Abstract:
Transistors ( 21, 41 ) employing floating buried layers (BL) ( 72 ) may exhibit transient breakdown voltage (BVdss) TR  significantly less than (BVdss) DC . It is found that this occurs because the floating BL ( 72 ) fails to rapidly follow the applied transient, causing the local electric field within the device to temporarily exceed avalanche conditions. (BVdss) TR  of such transistors ( 69. 69′ ) can be improved to equal or exceed (BVdss) DC  by including a charge pump capacitance ( 94, 94′ ) coupling the floating BL ( 72 ) to whichever high-side terminal ( 28, 47 ) receives the transient. The charge pump capacitance ( 94, 94′ ) may be external to the transistor ( 69, 69′ ), may be formed on the device surface ( 71 ) or, may be formed internally to the transistor ( 69 - 3, 69′ - 3 ) using a dielectric deep trench isolation wall ( 100 ) separating DC isolated sinker regions ( 86, 88 ) extending to the BL ( 72 ). The improvement is particularly useful for LDMOS devices.

Description:
FIELD OF THE INVENTION 
     The field of the invention generally relates to semiconductor devices and methods for fabricating semiconductor devices, and more particularly relates to insulated gate field effect transistor (IGFET) devices. 
     BACKGROUND OF THE INVENTION 
     Insulated gate field effect transistor (IGFET) devices are widely used in modern electronic applications. Metal-oxide-semiconductor field effect transistor (MOSFET) devices and lateral-(double)-diffused-metal-oxide-semiconductor (LDMOS) devices are well known examples of such IGFET devices. As used herein, the term metal-oxide-semiconductor and the abbreviation MOS are to be interpreted broadly. In particular, it should be understood that they are not limited merely to structures that use “metal” and “oxide”, but may employ any type of conductor, including “metal”, and any type of dielectric, including “oxide”. The term field effect transistor is abbreviated as “FET”. It is known that improved performance of LDMOS devices can be obtained by using reduced surface field (RESURF) structures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
         FIG. 1  is a simplified electrical schematic diagram of an N-channel LDMOS RESURF transistor including a MOSFET and parasitic bipolar transistor associated therewith, according to the prior art; 
         FIG. 2  is a simplified electrical schematic diagram of a P-channel LDMOS RESURF transistor including a MOSFET and parasitic bipolar transistors associated therewith; 
         FIG. 3  is a simplified cross-section view through a transistor of the type illustrated schematically in  FIG. 1  having a floating buried layer underlying the LDMOSFET, according to the prior art; 
         FIG. 4  is a simplified electrical schematic diagram showing the junction capacitances between the input electrodes and the underlying buried layer within the N channel devices of  FIGS. 1 and 3 , and the P-channel device of  FIG. 2 , that impact the breakdown voltage in response to very fast transients, and showing use of a charge pump capacitance to improve device behavior, according to an embodiment of the present invention; 
         FIG. 5  is a simplified electrical schematic diagram of the N channel LDMOSFET of  FIG. 3  employing a floating buried layer, illustrating how the charge pump capacitance of  FIG. 4  is provided to couple the floating buried layer to the drain, to reduce the adverse impact of rapid electrical transients appearing on the source-drain terminals, according to another embodiment of the present invention; 
         FIG. 6  is a simplified electrical schematic diagram of a P channel LDMOSFET employing a floating buried layer, illustrating how the charge pump capacitance of  FIG. 4  is provided to couple the floating buried layer to the source, to reduce the adverse impact of rapid electrical transients appearing on the source-drain terminals, according to still another embodiment of the present invention; 
         FIG. 7  is a simplified cross-section view, analogous to that of  FIG. 3 , through an N-channel LDMOSFET showing how the charge pump capacitance of  FIG. 5  may be provided, according to a yet further embodiment of the present invention; 
         FIG. 8  is a simplified cross-section view, analogous to that of  FIG. 7 , through an N-channel LDMOSFET showing how the charge pump capacitance of  FIG. 5  may be implemented on a monolithic substrate, according to a still yet further embodiment of the present invention; 
         FIG. 9  is a simplified cross-section view, analogous to that of  FIG. 7 , through an N-channel LDMOSFET showing how the charge pump capacitance of  FIG. 5  may be implemented in a monolithic substrate, according to a yet still further embodiment of the present invention; 
         FIG. 10  is a simplified cross-section view, analogous to that of  FIG. 9 , through a P-channel LDMOSFET showing how the charge pump capacitance of  FIG. 6  may be implemented in a monolithic substrate, according to a still yet another embodiment of the present invention; and 
         FIGS. 11-19  are simplified cross-sectional views through the device of  FIG. 9  at different stages of manufacture according to additional embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, or the following detailed description. 
     For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the invention. Additionally, elements in the drawing figures are not necessarily drawn to scale. For example, the dimensions of some of the elements or regions in the figures may be exaggerated relative to other elements or regions to help improve understanding of embodiments of the invention. 
     The terms “first,” “second,” “third,” “fourth” and the like in the description and the claims, if any, may be used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Furthermore, the terms “comprise,” “include,” “have” and any variations thereof, are intended to cover non-exclusive inclusions, such that a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. The term “coupled,” as used herein, is defined as directly or indirectly connected in an electrical or non-electrical manner. As used herein the terms “substantial” and “substantially” mean sufficient to accomplish the stated purpose in a practical manner and that minor imperfections, if any, are not significant for the stated purpose. 
     As used herein, the term “semiconductor” (abbreviated as “SC”) is intended to include any semiconductor whether single crystal, poly-crystalline or amorphous and to include type IV semiconductors, non-type IV semiconductors, compound semiconductors as well as organic and inorganic semiconductors. Further, the terms “substrate” and “semiconductor substrate” are intended to include single crystal structures, polycrystalline structures, amorphous structures, thin film structures, layered structures as for example and not intended to be limiting, semiconductor-on-insulator (SOI) structures, and combinations thereof The term “semiconductor” is abbreviated as “SC.” 
