Abstract:
Integrated power semiconductor devices having improved high frequency switching performance, improved edge termination characteristics and reduced on-state resistance include GD-UMOSFET unit cells with upper trench-based gate electrodes and lower trench-based source electrodes. The use of the trench-based source electrode instead of a larger gate electrode reduces the gate-to-drain capacitance (C GD ) of the UMOSFET and improves switching speed by reducing the amount of gate charging and discharging current that is needed during high frequency operation. Methods according to the present invention also include the steps of forming a trench in a semiconductor substrate and then forming an insulated source electrode in and adjacent a bottom of the trench by forming a first electrically insulating layer on a bottom and sidewall of the trench, then forming a first electrically conductive layer on the first electrically insulating layer and then forming a second electrically insulating layer on the first electrically conductive layer using a thermal oxidation technique, for example. An insulated gate electrode is then formed in and adjacent a top of trench. Steps are also provided to form a source region of first conductivity type in the semiconductor substrate and form a second source electrode electrically connected to the source region and the insulated source electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. application Ser. No. 09/178,845, filed Oct. 26, 1998, now U.S. Pat. No. 5,998,833, assigned to the present assignee, the disclosure of which is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to semiconductor switching devices, and more particularly to switching devices for high power applications and methods of forming same. 
     BACKGROUND OF THE INVENTION 
     The silicon bipolar transistor has been the device of choice for high power applications in motor drive circuits, appliance controls, robotics and lighting ballasts. This is because bipolar transistors can be designed to handle relatively large current densities in the range of 40-50 A/cm 2  and support relatively high blocking voltages in the range of 500-1000V. 
     Despite the attractive power ratings achieved by bipolar transistors, there exist several fundamental drawbacks to their suitability for all high power applications. First of all, bipolar transistors are current controlled devices which require relatively large base currents, typically one fifth to one tenth of the collector current, to maintain the transistor in an operating mode. Proportionally larger base currents can be expected for applications which also require high speed turn-off. Because of the large base current demands, the base drive circuitry for controlling turn-on and turn-off is relatively complex and expensive. Bipolar transistors are also vulnerable to premature breakdown if a high current and high voltage are simultaneously applied to the device, as commonly required in inductive power circuit applications. Furthermore, it is relatively difficult to operate bipolar transistors in parallel because current diversion to a single transistor typically occurs at high temperatures, making emitter ballasting schemes necessary. 
     The silicon power MOSFET was developed to address this base drive problem. In a power MOSFET, the gate electrode provides turn-on and turn-off control upon the application of an appropriate gate bias. For example, turn-on in an N-type enhancement MOSFET occurs when a conductive N-type inversion layer is formed in the P-type base region (also referred to as “channel region”) in response to the application of a positive gate bias. The inversion layer electrically connects the N-type source and drain regions and allows for majority carrier conduction therebetween. 
     The power MOSFET&#39;s gate electrode is separated from the base region by an intervening insulating layer, typically silicon dioxide. Because the gate is insulated from the base region, little if any gate current is required to maintain the MOSFET in a conductive state or to switch the MOSFET from an on-state to an off-state or vice-versa. The gate current is kept small during switching because the gate forms a capacitor with the MOSFET&#39;s base region. Thus, only charging and discharging current (“displacement current”) is required during switching. Because of the high input impedance associated with the insulated-gate electrode, minimal current demands are placed on the gate and the gate drive circuitry can be easily implemented. Moreover, because current conduction in the MOSFET occurs through majority carrier transport only, the delay associated with the recombination and storage of excess minority carriers is not present. Accordingly, the switching speed of power MOSFETs can be made orders of magnitude faster than that of bipolar transistors. Unlike bipolar transistors, power MOSFETs can be designed to withstand high current densities and the application of high voltages for relatively long durations, without encountering the destructive failure mechanism known as “second breakdown”. Power MOSFETs can also be easily paralleled, because the forward voltage drop across power MOSFETs increases with increasing temperature, thereby promoting an even current distribution in parallel connected devices. 
     In view of these desirable characteristics, many variations of power MOSFETs have been designed. Two popular types are the double-diffused MOSFET device (DMOSFET) and the UMOSFET device. These and other power MOSFETs are described in a textbook by B. J. Baliga entitled  Power Semiconductor Devices , PWS Publishing Co. (ISBN 0-534-94098-6) (1995), the disclosure of which is hereby incorporated herein by reference. Chapter 7 of this textbook describes power MOSFETs at pages 335-425. Examples of silicon power MOSFETs including accumulation, inversion and extended trench FETs having trench gate electrodes extending into the N+ drain region are also disclosed in an article by T. Syau, P. Venkatraman and B. J. Baliga, entitled  Comparison of Ultralow Specific On-Resistance UMOSFET Structures : The ACCUFET, EXTFET, INVFET, and Convention UMOSFETs, IEEE Transactions on Electron Devices, Vol. 41, No. 5, May (1994). As described by Syau et al., specific on-resistances in the range of 100-250 μΩcm 2  were experimentally demonstrated for devices capable of supporting a maximum of 25 volts. However, the performance of these devices was limited by the fact that the forward blocking voltage must be supported across the gate oxide at the bottom of the trench. 
