Abstract:
A LDMOS transistor is implemented in a first impurity region on a substrate. The LDMOS transistor has a source that includes a second impurity region. The second impurity region is implanted into the surface of the substrate within the first impurity region. Additionally, the LDMOS transistor has a drain that includes a third impurity region. The third impurity region is implanted into the surface of the substrate within the first impurity region. The third impurity region is spaced a predetermined distance away from a gate of the LDMOS transistor. The drain of the LDMOS transistor further includes a fourth impurity region within the third impurity region. The fourth impurity region provides an ohmic contact for the drain.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation and claims the benefit of priority under 35 U.S.C. Section 120 of U.S. patent application Ser. No. 13/027,132, filed Feb. 14, 2011, which is a divisional of U.S. patent application Ser. No. 11/677,724, filed Feb. 22, 2007, now U.S. Pat. No. 7,888,222, which is a divisional of U.S. patent application Ser. No. 10/713,749, filed on Nov. 13, 2003, now U.S. Pat. No. 7,220,633. The disclosure of each prior application is considered part of and is incorporated by reference in the disclosure of this application. 
    
    
     BACKGROUND 
     The following disclosure relates to semiconductor devices, and more particularly to a lateral double-diffused MOSFET (LDMOS) transistor. 
     Voltage regulators, such as DC to DC converters, are used to provide stable voltage sources for electronic systems. Efficient DC to DC converters are particularly needed for battery management in low power devices, such as laptop notebooks and cellular phones. Switching voltage regulators (or simply “switching regulators”) are known to be an efficient type of DC to DC converter. A switching regulator generates an output voltage by converting an input DC voltage into a high frequency voltage, and filtering the high frequency input voltage to generate the output DC voltage. Specifically, the switching regulator includes a switch for alternately coupling and decoupling an input DC voltage source, such as a battery, to a load, such as an integrated circuit. An output filter, typically including an inductor and a capacitor, is coupled between the input voltage source and the load to filter the output of the switch and thus provide the output DC voltage. A controller, such as a pulse width modulator or a pulse frequency modulator, controls the switch to maintain a substantially constant output DC voltage. 
     LDMOS transistors are commonly used in switching regulators as a result of their performance in terms of a tradeoff between their specific on-resistance (R dson ) and drain-to-source breakdown voltage (BV d     —     s ). Conventional LDMOS transistors are typically fabricated having optimized device performance characteristics through a complex process, such as a Bipolar-CMOS (BiCMOS) process or a Bipolar-CMOS-DMOS (BCD) process, that includes one or more process steps that are not compatible with sub-micron CMOS processes typically used by foundries specializing in production of large volumes of digital CMOS devices (e.g, 0.5 μm DRAM production technologies), as described in greater detail below. As a result, conventional LDMOS transistors are, therefore, not typically fabricated at such foundries. 
     A typical sub-micron CMOS process used by foundries specializing in production of large volumes of digital CMOS devices, referred to herein as sub-micron CMOS process, will now be described. A sub-micron CMOS process is generally used to fabricate sub-micron CMOS transistors—i.e., PMOS transistors and/or NMOS transistors having a channel length that is less than 1 μm.  FIG. 1  shows a PMOS transistor  100  and an NMOS transistor  102  fabricated through a sub-micron CMOS process on a p-type substrate  104 . The PMOS transistor  100  is implemented in a CMOS n-well  106 . The PMOS transistor  100  includes a source region  108  and a drain region  110  having p-doped p+ regions  112  and  114 , respectively. The PMOS transistor  100  further includes a gate  116  formed of a gate oxide  118  and a polysilicon layer  120 . The NMOS transistor  102  is implemented in a CMOS p-well  122 . The NMOS transistor  102  includes a source region  124  and a drain region  126  having n-doped n+ regions  128  and  130 , respectively. The NMOS transistor  102  further includes a gate  132  formed of a gate oxide  134  and a polysilicon layer  136 . 