     For convenience of explanation and not intended to be limiting, semiconductor devices and methods of fabrication are described herein for silicon semiconductors, but persons of skill in the art will understand that other semiconductor materials may also be used. Additionally, various device types and/or doped SC regions may be identified as being of N type or P type, but this is merely for convenience of description and not intended to be limiting, and such identification may be replaced by the more general description of being of a “first conductivity type” or a “second, opposite conductivity type” where the first type may be either N or P type and the second type then is either P or N type. 
       FIG. 1  is a simplified electrical schematic diagram of N-channel LDMOS RESURF transistor  20  including MOSFET  21  and parasitic bipolar transistor  30  associated therewith, according to the prior art. LDMOS FET  21  comprises N-type source  22  and drain  24 , and conductive gate  25  insulated from and overlying P-type body region  26 . Source  22  is coupled to source terminal  27  and drain  24  is coupled to drain terminal  28 . Parasitic bipolar transistor  30  exists between source  22  (and source terminal  27 ) and drain  24  (and drain terminal  28 ). Parasitic bipolar transistor  30  comprises N-type emitter  32  (e.g., associated with source  22 ), N-type collector  34  (e.g., associated with drain  24 ), P-type base region  36  (e.g., associated with body region  26 ) and internal body resistance  37 . Resistance  37  and emitter  32  are coupled to source terminal  27 . Collector  34  is coupled to drain terminal  28 . U.S. Pat. No. 6,882,023 describes a physical RESURF LDMOS structure that can be represented by the simplified electrical schematic diagram of  FIG. 1  including (e.g., N type) drift region and (e.g., P type) RESURF region under which is provided a floating buried layer (e.g., N type) identified in  FIG. 1  by the label “FLOATING”  39 , which has no external connection. 
       FIG. 2  is a simplified electrical schematic diagram of P-channel LDMOS RESURF transistor  40  with MOSFET  41 , parasitic bipolar transistor  50  associated therewith and further parasitic bipolar device  60 . Further parasitic bipolar device  60  arises because of the presence of an (e.g., N type) floating buried layer underlying MOSFET  41  and parasitic bipolar device  50  in LDMOS transistor  40 . In this respect, LDMOS transistor  40  of  FIG. 2  differs from what would be obtained by simply exchanging the N and P regions of LDMOS transistor  20  of  FIG. 1 . MOSFET  41  comprises P-type source  42  and drain  44 , and conductive gate  45  insulated from and overlying N-type body region  46 . Source  42  is coupled to source terminal  47 , and drain  44  is coupled to drain terminal  48 . Parasitic bipolar transistor  50  exists between source  42  (and source terminal  47 ) and drain  44  (and drain terminal  48 ). Parasitic bipolar transistor  50  comprises (e.g., P-type) emitter  52  (e.g., associated with source  42 ), (e.g., P-type) collector  54  (e.g., associated with drain  44 ), (e.g., N-type) base region  56  (e.g., associated with body region  46 ) and internal body resistance  57 . Resistance  57  and emitter  42  are coupled to source terminal  47 . Collector  54  is coupled to drain terminal  48 . P and N type RESURF regions and underlying (e.g., N type) floating buried layer are included in transistor  40 , thereby giving rise to further parasitic bipolar transistor  60 . Further parasitic bipolar transistor  60  has (e.g., P type) base  66  coupled to (e.g., P type) collector region  54  of parasitic bipolar  50  and (e.g., P type) drain  44 , (e.g., N type) collector  64  coupled to (e.g., N type) base region  56  of parasitic bipolar transistor  50 , and (e.g., N type) emitter  62  coupled to terminal  59 , identified in  FIG. 2  by the label “FLOATING”  59 , which has no external connection. 
     The use of a floating buried layer RESURF structures represented by the electrical schematic diagrams of  FIGS. 1 and 2  can provide substantially improved breakdown voltages BVdss and relatively low ON resistance Rdson. However, the relatively large area of floating buried layer regions in such devices that lie between the LDMOS device and the substrate may make them susceptible to degradation of the breakdown voltage in the presence of very fast transients, e.g., transient (TR) voltages appearing across source-drain terminals  27 ,  28 ;  47 ,  48  and/or regions  22 ,  24 ;  42 ,  44  with rise times of about 100 nanoseconds or less, especially pulses with rise times of about 10 nanoseconds or less. This can result in transient drain-source break-down voltages (BVdss) TR  that are substantially less than DC breakdown voltages (BVdss) DC , that is, (BVdss) TR &lt;&lt;(BVdss) DC , where “TR” is understood to refer to transient signals of the type noted above and “DC” is understood to refer to zero frequency or low frequency signals. This situation is undesirable. As an aid to understanding how such (BVdss) TR . degradation can come about, it is useful to consider the physical structure of a typical LDMOSFET device employing a floating buried layer. 
       FIG. 3  is a simplified cross-section view through transistor  20  of the type illustrated schematically in  FIG. 1  having floating buried layer  72  underlying MOSFET  21 , according to the prior art. Where appropriate, the same reference numbers have been used in  FIG. 3  as in  FIG. 1  to facilitate correlation between  FIGS. 1 and 3 . Transistor  20  of  FIG. 3  comprises semiconductor (SC) substrate  70  (e.g., P type) with overlying buried layer  72  (e.g., N type, abbreviated as “NBL”). Above buried layer  72  is overlying further (e.g., P type epi) SC region  74  extending to surface  71 . Located within overlying further region  74  is body region  76  (e.g., P type). Within body region  76  are (e.g., N+) source region  22  and (e.g., P+) body contact region  78 . Also located within overlying SC region  74  are (e.g., N type) drift region  80  and (e.g., P type) RESURF region  82 , which generally underlies drift region  80 . As is well known in the art, to obtain RESURF action, charge balancing should be provided between regions  80 , and  82  and is hereafter presumed in the device of  FIG. 3  and subsequent LDMOS devices. Doped (e.g., N+) drain region  24  is provided within drift region  80  extending to surface  71 . N type buried layer  72  is DC isolated from overlying MOSFET  21  by PN junction  92 - 1  between (e.g., N type) buried layer  72  and overlying further (e.g., P type) layer or region  74 . Shallow Trench Isolation (STI) regions  84  are provided extending from surface  71  into SC region  74  in the locations indicated. Sinker region  86  (e.g., N type) extends from beneath STI region  84  through further SC region  74  to make Ohmic contact to buried layer  72 . Conventional gate conductor  25  is provided overlying and insulated from surface  71  between source region  22  and drift region  80  and extending somewhat there-over. When source  22 , gate  25  and drain  24  are appropriately biased, channel  90  forms between source  22  and drain  24 . Conductors are also conventionally provided to couple source region  22 , drain region  24  and gate  25  to their respective terminals  27 ,  28  and  29 . 
