Abstract:
A semiconductor device includes a source, a drain, and a gate configured to selectively enable a current to pass between the source and the drain. The semiconductor device includes a drift zone between the source and the drain and a first field plate adjacent the drift zone. The semiconductor device includes a dielectric layer electrically isolating the first field plate from the drift zone and charges within the dielectric layer close to an interface of the dielectric layer adjacent the drift zone.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Utility Patent Application is a divisional application of U.S. application Ser. No. 12/633,956, filed Dec. 9, 2009, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     The on-resistance of a typical high voltage power metal-oxide semiconductor field effect transistor (MOSFET) is dominated by the resistance of the voltage sustaining drift zone. The blocking voltage capability of the drift zone is typically based on its thickness and doping. To increase the blocking voltage, the doping of the drift zone is typically reduced and the layer thickness is increased. The on-resistance of the typical transistor therefore increases disproportionately strongly as a function of its blocking voltage capability. For a 600V transistor, for example, the drift zone contributes over 95% of the total on-resistance. Thus, to improve the performance of power MOSFETs, the drift region resistance should be reduced. 
     For these and other reasons, there is a need for the present invention. 
     SUMMARY 
     One embodiment provides a semiconductor device. The semiconductor device includes a source, a drain, and a gate configured to selectively enable a current to pass between the source and the drain. The semiconductor device includes a drift zone between the source and the drain and a first field plate adjacent the drift zone. The semiconductor device includes a dielectric layer electrically isolating the first field plate from the drift zone and charges within the dielectric layer close to an interface of the dielectric layer adjacent the drift zone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and together with the description serve to explain principles of embodiments. Other embodiments and many of the intended advantages of embodiments will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. 
         FIG. 1  illustrates a cross-sectional view of one embodiment of a power metal-oxide semiconductor field effect transistor (MOSFET). 
         FIG. 2  illustrates a cross-sectional view of another embodiment of a power MOSFET. 
         FIG. 3  illustrates a cross-sectional view of one embodiment of a drain and a substrate including a trench. 
         FIG. 4  illustrates a cross-sectional view of one embodiment of the drain, the substrate, a first dielectric material layer, and positive charges. 
         FIG. 5  illustrates a cross-sectional view of one embodiment of the drain, the substrate, the first dielectric material layer, the positive charges, and a second dielectric material layer. 
         FIG. 6  illustrates a cross-sectional view of one embodiment of the drain, the substrate, the first dielectric material layer, the positive charges, the second dielectric material layer, and a hard mask material layer. 
         FIG. 7  illustrates a cross-sectional view of one embodiment of the drain, the substrate, a dielectric material layer, positive charges, and the hard mask material layer after etching portions of the second dielectric material layer, portions of the positive charges, and portions of the first dielectric material layer. 
         FIG. 8  illustrates a cross-sectional view of one embodiment of the drain, the substrate, a dielectric material layer, positive charges, and a field plate. 
         FIG. 9  illustrates a cross-sectional view of one embodiment of the drain, the substrate, the first dielectric material layer, the positive charges, and an etch stop material layer. 
         FIG. 10  illustrates a cross-sectional view of one embodiment of the drain, the substrate, the first dielectric material layer, the positive charges, the etch stop material layer, a silicon layer, and a second dielectric material layer. 
         FIG. 11  illustrates a cross-sectional view of one embodiment of the drain, the substrate, the first dielectric material layer, the positive charges, the etch stop material layer, the silicon layer, the second dielectric material layer, and a hard mask material layer. 
         FIG. 12  illustrates a cross-sectional view of one embodiment of the drain, the substrate, the first dielectric material layer, the positive charges, the etch stop material layer, the silicon layer, and the hard mask material layer after etching the second dielectric material layer and portions of the silicon layer. 
         FIG. 13  illustrates a cross-sectional view of one embodiment of the drain, the substrate, the first dielectric material layer, the positive charges, the etch stop material layer, and a third dielectric material layer after oxidizing the silicon layer. 
         FIG. 14  illustrates a cross-sectional view of one embodiment of the drain, the substrate, a dielectric material layer, positive charges, and a field plate. 