       FIG. 2  illustrates a sub-micron CMOS process  200  that can be used to fabricate large volumes of sub-micron CMOS transistors (such as the CMOS transistors shown in  FIG. 1 ). The process  200  begins with forming a substrate (step  202 ). The substrate can be a p-type substrate or an n-type substrate. Referring to  FIG. 1 , the CMOS transistors are fabricated on a p-type substrate  104 . A CMOS n-well  106  for the PMOS transistor and a CMOS p-well  122  for the NMOS transistor are implanted into the substrate (step  204 ). The gate oxide  118 ,  134  of each CMOS transistor is formed, and a CMOS channel adjustment implant to control threshold voltages of each CMOS transistor is performed (step  206 ). A polysilicon layer  120 ,  136  is deposited over the gate oxide  118 ,  134 , respectively (step  208 ). The p+ regions of the PMOS transistor and the n+ regions of the NMOS transistor are implanted (step  210 ). The p+ regions  112 ,  114  and n+ regions  128 ,  130  are highly doped, and provide low-resistivity ohmic contacts. In a sub-micron CMOS process, formation of an n+ region typically occurs through a three-step process in a single masking and photolithography step as follows: 1) a lightly doped n-type impurity region is implanted, 2) an oxide spacer is formed, and 3) a heavily doped n+ impurity region is implanted. Formation of a p+ region occurs in a similar manner. The formation such n+ and p+ regions allow transistors to have an improved hot carrier performance. 
     Foundries specializing in production of large volumes of digital CMOS devices generally have fixed parameters associated with the foundries&#39; sub-micron CMOS process. These fixed parameters are typically optimized for the mass production of digital sub-micron CMOS transistors. For example, in process step  206 , the CMOS channel adjustment implant generally has an associated thermal budget that is typically fixed, and has parameters optimized for mass production of sub-micron CMOS transistors. 
     As discussed above, conventional LDMOS transistors typically achieve optimized device performance through a complex process, such as a BiCMOS process or a BCD process, that includes one or more process steps that are not compatible with a sub-micron CMOS process optimized for the mass production of digital sub-micron CMOS transistors. 
       FIG. 3A  shows a conventional LDMOS transistor  300  fabricated through a BiCMOS process on a p-type substrate  302 . The LDMOS transistor  300  includes source region  304  with an n-doped n+ region  306 , a p-doped p+ region  308 , and a p-doped P-body diffusion (P-body)  310 . The LDMOS transistor  300  also includes a drain region  312  with an n-doped n+ region  314  and an n-type well (HV n-well)  316 , and a gate  318 , including a gate oxide  320  and a polysilicon layer  322 . 
     In the BiCMOS process, the gate oxide  320 , and gate oxide of any CMOS transistors fabricated in the BiCMOS process, is formed prior to implantation of the n+ region  306  and the P-body  310 . The BiCMOS process, therefore, allows the gate  318  to serve as a mask during implantation of the n+ region  306  and the P-body  310 —i.e., the n+ region  306  and the P-body  310  are self aligned with respect to the gate  318 . The self aligned lateral double diffusion of the n+ region  306  and the P-body  310  forms the channel of the LDMOS transistor  300 . 
     Such kinds of self aligned double diffusions are not easily integrated into a sub-micron CMOS process because the subsequent drive-in step (or thermal budget) associated with self aligned double diffusions disrupts the fixed thermal budget associated with sub-micron CMOS process steps (e.g., process step  206 ) and requires a redesign of the thermal budget allocated to the sub-micron CMOS process steps. That is, the self aligned double diffusions generally includes a drive-in step with a long duration and a high temperature that can cause the characteristics of sub-micron CMOS transistors (e.g., threshold voltages) to shift. 
     The lateral doping profile in region (a) of the LDMOS transistor  300  controls the tradeoff between the on-resistance R dson  and the drain-to-source breakdown voltage BV d     —     s . The vertical doping profile in region (b) determines the drain-to-substrate breakdown voltage BV d     —     sub  of the LDMOS transistor, and the pinch-off doping profile in region (c) determines the source-to-substrate punch-through breakdown voltage BV s     —     sub  of the LDMOS transistor. The source-to-substrate punch-through breakdown voltage BV s     —     sub  is an important parameter for an LDMOS transistor with a floating operation requirement, e.g, an LDMOS transistor implemented as a high-side control switch in a synchronous buck circuit configuration. 
