conductivity of this layer, which functions to control the current flow between drain and source. 
     A power MOSFET is a specific type of MOSFET widely used as a low-voltage switch, for example less than 200V. A lateral power MOSFET refers to a configuration where both the drain and the source are positioned lateral of each other, such as both at the top surface of the substrate. This is in contrast to a vertical power MOSFET where the drain and source are stacked vertically relative to each other, such as the source at the top surface of the substrate and the drain at the bottom surface. 
     One limiting factor in how fast the power MOSFET can be switched on and off is the amount of gate charge needed to turn the transistor on and off. The gate charge refers to the number of electrons that are moved into and out of the gate to turn the transistor on and off, respectively. The larger the needed gate charge, the more time to switch the transistor on and off. There is an advantage to quickly switching the power transistor in a switch-mode power supply. The higher the frequency, the smaller the size of the discrete components used in the gate drive circuit of the SMPS. Smaller components are less expensive than larger components. 
       FIG. 1  illustrates a cut-out side view of an exemplary configuration of a conventional lateral power MOSFET. In this exemplary configuration, the substrate  10  is doped to form a P-type region, or well,  12  and a N-type region, or well,  14 . The P-type well  12  includes a double diffused source  16  having a merged contact  24  between a P+ region  20  and a N+ region  22 . The contact  24  shorts the P+ region  20  and the N+ region  22  together. The contact  24  functions as a source contact of the power transistor, and the source is shorted to the body of the substrate, which is P-type in this exemplary configuration. A source contact terminal  42  is coupled to the contact  24 , and therefore to the source  16 . The substrate  10  is also doped to form a N+ region  18  within the N-type region  14 . The N+ region  18  functions as the drain of the power transistor. A drain contact terminal  40  is coupled to the drain  18 . A trench  26  is formed in a top surface of the substrate  10 . The trench  26  is filled with field oxide. The trench  26  can be formed using Shallow Trench Isolation (STI) and in this case the field oxide filled trench is referred to as a shallow trench isolation (STI) region. 
     A gate oxide  28  is formed on the top surface of the substrate  10 . A polysilicon gate  30  is formed over the gate oxide  28 . As shown in  FIG. 1 , the gate oxide layer  28  between the polysilicon gate  30  and the substrate  10  is a thin oxide layer. The polysilicon gate  30  extends over the STI region to support high drain-to-gate voltage. 
     There are three main regions in the substrate  10  relative to the operation of the power transistor: a channel region, a transition region, and a drift region. The channel region is formed underneath the polysilicon gate  30  and in the P-type region  12  of the substrate  10 . In other words, the channel region is formed where the polysilicon gate  30  overlaps the P-type region  12 . The drift region is the portion of the N-type region  14  underneath the trench  26 , or the STI region. The drift region is where most of the drain-to-gate voltage is dropped in the transistor off state. The STI region is necessary to achieve a high drain-to-gate voltage. If the polysilicon gate  30  were to instead terminate over the thin gate oxide, this would result in too high a voltage across the gate oxide and the power transistor would not function. As such, the STI region and the polysilicon gate extension over the STI region are necessary to drop the high gate-to-drain voltage. 
     The transition region is the portion of the N-type region  14  underneath the gate oxide  28  and the polysilicon gate  30 . The transition region provides a current flow path from the channel region to the drift region when the power transistor is turned on. The transition region is also referred to as the accumulation region or the neck region. In many applications, the transition region accounts for the largest single component of on-resistance in a low-voltage power MOSFET. The length of the transition region is an important design consideration, where the length refers to the horizontal direction in  FIG. 1 . If the length is too short, the on-resistance of the power MOSFET increases, and the device suffers from early quasi-saturation when turned on hard. If the length is too long, the on-resistance saturates, the specific on-resistance increases, and the breakdown voltage drops. The portion of the polysilicon gate  30  positioned over the transition region accounts for a significant portion of the gate capacitance, and therefore the gate charge. 