         FIG. 15  illustrates a cross-sectional view of another embodiment of a power MOSFET. 
         FIG. 16  illustrates a cross-sectional view of another embodiment of a power MOSFET. 
         FIG. 17  illustrates a cross-sectional view of another embodiment of a power MOSFET. 
         FIG. 18  illustrates a cross-sectional view of another embodiment of a power MOSFET. 
         FIG. 19  illustrates a cross-sectional view of another embodiment of a power MOSFET. 
         FIG. 20  illustrates a cross-sectional view of another embodiment of a power MOSFET. 
         FIG. 21  illustrates a cross-sectional view of another embodiment of a power MOSFET. 
     
    
    
     DETAILED DESCRIPTION 
     In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     It is to be understood that the features of the various exemplary embodiments described herein may be combined with each other, unless specifically noted otherwise. 
     While the following embodiments are illustrated and described with reference to n-channel metal-oxide semiconductor field effect transistors (MOSFETs), the embodiments are also applicable to p-channel MOSFETs having opposite dopings and charges. 
       FIG. 1  illustrates a cross-sectional view of one embodiment of a power MOSFET  100   a . Power MOSFET  100   a  includes a drain  102 , a doped substrate  104 , a field stop region  106 , a drift zone  108 , positive charges  110 , a dielectric material  112 , a field plate  114 , a channel or body region  116 , a gate  118 , a body contact region  120 , a source region  122 , dielectric material  126  and optional  124 , source contacts  128 , and a gate contact  130 . In one embodiment, positive charges  110  provide for a vertical accumulation channel through drift zone  108  with a carrier density above approximately 10 11 /cm 2 . The vertical accumulation channel reduces the on-resistance compared to typical power MOSFETs that do not include positive charges  110 . 
     In one embodiment, drain  102  includes Cu, Al, W, or another suitable conductive material. The top of drain  102  contacts the bottom of doped substrate  104 . In one embodiment, doped substrate  104  includes n+ doped Si or another suitable semiconductor material. A first portion of the top of doped substrate  104  contacts the bottom of field stop layer  106 . In one embodiment, field stop layer  106  is excluded and the first portion of the top of doped substrate  104  contacts the bottom of drift zone  108 . A second portion of the top of doped substrate  104  may contact the bottom of dielectric material  112 . 
     In one embodiment, dielectric material  112  includes SiO 2  or another suitable dielectric material. Positive charges  110  are located within dielectric material  112  preferably near the outer surface or interface of dielectric material  112  adjacent drift zone  108 , field stop region  106 , and doped substrate  104 . In one embodiment, positive charges  110  are provided by Cs or another suitable electropositive material. Dielectric material  112  contacts the top, bottom, and sidewalls of field plate  114  and electrically isolates field plate  114  from drift zone  108  and gate  118 . In one embodiment, field plate  114  includes polysilicon or another suitable conductive material. 
     Drift zone  108  laterally surrounds dielectric material  112 . In one embodiment, drift zone  108  includes n doped Si. The top of drift zone  108  contacts the bottom of channel region  116 . In one embodiment, channel region  116  includes p+ doped Si. The top of channel region  116  contacts the bottom of body contact region  120  and source region  122 . In one embodiment, body contact region  120  includes p+ doped Si. In one embodiment, source region  122  includes n+ doped Si. 
     Dielectric material  124  electrically isolates source region  122  from gate  118 . In one embodiment, dielectric material  124  includes SiO 2 , SiN, or another suitable dielectric material. Dielectric material  124  being different from dielectric material  126  as shown in  FIG. 1  is optional, dielectric materials  124  and  126  may be identical and thus not distinguishable. The top of body contact region  120  and source region  122  contact the bottom of source contacts  128 . In one embodiment, source contacts  128  include Cu, Al, W, or another suitable contact material. Source contacts  128  are electrically coupled together via source signal path  132 . In one embodiment, field plate  114  is electrically coupled to source signal path  132 . 