     FIG. 1, which is a reproduction of FIG.  1 ( d ) from the aforementioned Syau et al. article, discloses a conventional UMOSFET structure. In the blocking mode of operation, this UMOSFET supports most of the forward blocking voltage across the N-type drift layer which must be doped at relatively low levels to obtain a high maximum blocking voltage capability, however low doping levels typically increase the on-state series resistance. Based on these competing design requirements of high blocking voltage and low on-state resistance, a fundamental figure of merit for power devices has been derived which relates specific on-resistance (R on,sp ) to the maximum blocking voltage (BV). As explained at page 373 of the aforementioned textbook to B. J. Baliga, the ideal specific on-resistance for an N-type silicon drift region is given by the following relation: 
     
       
         R on,sp =5.93×10 −9 (BV) 2.5   (1) 
       
     
     Thus, for a device with 60 volt blocking capability, the ideal specific on-resistance is 170 μΩcm 2 . However, because of the additional resistance contribution from the base region (e.g., P-type base region in an N-channel MOSFET), reported specific on-resistances for UMOSFETs are typically much higher. For example, a UMOSFET having a specific on-resistance of 730 μΩcm 2  is disclosed in an article by H. Chang, entitled  Numerical and Experimental Comparison of  60V  Vertical Double-Diffused MOSFETs and MOSFETs With A Trench-Gate Structure , Solid-State Electronics, Vol. 32, No. 3, pp. 247-251, (1989). However, in this device a lower-than-ideal uniform doping concentration in the drift region was required to compensate for the high concentration of field lines near the bottom corner of the trench when blocking high forward voltages. U.S. Pat. Nos. 5,637,989, 5,742,076 and 5,912,497, the disclosures of which are hereby incorporated herein be reference, also disclose popular power semiconductor devices having vertical current carrying capability. 
     In particular, U.S. Pat. No. 5,637,898 to Baliga discloses a preferred silicon field effect transistor which is commonly referred to as a graded-doped (GD) UMOSFET. As illustrated by FIG. 2, which is a reproduction of FIG. 3 from the &#39;898 patent, a unit cell  100  of an integrated power semiconductor device field effect transistor may have a width “W c ” of 1 μm and comprise a highly doped drain layer  114  of first conductivity type (e.g., N+) substrate, a drift layer  112  of first conductivity type having a linearly graded doping concentration therein, a relatively thin base layer  116  of second conductivity type (e.g., P-type) and a highly doped source layer  118  of first conductivity type (e.g., N+). The drift layer  112  may be formed by epitaxially growing an N-type in-situ doped monocrystalline silicon layer having a thickness of 4 μm on an N-type drain layer  114  having a thickness of 100 μm and a doping concentration of greater than 1×10 18  cm −3  (e.g. 1×10 19  cm −3 ) therein. The drift layer  112  also has a linearly graded doping concentration therein with a maximum concentration of 3×10 17  cm −3  at the N+/N junction with the drain layer  114 , and a minimum concentration of 1×10 16  cm −3  beginning at a distance 3 μm from the N+/N junction (i.e., at a depth of 1 μm) and continuing at a uniform level to the upper face. The base layer  116  may be formed by implanting a P-type dopant such as boron into the drift layer  112  at an energy of 100 kEV and at a dose level of 1×10 14  cm −2 . The P-type dopant may then be diffused to a depth of 0.5 μm into the drift layer  112 . An N-type dopant such as arsenic may also be implanted at an energy of 50 kEV and at dose level of 1×10 15  cm −2 . The N-type and P-type dopants can then be diffused simultaneously to a depth of 0.5 μm and 1.0 μm, respectively, to form a composite semiconductor substrate containing the drain, drift, base and source layers. 
     A stripe-shaped trench having a pair of opposing sidewalls  120   a  which extend in a third dimension (not shown) and a bottom  120   b  is then formed in the substrate. For a unit cell  100  having a width W c  of 1 μm, the trench is preferably formed to have a width “W t ” of 0.5 μm at the end of processing. An insulated gate electrode, comprising a gate insulating region  124  and an electrically conductive gate  126  (e.g., polysilicon), is then formed in the trench. The portion of the gate insulating region  124  extending adjacent the trench bottom  120   b  and the drift layer  112  may have a thickness “T 1 ” of about 2000 Å to inhibit the occurrence of high electric fields at the bottom of the trench and to provide a substantially uniform potential gradient along the trench sidewalls  120   a . The portion of the gate insulating region  124  extending opposite the base layer  116  and the source layer  118  may have a thickness “T 2 ” of about 500 Å to maintain the threshold voltage of the device at about 2-3 volts. Simulations of the unit cell  100  at a gate bias of 15 Volts confirm that a vertical silicon field effect transistor having a maximum blocking voltage capability of 60 Volts and a specific on-resistance (R sp,on ) of 40 μΩcm 2 , which is four (4) times smaller than the ideal specific on-resistance of 170 μΩcm 2  for a 60 volt power UMOSFET, can be achieved. Notwithstanding these excellent characteristics, the transistor of FIG. 2 may suffer from a relatively low high-frequency figure-of-merit (HFOM) if the overall gate-to-drain capacitance (C GD ) is too large. Improper edge termination of the MOSFET may also prevent the maximum blocking voltage from being achieved. 