       FIG. 4  shows simplified electrical schematic diagram  68  of internal capacitances associated with transistor  20  of  FIG. 3  and transistor  69  of  FIGS. 5  (“N channel”) and associated with transistor  40  of  FIG. 2  and transistor  69 ′ of  FIG. 6  (“P-channel”). Schematic diagram  68  illustrates: (i) how floating buried layer (BL)  72  is capacitively coupled to source terminal  27  (and source region  22  of  FIG. 1 ) by junction capacitance  93  and to drain terminal  28  (and to drain region  24 ) by junction capacitances  91 ,  92  in N channel device  20 ,  69  of  FIGS. 1 ,  3  and  5 , and (ii) how floating buried layer (BL)  72  is capacitively coupled to drain terminal  48  (and drain region  44  of  FIG. 2 ) by junction capacitance  93 ′ and to source terminal  47  (and source region  42 ) by junction capacitances  91 ′,  92 ′ in P channel device  40 ,  69 ′ of  FIGS. 2 and 6 . Schematic diagram  68  also illustrates how the transient breakdown voltage (BVdss) TR  can be improved by use of charge pump capacitance  94  between floating buried layer  72  and drain terminal  28  (or drain  24 ) of N channel device  69  of  FIG. 5 , and by use of charge pump capacitance  94 ′ between floating buried layer  72  and source terminal  47  (or source  27 ) of P channel device  69 ′ of  FIG. 6 . 
     Referring to both  FIGS. 3 and 4 , N Channel device capacitance  93  is associated with junction  93 - 1 , capacitance  91  is associated with junction  91 - 1  and capacitance  92  is associated with junction  92 - 1 . (Analogous P channel device capacitance  93 ′,  91 ′ and  92 ′ are associated with junctions  93 - 1 ′,  91 - 1 ′ and  92 - 1 ′ shown in  FIG. 10 .) Under DC (e.g., low frequency) conditions, the applied voltage is distributed across these capacitances to floating buried layer (BL)  72 , and the drain-source breakdown voltage BVdss is substantially improved compared to an otherwise similar device without floating BL  72 . However, it has been found that when the applied voltage is in the form of fast transient  95  (e.g., see  FIG. 4 ) having the fast rise times noted earlier, the space-charge regions associated with the several junctions between, for example, terminals  27 ,  28  (or terminals  48 ,  47 ) and BL  72 , represented by capacitances  93 ,  91 ,  92  (or  93 ′,  91 ′,  92 ′) do not have time to adjust, with the result that the applied voltage is concentrated across a smaller region of the semiconductor (SC) thereby increasing the local electric field so that premature breakdown can occur at voltages (BVdss) TR  much lower than (BVdss) DC  observed with a substantially DC signal, so that (BVdss) TR &lt;&lt;(BVdss) DC . 
     It has further been determined that this condition can be avoided by providing a circuit path by which buried layer  72  can be charge pumped, so that its voltage can also rise rapidly in response to fast transient  95 , thereby preventing the localized electric field from rising above that necessary to induce avalanche and premature breakdown. This is accomplished by providing shunt capacitance  94 ,  94 ′ between the appropriate source or drain terminal (or source or drain region) and buried layer  72 . In the N channel device (see also  FIG. 5 ), charge pump capacitance  94  is provided between drain terminal  28  (or drain region  24 ) and BL  72  and in the P channel device (see also  FIG. 6 ), charge pump capacitance  94 ′ is provided between source terminal  47  (or source region  42 ) and BL  72 . 
     Rapid rise time pulses can be readily obtained for test purposes using transmission lines. Such transmission line pulse (TLP) tests are well known in the art. It is found that providing shunt capacitance  94 ,  94 ′ improves the transient breakdown voltage so that it equals or exceeds the DC breakdown voltage. This is a much desired result and a significant improvement in the art. The desired magnitude of charge pump capacitance  94 ,  94 ′ is discussed later. 
       FIG. 5  shows a simplified electrical schematic diagram of N-channel LDMOS RESURF transistor  69  including MOSFET  63 , parasitic bipolar transistor  30  associated therewith and further capacitance  94 , according to another embodiment of the present invention. The same reference numbers are used in  FIG. 5  as in  FIG. 1  to refer to analogous regions or elements, and reference should be had to the discussion of  FIG. 1  for further details. Further capacitance  94  is coupled in  FIG. 5  from lead  38  of the floating buried layer in  FIG. 1  (identified as “FLOATING  39 ” in  FIG. 1 ) to drain terminal  28  so that a rapidly rising pulse applied to terminal  28  can pump charge onto floating BL  72  (see also  FIGS. 3-4 ), thereby reducing the peak electric field that must be sustained within the SC of LDMOSFET  69 . Reference number  69  is also intended to refer collectively to specific embodiments  69 - 1 ,  69 - 2 ,  69 - 3  described later. 
       FIG. 6  shows a simplified electrical schematic diagram of P-channel LDMOS RESURF transistor  69 ′ including MOSFET  65 , parasitic bipolar transistor  50  associated therewith, additional parasitic device  60  as noted in connection with  FIG. 2  and further capacitance  94 ′, according to still another embodiment of the present invention. The same reference numbers are used in  FIG. 6  as in  FIG. 2  to refer to analogous regions and elements, and reference should be had to the discussion of  FIG. 2  for further details. Further capacitance  94 ′ is coupled in  FIG. 6  from emitter  64  of further parasitic transistor  60  associated with the floating buried layer (identified as “FLOATING  59 ” in  FIG. 2 ) to source terminal  47  so that a rapidly rising pulse applied to terminal  47  can pump charge onto floating BL  72  (see  FIGS. 3-4 ), thereby reducing the peak electric field that must be sustained within the SC of LDMOSFET  69 ′. Reference number  69 ′ is also intended to refer collectively to specific embodiments  69 ′- 1 ,  69 ′- 2 ,  69 ′- 3  described later. 