     In one embodiment, gate  118  includes polysilicon or another suitable conductive material. The top of gate  118  contacts the bottom of gate contact  130 . In one embodiment, gate contact  130  includes Cu, Al, W, or another suitable conductive material. Dielectric material  126  laterally surrounds source contacts  128  and gate contact  130 . In one embodiment, dielectric material  126  includes SiO 2 , SiN, or another suitable dielectric material. 
     In one embodiment, Cs or another suitable electropositive material is deposited over a dielectric layer to provide positive charges  110  within dielectric material  112 . In another embodiment, Cs or another suitable electropositive material is implanted into dielectric material  112  to provide positive charges  110 . Positive charges  110  are preferably provided near the interface between dielectric material  112  and drift zone  108 , but preferably not at the interface to prevent a reduction in carrier mobility in the accumulation channel through drift zone  108 . The distance between the positive charges  110  and drift zone  108  is preferably selected such that carriers having a typical kinetic energy have a small probability of tunneling such that positive charges  110  cannot neutralize them. In one embodiment, positive charges  110  are located adjacent drift zone  108  and do not affect channel region  116 . Therefore, increasing the surface charge density due to positive charges  110  reduces the on-resistance of power MOSFET  100   a . In another embodiment, for a p-channel MOSFET, the positive charges are replaced with negative charges. 
     In one embodiment, where dielectric material  112  is SiO 2 , the charge (Q) in the accumulation channel for a voltage (U) across dielectric material  112  of 10V and a thickness (d Oxide ) of dielectric material  112  of 80 nm is given by: 
     
       
         
           
             
               
                 
                   
                     
                       
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     where:
         e 0  is the elementary charge;   C is the capacitance of dielectric material  112 ;   ∈ Oxide  is the dielectric constant of SiO 2 ; and   ∈ 0  is permittivity in a vacuum.       

     In the blocking state, field plate  114  compensates for positive charges  110 . In one embodiment, the source potential is applied to field plate  114  and the thickness of dielectric material  112  is sufficient to maintain stability. In one embodiment, with a maximum permissible electrical field strength of approximately 2 MV/cm and a blocking capability of approximately 200V, the thickness of dielectric material  112  is at least approximately 1 μm (at least in the proximity of doped substrate  104 ). Therefore in one embodiment, for a charge density of 3·10 12 /cm 2  provided by positive charges  110 , U is provided as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
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     In operation in the on-state, gate  118  is selected to pass a current through channel region  116 , drift zone  108 , field stop region  106 , and doped substrate  102  between source contacts  128  and drain  102 . In the on-state, positive charges  110  generate a vertical accumulation region through drift zone  108 , which reduces the on-resistance compared to typical power MOSFETs. In the off-state, field plate  114  compensates for positive charges  110 , which increases the blocking voltage compared to typical power MOSFETs with the same doping and/or amount of fixed positive charges in the drift region. 
       FIG. 2  illustrates a cross-sectional view of another embodiment of a power MOSFET  100   b . Power MOSFET  100   b  is similar to power MOSFET  100   a  previously described and illustrated with reference to  FIG. 1 , except that in power MOSFET  100   b  dielectric material  113  and field plate  115  are used in place of dielectric material  112  and field plate  114 . In this embodiment, field plate  115  is tapered such that it is wider near gate  118  and narrower near drain  102 . Dielectric material  113  contacts the bottom and sidewalls of tapered field plate  115  and electrically isolates tapered field plate  115  from drift zone  108  and gate  118 . Dielectric material  113  gradually increases in thickness such that dielectric material  113  is thinner near gate  118  and thicker near drain  102 . Since the potential in drift zone  108  near channel region  116  is lower than the potential near drain  102 , the thickness of dielectric material  113  is gradually increased toward drain  102  to improve the blocking capability of power MOSFET  100   b.    
     In another embodiment, dielectric material  113  increases in thickness in a step-like manner in place of the gradual transition. In this case, field plate  115  correspondingly decreases in width in a step-like manner in place of the gradual transition of the tapered field plate. Power MOSFET  100   b  operates similarly to power MOSFET  100   a  previously described and illustrated with reference to  FIG. 2 . 
     The following  FIGS. 3-8  illustrate one embodiment of a method for fabricating a power MOSFET, such as power MOSFET  100   b  previously described and illustrated with reference to  FIG. 2 . 