     Thus, notwithstanding these attempts to develop power semiconductor devices that can be switched at high speed and have high maximum blocking voltage capability and low specific on-resistance, there still continues to be a need to develop power devices having improved electrical characteristics. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide integrated circuit power devices having low on-state resistance and high maximum blocking voltage capability, and methods of forming same. 
     It is another object of the present invention to provide integrated circuit power devices having excellent high frequency switching characteristics, and methods of forming same. 
     It is still another object of the present invention to provide integrated circuit power devices having reduced susceptibility to parasitic oxide breakdown, and methods of forming same. 
     These and other objects, advantages and features of the present invention are provided by integrated power semiconductor device which may comprise a plurality of graded-doped (GD) UMOSFET unit cells having, among other things, improved high frequency switching performance, improved edge termination characteristics and reduced on-state resistance. The preferred integrated power semiconductor devices may also include integral Schottky barrier flyback diodes and shielded gate insulating regions. 
     According to one embodiment of the present invention, a GD-UMOSFET is provided having an upper trench-based gate electrode and a lower trench-based source electrode. The use of the trench-based source electrode instead of a larger gate electrode which occupies the entire trench reduces the gate-to-drain capacitance (C GD ) of the UMOSFET and thereby improves switching speed by reducing the amount of gate charging and discharging current that is needed during high frequency operation. In this embodiment of an integrated power semiconductor device, a plurality of GD-UMOSFET unit cells may be provided side-by-side in a semiconductor substrate having first and second opposing faces. Source and drain regions of first conductivity type (e.g., N+) are also provided in the substrate. The source region may extend adjacent the first face and the drain region may extend adjacent the second face. A drift region of first conductivity type is also provided in the substrate. The drift region, which forms a non-rectifying junction with the drain region, may be formed as an epitaxial layer of predetermined thickness and the doping profile in the drift region may be linearly graded and decrease in a direction from the drain region to the first face. For an enhancement mode UMOSFET device, a base region of second conductivity type (e.g., P-type) is formed in the substrate. The base region extends between the source region and the drift region and forms first and second P-N junctions therewith, respectively. 
     A plurality of trenches are also provided in the substrate, at the first face. These trenches may be formed as parallel stripe-shaped trenches. With respect to a particular unit cell, a first trench may be provided having opposing sidewalls when viewed in transverse cross-section. One of these sidewalls preferably extends adjacent the drift region and the base region. The first trench may also have a bottom which extends opposite the drain region. In particular, the bottom of the first trench may define an interface between an interior of the first trench and the drift region or an interior of the trench and the drain region, depending on the depth of the first trench and the thickness of the drift region. A gate electrode is also provided in the first trench. This gate electrode preferably extends opposite the base region so that an inversion layer channel can be formed in the base region when an appropriate bias is applied to the gate electrode. 
     According to a preferred aspect of this embodiment of the present invention, a first source electrode is also provided in the first trench and this first source electrode extends between the gate electrode and the bottom of the first trench. An electrically insulating region is also provided in the first trench. This electrically insulating region extends along the sidewalls of the first trench, between the gate electrode and the first source electrode and between the first source electrode and the bottom of the trench. The inclusion of this source electrode adjacent the bottom of the first trench improves the breakdown and high frequency switching characteristics of the UMOSFET with only minimal impact on specific on-state resistance. 
     According to another preferred aspect of this invention, the electrically insulating region includes a gate insulating region having a first thickness (e.g., T 2 ≦750 Å) as measured between the gate electrode and the sidewall of the first trench, and a source insulating region having a second thickness (e.g., T 1 ≧1500 Å) as measured between the first source electrode and the same sidewall. In addition, a second source electrode is provided on the first face, in ohmic contact with the source region. The first and second source electrodes are also electrically connected together. 
     Improved edge termination characteristics can also be achieved by forming a second trench which extends adjacent the first trench and defines an edge of the integrated power device comprising the plurality of side-by-side GD-UMOSFET unit cells. According to this aspect of the present invention, a uniformly thick first field plate insulating region is provided which lines the sidewalls and bottom of the second trench and a field plate is provided on the first field plate insulating region. This field plate is preferably connected to the source electrode or the gate electrode. In addition, a second field plate insulating region is provided on the first face and this second field plate insulating region is contiguous with the first field plate insulating region. A field plate extension is provided on the second field plate insulating region and extends opposite the first face. This field plate extension is electrically connected to the field plate in the second trench. 