     SUMMARY OF THE INVENTION 
     A split gate power transistor includes a laterally configured power MOSFET including a doped silicon substrate, a gate oxide layer formed on a surface of the substrate, and a split polysilicon layer formed over the gate oxide layer. The polysilicon layer is cut into two electrically isolated portions, a first portion forming a polysilicon gate positioned over a channel region of the substrate, and a second portion forming a polysilicon field plate formed over a portion of a transition region of the substrate. The two polysilicon portions are separated by a gap. A lightly doped region is implanted in the substrate below the gap, thereby forming a bridge having the same doping type as the substrate body. The field plate also extends over a drift region of the substrate, where the drift region is under a field oxide filled trench formed in the substrate. The field plate is electrically coupled to a source of the split gate power transistor. 
     In an aspect, a power transistor includes a doped substrate comprising a source and a channel region within a first doped region, a drain, a bridge, a first transition region, and a second transition region within a second doped region, and a trench within the second doped region, wherein the trench is formed in a first surface of the substrate and the trench is filled with insulating material, further wherein the channel region is positioned between the source and the first transition region, the first transition region is positioned between the channel region and the bridge, the bridge is positioned between the first transition region and the second transition region, the second transition region is positioned between the bridge and the trench, and the trench is positioned between the second transition region and the drain. The power transistor also includes a gate oxide layer positioned on the first surface of the substrate; a gate positioned on the gate oxide layer and over the channel region and the first transition region; and a field plate positioned on the gate oxide layer and over the second transition region and a portion of the trench, wherein the gate is separated from the field plate by a gap positioned over the bridge, further wherein the field plate is electrically coupled to the source via a conductive trace. 
     The gate and the field plate can be polysilicon. In some embodiments, the first doped region is a P-type region and the second doped region is a N-type region, and the bridge includes a P-type region. The bridge can be a lightly doped P-type implant. In some embodiments, the bridge can also a N-type region positioned as a halo between the P-type region within the bridge and the N-type first transition region and the N-type second transition region. The N-type region in the bridge can be a lightly doped N-type implant. In some embodiments, the bridge is continuous across a width of the gap and the bridge is continuous along a length of the gap. In some embodiments the halo N-type region is continuous along the length of the gap and in other embodiments the halo N-type region is discontinuous along the length of the gap. In some embodiments, the bridge is a plurality of discontinuous sections along a length of the gap, each section is continuous across a width of the gap and each section is separated from an adjacent section along the length of the gap. In some embodiments, each section has a P-type region and a halo N-type region. In other embodiments, each of the sections includes the P-type region and only select sections include the halo N-type region. 
     In some embodiments, the power transistor comprises a lateral double-diffused metal-oxide-semiconductor field-effect transistor. In some embodiments, the doped substrate also includes a drift region within the second doped region, wherein the drift region is positioned under the trench. In some embodiments, the power transistor also includes a conductive drain terminal coupled to the drain and a conductive source terminal coupled to the source, wherein the source terminal is coupled to the field plate via the conductive trace. In some embodiments, the substrate comprises a silicon substrate. In some embodiments, the source comprises a double-diffused region. 
     In another aspect, a method of fabricating a power transistor is disclosed. The method includes doping a substrate to form a source and a channel region within a first doped region, and a drain and a transition region within a second doped region, wherein the channel region is positioned between the source and the transition region, and the transition region is positioned between the channel region and the drain; forming a trench within a portion of the transition region proximate the drain; filling the trench with a field oxide; applying a gate oxide layer to a top surface of the substrate; forming a conductive layer over the channel region, the transition region, and a portion of the trench; removing a portion of the conductive layer over a first portion of the transition region, thereby forming two separate conductive layer portions separated by a gap including a first conductive layer portion positioned over the channel region and a first portion of the transition region, and a second conductive layer portion positioned over a second portion of the transition region and the portion of the trench; forming a conductive trace to electrically couple the second conductive layer portion to the source; and doping a third portion of the transition region exposed at the gap where the portion of the conductive layer is removed, thereby forming a doped bridge region between the first portion of the transition region and the second portion of the transition region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Several example embodiments are described with reference to the drawings, wherein like components are provided with like reference numerals. The example embodiments are intended to illustrate, but not to limit, the invention. The drawings include the following figures: 
         FIG. 1  illustrates a cut-out side view of an exemplary configuration of a conventional lateral power MOSFET. 