       FIG. 3B  shows a conventional LDMOS transistor  330  fabricated through a BCD process on a p-type substrate  332 . The LDMOS transistor  330  includes source region  334  with an n-doped n+ region  336 , a p-doped p+ region  338 , and a p-doped P-body  340 . The LDMOS transistor  330  also includes a drain region  342  with an n-doped n+ region  344  and an n-type layer (HV n-Epi)  346 , and a gate  348 , including a gate oxide  350  and a polysilicon layer  352 . As with the BiCMOS process, in the BCD process, the gate oxide  350 , and gate oxide of any CMOS transistors fabricated in the BCD process, is formed prior to implantation of the n+ region  336  and the P-body  340 . 
     In the BCD process, an n+ buried layer  354  can be grown on the p-type substrate  332  to improve the source-to-substrate punch-through breakdown characteristics of the LDMOS transistor. Such an approach offers an improved tradeoff between the on-resistance R dson  and drain-to-source breakdown voltage BV d     —     s  of the LDMOS transistor as the lateral doping profile of the LDMOS transistor can be optimized without constrain on the vertical doping profiles. However, such a BCD process includes the growth of the HV n− Epi layer  346 , and this step is generally not compatible with a sub-micron CMOS process. 
     Another approach used in a BCD process is to utilize an n− layer  360  implanted in the drain region  362  of the LDMOS transistor  364  as shown in  FIG. 3C . The n− layer  360 , n+ region  366 , and P-body  368  are self aligned with respect to the gate  370 —i.e., the n− layer  360 , n+ region  366 , and P-body  368  are implanted after formation of gate oxide  372 . The inclusion of the n− layer  360  provides an additional parameter to further optimize the tradeoff between the on-resistance R dson  and drain-to-source breakdown voltage BV d     —     s  of the LDMOS transistor. Similar to the n+ buried layer approach of  FIG. 3B , the inclusion of the n− layer  360  at the surface provides a method to decouple vertical and horizontal doping constraints. 
     SUMMARY 
     In one aspect, this specification describes a method for fabricating a transistor having a source, drain, and a gate on a substrate. A first impurity region is implanted into a surface of the substrate. The first impurity region has a first volume and a first surface area, and is of a first type. A second impurity region is implanted into the source region of the transistor. The second impurity region has a second volume and a second surface area in the first surface area of the first impurity region, and is of an opposite second type relative to the first type. A gate oxide is formed between the source region and a drain region of the transistor. The gate oxide of the transistor is formed after implantation of the second impurity region. The gate oxide is covered with a conductive material. A third impurity region and a fourth impurity region are implanted into the source region of the transistor. The third impurity region has a third volume and a third surface area in the second surface area of the second impurity region, and is of the opposite second type. The fourth impurity region has a fourth volume and a fourth surface area in the second surface area of the second impurity region, and is of the first type. A fifth impurity region is implanted into the drain region of the transistor. The fifth impurity region has a fifth volume and a fifth surface area, and is of the first type. 
     Implementations may include one or more of the following features. A sixth impurity region can be implanted into the drain region of the transistor. The sixth impurity region can have a sixth volume and a sixth surface area in the first surface area of the first impurity region, and be of the first type. The sixth impurity region can be implanted with a spacing from the second impurity region. The sixth impurity region can be self aligned to the gate of the transistor (i.e., implanted after gate formation of the transistor). The sixth impurity region can be non-self aligned to the gate of the transistor (i.e., implanted prior to gate formation of the transistor). The sixth impurity region can be a double doped drain implant or a conventional CMOS well implant. The spacing of the second impurity region from the sixth impurity region can be sized such that the sixth impurity region is spaced a predetermined distance away from the gate of the transistor as measured along a surface of the transistor. The first impurity region and the sixth impurity region can be implanted using a same mask. The implantation of the fifth impurity region can be defined by a slit mask. The fifth impurity region can be formed by multiple implants spaced apart relative to each other along a surface of the transistor in the drain region of the transistor. The third impurity region, the fifth impurity region, and the sixth impurity region can be implanted simultaneously using the slit mask. The second impurity region can be implanted using a first implant and a second implant. The first implant can be a high energy implant. The first implant can also be a large angle tilt implant. 