       FIG. 3  illustrates a cross-sectional view of one embodiment of a drain  102  and a substrate  103  including a trench  140 . In one embodiment, substrate  103  includes an n+ doped Si substrate  104 , a field stop region  106  over n+ doped Si substrate  104 , and a drift zone  108  over field stop region  106 . In another embodiment, field stop region  106  is excluded such that drift zone  108  is over n+ doped substrate  104 . Substrate  103  is etched to provide trench  140  extending through drift zone  108  and field stop region  106  into n+ doped substrate  104 . In other embodiments, trench  140  does not reach Si substrate  104  and ends in field stop region  106  or in drift zone  108 . 
       FIG. 4  illustrates a cross-sectional view of one embodiment of drain  102 , substrate  103 , a first dielectric material layer  113   a , and positive charges  110   a . In one embodiment, the exposed surface of substrate  103  is thermally oxidized to provide oxide or first dielectric material layer  113   a . In another embodiment, a dielectric material, such as SiO 2  or another suitable dielectric material is deposited over exposed portions of substrate  103  to provide first dielectric material layer  113   a . In this case, first dielectric material layer  113   a  is deposited using chemical vapor deposition (CVD), high density plasma-chemical vapor deposition (HDP-CVD), atomic layer deposition (ALD), metal organic chemical vapor deposition (MOCVD), plasma vapor deposition (PVD), jet vapor deposition (JVD), or other suitable deposition technique. 
     In one embodiment, Cs or another suitable electropositive material is then deposited on first dielectric material layer  113   a  to provide positive charges  110   a . The electropositive material is deposited using CVD, HDP-CVD, ALD, MOCVD, PVD, JVD, or other suitable deposition technique. In another embodiment, an electropositive material is implanted into first dielectric material layer  113   a  to provide positive charges  110   a . In another embodiment, a SiO 2  first dielectric material layer  113   a  is nitrated to provide positive charges  110   a . The charge density is adjustable based on the surface concentration of positive charges  110   a.    
       FIG. 5  illustrates a cross-sectional view of one embodiment of drain  102 , substrate  103 , first dielectric material layer  113   a , positive charges  110   a , and a second dielectric material layer  113   b . In one embodiment, a dielectric material having a faster etch rate than first dielectric material layer  113   a  is deposited over positive charges  110   a  and first dielectric material layer  113   a  to provide second dielectric material layer  113   b . Second dielectric material layer  113   b  is deposited using CVD, HDP-CVD, ALD, MOCVD, PVD, JVD, or other suitable deposition technique. 
     In another embodiment, the same dielectric material as first dielectric material layer  113   a  is deposited over positive charges  110   a  and first dielectric material layer  113   a  to provide a dielectric material layer. The dielectric material layer is then damaged by implanting the dielectric material layer with heavy ions or by another suitable technique to provide second dielectric material layer  113   b . In another embodiment, second dielectric material layer  113   b  consists of more than one dielectric layer with the upper layer exhibiting a higher etch rate than dielectric material layer  113   a  and lower parts of dielectric material layer  113   b . By damaging the surface of second dielectric material layer  113   b  or by adding a material of higher etch rate, the surface of second dielectric material layer  113   b  has a faster etch rate than first dielectric material layer  113   a.    
       FIG. 6  illustrates a cross-sectional view of one embodiment of drain  102 , substrate  103 , first dielectric material layer  113   a , positive charges  110   a , second dielectric material layer  113   b , and a hard mask material layer  142 . A hard mask material, such a C or another suitable hard mask material is deposited over second dielectric material layer  113   b . The hard mask material is deposited using CVD, HDP-CVD, ALD, MOCVD, PVD, JVD, or other suitable deposition technique. The hard mask material is then recess etched to expose portions of the sidewalls of second dielectric material layer  113   b  within trench  140  to provide hard mask material layer  142 . 