         FIG. 2  illustrates a cut-out side view of a split gate laterally-configured power transistor according to an embodiment. 
         FIG. 3  illustrates a gate charge curve for a conventional power MOSFET, such as that shown in  FIG. 1 , and the split gate power MOSFET of  FIG. 2 . 
         FIG. 4  illustrates a cut-out side view of a split gate laterally-configured power transistor according to another embodiment. 
         FIG. 5  illustrates a table of characteristics corresponding to comparable power transistors. 
         FIG. 6  illustrates a top down view of a portion of a split gate power transistor having a discontinuous bridge according to an embodiment. 
         FIG. 7  illustrates a table of characteristics corresponding to comparable power transistors. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present application are directed to a split gate power transistor. Those of ordinary skill in the art will realize that the following detailed description of the split gate power transistor is illustrative only and is not intended to be in any way limiting. Other embodiments of the split gate power transistor will readily suggest themselves to such skilled persons having the benefit of this disclosure. 
     Reference will now be made in detail to implementations of the split gate power transistor as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or like parts. In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will, of course, be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer&#39;s specific goals, such as compliance with application and business related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure. 
     Embodiments of a split gate power transistor include a laterally configured power MOSFET including a doped silicon substrate, a gate oxide layer formed on a surface of the substrate, and a split polysilicon layer formed over the gate oxide layer. The polysilicon layer is cut into two electrically isolated portions separated by a gap, a first portion forming a polysilicon gate positioned over a channel region of the substrate and a first portion of a transition region, and a second portion forming a polysilicon field plate formed over a second portion of a transition region of the substrate. The field plate also extends over a drift region of the substrate, where the drift region is under a field oxide filled trench formed in the substrate. The field plate is electrically coupled to a source of the power transistor. 
     The polysilicon layer is cut over the transition region. As a significant portion of the gate capacitance is due to the portion of the polysilicon gate formed over the transition region, removal of the cut polysilicon over the transition region reduces the gate capacitance, and therefore the gate charge. For a given on-resistance, the split gate configuration reduces the gate charge per cycle by about 50%. The gate charge determines how fast a switch is turned on and off. Reducing the gate charge allows for faster switching, and therefore higher frequency, at the same efficiency for the entire system. The higher frequency allows for the use of smaller discrete components which reduces costs. The split gate power transistor configuration is applicable to all switchable power supply integrated circuits that have internal switches. This configuration is not limited to integrated MOSFETs. The split gate power transistor configuration can be applied to any lateral power MOSFET, either integrated or discrete. 
       FIG. 2  illustrates a cut-out side view of a split gate laterally-configured power transistor according to an embodiment. In this exemplary configuration, the power transistor is a N-channel double-diffused MOSFET (N-channel DMOSFET). The substrate  110  is doped to form a P-type region  112  and a N-type region  114 . The P-type region  112  includes a double-diffused source  116  having a merged contact  124  between a P+ region  120  and a N+ region  122 . The contact  124  shorts the P+ region  120  and the N+ region  122  together. The contact  124  functions as a source contact of the split gate power transistor, and the source is shorted to the body of the substrate, which is P-type. The P-type region extends across the entire width of the lower portion of the substrate  110 , including underneath the N-type region  114  on the right hand side of  FIG. 2 . A source contact terminal  142  is coupled to the contact  124 , and therefore to the source  116 . The substrate  110  is also doped to form a N+ region  118  within the N-type region  114 . The N+ region  118  functions as the drain of the split gate power transistor. A drain contact terminal  140  is coupled to the drain  118 . A trench  126  is formed in a top surface of the substrate  110 . The trench  126  is filled with field oxide. In some embodiments, the trench  126  is formed using a Shallow Trench Isolation (STI) process, and the field oxide filled trench is referred to as a STI region. In other embodiments, the trench  126  is formed using any conventional semiconductor fabrication technique capable of removing a portion of the substrate to form a thick field oxide region. 