     Additional implementations may include one or more of the following features. A seventh impurity region can be implanted into the source region of the transistor. The seventh impurity region can have a seventh volume and a seventh surface area, and be of the first type. An eighth impurity region can be implanted into the drain region of the transistor. The eighth impurity region can have an eighth volume and an eighth surface area, and be of the first type. A field oxide can be formed on the drain region of the transistor. The transistor can be a lateral double-diffused MOSFET. 
     In another aspect, this specification describes a method for fabricating an LDMOS transistor. A P-body is implanted into a source region of the LDMOS transistor. A gate oxide for the LDMOS transistor is formed after implantation of the P-body. An n+ region is implanted into the source region of the LDMOS transistor. The n+ region provides an ohmic contact. Implementations may include one or more of the following features. The P-body for the LDMOS transistor can be implanted using a first implant and a second implant. The first implant can be a high energy implant. The first implant can also be a large angle tilt implant. 
     In another aspect, this specification describes an LDMOS transistor. The LDMOS transistor is implemented in a first impurity region with a first volume and a first surface area. The first impurity region is implanted into a surface of a substrate. The LDMOS transistor has a source that includes a second impurity region with a second volume and a second surface area. The second impurity region is implanted into the surface of the substrate within the first impurity region. The LDMOS transistor also has a gate. Additionally, the LDMOS transistor has a drain that includes a third impurity region with a third volume and a third surface area. The third impurity region is implanted into the surface of the substrate within the first impurity region. The third impurity region is spaced a predetermined distance away from the gate of the LDMOS transistor as measured along a surface of the substrate. The drain of the LDMOS transistor further includes a fourth impurity region with a fourth volume and a fourth surface area within the third impurity region. The fourth impurity region provides an ohmic contact for the drain. Implementations may include the following features. The third impurity region can be a shallow drain implant. 
     Advantages of the invention may include the following. The method of fabricating a transistor having a double-diffused source region is compatible with mainstream sub-micron CMOS fabrication process technologies offered by foundries specializing in mass volume production (e.g., foundries specializing in mass production of digital sub-micron CMOS devices). That is, foundries specializing in mass production of sub-micron CMOS technologies do not have to disrupt (or change) fixed CMOS process parameters that have been optimized for the production of mass volumes the digital sub-micron CMOS devices. Production of conventional LDMOS transistors can, therefore, be seamlessly integrated into sub-micron CMOS production technologies. The LDMOS transistor can be fabricated in a process that is compatible with a sub-micron CMOS process, using a lower mask count than conventional BiCMOS and BCD processes. Integrated circuits including LDMOS transistors, e.g., a switching regulator, can be monolithically integrated onto a single chip using a sub-micron CMOS process. An input voltage source to a switching regulator having one or more LDMOS transistors can be optimized for different applications, and the fabrication process for the LDMOS transistors can be adjusted accordingly. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a conventional PMOS transistor and NMOS transistor formed on a p-type substrate. 
         FIG. 2  is a flow diagram illustrating a conventional sub-micron CMOS process for manufacturing CMOS transistors. 
         FIGS. 3A ,  3 B, and  3 C are schematic cross-sectional views of conventional LDMOS transistors. 
         FIG. 4  is a block diagram of a buck switching regulator. 
         FIGS. 5A-5B  are a schematic cross-sectional view of an LDMOS transistor and a three-dimensional view of the surface area of the LDMOS transistor source and drain regions, respectively. 