       FIG. 7  illustrates a cross-sectional view of one embodiment of drain  102 , substrate  103 , dielectric material layer  113   c , positive charges  110 , and hard mask material layer  142  after etching portions of second dielectric material layer  113   b , portions of positive charges  110   a , and portions of first dielectric material layer  113   a . Second dielectric material layer  113   b , positive charges  110   a , and first dielectric material layer  113   a  are preferably etched using an isotropic wet etch. During the etching process, the portions of second dielectric material layer  113   b , the portions of positive charges  110   a , and the portions of first dielectric material layer  113   a  above hard mask material layer  142  are removed to expose portions of the sidewalls of drift zone  108  within trench  140 . The remaining portions of the dielectric material adjacent to hard mask material layer  142  provide dielectric layer  113   c , which due to the etching gradually increases in thickness toward the bottom of trench  140 . By adjusting the surface damage or the thickness and/or material of the upper part of dielectric layer  113   b , the taper of final dielectric layer  113   c  can be adjusted. In another embodiment, without tapered dielectric material  113  as stated before, no isotropic etching of dielectric material  113  is performed and hard mask material  142  may be identical with later field plate material and field plate  115  resulting in a MOSFET as illustrated in  FIG. 1 . 
       FIG. 8  illustrates a cross-sectional view of one embodiment of drain  102 , substrate  103 , dielectric material layer  113 , positive charges  110 , and a field plate  115 . Hard mask material layer  142  is removed. In one embodiment, a dielectric material, such as SiO 2  or another suitable dielectric material is deposited over exposed portions of substrate  103  and dielectric material layer  113   c  to provide a dielectric material layer. The dielectric material layer is deposited using CVD, HDP-CVD, ALD, MOCVD, PVD, JVD, or other suitable deposition technique. In another embodiment, this dielectric material layer is excluded. 
     A field plate material, such as polysilicon or another suitable conductive material is then deposited over the dielectric material layer or exposed portions of the substrate  103  and dielectric material layer  113   c . The field plate material is deposited using CVD, HDP-CVD, ALD, MOCVD, PVD, JVD, or other suitable deposition technique. The field plate material is recess etched to expose portions of the sidewalls of substrate  103  within trench  140  to provide tapered field plate  115 . A dielectric material is then deposited or formed over exposed portions of field plate  115  and substrate  103  to provide dielectric material layer  113 . Additional processes are performed to provide channel region  116 , gate  118 , body contact region  120 , source region  120 , source contacts  128 , gate contact  130 , and dielectric material  124  and  126  as previously described and illustrated with reference to  FIGS. 1 and 2 . 
     The following  FIGS. 9-14  illustrate another embodiment of a method for fabricating a power MOSFET, such as power MOSFET  100   b  previously described and illustrated with reference to  FIG. 2 . To begin, the process previously described and illustrated with reference to  FIGS. 3 and 4  is performed. 
       FIG. 9  illustrates a cross-sectional view of one embodiment of drain  102 , substrate  103 , first dielectric material layer  113   a , positive charges  110   a , and an etch stop material layer  150 . An etch stop material, such as Si 3 N 4  or another suitable etch stop material is deposited over first dielectric material layer  113   a  and positive charges  110   a  to provide etch stop material layer  150 . Etch stop material layer  150  is deposited using CVD, HDP-CVD, ALD, MOCVD, PVD, JVD, or other suitable deposition technique. 
       FIG. 10  illustrates a cross-sectional view of one embodiment of drain  102 , substrate  103 , first dielectric material layer  113   a , positive charges  110   a , etch stop material layer  150 , a silicon layer  152   a , and a second dielectric material layer  154   a . Polysilicon or amorphous silicon is deposited over etch stop material layer  150  to provide silicon layer  152   a . Silicon layer  152   a  is deposited using CVD, HDP-CVD, ALD, MOCVD, PVD, JVD, or other suitable deposition technique. In one embodiment, a portion of silicon layer  152   a  is thermally oxidized to provide second dielectric material layer  154   a . In another embodiment, a dielectric material, such as SiO 2  or another suitable dielectric material is deposited over silicon layer  152   a  to provide second dielectric material layer  154   a.    