     A gate oxide  128  is formed on the top surface of the substrate  110 . In some embodiments, the gate oxide layer is deposited using conventional semiconductor deposition processes. A polysilicon layer is formed over the gate oxide  128 . A slice of the polysilicon layer is then removed, forming two electrically isolated polysilicon portions. In some embodiments, the polysilicon portions are formed using conventional semiconductor deposition and etching processes. A first polysilicon portion forms a polysilicon gate  130 . A second polysilicon portion forms a field plate  132 . The polysilicon gate  130  and the field plate  132  are physically separated by a gap  134 , which corresponds to the removed slice of polysilicon. An insulating oxide  138  covers the polysilicon gate  130  and the field plate  132 . As shown in  FIG. 2 , the gate oxide layer  128  between the polysilicon gate  130  and the substrate  110 , and the gate oxide layer  128  between the field plate  132  and the substrate  110  is a thin oxide layer. The field plate  132  is electrically isolated from the polysilicon gate  130  by the gap  134 , and the field plate  132  is electrically coupled to the source  116 . In many applications, power transistors are laid out having many interdigitated stripes, for example a source stripe, a gate stripe, and a drain stripe. For example, the drain stripe functions as the drain contact terminal  140 , and the source stripe functions as the source contact terminal  142 . In the split gate power transistor, the gate and the field plate can also be laid out in stripes, separated by the gap. For example, the field plate stripe functions as a field plate contact terminal, schematically illustrated in  FIG. 2  as field plate contact terminal  144 . In reference to  FIG. 2 , the stripes are oriented into and out of the plane of the page. If the gate is normally connected at end of its stripe, which can be hundreds of microns long, the field plate similarly extends as a stripe, the end of which is electrically connected to the source stripe by a conductive trace.  FIG. 2  conceptually illustrates this point as a conductive trace  146  coupling the field plate contact terminal  144  and the source contact terminal  142 . Alternatively, the field plate  132  and the source  116  can be electrically coupled along an entire length of the device, or along periodic contact points along the device length, where the length of the device is into and out of the page of  FIG. 2 . In these alternative configurations, a gap is cut into the oxide  138  to provide a contact access point to the field plate  132 . A gap is cut in the oxide  138  at each desired contact point or region. 
     The field plate  132  extends over the field oxide filled trench  126  to support high gate-to-drain voltage. The field plate  132  is necessary to maintain the breakdown voltage. If the field plate is removed, for example the entire polysilicon gate portion above the transition region is removed, the breakdown voltage suffers. In this case, almost all the gate-to-drain voltage is dropped across the thin gate oxide, which does not enable the power transistor to meet the rated voltage. 
     There are three main regions in the substrate  110  relative to the operation of the split gate power transistor: a channel region, a transition region, and a drift region. The channel region is formed underneath the polysilicon gate  130  and in the P-type region  112  of the substrate  110 . In other words, the channel region is formed where the polysilicon gate  130  overlaps the P-type region  112 . The drift region is the portion of the N-type region  114  underneath the trench  126 , or the STI region. The drift region is necessary to support a high gate-to-drain voltage. If the field plate  132  were to instead terminate over the thin gate oxide, this would result in too high a voltage over the gate oxide and the split gate power transistor would not function. As such, the STI region and the field plate extension over the STI region are necessary to drop the high gate-to-drain voltage. 
     The transition region is the portion of the N-type region  114  underneath the gate oxide  128 , the gap  134 , and the field plate  132 . The transition region provides a current flow path from the channel region to the drift region when the split gate power transistor is turned on. The transition region is also referred to as the accumulation region or the neck region. 
       FIG. 3  illustrates a gate charge curve for a conventional power MOSFET, such as that shown in  FIG. 1 , and the split gate power MOSFET of  FIG. 2 . The gate charge curve is a common figure of merit for MOSFETs. To determine the gate charge, the drain is connected to a nominal supply voltage through a load resistance, the source is grounded, and the gate is grounded. A constant current is forced into the gate, and the gate-to-source voltage Vgs is measured. As the supply voltage is applied to the gate, the gate-to-source voltage Vgs starts to rise until the threshold voltage is reached, which is 1.5V in this example. The threshold voltage corresponds to the flat portion of the curve, which is where the power transistor begins to turn on. When the gate-to-source voltage Vgs reaches the fully rated voltage, which is 5V in this example, the trace is stopped. The gate charge is determined as the integration of the measured voltage. In the example shown in  FIG. 3 , the gate charge curves are measured for power MOSFETS having a rated gain-to-source voltage of 5V and an operating voltage of 24V. In general, the operating voltage range is 14V to 60V without having to increase the footprint of the polysilicon that forms the active gate and the field plate of the split gate power transistor. 