         FIG. 6  is a flow diagram illustrating a process for manufacturing a semiconductor transistor, including an LDMOS transistor, that is compatible with a sub-micron CMOS process. 
         FIGS. 7A-7H  illustrate the process of manufacturing an LDMOS transistor, a PMOS transistor, and an NMOS transistor according to the process of  FIG. 6 . 
         FIGS. 8A-8C  illustrate a P-body implant step of the process of  FIG. 6  according to one implementation. 
         FIG. 9  illustrates a shallow drain implant according to one implementation. 
         FIGS. 10A-10B  shows a graph of current conductance as a function of voltage difference between the drain and source of a PMOS transistor implemented in an HV n-well and a conventional CMOS n-well, respectively. 
         FIGS. 11A-11B  shows a graph of current conductance as a function of voltage difference between the drain and source of an NMOS transistor implemented in a P-body implant and a conventional NMOS transistor implemented in a CMOS p-well, respectively. 
         FIG. 12  is a flow diagram illustrating an alternative process for manufacturing a semiconductor transistor including an LDMOS transistor according to a process that is compatible with a sub-micron CMOS process. 
         FIGS. 13A-13H  illustrate the process of manufacturing an LDMOS transistor according to the process of  FIG. 12 . 
         FIG. 14  is a schematic cross-sectional view of an LDMOS transistor having a CMOS n-well implant. 
         FIG. 15  is a schematic cross-sectional view of an LDMOS transistor having a CMOS n-well implant as a shallow drain. 
         FIG. 16  is a schematic cross-sectional view of an LDMOS transistor having a DDD implant as a shallow drain. 
         FIG. 17  is a schematic cross-sectional view of an LDMOS transistor having an LDD diffused into source and drains regions of the transistor. 
         FIG. 18  is a schematic cross-sectional view of an LDMOS transistor having a graded shallow drain implant. 
         FIG. 19  is a schematic cross-sectional view of a p-type LDMOS transistor. 
         FIG. 20  shows a graph of current conductance as a function of voltage difference between the drain and source of a p-type LDMOS transistor. 
         FIG. 21  is a schematic cross-sectional view of a switching circuit including a switching circuit having a high-side LDMOS transistor and a low-side LDMOS transistor. 
         FIG. 22  is a schematic cross-sectional view of a NPN transistor. 
         FIG. 23  is a flow diagram illustrating a process for manufacturing the NPN transistor of  FIG. 22 . 
         FIG. 24  shows a graph of current conductance as a function of voltage of the NPN transistor of  FIG. 22 . 
         FIGS. 25A and 25B  are a schematic cross-sectional view of an implementation of high-side drive (HSD) circuits with CMOS logic and a circuit diagram of the HSD circuits with CMOS logic, respectively. 
         FIGS. 26A and 26B  show a graph of conductance of the CMOS transistors of  FIGS. 25A and 25B . 
         FIG. 27  is a schematic cross-sectional view of a LDMOS transistor with LOCOS on the drain region of the transistor. 
         FIG. 28  is a flow diagram illustrating a process for implanting a P-body of an LDMOS transistor. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 4  is a block diagram of a switching regulator  400  including an LDMOS transistor according to one implementation. Conventional LDMOS transistors typically achieve optimized device performance through a complex process, such as a BiCMOS process or a BCD process, that includes one or more process steps not compatible with a sub-micron CMOS process optimized for the mass production of digital sub-micron CMOS transistors. According to one aspect, an LDMOS transistor is provided that can be fabricated through a process that can be seamlessly integrated into a typical sub-micron CMOS process. 