     To improve the edge termination and breakdown characteristics of the integrated power device even further, the second trench is positioned so that the first and second trenches define a transition mesa region therebetween. However, unlike the mesa regions which may be defined between trenches within the active area of the integrated power device, the transition mesa region is preferably formed to be devoid of a source region of first conductivity type. Instead, a preferred breakdown shielding region of second conductivity type (e.g., P+) is provided which extends to the first face and forms a third P-N junction with the drift region. Here, the breakdown shielding region may be formed deeper (and more highly doped) than the base region to increase the likelihood that avalanche breakdown will occur in the transition mesa region instead of within the active area, to thereby improve device reliability. According to still further aspects of the present invention, an integrated power semiconductor device may include an integral Schottky barrier diode (SBD) along one sidewall of a trench and a GD-UMOSFET along an opposing sidewall of the trench. In particular, a Schottky rectifying junction can be formed to a uniformly doped portion of the drift region which extends adjacent the first face. This composite SBD and GD-UMOSFET is valuable for motor control and synchronous rectifier applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a prior art power device. 
     FIG. 2 is a cross-sectional view of another prior art power semiconductor device and doping profile therein. 
     FIG.3 is a cross-sectional view of a unit cell of an integrated power semiconductor device and doping profile therein, according to a first embodiment of the present invention. 
     FIGS. 4A-4K are cross-sectional views of intermediate structures which illustrate preferred methods of forming the integrated power semiconductor device of FIG.  3 . 
     FIG. 5 is a cross-sectional view of an integrated power semiconductor device according to a second embodiment of the present invention. 
     FIG. 6 is a cross-sectional view of an integrated power semiconductor device according to a third embodiment of the present invention. 
     FIG. 7 is a cross-sectional view of an integrated power semiconductor device according to a fourth embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Moreover, the terms “first conductivity type” and “second conductivity type” refer to opposite conductivity types such as N or P-type, however, each embodiment described and illustrated herein includes its complementary embodiment as well. Like numbers refer to like elements throughout. 
     Referring now to FIG. 3, an integrated power semiconductor device according to a first embodiment of the present invention will be described. In particular, a unit cell  200  of a preferred integrated power semiconductor device has a predetermined width “W c ” (e.g., 1 μm) and comprises a highly doped drain layer  114  of first conductivity type (e.g., N+), a drift layer  112  of first conductivity type having a linearly graded doping concentration therein, a relatively thin base layer  116  of second conductivity type (e.g., P-type) and a highly doped source layer  118  of first conductivity type (e.g., N+). A source electrode  128   b  and drain electrode  130  may also be provided at the first and second faces, in ohmic contact with the source layer  118  and drain layer  114 , respectively. The source electrode  128   b  also preferably forms an ohmic contact with the base layer  116  in a third dimension (not shown). The drift layer  112  may be formed by epitaxially growing an N-type in-situ doped monocrystalline silicon layer having a thickness of about 4 μm on an N-type drain layer  114  (e.g., N+substrate) having a thickness of 100 μm and a first conductivity type doping concentration of greater than about 1×10 18  cm −3  (e.g. 1×10 19  cm −3 ) therein. As illustrated, the drift layer  112  may have a linearly graded doping concentration therein with a maximum concentration of greater than about 5×10 16  cm −3  (e.g., 3×10 17  cm −3 ) at the N+/N non-rectifying junction with the drain layer  114  and a minimum concentration of 1×10 16  cm −3  at a depth of 1 μm and continuing at a uniform level to the upper face. The base layer  116  may be formed by implanting P-type dopants such as boron into the drift layer  112  at an energy of 100 kEV and at a dose level of 1×10 14  cm −2 , for example. The P-type dopants may then be diffused to a depth of 0.5 μm into the drift layer  112 . An N-type dopant such as arsenic may then be implanted at an energy of 50 kEV and at dose level of 1×10 15  cm −2 . The N-type and P-type dopants are then diffused simultaneously to a depth of 0.5 μm and 1.0 μm, respectively, to form a composite semiconductor substrate containing the drain, drift, base and source layers. As illustrated by FIG. 3, the first conductivity type (e.g., N-type) doping concentration in the drift layer  112  is preferably less than about 5×10 16  cm −3  at the P-N junction with the base layer  116  (i.e., second P-N junction), and more preferably only about 1×10 16  cm −3  at the P-N junction with the base layer  116 . The second conductivity type (e.g., P-type) doping concentration in the base layer  116  is also preferably greater than about 5×10 16  cm −3  at the P-N junction with the source layer  118  (i.e., first P-N junction). Furthermore, according to a preferred aspect of the present invention, the second conductivity type doping concentration in the base layer  116  at the first P-N junction (e.g., 1×10 17  cm −3  is about ten times greater than the first conductivity type doping concentration in the drift region at the second P-N junction (e.g., 1×10 16  cm −3 ). 