     The curve  200  is the gate charge curve of the split gate power transistor of  FIG. 2 , and the curve  210  is for a similar conventional power transistor, such as the power transistor of  FIG. 1 . It is seen in  FIG. 3  that the gate charge of the split gate power transistor is reduced by about 50% compared to the conventional power transistor. Reducing the size of the active gate, by removing the slice of polysilicon, reduces the gate charge. It is still necessary to prevent the breakdown of the split gate power transistor, which is accomplished using the field plate. The active polysilicon gate and the field plate are electrically isolated so that the charge that effects the active gate is reduced to the lowest possible level. 
     It can also be seen that the flat portion of the curve  200  is reduced by approximately 75% compared to the flat portion of the curve  210 . The flat portion represents the gate-to-drain charge Qgd, which is the integral of the gate-to-drain voltage across the flat region. Within the flat region, more and more current is forced into the gate, but the gate-to-source voltage remains constant. The gate-to-drain charge Qgd is related to the feedback capacitance between the drain and the gate. In general, the portion of the gate that is positioned over the drain well is amplified and has more of an effect on the gate charge than the portion of the gate that is over the source well. Electrically connecting the field plate to the source, as is done in the split gate power transistor, effectively puts a conductive shield in between the gate and the drain. This reduces the feedback capacitance related to the Miller effect. The reduced flat portion on the gate charge curve reflects this reduction in the feedback capacitance. 
     The split gate power transistor provides a reduction in the product of on-resistance (R) and gate charge (Qg). An on-resistance of the power MOSFET is the resistance between the drain and the source while the transistor is turned on. However, there is a slight increase in the product of on-resistance (R) and gate area (A), referred to as the specific on-resistance. The specific on-resistance provides a conceptual measure of the size of the power transistor. The specific on-resistance of the split gate configuration rises not due to an increase in the physical gate area A, as the half-pitch of the split gate power transistor having the two polysilicon stripes remains the same as that of the comparable conventional power transistor having a single polysilicon stripe. Instead, the specific on-resistance increases due to an increase in the on-resistance R. When the split gate power transistor is turned completely on, for example when the gate-to-source voltage Vgs=5V, the current flows through the channel region, across the transistor region and the drift region, which is under the field oxide filled trench, and back up to the N+ drain. In the conventional configuration where the polysilicon gate covers the entire transition region, the polysilicon gate above the transition region is at 5V, which accumulates electrons in the transition region. When the gate-to-source voltage Vgs is positive, the transition region is considered accumulated, not inverted. With more electrons accumulated in the transition region, the resistance is reduced. However, in the split gate configuration, a portion of the polysilicon gate over the transition region is removed, and the remaining portion (the field plate) is connected to the source, not the 5V of the active gate. As such, electrons are accumulated in the transition region only in the portion that is directly overlapping with the polysilicon gate. The portion of the transition region that is under the polysilicon field plate and split gap does not accumulate and simply has its natural equilibrium concentration of electrons. As compared to the non-split gate configuration, there are fewer electrons in the transition region, which results in a higher resistance. In an exemplary application, there is an approximate 44% reduction in the R*Qg product, and an approximate 12% increase in the R*A product compared to the comparable conventional power transistor that does not have a split gate configuration, such as the lateral power MOSFET of  FIG. 1 . 
     The split gate power transistor also improves the hot carrier lifetime since the grounded field plate directs on-current away from the gate oxide thereby reducing the impact of injected charge on the on-state current flow inside the device. The field plate reduces the electric field for any given supply voltage, which effectively maintains the breakdown voltage of the split gate power transistor. In general, the split gate configuration and field oxide filled trench avoids premature breakdown of the split gate power transistor. In the split gate configuration, the field plate extends over the STI region, and the field plate is electrically connected to the source. The resulting source-to-drain capacitance increases by an amount that is slightly less than the reduction in the gate-to-drain capacitance. So the source-to-drain capacitance is higher, but overall there is an efficiency improvement. 