       FIG. 7  is a simplified cross-sectional view, analogous to that of  FIG. 3 , through N-channel LDMOSFET  69 - 1  showing how charge pump capacitance  94  of  FIG. 5  may be provided as external capacitance  94 - 1 , according to a yet further embodiment of the present invention. For convenience of explanation and not intended to be limiting, in  FIG. 7  and following, illustrative N and P conductivity types are included in the description and the drawings with the various reference numbers by way of example and not limitation. Persons of skill in the art will understand that such conductivity types may be interchanged in other embodiments or referred to as of a first conductivity type, which may be either N or P, and of a second opposite conductivity type which is then either P or N. The same reference numbers are used in  FIG. 7  as in  FIG. 3  for analogous regions and reference should be had to the discussion of  FIG. 3  for further details. Device  69 - 1  of  FIG. 7  differs from device  20  of  FIG. 3  in that (e.g., N+) contact region  87  is provided to sinker region  86  and external capacitance  94 - 1  is coupled between drain terminal  28  (or drain region  24 ) and contact region  87  to sinker region  86 , which is in turn coupled to buried layer (BL)  72 . Thus, a charge pump path to BL  72  is provided via capacitance  94 - 1 . The use of capacitance  94 - 1  means that BL  72  can continue to be floating for DC and slow AC purposes, so that the advantages of a floating RESURF BL are preserved and there is no adverse affect on the DC breakdown voltage (BVdss) DC  or series-ON resistance, while the transient breakdown voltage (BVdss) TR  is substantially increased. 
       FIG. 8  is a simplified cross-section view, analogous to that of  FIG. 7 , through N-channel LDMOSFET  69 - 2  showing how charge pump capacitance  94  of  FIG. 5  may be implemented on a monolithic substrate as capacitance  94 - 2 , according to a still yet further embodiment of the present invention. Device  69 - 2  of  FIG. 8  differs from device  20  of  FIG. 3  in that (e.g., N+) contact region  87  is provided to sinker region  86  and monolithic capacitance  94 - 2  formed on surface  71  of substrate  70  is coupled between drain terminal  28  (or drain region  24 ) and contact region  87  to sinker region  86 , which is in turn coupled to buried layer (BL)  72 . Capacitance  94 - 2  comprises: (i) lower electrically conductive electrode (e.g., metal or metal-SC alloy, etc.)  96  desirably making Ohmic connection to contact  87  to sinker region  86 , (ii) interlayer dielectric  97  of relatively low loss insulator, (e.g., silicon oxide) overlying lower conductor  96 , and (iii) upper electrically conductive electrode (e.g., metal or metal-SC alloy, etc.)  97  which is in turn coupled to drain terminal  28  (or drain region  24 ). Thus, a charge pump path is provided to BL  72  via capacitance  94 - 2 . The use of capacitance  94 - 2  means that BL  72  can continue to be floating for DC and slow AC purposes, so that the advantages of a floating RESURF BL are preserved and there is no adverse affect on the DC breakdown voltage (BVdss) DC  or series-ON resistance, while the transient breakdown voltage (BVdss) TR  is substantially increased. 
       FIG. 9  is a simplified cross-section view, analogous to that of  FIG. 7 , through N-channel LDMOSFET  69 - 3  showing how supplementary charge pump capacitance  94  of  FIG. 5  may be implemented by capacitance  94 - 3  within monolithic substrate  70 , according to a yet still further embodiment of the present invention. Device  69 - 3  of  FIG. 9  differs from device  20  of  FIG. 3  in that: (i) deep lateral dielectric isolation wall  100  is provided, in this example, adjacent sinker region  86  and extending from surface  71  (or from STI region  84 ) through SC region  74  and BL region  72  into underlying portion  701  of substrate  70 , (ii) further sinker region  88  is provided extending from surface  71  through region  74  to further (e.g., N type) region  722  to make Ohmic contact thereto, and (iii) (e.g., N+) contact region  89  is provided to further sinker region  88 . STI regions  84  may be omitted in other embodiments. Contact region  89  is electrically coupled to drain terminal  28  (or drain region  24 ). Further sinker region  88  and underlying region  722  may be a single doped region or separately formed, ohmically coupled doped regions of the same conductivity type. Either arrangement is useful. 
     Capacitance  94 - 3  is formed by relatively deep dielectric isolation wall  100 , which DC isolates sinker region  86  and BL  72  from sinker region  88  and doped region  722 . Dielectric isolation wall  100  has lateral thickness  101  and vertical extent  102  between substrate  70  and STI region  84 , and functions as the dielectric layer of capacitance  94 - 3  between the opposed conductors formed, on the left, by sinker  88  and doped region  722  and, on the right, by sinker  86  and BL  72 . Silicon dioxide is a non-limiting example of a suitable dielectric material for capacitance  94 - 3 , but other substantially insulating materials may also be used. Means and methods for providing such dielectric isolation walls are well known in the art, and any convenient means that fulfills the desired characteristics described below may be used. In some embodiments, dielectric isolation wall  100  may comprise a sandwich of dielectric material (e.g., silicon oxide) with a polycrystalline SC (e.g., polysilicon) or other conductive inclusion  103  substantially in the center of the dielectric making up isolation wall  100 . When centrally located conductive inclusion  103  is floating, its presence does no harm. Lateral thickness  101  of isolation wall  100  is desirably in the range of about 0.5 to 2.0 micrometers, more conveniently in the range of about 1.0 to 2.0 micrometers and preferably about 1.5 micrometers, although larger or smaller values can also be used. Vertical height  102  of isolation wall  100  approximately from substrate region  701  to the top of sinker  86  is desirably in the range of about 3 to 10 micrometers, more conveniently in the range of about 5 to 9 micrometers and preferably about 8 micrometers, although larger or smaller values can also be used. 