     A stripe-shaped trench having a pair of opposing sidewalls  120   a  which extend in a third dimension (not shown) and a bottom  120   b  is then formed in the substrate. For a unit cell  100  having a width W c  of 1 μm, the trench is preferably formed to have a width “W t ” of 0.5 μm at the end of processing. A gate electrode/source electrode insulating region  125 , a gate electrode  127  (e.g., polysilicon) and a trench-based source electrode  128   a  (e.g., polysilicon) are also formed in the trench. Because the gate electrode  127  is made relatively small and does not occupy the entire trench, the amount of gate charge required to drive the unit cell  200  during switching is much smaller than the amount of gate charge required to drive the unit cell  100  of FIG. 2 (assuming all other parameters and dimensions are equal), as described more fully hereinbelow. As will be understood by those skilled in the art, the trench may also be formed as a cylindrical or annular-shaped trench which when viewed in transverse-cross section appears as a trench having opposing sidewalls. 
     Here, the trench-based source electrode  128   a  is electrically connected to the source electrode  128   b  in a third dimension (not shown). The portion of the gate electrode/source electrode insulating region  125  extending adjacent the trench bottom  120   b  and the drift layer  112  may also have a thickness “T 1 ” in a range between about 1500 Å and 3000 Å, for example, to inhibit the occurrence of high electric field crowding at the bottom corners of the trench and to provide a substantially uniform potential gradient along the trench sidewalls  120   a . However, the portion of the gate electrode/source electrode insulating region  125  extending opposite the base layer  116  and the source layer  118  preferably has a thickness “T 2 ” of less than about 750 Å, and more preferably about 500 Å to maintain the threshold voltage of the device at about 2-3 volts. The use of tapered trench-based insulating regions to improve electric field profiles in highly doped drift region mesas is also described in U.S. application Ser. No. 09/322,424, to B. Jayant Baliga, entitled “Power Semiconductor Devices That Utilize Tapered Trench-Based Insulating Regions to Improve Electric Field Profiles in Highly Doped Drift Region Mesas and Methods of Forming Same”, filed May 28, 1999, the disclosure of which is hereby incorporated herein by reference. In particular, these tapered trenchbased insulating regions are most preferably designed to have a nonuniform thickness T ins (y) in a range between about 0.5 and 1.5 times T ideal (y), where T ideal (y)| y≧α =ε ins ((2ε s E cr /qW m N d ) (y−-α)+¼W m )/ε s  and ε ins  is the permittivity of the electrically insulating region, ε s  is the permittivity of the drift region, E cr  is the breakdown electric field strength of the drift region, q is the electron charge, N d  is the first conductivity type doping concentration in the drift region, W m  is a width of the mesa, y is the depth, relative to a top of the first trench, at which the thickness of the electrically insulating region is being determined and α is a non-negative constant. The constant α may equal zero in the event the power device is a Schottky rectifier and may equal the depth of the P-base region/N-drift region junction in the event the power device is a vertical MOSFET, for example. 
     Numerical simulations of the unit cell  200  of FIG. 3 were performed using a drift region doping concentration which increased from a value of 1×10 16  cm −3  at a depth of 1 micron to a value of 2×10 17  cm −3  at a trench depth (T D ) of 5 microns. The thin portion of the gate/source electrode insulating region  125  extended to 1.2 microns of a total trench depth of 4.7 microns and had a thickness of 500 Å. The thick portion of the gate/source electrode insulating region  125  had a thickness of 3000 Å. The N+ source layer  118  depth was set to 0.3 microns, and the depth of the P-type base region  116  was set to 0.9 microns. A half-cell width of 1 micron was used based on 1 micron design rules. The polysilicon gate electrode  127  extended to 1.2 microns and the polysilicon source electrode  128   a  extended from 1.5 microns to 4.4 microns. Based on these parameters, the specific on-resistance (R on,sp ) at a gate bias of 15 volts was found to be 114 microOhm cm 2  and the device blocked more than 60 volts. Comparisons of the potential distributions and contours in the device of FIG. 3 at a drain bias of 60 volts, against the device of FIG. 2, indicated essentially no change in the electric field profile within the drift region. This finding suggests that the trench-based source electrode  128   a  does not upset the degree of charge coupling and field distribution required to obtain high performance operation. Moreover, even though the specific on-resistance of the device of FIG. 3 was about 20% greater than the specific on-resistance of the device of FIG. 2, the high frequency figure-of-merit (HFOM), defined as (R on,sp (Q GS +Q GD )) −1 , where Q GS  and Q GD  represent the gate-source and gate-drain charge per unit area, was also calculated to be three (3) times better than the HFOM for the device of FIG.  2 . This result means the device of FIG. 3 is very suitable for high frequency operation. 