     In an exemplary application, the cut gap  134  ( FIG. 2 ) between the polysilicon gate  128  and the field plate  132  is fabricated using 0.18 micron semiconductor processing technology, resulting in a 0.25 micron wide gap. However, the gap can be larger or smaller than 0.25 microns, limited in size only by the available technology. For example, utilization of 0.13 micron semiconductor fabrication technology can achieve a gap width of 0.2 microns. In practice, the gap can be as small as technology allows, thereby minimizing the overall size of the transistor, such as the half-pitch. In general, formation of the split gate power transistor is accomplished without increasing the half-pitch, as compared to a comparable power transistor without the split gate configuration. 
     The following highlight some of the electrical properties of the split gate power transistor of  FIG. 2 , especially as compared to a comparable power transistor. First, the on-resistance is slightly higher (about 12% higher for a 24 V device) because the transition region is no longer accumulated when the device is turned on. The field plate is connected to the source so the field plate is grounded, and the transition region does not have as high a concentration of electrons. Second, the gate capacitance and the gate charge are reduced because of smaller gate area. Third, because the source-connected field plate is positioned between the gate and the drain, the gate-to-drain feedback capacitance is greatly reduced. This further reduces the gate charge because during switching, the gate-to-drain capacitance is amplified by the Miller effect. Fourth, peak impact ionization is reduced so that the hot carrier lifetime is improved. Or, for a given hot carrier lifetime, the half-pitch is reduced. Fifth, switch mode power supply (SMPS) efficiency is improved. 
     The embodiment of the split gate power transistor described above in  FIG. 2  provides the aforementioned properties by separating the field plate poly from the gate poly and connecting the field plate to the source. During operation with high drain voltage, an inversion layer of holes builds up under the field plate poly. With thin gate oxide, the voltage drop this inversion layer induces across the gate oxide under the field plate is larger than the maximum permitted for reliable operation. In an exemplary application where the split gate power transistor is rated at 14V operating voltage, there is a 4.1V drop across the gate oxide below the field plate  132  when the drain to source voltage Vds is 14V. Although such a voltage drop is fine for sufficiently thick gate oxides, this voltage drop is much higher than is allowed for reliable 10 year operation for thinner gate oxides, such as 85A. For a gate oxide thickness of 85A, for example, a maximum voltage drop across the gate oxide is less than about 3.3V. 
     A reason for the voltage drop across the gate oxide in the configuration of  FIG. 2  is there is an inversion layer of holes under the field plate  132 . These holes could be due to thermal generation as seen in a conventional Metal-Oxide-Silicon (MOS) interface. 
     To eliminate this inversion layer of holes, a lightly doped region is implanted below the gap between the field plate and the polysilicon gate. As applied to the exemplary N-channel configuration shown in  FIG. 2 , a lightly doped P-type region, also referred to as a P− bridge, is implanted in the N-type region below the gap. This P-type bridge forces the portion of transition region below the field plate  132  to go into deep-depletion with depletion assistance from the inherent P-N junction formed between the P-type bridge and the transition region. Formation of deep-depletion blocks any formation of inversion charge under the field plate thereby relieving the voltage in that area. Maximum gate oxide voltage is reduced to a level safe for use with thinner gate oxides. 
       FIG. 4  illustrates a cut-out side view of a split gate laterally-configured power transistor according to another embodiment. The exemplary split gate power converter of  FIG. 4  is similar to the split gate power converter of  FIG. 2  except that the split gate power converter of  FIG. 4  includes a bridge in the substrate portion underneath the gap in the polysilicon gate and field plate. As shown in  FIG. 4 , the substrate  210  is doped to include a bridge  236 . In some embodiments, the bridge  236  is a lightly doped P-type region. The bridge  236  functions to force the portion underneath the field plate  232  into deep-depletion and prevent the formation of an inversion layer. Inclusion of the P-type bridge  236  reduces the electrostatic potential directly underneath the field plate when compared to the split gate power transistor that does not include the bridge, as in  FIG. 2 . A reduced electrostatic creates a path for the holes away from underneath the field plate, and therefore there is not as large a voltage drop across the gate oxide. In some embodiments, the bridge is implanted. It is understood that other conventional doping techniques can be used, such as diffusion, as long as the technique enables proper application of a lightly doped region. In some embodiments, the bridge is doped using processing steps already performed as part of the power transistor fabrication process. Alternatively, additional processing steps can be performed to customize the doping concentration of the bridge. 