     The effectiveness of charge pumping into BL  72  using capacitance  94 - 3  depends upon the magnitude of capacitance  94 - 3 . Persons of skill in the art will understand based on the description herein, that capacitance  94 - 3  may be increased by decreasing thickness (X)  101 , increasing vertical height (Y)  102  and/or increasing the plan view perimeters (Z) of isolation wall  100  forming capacitance  94 - 3 . Stated another way, capacitance C 94-3 =f((Y)*(Z)/(X)), and any or all of these parameters may be adjusted to obtain the desired magnitude of capacitance. The use of capacitance  94 - 3  means that BL  72  can continue to be floating for DC and slow AC purposes, so that the advantages of a floating RESURF BL are preserved and there is no adverse affect on the DC breakdown voltage (BVdss) DC  or series-ON resistance Rdson while the transient breakdown voltage (BVdss) TR  is substantially increased. The arrangement of  FIG. 9  has the further advantage that it uses chip area that would otherwise be substantially occupied by a lateral isolation wall and so has the least adverse impact on die per wafer and manufacturing cost. The arrangement of  FIG. 9  is a significant and valuable advance in the art and is preferred. 
       FIG. 10  is a simplified cross-section view, corresponding to that of  FIG. 6  and analogous to that of  FIG. 9 , through P-channel LDMOSFET  69 ′- 3  showing how supplementary capacitance  94 ′ of  FIG. 6  may be implemented by capacitance  94 ′- 3  within monolithic substrate  70 , according to a still yet further embodiment of the present invention. Device  69 ′- 3  of  FIG. 10  comprises semiconductor (SC) substrate  70  (e.g., P type) with overlying buried layer  72  (e.g., N type, abbreviated as “NBL” or “BL  72 ”). Above buried layer  72  is further overlying (e.g., P type epi) SC region  74  extending to surface  71 . Located within overlying region  74  is (e.g., N type) body region  154 . Within body region  154  are (e.g., P+) source region  42  and (e.g., N+) body contact region  46 . Also located within overlying SC region  74  is (e.g., P type) RESURF region  156 , which generally underlies body region  154 . Also located in further SC region  74  is (e.g., P type) drift region  148 . Doped (e.g., P+) drain region  44  is provided within drift region  148  extending to surface  71 . Shallow Trench Isolation (STI) regions  84  are desirably provided extending from surface  71  into SC region  74  in the locations indicated. STI regions  84  may be omitted in other embodiments. 
     Sinker region  86  (e.g., N type) extends from beneath STI region  84  through further SC region  74  to make Ohmic contact to buried layer  72 . Conventional gate conductor  45  is provided overlying and insulated from surface  71  between source region  42  and drift region  148  and extending somewhat there-over. Conductors are conventionally provided to couple source region  42 , drain region  44  and gate  45  to their respective terminals  47 ,  48  and  49 . When source  42 , gate  45  and drain  44  are appropriately biased, channel  90 ′ forms between source  42  and drain  44 . Device  69 ′- 3  has: (i) relatively deep lateral dielectric isolation wall  100 , in this example, adjacent sinker region  86  and extending from surface  71  (or from STI region  84 ) through SC region  74  and BL region  72  into underlying portion  701  of substrate  70 , (ii) further sinker region  88  is provided extending from surface  71  through region  74  to further (e.g., N type) region  722  to make Ohmic contact thereto, and (iii) (e.g., N+) contact region  89  is provided to further sinker region  88 . Contact region  89  is electrically coupled to source terminal  47  (or source region  42 ). Further sinker region  88  and underlying region  722  may be a single doped region or may be separately formed, ohmically coupled doped regions of the same conductivity type. Either arrangement is useful. 
     Capacitance  94 ′- 3  is formed by dielectric isolation wall  100 , which DC isolates sinker region  86  and BL  72  from sinker region  88  and doped region  722 . The discussion of dielectric isolation wall  100  in connection with  FIG. 9  should be referred to for further details. The use of capacitance  94 ′- 3  means that BL  72  can continue to be floating for DC and slow AC purposes, so that the advantages of a floating RESURF BL are preserved and there is no adverse affect on the DC breakdown voltage (BVdss) DC  or series-ON resistance Rdson while the transient breakdown voltage (BVdss) TR  is substantially increased. The arrangement of  FIG. 10  has the further advantage in that it uses chip area that would otherwise be substantially occupied by a lateral isolation wall and so has the least adverse impact on die per wafer and manufacturing cost. The arrangement of  FIG. 10  is a significant and valuable advance in the art and is preferred. 
     Persons of skill in the art will understand based on the description herein, that charge pump capacitance  94 ′- 3  of P channel device  69 ′- 3  of  FIG. 10  employing dielectric trench isolation wall  100  between sinkers  88  and  86  may be replaced by charge pump capacitances  94 ′- 1  corresponding to capacitance  94 - 1  of N channel device  69 - 1  of  FIG. 7  or by charge pump capacitances  94 ′- 2  corresponding to capacitance  94 - 2  of N channel device  69 - 2  of  FIG. 8 . Any of these P channel device arrangements is useful and a significant advance in the art. 
     Further, with respect to the embodiments of  FIGS. 7-10 , capacitances  94 - 1 ,  94 - 2 ,  94 - 3  for N channel devices, and equivalent capacitances  94 ′- 1 ,  94 ′- 2 ,  94 ′- 3  for P channel devices, should be large enough so that, usefully at least 5% of the voltage of fast transient  95  is coupled from terminals  27 ,  28  (or  47 ,  48 ) to buried layer  72 , more conveniently at least about 10% of fast transient voltage  95  is coupled from terminals  27 ,  28  (or  47 ,  48 ) to buried layer  72 , and preferably at least about 20% of fast transient voltage  95  is coupled from terminals  27 ,  28  (or  47 ,  48 ) to buried layer  72 , but other values may also be used. In the examples of N channel and P channel devices described above, both use N type buried layers, and the charge pump capacitance  94 ,  94 ′ is coupled to the high side terminal receiving the fast transient, for example, drain  24  (or drain terminal  28 ) of N channel device  69 , or source  42  (or source terminal  47 ) of P channel device  69 ′. 
       FIGS. 11-19  are simplified cross-sectional views through device  69 - 3  of  FIG. 9  at different stages  211 - 219  of manufacture showing structures  311 - 319 , according to additional embodiments of the present invention. Persons of skill in the art will understand that the manufacturing sequence illustrated hereafter can generally also be used to form those devices illustrated in cross-sections in  FIGS. 7-10 . Modifications needed to provide regions of somewhat different lateral extent, thickness and/or doping, if needed, are within the capabilities of those of skill in the art. 