     Referring now to FIGS. 4A-4K, a preferred method of forming the integrated power semiconductor device of FIG. 3 will be described. As illustrated best by FIG. 4A, the method begins with the step of forming a semiconductor substrate  10  by epitaxially growing a drift region  12  of first conductivity type (e.g., N-type) on a highly doped drain region  14  of first conductivity type which has a doping concentration greater than 1×10 18  cm −3  therein. Computer controlled in-situ doping of the drift region  12  is also preferably performed during the epitaxial growth step so that the drift region  12  has a linearly graded (or step graded) first conductivity type doping concentration therein which decreases in a direction away from the drain region  14 . In particular, the drift region  12  is preferably doped so that the doping concentration of the drift region  12  at the abrupt non-rectifying junction (J 3 ) is greater than about 1×10 17  cm −3  and more preferably about 3×10 17  cm −3 , but less than 5×10 16  cm −3  at the first face  15   a  and more preferably only about 1×10 16  cm −3  at the first face  15   a.    
     Referring now to FIG. 4B, a thin base region  16  is then formed in the substrate  10  by patterning a first implant mask (not shown) on the first face  15   a  and then performing an implant of second conductivity type dopants through the first mask. The implanted second conductivity type dopants can then be diffused into the drift region  12  to an initial depth of about 0.5 μm, for example. These steps are then preferably followed by the steps of patterning a second implant mask (not shown) on the first face  15   a  and performing an implant of first conductivity type dopants through the second mask. The second mask is also preferably patterned in a third dimension (not shown) so that portions of the underlying base region  16  do not receive the implanted source dopants. These portions of the base region  16  which extend to the first face  15   a  can be ohmically contacted by a source electrode at the end of processing. The implanted first conductivity type dopants and the second conductivity type dopants can then be diffused to a depth of about 0.5 μm and 1.0 μm, respectively, to provide a base region thickness “T c ” of about 0.5 μm. Preferably, boron (B) is used as a P-type dopant and is implanted at a dose level of about 1×10 14  cm −2  and at an energy of 100 keV. Arsenic (As) is preferably used as an N-type dopant and is implanted at a dose level of about 1×10 15  cm −2  and at an energy of 50 KeV. 
     As will be understood by those skilled in the art, the implant and diffusion steps will cause the doping profile of the second conductivity type dopants in the substrate  10  to be generally gaussian in shape and have a maximum value at the first face  15   a . The doping concentration in the base region  16  will also have a maximum value adjacent the source region  18  and a minimum value adjacent the drift region  12 . In particular, the implant and diffusion steps may be performed so that the first conductivity type dopant concentration of the source region  18  at the first face  15   a  is greater than 1×10 18  cm −3  and the second conductivity type dopant concentration in the base region  16  is greater than about 1×10 17  cm −3  at a first P-N junction (J 1 ) with the source region  18 , but less than about 5×10 16  cm −3  and more preferably only about 1×10 16  cm −3  at the second P-N junction (J 2 ) with the drift region  12 . To meet these criteria, the first conductivity type doping concentration in the drift region  12  should be about 1×10 16  cm −3  at the second P-N junction (J 2 ). Because of this relatively low value of 1×10 16  cm −3  in the drift region  12 , the base region  16  can be made thin (e.g., 0.5 μm) without being susceptible to parasitic reach-through breakdown and can be doped relatively low to maintain the transistor&#39;s threshold voltage at about 2-3 volts. 
     Referring now to FIG. 4C, an etching mask  22  including a stress relief oxide layer  22   a  and an oxidation barrier layer  22   b  (e.g., Si 3 N 4 ) is then patterned on the first face  15   a  to define openings which expose adjacent portions of the first face extending over the source and base regions  18  and  16 . The source, base and drift regions are then chemically etched to form a plurality of trenches  20 . As illustrated, each of the trenches has opposing vertical sidewalls  20   a  which define interfaces between the source, base and drift regions and an interior of the trench, and a trench bottom  20   b  in the drift region. Facing sidewalls  20   a  of adjacent trenches also define respective mesas  17  which may have uniform widths of about 0.5 μm at the end of processing. The trenches  20  and mesas  17 , which extend in a third dimension, not shown, can be of stripe or similar geometry. The mesas  17  can also be polygonal (e.g., hexagonal) in shaped with the trenches  20  defining a continuous mesh when viewed from a direction normal to the face  15   a . As explained more fully hereinbelow, the widths of the mesas  17  can be selected to improve the blocking voltage capability of the transistor. In particular, the width of the mesas  17  (at the end of processing) and the doping concentration in the drift region  12  at J 3  should be selected so that their product is within the range of 1×10 13 -2×10 13  cm −2  to obtain a preferred charge concentration in the drift region  12 . In addition, the width of the mesas  17  and the doping concentration in the drift region  12  at J 2  should be selected so that their product is within the range of 1×10 11 -2×10 12  cm −2 . 
     Referring now to FIG. 4D which illustrates a cross-section of three adjacent mesas  17 , the method continues with the formation of a first electrically insulating region  24  (e.g., SiO 2 ) having a first thickness greater than 1000 Å and more preferably about 3000 Å, on the trench sidewalls  20   a  and the trench bottom  20   b  of each trench  20 . This step is preferably performed by oxidizing the etched source, base and drift regions, using the oxidation barrier layer  22   b  as an oxidation mask. As will be understood by those skilled in the art, the growth of an oxide having a thickness of about 3000 Å will typically consume about 0.1 μm or more of semiconductor material. Accordingly, the initial widths of the mesas  17  should be selected so that at the end of processing the widths are at the desired value of about 0.5 μm. 