       FIG. 5  illustrates a table of characteristics corresponding to comparable power transistors. The first row is directed to a conventional power transistor having a continuous gate, such as the power transistor shown in  FIG. 1 . The second row is directed to a power transistor having a split gate, such as the split gate power transistor shown in  FIG. 2 . The third row is directed to a power transistor having a split gate with bridge, such as the split gate power transistor shown in  FIG. 4 . The column labeled “R*A” is the specific on-resistance, where R is the on-resistance and A is the gate area. The column labeled “ΔR*A” is the percentage change in specific on-resistance relative to the continuous gate power transistor of the first row. The column labeled “Q G ” is the gate charge. The column labeled “R*Q G ” is the product of the on-resistance and the gate charge. The column labeled “ΔR*Q G ” is the percentage change in the on-resistance and gate charge product relative to the continuous gate power transistor of the first row. The column labeled “BV DSS ” is the breakdown voltage. The column labeled “V OX @BV” is the voltage drop across the gate oxide at the breakdown voltage. The column labeled “V OX @hot” is the voltage drop across the gate oxide at the worst case hot carrier conditions. 
     The values shown in the table of  FIG. 5  are for simulated power transistors having identical dimensions, each rated at 14V operating voltage. For a power transistor configuration having a gate oxide thickness of 85A, the maximum rated voltage drop across the gate oxide is 3.6V. As shown in the “V OX @BV” column of  FIG. 5 , the “split” gate power transistor, which has a “V OX @BV” of 4.1V is not acceptable. However, either the “continuous” power transistor or the “split with bridge” power transistor would be acceptable for 14V Vds applications. The “split with bridge” power transistor reduces the voltage drop across the gate oxide and therefore can be applied to thinner gate oxide thicknesses. However, despite providing a reduction in gate charge and the R*Q G  product compared to the conventional “continuous” power transistor, the “split with bridge” power transistor has a large specific on-resistance penalty of +30%. For a 30% increase in specific on-resistance, the “split with bridge” power transistor reduces R*Q G  product by 27%. 
     In an alternative embodiment, the bridge  236  of  FIG. 4  is additionally doped with a lightly doped region, such as N−, positioned as a halo between the lightly doped P-type portion and the N-type region  214 . In some embodiments, the light doped halo N-type region is implanted. It is understood that other conventional doping techniques can be used, such as diffusion, as long as the technique enables proper application of a lightly doped region. In some embodiments, the doping concentration of the lightly doped P-type portion of the bridge is in the range of about 10 18 , and the doping concentration of the lightly doped halo N-type region of the bridge is in the range of about 10 17 . Addition of the light doped halo N-type region eliminates the specific on-resistance penalty seen in the split gate with bridge configuration. Referring to the table of  FIG. 5 , the fourth row is directed to a split gate power transistor having a bridge having the halo. As shown in  FIG. 5 , the “split with bridge/halo” power transistor actually reduces the specific on-resistance by 3%. The “split with bridge/halo” power transistor additionally reduces gate charge and the R*Q G  product compared to the “continuous” power transistor, the “split” power transistor, and the “split with bridge” power transistor. 
     The bridge described in the context of  FIG. 5 , whether the bridge does or does not include the halo, is configured to be continuous in both the width that spans the gap between the field plate and the gate and the length of the gap. The length of the gap is into and out of the page of  FIG. 4 . In alternative configurations, the width of the bridge across gap remains continuous, but the length is discontinuous. For example, 0.25 μm bridges can be disposed every 1-2 μm along the length direction of the gap. The use of discontinuous bridges reduces the specific on-resistance penalty, while still preventing the buildup of a hole inversion layer. 