     Referring now to manufacturing stage  211  of  FIG. 11 , semiconductor (SC) containing substrate  70  is provided. Buried layer  72  of thickness  721  is formed in or on substrate  70 , for example by ion implantation, but other doping means well known in the art may also be used. In preferred embodiments, at least the upper portion of substrate  70  is P type with doping density usefully in the range of about 1E15 to 1E18 cm -3 , more conveniently in the range of about 1E15 to 1E16 cm -3  and preferably about 2E15 cm -3 , although higher and lower values can also be used as well as other doping types. Boron is a suitable dopant for substrate  70 , but other dopants may also be used. Buried layer  72  is desirably N type with doping density usefully in the range of about 5E18 to 1E20 cm -3 , more conveniently in the range of about 1E19-to 5E19 cm -3  and preferably about 2E19 cm -3 , although higher and lower values can also be used and other doping types. Thickness  202  is usefully in the range of about 0.5 to 3.0 micrometers, more conveniently in the range of about 1.0 to 2.0 micrometers and preferably about 1.5 micrometers, but larger and smaller values may also be used. Further SC region or layer  74  of thickness  741  with upper surface  71  is formed above buried layer  72 . Epitaxial growth is a useful means for providing further SC region of layer  74 , but other well known techniques may also be used to form structure  311  resulting from manufacturing stage  211 . Layer or region  74  is desirably P type with doping density usefully in the range of about 5E14 to 5E16 cm -3 , more conveniently in the range of about 1E15 to 1E16 cm -3  and preferably about 2E15 cm -3 , although higher and lower values can also be used and other doping types. Thickness  741  is usefully in the range of about 1.0 to 10.0 micrometers, more conveniently in the range of about 2.0 to 5.0 micrometers and preferably about 4.0 micrometers, but larger and smaller values may also be used. Structure  311  results. The combination of substrate  70 , buried layer  72  and further SC region or layer  74  is also referred to as semiconductor body  70 ,  72 ,  74  or semiconductor containing body  70 ,  72 ,  74  having an upper surface  71 . 
     Referring now to manufacturing stage  212  of  FIG. 12 , mask  612  is applied above surface  71  with closed portion  612 - 2  and opening  612 - 1 . Ion implant  512  is desirably used to form superposed doped region  80  of thickness or depth  801  and doped region  82  of thickness or depth  821  through opening  612 - 1 . A chain implant is preferred although separate implants may also be used in other embodiments. Region  80  is conveniently N type and region  82  is conveniently P type, but other doping types may be used in other embodiments. Phosphorus is a suitable dopant for forming region  80  and boron is a suitable dopant for forming regions  82 , with the implant energies being selected to provide depths  801 ,  821  respectively. Region  80  has a peak doping density usefully in the range of about 1E16 to 1E17 cm -3 , more conveniently in the range of about 2E16 to 5E16 cm -3  and preferably about 4E16 cm -3 , although higher and lower values can also be used and other doping types. Depth  801  is usefully in the range of about 0.5 to 2.5 micrometers, more conveniently in the range of about 1.0 to 2.0 micrometers and preferably about 1.0 micrometers, but larger and smaller values may also be used. Region  82  has a peak doping density usefully in the range of about 1E16 to 1E17 cm -3 , more conveniently in the range of about 2E16-to 5E16 cm -3  and preferably about 4E16 cm -3 , although higher and lower values can also be used and other doping types. Depth  821  usefully in the range of about 0.5 to 2.5 micrometers, more conveniently in the range of about 1.0 to 2.0 micrometers and preferably about 1.0 micrometers, but larger and smaller values may also be used. Structure  312  results. Analogous process steps may be used to form doped regions  154  (e.g., N type) and  156  (e.g., P type) of  FIG. 10 . 
     Referring now to manufacturing stage  213  of  FIG. 13 , mask  612  is removed and shallow trench isolation (STI) regions  84  of thickness or depth  841  from surface  71  are desirably formed at the indicated location using teachings well known in the art. STI regions  84  may be omitted in whole or in part in other embodiments. Silicon dioxide is a non-limiting example of a suitable dielectric for STI regions  84  but other well known insulators may also be used. Thicknesses or depth  841  is usefully in the range of about 0.1 to 0.6 micrometers, more conveniently in the range of about 0.2 to 0.5 micrometers and preferably about 0.35 micrometers, but larger and smaller values may also be used. Before, during or after the formation of STI regions  84 , relatively deep dielectric trench isolation (DTI) wall  100  of depth  104  from surface  71  and width  101 , with or without poly inclusions  103 , is formed, also using teachings well known in the art. While DTI wall  100  is shown as extending from beneath STI region  84 , in other embodiments, DTI wall  100  may extend from surface  71 . Either arrangement is useful. DTI wall  100  extends into portion  701  of substrate  70  beneath BL  72 , so to DC isolate (e.g., N type) portion  722  of BL  72  of  FIG. 13  to the left of DTI wall  100  from portion  723  of BL  72  of  FIG. 13  to the right of DTI wall  100 . In a preferred embodiment, in plan view (not shown), DTI wall  100  laterally encloses the active regions of LDMOS device  69 - 3 , but other plan view layouts may also be used in other embodiments. Structure  313  results. 
     Referring now to manufacturing stage  214  of  FIG. 14 , mask  614  is applied having opening  614 - 1  and closed portions  614 - 2 ,  614 - 3 . Ion implant  514  is desirably provided to form (e.g., P type) body region  76  of depth or thickness  761 . Region  76  has a peak doping density usefully in the range of about 1E17 to 5E18 cm -3 , more conveniently in the range of about 5E17 to 2E18 cm -3  and preferably about 1E18 cm -3 , although higher and lower values can also be used as well as other doping types. Depth  761  usefully in the range of about 0.5 to 2.0 micrometers, more conveniently in the range of about 1.0 to 1.5 micrometers and preferably about 1.0 micrometers, but larger and smaller values may also be used. Structure  314  results. Region  148  (e.g., P type) of  FIG. 10  can be formed in an analogous manner, having similar depth or thickness and doping usefully in the range of about 1E16 cm -3  to 1E 17 cm -3 , more conveniently in the range of about 2E16 cm -3  to 8E16 cm -3  and preferably about 5E 16 cm -3 , but other values may also be used. 