     Referring now to FIGS. 4E-4K, a conformal N-type first polycrystalline silicon region  26  is then deposited and etched until it is recessed in the trench to a depth just below the second P-N junction (J 2 ). This first polysilicon region  26  defines a trench-based source electrode. An oxide etching step is then performed to remove the first electrically insulating region  24  from the portions of the sidewalls  20   a  which extend adjacent the etched source and base regions  18  and  16 , as illustrated best by FIG.  2 F. Referring now to FIG. 4G, a second electrically insulating region  28  (e.g., SiO 2 ) having a second thickness less than about 1000 Å and more preferably about 500 Å, is then formed on the exposed trench sidewalls  20   a  and on the polysilicon region  26 . Referring now to FIG. 2H, a conformal second polycrystalline silicon region  30  is then deposited on the second electrically insulating region  28 . The second polycrystalline silicon region  30  is then etched until it is recessed in the trench to a depth just below the first face  15   a . Steps are also preferably performed to expose the first polysilicon region  26  in a third dimension (not shown) so that subsequent contact can be made to a source electrode on the face  15   a.    
     A third electrically insulating region  32  is then formed on the etched second polycrystalline silicon region  30  by oxidizing the second polycrystalline silicon region  30 , as illustrated by FIG.  21 . The stress relief oxide layer  22   a  and the oxidation barrier layer  22   b  (e.g., Si 3 N 4 ) are then etched to expose the source region  18  and base region  16  at the first face  15   a , as illustrated by FIG. 4J. A source metal contact layer  34  is then deposited on the first face  15   a  and a drain metal contact layer  36  is deposited on an opposing second face  15   b  to form ohmic contacts to the source, base and first polysilicon regions ( 18 ,  16  and  26 ) and drain region  14 , respectively, as illustrated by FIG.  4 K. 
     Referring now to FIG. 5, an integrated power semiconductor device  300  according to a second embodiment of the present invention will be described. As illustrated, this power device  300  includes an active device region and an edge termination region which may extend adjacent an outermost edge of a semiconductor substrate containing the power device  300 . According to one aspect of this embodiment, the active device region may include a plurality of unit cells which are similar to the unit cells  100  and  200  of FIGS. 2 and 3, respectively. However, to improve edge termination characteristics, an edge termination trench is provided in the edge termination region and a uniformly thick first field plate insulating region  134  is provided which lines the sidewalls and bottom of the edge termination trench. A field plate  136 , comprising a material such as N-type polysilicon, is also provided on the first field plate insulating region  134 . In addition, a second field plate insulating region  138  is provided on the first face and this second field plate insulating region overlaps the first field plate insulating region  134 . To complete the field plate structure, a field plate extension  140  is provided. This field plate extension  140 , which is electrically connected to the field plate  136 , is provided on the second field plate insulating region  138  and extends opposite the face of the substrate, as illustrated. This field plate extension  140  may also comprise N-type polysilicon. The field plate  136  is also preferably connected to the source electrode  128   b  or gate electrode  126 . 
     To improve the edge termination and breakdown characteristics of the integrated power device  300  even further, the edge termination trench is preferably positioned so that a transition mesa region is defined between opposing sidewalls of the edge termination trench and the trench corresponding to the outermost unit cell of the device  300 . However, unlike the mesa regions which are defined between trenches within the active device region of the integrated power device  300 , the transition mesa region is preferably formed to be devoid of a source region of first conductivity type. Instead, a preferred breakdown shielding region  117  of second conductivity type (e.g., P-type) is provided. The breakdown shielding region  117  may be formed at the same time the base region  116  is formed, for example. However, as illustrated best by FIG. 6 which is a cross-sectional view of an integrated power semiconductor device  300 ′ according to a third embodiment of the present invention, the breakdown shielding region  117 ′ may also be formed deeper (and more highly doped) than the base region  116  to further increase the likelihood that avalanche breakdown will occur in the transition mesa region instead of within the active region. The use of breakdown shielding regions to improve the breakdown characteristics of power semiconductor devices is also described in commonly assigned U.S. application Ser. No. 09/167,298, filed Oct. 6, 1998, entitled “Rugged Schottky Barrier Rectifiers Having Improved Avalanche Breakdown Characteristics”, the disclosure of which is hereby incorporated herein by reference. 