       FIG. 6  illustrates a top down view of a portion of a split gate power transistor having a discontinuous bridge according to an embodiment. The exemplary split gate power transistor of  FIG. 6  is similar to the split gate power transistor of  FIG. 4  except that the bridge of  FIG. 4  is replaced by a discontinuous bridge.  FIG. 6  shows a portion of a gate  330 , a field plate  332 , and a gap  334 . Two bridge segments  336 A and  336 B represent the discontinuous bridge. It is understood that the discontinuous bridge can include more than two bridge segments. Each bridge segment  336 A,  336 B spans an entire width W of the gap  334 , but are discontinuous across a length L of the gap  334 . Each bridge segment  336 A,  336 B is similar in composition and function as the bridge  236  of  FIG. 4 . For example, each bridge segment  336 A,  336 B can include a lightly doped P-type region or both a lightly doped P-type region and a light doped halo N-type region. In some embodiments, all of the bridge segments are doped the same, for example all are either doped with the lightly doped P-type region or all are doped with both the lightly doped P-type region and the lightly doped halo N-type region. In other embodiments, the bridge segments can be doped differently, for example some bridge segments are doped with the lightly doped P-type region and some of the bridge segments are doped with both the light doped P-type region and the halo N-type region. In other alternative embodiments, the continuous bridge of  FIG. 4  can be modified to have discontinuous halo N-type regions. In this configuration, the gap length has a continuous lightly doped P-type region along the entire length of the gap, and segments of the bridge also include the halo N-type region. 
     The discontinuous bridge prevents the formation of a hole inversion layer underneath the field plate, but without the specific on-resistance penalty of the split gate without bridge configuration.  FIG. 7  illustrates a table of characteristics corresponding to comparable power transistors. The table of  FIG. 7  shows the “continuous” power transistor, the “split” power transistor, and the “split with bridge” power transistor similarly shown in the table of  FIG. 5  plus the “split with discontinuous bridges” power transistor corresponding to the split gate power transistor of  FIG. 6 . Although similar, the results shown in  FIG. 7  are not intended to be compared to the results of  FIG. 5 , as each represents a separate simulation. The values of the “split with discontinuous bridges” power transistor shown in  FIG. 7  correspond to a discontinuous bridge having bridge segments of the lightly doped P-type region, but no halo N-type region. Each bridge segment has a 0.25 μm length on a 1.5 μm pitch. As shown in the table of  FIG. 7 , instead of the 24% specific on-resistance penalty of the “split with bridge” power transistor, the penalty is much lower. One-sixth of the device L has R*L product that is 24% higher than the rest, so the resistance per unit length is only 3.4% higher than the device with no bridge at all. The “split with discontinuous bridges” power transistor has a 36% reduction in the R*Q G  product with only a 9% specific on-resistance penalty compared to the “continuous” gate power transistor. 
     In general, the discontinuous bridge without the bridge halo configuration provides the benefits of the continuous bridge configuration with the bridge halo. 
     In the presented embodiments, the bridge or bridge segments are floating. In other words, the bridge is not coupled to a contact. In other embodiments, the bridge or bridge segments are coupled to a contact. The contact can be coupled to ground or to a potential. 
     Embodiments of the split gate power transistor are described above as N-channel MOSFETs. Alternative embodiments are also contemplated, for example a P-channel MOSFET. Application to a P-channel MOSFET requires a slightly different configuration. Alternative configurations can be implemented where the split gate power transistor is configured with all aspects having opposite polarities than those shown in the described embodiments. 
     The gate material is described above as being polysilicon. Alternatively, the gate can be made of any conventional material used in the fabrication of semiconductor transistors including, but not limited to, polysilicon and/or metal. The substrate is described above as being silicon. Alternatively, the substrate can be a silicon-based compound, for example silicon germanium (SiGe). 
     The present application has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the split gate power transistor. Many of the components shown and described in the various figures can be interchanged to achieve the results necessary, and this description should be read to encompass such interchange as well. As such, references herein to specific embodiments and details thereof are not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications can be made to the embodiments chosen for illustration without departing from the spirit and scope of the application.