       FIG. 11  illustrates a cross-sectional view of one embodiment of drain  102 , substrate  103 , first dielectric material layer  113   a , positive charges  110   a , etch stop material layer  150 , silicon layer  152   a , second dielectric material layer  154   b , and a hard mask material layer  156 . A hard mask material, such a C or another suitable hard mask material is deposited over second dielectric material layer  154   a . The hard mask material is deposited using CVD, HDP-CVD, ALD, MOCVD, PVD, JVD, or other suitable deposition technique. The hark mask material is then recess etched to provide hard mask material layer  156  within trench  140 . Second dielectric material layer  154   a  is etched to expose portions of silicon  152   a  outside trench  140  to provide second dielectric material layer  154   b.    
       FIG. 12  illustrates a cross-sectional view of one embodiment of drain  102 , substrate  103 , first dielectric material layer  113   a , positive charges  110   a , etch stop material layer  150 , silicon layer  152   b , and hard mask material layer  156  after etching second dielectric material layer  154   b  and portions of silicon layer  152   a . Second dielectric material layer  154   b  is etched. During the etching process, portions of silicon layer  152   a  are also etched such that silicon layer  152   b  is provided. Silicon layer  152   b  increases in thickness toward the bottom of trench  140 . 
       FIG. 13  illustrates a cross-sectional view of one embodiment of drain  102 , substrate  103 , first dielectric material layer  113   a , positive charges  110   a , etch stop material layer  150 , and a third dielectric material layer  152   c  after oxidizing silicon layer  152   b . Hard mask material layer  156  is removed. Silicon layer  152   b  is then thermally oxidized to provide oxide or third dielectric material layer  152   c.    
       FIG. 14  illustrates a cross-sectional view of one embodiment of drain  102 , substrate  103 , a dielectric material layer  113 , positive charges  110 , and a field plate  115 . A field plate material, such as polysilicon or another suitable conductive material is deposited over third dielectric material layer  152   c . The field plate material is deposited using CVD, HDP-CVD, ALD, MOCVD, PVD, JVD, or other suitable deposition technique. The field plate material is recess etched to expose portions of the sidewalls of substrate  103  within trench  140  to provide tapered field plate  115 . 
     The portions of third dielectric material layer  152   c , the portions of positive charges  110   a , and the portions of first dielectric material layer  113   a  above field plate  115  are also removed. A dielectric material is then deposited or formed over exposed portions of field plate  115  and substrate  103  to provide dielectric material layer  113 . Additional processes are performed to provide channel region  116 , gate  118 , body contact region  120 , source region  120 , source contacts  128 , gate contact  130 , and dielectric material  124  and  126  as previously described and illustrated with reference to  FIGS. 1 and 2 . 
       FIG. 15  illustrates a cross-sectional view of another embodiment of a power MOSFET  100   c . Power MOSFET  100   c  is similar to power MOSFET  100   b  previously described and illustrated with reference to  FIG. 2 , except that in power MOSFET  100   c  dielectric material  113  and field plate  115  are replaced with dielectric material  160  and field plates  162   a - 162   d . In other embodiments, other suitable numbers of field plates are used. Field plates  162   a - 162   d  preferably decrease in width from field plate  162   a  to field plate  162   d.    
     In one embodiment, a fixed potential, such as the source potential is applied to each field plate  162   a - 162   d . In another embodiment, a different potential is applied to each field plate  162   a - 162   d . In another embodiment, each field plate  162   a - 162   d  is floating. Field plates  162   a - 162   d  may be capacitively or resistively coupled together. For purely capacitively coupled field plates  162   a - 162   d , the dielectric material  160  between the field plates has a high dielectric constant, which may lead to higher leakage currents. For purely resistively coupled field plates  162   a - 162   d , the potentials applied by a voltage divider to each individual field plate ensure lower leakage currents but may increase switching times. Therefore, a combination of capacitive and resistive coupling can be used to provide fast switching times with a high impedance voltage divider, which leads to a balancing of the leakage currents and thus to stable potential conditions during the blocking state. 