     Referring now to manufacturing stage  215  of  FIG. 15 , mask  614  is removed and mask  615  is applied having opening  615 - 1  and closed portion  615 - 2 . Ion implant  515  is desirably used to form (e.g., N type) sinker regions  86 ,  88  of depth sufficient to provide Ohmic (non-rectifying) contact to buried layer  72 . Other doping means well known in the art may also be used in other embodiments. Phosphorus is a non-limiting example of a suitable dopant. Sinker regions  86 ,  88  have a doping density usefully in the range of about 1E18 to 5E19 cm -3 , more conveniently in the range of about 2E18 to 1E19 cm -3  and preferably about 5E18 cm -3 , although higher and lower values can also be used as well as other doping types. Structure  315  results. Referring now to manufacturing stage  216  of  FIG. 16 , mask  614  is removed and gate  25  provided overlying a suitable gate insulator on surface  71  in the indicated location, using means well known in the art. Gate  25  of  FIGS. 16-18  is analogous to gate  45  of  FIG. 10 . Structure  316  results. 
     Referring now to manufacturing stage  217  of  FIG. 17 , mask  617  is provided on surface  71 , having openings  617 - 1 ,  617 - 2 ,  617 - 3  and closed portions  617 - 4 ,  617 - 5 ,  617 - 6 . Implant  517  is provided through openings  617 - 1 ,  617 - 2 ,  617 - 3  so as to form (e.g., N+) source region  22  in body region  76 , drain region  24  in drift region  80  and contact region  89  in sinker region  88 . Phosphorus is a non-limiting example of a suitable dopant for regions  22 ,  24 ,  89  with a doping density usefully in the range of about 1E19 to 1E21 cm -3 , more conveniently in the range of about 5E19 to 5E20 cm -3  and preferably about 1E20cm -3 , although higher and lower values can also be used and other doping types. Regions  22 ,  24 ,  89  may be relatively shallow, with depth  891  usefully in the range of about 0.1 to 0.5 micrometers, more conveniently in the range of about 0.2 to 0.4 micrometers and preferably about 0.2 micrometers, but larger and smaller values may also be used. Structure  317  results. Region  46  of  FIG. 10  may be formed in substantially the same way. 
     Referring now to manufacturing stage  218  of  FIG. 18 , mask  617  is removed and mask  618  provided on surface  71 , having opening  618 - 1  and closed portions  618 - 2 ,  618 - 3 . Implant  518  is provided through opening  618 - 1  to form (e.g., P+) body contact region  78  in body region  76 . Boron is a non-limiting example of a suitable dopant for region  78  with a doping density usefully in the range of about 1E19 to 1E21 cm -3 , more conveniently in the range of about 5E19 to 5E20 cm -3  and preferably about 1E20cm -3 , although higher and lower values can also be used as well as other doping types. Depth  781  is usefully in the range of about 0.1 to 0.5 micrometers, more conveniently in the range of about 0.2 to 0.4 micrometers and preferably about 0.2 micrometers, but larger and smaller values may also be used. Structure  318  results. Regions  42 ,  44  (e.g., P+) of  FIG. 10  may be formed in substantially the same way. 
     Referring now to manufacturing stage  219 , mask  618  is removed. Structure  319  results. Conductive contacts are then made to regions  22 ,  24 ,  89 , and  78  using teachings well known in the art. The interconnections to couple such regions to source, drain and gate terminals and to couple contact  89  of sinker region  88  to drain region  24  or drain terminal  28  are also formed using teachings well known in the art, thereby providing substantially finished device  69 - 3  of  FIG. 9 . Substantially finished device  69 ′- 3  of  FIG. 10  is similarly provided by making connections to and between the analogous regions of device  69 ′- 3  of  FIG. 10 . 
     According to a first embodiment, there is provided an electronic device ( 69 ,  69 ′), comprising, an MOS transistor ( 63 ,  65 ) having current carrying terminals including a source ( 22 ,  42 ) and a drain ( 24 ,  44 ) in a semiconductor containing body ( 70 ,  72 ,  74 ) having an upper surface ( 71 ), a DC isolated buried layer ( 72 ) underlying the MOS transistor ( 63 ,  65 ), and a charge pump capacitance ( 94 ,  94 ′) coupled between one of the current carrying terminals ( 22 ,  42 ;  24 ,  44 ) and the DC isolated buried layer ( 72 ). According to a further embodiment, the MOS transistor ( 63 ) is an N channel transistor and the DC isolated buried layer ( 72 ) is N type. According to a still further embodiment, the MOS transistor ( 65 ) is a P channel transistor and the DC isolated buried layer ( 72 ) is N type. According to a yet further embodiment, the charge pump capacitance ( 94 - 1 ,  94 ′- 1 ) is external to the MOS transistor ( 63 ,  65 ). According to a still yet further embodiment, the charge pump capacitance ( 94 - 2 ,  94 ′- 2 ) is formed over the upper surface ( 71 ). According to a yet still further embodiment, the charge pump capacitance ( 94 - 2 ,  94 ′- 2 ) is a deposited capacitance. According to another embodiment, the charge pump capacitance ( 94 - 3 ,  94 ′- 3 ) is formed under the upper surface ( 71 ). According to a still another embodiment, the charge pump capacitance ( 94 - 3 ,  94 ′- 3 ) comprises a dielectric trench isolation wall ( 100 ) penetrating substantially from the upper surface ( 71 ) through the DC isolated buried layer ( 72 ) underlying the MOS transistor ( 63 ,  65 ). According to a yet another embodiment, the dielectric trench isolation wall ( 100 ) has a first sinker region ( 86 ) on a first side thereof facing toward the MOS transistor ( 63 ,  65 ) and a second sinker region ( 88 ) on a second side thereof facing away from the MOS transistor ( 63 ,  65 ), wherein the first sinker region ( 86 ) is Ohmically coupled to the DC isolated buried layer ( 72 ) and the second sinker region ( 88 ) is Ohmically coupled to one of the source ( 42 ) and drain ( 24 ) of the MOS transistor ( 63 ,  65 ) and the first ( 86 ) and second ( 88 ) sinker regions are DC isolated from each other by the dielectric trench isolation wall ( 100 ). According to a till yet another embodiment, the MOS transistor ( 63 ,  65 ) is an LDMOS transistor ( 69 ,  69 ′). 