     Numerical simulations of the unit cell  300  of FIG. 5 were performed using a drift region doping concentration which increased from a value of 1×10 16  cm −3  at a depth of 1 micron to a value of 2×10 17  cm −3  at a trench depth (TD) of 5 microns. The thin portion of the gate electrode insulating region  124  had a thickness of 500 Å, the thick portion of the gate electrode insulating region  124  had a thickness of 3000 Å and the first field plate insulating region  134  had a uniform thickness of 3000 Å. Based on these parameters, the simulated potential contours were shown to be uniformly spaced in both the active and termination regions. The simulated current flowlines also indicated that breakdown would occur simultaneously in both the active and termination regions so long as the breakdown shielding region  117  is electrically connected to the source electrode  128   b . Accordingly, breakdown in the integrated power device  300  of FIG. 5 is not expected to be edge limited. Moreover, in applications where the GD-MOSFET unit cells in the active region are expected to be frequently driven into avalanche breakdown, it is preferable to move the location of avalanche breakdown to the more highly doped and deeper breakdown shielding region  117 ′ within the device  300 ′ of FIG.  6 . In particular, by increasing the depth of the breakdown shielding region  117 ′, the breakdown voltage can be decreased to a level which will provide sufficient protection to the unit cells in the active device area and improve the lifetime and reliability of the overall device  300 ′. In addition, as described more fully hereinbelow with respect to FIG. 7, the breakdown voltage and the specific on-resistance R sp,on  can also be scaled downward by decreasing the epitaxial layer thickness of the drift region  112 . 
     Referring now to FIG. 7, an integrated power semiconductor device  400  according to a fourth embodiment of the present invention will be described. As illustrated, this power device  400  is a hybrid device which may contain the GD-UMOSFET unit cells of FIGS. 2 and 3 (having non-uniformly thick gate insulating regions therein) extending adjacent one sidewall of each trench in the active region and a modified TMBS Schottky rectifier (acting as a flyback diode) extending adjacent an opposing sidewall of each trench. In the modified TMBS Schottky rectifier illustrated on the left side of the unit cell of FIG. 7, preferred charge coupling is provided by a gate electrode within a trench having a non-uniformly thick gate insulating region therein, instead of an anode electrode and a uniformly thick insulating region. Moreover, whereas conventional TMBS rectifiers, such as those disclosed in U.S. Pat. No. 5,612,567 to Baliga which is hereby incorporated herein by reference, include a linearly graded drift region doping concentration which extends all the way to the Schottky rectifying junction at the face, a uniformly doped region extends adjacent the Schottky rectifying junction in the modified TMBS rectifier of FIG.  7 . As illustrated, this uniformly doped portion of the drift region has a doping concentration of 1×10 16  cm −3  therein. The advantages of including a uniformly doped region adjacent the Schottky rectifying junction in a TMBS device are more fully described in the aforementioned application entitled “Rugged Schottky Barrier Rectifiers Having Improved Avalanche Breakdown Characteristics”. 
     This hybrid power device  400  is designed so that the GD-MOSFET unit cells have very low specific on-state resistance, the modified TMBS structure has very low leakage current and low on-state voltage drop and the combined hybrid structure exhibits very low parasitic inductance. In particular, simulations of the hybrid device of FIG. 7 illustrate that the on-state voltage drop of both the GD-MOSFET and modified TMBS are reduced because of improved current spreading in the N+ substrate region  114 . The specific on-resistance R sp,on  can also be scaled downward by decreasing the epitaxial layer thickness of the drift region  112 . This reduction in epitaxial layer thickness causes the nonrectifying junction formed between the drift region  112  and the drain region  114 ′ (illustrated by the dotted line in FIG. 7) to move up along the sidewalls of each trench, without any other modification in the process. Accordingly, the reduction in epitaxial layer thickness (or increase in trench depth) results in the formation of an interface between the bottom of the trench  120   b  and the drain region  114 ′. The simulations also indicate the possibility of higher temperature operation with smaller heat sinks because of an improvement in the leakage current characteristics. The hybrid device also limits the amount of parasitic inductance between the GD-MOSFET and modified TMBS rectifiers within each unit cell. 
     As further illustrated by FIG. 7, an increase in the depth of the base region  116  to the level illustrated by the dotted line  116 ′ can also be used advantageously to suppress the degree of any impact ionization near the gate insulating region  124  which may arise in response to hot electron injection during avalanche breakdown. In particular, steps to form the gate insulating region so that there is an overlap between the thick portion of the gate insulating region  124  (extending upward from the bottom of the trench) and the base region  116  can be used to enhance the electric field contours at the corner of the gate between the thick and thin portions and thereby shield the gate insulating region from the effects of hot electron induced instabilities during avalanche breakdown. Accordingly, the gate insulating region  124  may have a first thickness (shown as T 2 ) of less than about 750 Å as measured between the gate electrode  126  and a first intersection between the first sidewall and the P-N junction formed between the source region  118  and P-base region  116 . In addition, the gate insulating region  124  may have a second thickness (shown as T 1 ) of greater than about 1500 Å as measured between the gate electrode  126  and a second intersection between the first sidewall and the P-N junction formed between the P-base region  116  and the drift region  112 . Moreover, because of the illustrated graded doping profile of the base region  116  which falls off near the base/drift region junction, it is still possible to form an inversion layer channel across the entire base region  116  even though the gate oxide thickness is relatively large (e.g., 3000 Å) at the drain side of the base region  116 . Designing the unit cell to provide this gate shielding advantage may, however, result in some increase in the specific on-state resistance of the device. 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.