       FIG. 16  illustrates a cross-sectional view of another embodiment of a power MOSFET  100   d . Power MOSFET  100   d  is similar to power MOSFET  100   a  previously described and illustrated with reference to  FIG. 1 , except that power MOSFET  100   d  includes negative charges  170 . Negative charges, provided by Al or another suitable electronegative material are located within dielectric material  112  near the inner surface or interface of dielectric material  112  adjacent field plate  114 . Negative charges  170  compensate for positive charges  110  in the off-state. In one embodiment, negative charges  170  are provided by depositing a thin Al 2 O 3  layer over dielectric layer  112 . The Al 2 O 3  layer is deposited using ALD or another suitable deposition technique. Over the thin Al 2 O 3  layer another thin SiO 2  layer is formed by deposition or by thermally oxidizing a thin deposited Si material. Doping of SiO 2  with small amounts of Al coming from the thin Al 2 O 3  layer leads to negative oxide charges. In one embodiment, field plate  114  is excluded and replaced by a dielectric material, such as SiO 2  or another suitable dielectric material. 
       FIG. 17  illustrates a cross-sectional view of another embodiment of a power MOSFET  100   e . Power MOSFET  100   e  is similar to power MOSFET  100   d  previously described and illustrated with reference to  FIG. 16 , except that power MOSFET  100   e  includes dielectric material layer  172  including negative charges  174  and positive charges  176  and excludes field plate  114 . Positive charges  176  are provided by Cs or another suitable electropositive material. Positive charges  176  are located within dielectric material layer  172  near the surface or interface of dielectric material layer  172  adjacent drift zone  108 . 
     Negative charges  174  are provided by Al or another suitable electronegative material. Negative charges  174  are located within dielectric material  172  near the surface or interface of dielectric material layer  172  opposite positive charges  176 . In the on-state, positive charges  176  provide a vertical accumulation region through drift zone  108 , which reduces the on-resistance of power MOSFET  100   e . In the off-state, negative charges  174  compensate for positive charges  176  to improve the blocking capability of power MOSFET  100   e.    
       FIG. 18  illustrates a cross-sectional view of another embodiment of a power MOSFET  100   f . Power MOSFET  100   f  is similar to power MOSFET  100   e  previously described and illustrated with reference to  FIG. 17 , except that power MOSFET  100   f  includes field plates  178  and  182  and dielectric material  180 . In other embodiments, other suitable numbers of field plates are used. Field plate  178  is electrically isolated from field plate  182  by dielectric material  180 . The top of field plate  178  contacts source contact  128  and a sidewall of field plate  178  contacts dielectric material layer  172 . A sidewall of field plate  182  contacts dielectric material layer  172 . In one embodiment, the drain potential is applied to field plate  182 . Field plates  178  and  182  provide additional compensation for positive charges  176  during the blocking state. 
       FIG. 19  illustrates a cross-sectional view of another embodiment of a power MOSFET  200   a . Power MOSFET  200   a  includes a drain  202 , a doped substrate  204 , a drift zone  206 , a dielectric material  208 , a field plate  210 , a gate  212 , a channel or body region  214 , a source region  216 , and a source contact  218 . In one embodiment, dielectric material  208  includes positive charges distributed therein to provide a vertical accumulation channel through drift zone  206  in the on-state. The vertical accumulation channel reduces the on-resistance compared to typical power MOSFETs that do not include the positive charges. 
     In one embodiment, drain  202  includes Cu, Al, W, or another suitable conductive material. The top of drain  202  contacts the bottom of doped substrate  204 . In one embodiment, doped substrate  204  includes n+ doped Si or another suitable semiconductor material. The top of doped substrate  204  contacts the bottom of drift zone  206 . Drift zone  206  contacts the bottom and sidewalls of dielectric material  208 . In one embodiment, dielectric material  208  is positively charged and includes Al 2 O 3 , SiN, or another suitable positively charged dielectric material. Dielectric material  208  contacts the top, bottom, and sidewalls of field plate  210  and electrically isolates field plate  210  from drift zone  206  and gate  212 . In one embodiment, field plate  210  includes polysilicon or another suitable conductive material. 