     According to a second embodiment, there is provided an LDMOS transistor ( 69 ,  69 ′), comprising, a buried SC layer region ( 72 ), a further SC region ( 74 ) overlying the buried layer region ( 72 ) and having an upper surface ( 71 ), a MOSFET ( 63 ,  65 ) formed in the further SC region ( 74 ), wherein the MOSFET ( 63 ,  65 ) comprises, a body region ( 76 ,  154 ) having therein a source region ( 22 ,  42 ) of the MOSFET ( 63 ,  65 ), and a carrier drift region ( 80 ,  148 ) laterally separated from the body region ( 76 ,  154 ) and having therein a drain region ( 24 ,  44 ) of the MOSFET ( 63 ,  65 ), and a charge pump capacitance ( 94 ,  94 ′) coupled between the buried layer region ( 72 ) and one of the drain region ( 24 ) and the source region ( 42 ) of the MOSFET ( 63 ,  65 ). According to a further embodiment, the charge pump capacitance ( 94 - 1 ,  94 ′- 1 ;  94 - 2 ,  94 ′- 2 ) is formed substantially over the upper surface ( 71 ). According to a still further embodiment, the charge pump capacitance ( 94 - 1 ,  94 ′- 1 ;  94 - 2 ,  94 ′- 2 ) is formed substantially on the upper surface ( 71 ). According to a yet further embodiment, the charge pump capacitance ( 94 - 3 ,  94 ′- 3 ) is formed substantially beneath the upper surface ( 71 ). According to a still yet further embodiment, the charge pump capacitance ( 94 ,  94 ′) has a capacitance value adapted to pump charge into the buried layer ( 72 ) in response to a fast voltage transient voltage ( 95 ) applied between the source ( 22 ,  42 ) and drain ( 24 ,  44 ) so as to temporarily raise a voltage of the buried layer ( 72 ) by at least 5% of the magnitude of fast voltage transient voltage ( 95 ). According to a yet still further embodiment, the charge pump capacitance ( 94 ,  94 ′) has a capacitance value adapted to pump charge into the buried layer ( 72 ) in response to a fast voltage transient ( 95 ) applied between the source ( 22 ,  42 ) and drain ( 24 ,  44 ) so as to temporarily raise the voltage of the buried layer ( 72 ) by at least 10% of the magnitude of fast voltage transient voltage ( 95 ). 
     According to a third embodiment, there is provided a method for providing an LDMOS transistor ( 69 ,  69 ′), comprising, forming a buried layer ( 72 ) of a first conductivity type, forming a further SC region ( 74 ) of a second, opposite, conductivity type on the buried layer ( 72 ), and having an upper surface ( 71 ), forming a first doped region ( 80 ,  154 ) of the first conductivity type in a first portion of the further SC region ( 74 ) extending at least in part to the upper surface ( 71 ) and overlying at least part of the buried layer ( 72 ), forming a dielectric trench isolation wall ( 100 ) extending though the further SC region ( 74 ) and the buried layer ( 72 ), and laterally separated from the first doped region ( 80 ,  154 ), forming another doped region ( 76 ,  148 ) of the second conductivity type extending into the further semiconductor region ( 74 ) between the first doped region ( 80 ,  154 ) and the dielectric trench isolation wall ( 100 ) and laterally separated from the first doped region ( 80 ,  154 ) by a portion of the further semiconductor region ( 74 ), forming first ( 86 ) and second ( 88 ) sinker regions of the first conductivity type extending substantially from the surface ( 71 ) through the further semiconductor region ( 74 ) to make Ohmic contact to the buried layer ( 72 ), the first sinker region ( 86 ) located on a first side of the dielectric trench isolation wall ( 100 ) toward the first doped region ( 80 ,  154 ) and the second sinker region ( 88 ) located on a second side of the dielectric trench isolation wall ( 100 ) facing away from the first doped region ( 80 ,  154 ) so that, (i) the first ( 86 ) and second ( 88 ) sinker regions and (ii) portions ( 722 ,  723 ) of the buried layer ( 72 ) lying on either side of the dielectric trench isolation wall ( 100 ) are DC isolated from each other, providing a second sinker Ohmic contact region ( 89 ) of the first conductivity type in the second sinker region ( 88 ), wherein if the LDMOS transistor ( 69 ,  69 ′) is an N channel LDMOS transistor ( 69 ), providing a drain region ( 24 ) of the first conductivity type in the first doped region ( 80 ) and Ohmically connecting the second sinker contact region ( 89 ) to the drain region ( 24 ), and wherein if the LDMOS transistor ( 69 ,  69 ′) is a P channel LDMOS transistor ( 69 ′), providing a source region ( 42 ) of the second conductivity type in the first doped region ( 154 ) and Ohmically connecting the second sinker contact region ( 89 ) to the source region ( 42 ). According to a further embodiment, the method further comprises, forming a gate insulator with an overlying gate conductor ( 25 ,  45 ) on the upper surface ( 71 ) above at least the portion of the further semiconductor region ( 74 ) between the first doped region ( 80 ,  154 ) and the another doped region ( 76 ,  148 ). According to a still further embodiment, the LDMOS transistor ( 69 ,  69 ′) is an N channel LDMOS transistor ( 69 ) and the source region ( 22 ), the drain region ( 24 ) and the second sinker Ohmic contact region ( 89 ) are formed substantially at the same time. According to a yet further embodiment, the LDMOS transistor ( 69 ,  69 ′) is a P channel LDMOS transistor ( 69 ′), and the second sinker Ohmic contact region ( 89 ) and a body contact region ( 46 ) to the first doped region ( 154 ) are formed at substantially the same time. 
     While at least one exemplary embodiment and method of fabrication has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.