     In one embodiment, drift zone  206  includes n doped Si. The top of drift zone  206  contacts the bottom of channel region  214 . In one embodiment, channel region  214  includes p+ doped Si. In another embodiment p-doping of a portion of channel region  214  adjacent to dielectric material  208  is significantly reduced compared to part of the body or channel region being situated below the source contact  218 . The top of channel region  214  contacts the bottom of source region  216  and source contact  218 . In one embodiment, source region  216  includes n+ doped Si. In one embodiment, source contact  218  includes polysilicon or another suitable conductive material. 
     In one embodiment, dielectric material  208  is positively charged by using electron irradiation followed by an annealing process. In one embodiment, the annealing process is performed at about 350° C. In another embodiment, dielectric material  208  inherently includes positive charges, such that electron irradiation and annealing of the dielectric material is unnecessary. 
     In operation, in the on-state, positively charged dielectric material  208  generates a vertical accumulation region through drift zone  206 , which reduces the on-resistance compared to typical power MOSFETs. In the off-state, field plate  210  compensates for positively charged dielectric material  208 , which increases the blocking voltage compared to typical power MOSFETs. 
       FIG. 20  illustrates a cross-sectional view of another embodiment of a power MOSFET  200   b . Power MOSFET  200   b  is similar to power MOSFET  200   a  previously described and illustrated with reference to  FIG. 19 , except that power MOSFET  200   b  includes dielectric material  220  and  222  in place of dielectric material  208 . In this embodiment, dielectric material  222  includes positive charges provided by Al 2 O 3  bulk material or another suitable electropositive dielectric material. Dielectric material  222  is surrounded by dielectric material  220 , which provides the interface between dielectric material  222  and field plate  210  and between dielectric material  222  and drift zone  206 . Dielectric material  222  is not positively charged or has only a small positive charge (e.g., below an area charge density of 10 11 /cm 2 ) and includes SiO 2  or another suitable dielectric material. By not having the positive charges at the interface between drift zone  206  and the dielectric material, carrier mobility in the accumulation region through drift zone  206  is improved. 
       FIG. 21  illustrates a cross-sectional view of another embodiment of a power MOSFET  240 . Power MOSFET  240  includes a substrate/drain region  242 , a drift zone  244 , a channel or body region  246 , a source region  248 , a source contact  250 , dielectric material  252 , a gate  254 , a dielectric material  256 , and field plates  258   a - 258   c . In one embodiment, dielectric material  256  is positively charged to provide a vertical accumulation channel through drift zone  244 . The vertical accumulation channel reduces the on-resistance compared to typical power MOSFETs that do not include the positively charged dielectric material. 
     In one embodiment, substrate/drain region  242  includes n+ doped Si or another suitable semiconductor material. A first portion of the top of substrate/drain region  242  contacts the bottom of drift zone  244 . A second portion of the top of substrate/drain region  242  contacts the bottom of dielectric material  256 . Drift zone  244  contacts the sidewalls of dielectric material  256 . In one embodiment, dielectric material  256  includes positive charges provided by Al 2 O 3 , SiN, or another suitable electropositive dielectric material. Dielectric material  256  contacts the top, bottom, and sidewalls of field plates  258   a - 258   c  and electrically isolates each field plate from each other and from drift zone  244  and gate  254 . In one embodiment, field plates  258   a - 258   c  include polysilicon or another suitable conductive material. 
     In one embodiment, drift zone  244  includes n doped Si. The top of drift zone  244  contacts the bottom of channel region  246 . In one embodiment, channel region  246  includes p doped Si. Channel region  246  contacts source region  248  and source contact  250 . In one embodiment, source region  248  includes n+ doped Si. In one embodiment, source  250  includes Cu, Al, W, or another suitable conductive material. 
     In operation, in the on-state, the positive charges within dielectric material  256  generate a vertical accumulation region through drift zone  244 , which reduces the on-resistance compared to typical power MOSFETs. In the off-state, field plates  258   a - 258   c  compensate for the positive charges within dielectric material  256 , which increases the blocking voltage compared to typical power MOSFETs. In one embodiment, the potentials applied to field plates  258   a - 258   c  and the coupling between field plates  258   a - 258   c  are similar to the field plates  162   a - 162   d  previously described and illustrated with reference to  FIG. 15 . 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.