     Referring to  FIG. 4 , an exemplary switching regulator  400  is coupled to a first high DC input voltage source  402 , such as a battery, by an input terminal  404 . The switching regulator  400  is also coupled to a load  406 , such as an integrated circuit, by an output terminal  408 . The switching regulator  400  serves as a DC-to-DC converter between the input terminal  404  and the output terminal  408 . The switching regulator  400  includes a switching circuit  410  which serves as a power switch for alternately coupling and decoupling the input terminal  404  to an intermediate terminal  412 . The switching circuit  410  includes a rectifier, such as a switch or diode, coupling the intermediate terminal  412  to ground. Specifically, the switching circuit  410  may include a first transistor  414  having a source connected to the input terminal  404  and a drain connected to the intermediate terminal  412  and a second transistor  416  having a source connected to ground and a drain connected to the intermediate terminal  412 . The first transistor  414  may be a Positive-Channel Metal Oxide Semiconductor (PMOS) transistor, whereas the second transistor  416  may be an LDMOS transistor. 
     The intermediate terminal  412  is coupled to the output terminal  408  by an output filter  418 . The output filter  418  converts the rectangular waveform of the intermediate voltage at the intermediate terminal  412  into a substantially DC output voltage at the output terminal  408 . Specifically, in a buck-converter topology, the output filter  418  includes an inductor  420  connected between the intermediate terminal  412  and the output terminal  408  and a capacitor  422  connected in parallel with the load  406 . During a PMOS conduction period, the first transistor is closed, and the voltage source  402  supplies energy to the load  406  and the inductor  420  through the first transistor  414 . On the other hand, during an LDMOS transistor conduction period, the second transistor  416  is closed, and current flows through the second transistor  416  as energy is supplied by the inductor  420 . The resulting output voltage V out  is a substantially DC voltage. 
     The switching regulator also includes a controller  424 , a PMOS driver  426  and an LDMOS driver  428  for controlling the operation of the switching circuit  400 . The PMOS driver  426  and the LDMOS driver are coupled to voltage source  430 . A first control line  432  connects the PMOS transistor  414  to the PMOS driver  426 , and a second control line  434  connects the LDMOS transistor  416  to the LDMOS driver  428 . The PMOS and NMOS drivers are connected to the controller  424  by control lines  436  and  438 , respectively. The controller  424  causes the switching circuit  400  to alternate between PMOS and LDMOS conduction periods so as to generate an intermediate voltage V int  at the intermediate terminal  412  that has a rectangular waveform. The controller  424  can also include a feedback circuit (not shown), which measures the output voltage and the current passing through the output terminal. Although the controller  424  is typically a pulse width modulator, the invention is also applicable to other modulation schemes, such as pulse frequency modulation. 
     Although the switching regulator discussed above has a buck converter topology, the invention is also applicable to other voltage regulator topologies, such as a boost converter or a buck-boost converter, and to RF output amplifiers. 
       FIG. 5A  shows a schematic cross-sectional view of the LDMOS transistor  416 . The LDMOS transistor  416  can be fabricated on a high voltage n-type well (HV n-well)  500 A implanted in a p-type substrate  502 . An HV n-well implant is typically a deep implant and is generally more lightly doped relative to a CMOS n-well. HV n-well  500 A can have a retrograded vertical doping profile. The LDMOS transistor  416  includes a drain region  504 , a source region  506 , and a gate  508 . The drain region  504  includes an n-doped n+ region  510  and an n-doped shallow drain (N-LD)  512 . The source region  506  includes an n-doped n+ region  514 , a p-doped p+ region  516 , and a p-doped P-body  518 . The HV n-well  500 A, the N-LD  512 , and the n+ region  510  in drain region  504  are volumes composed of doped material. Both the N-LD  512  and the HV n-well  500 A have a lower concentration of impurities than the n+ regions  510 ,  514 . However, portions at which these volumes overlap have a higher doping concentration than the individual volumes separately. A portion  520  that contains the overlapping volumes of the n+ region  510 , the N-LD  512 , and the HV n-well  500 A has the highest doping concentration of all the overlapping volume portions. A portion  522  that contains the overlapping volumes of the N-LD  512  and the HV n-well  500 A, but not the n+ region  510 , has a lower doping concentration than portion  520 . A portion  524  that only includes the HV n-well  500 A has a lower doping concentration than either portions  520  or  522  because it does not include multiple overlapping doped volumes. Likewise, the n+ region  514 , the p+ region  516 , and the P-body  518  in source region  506  are volumes ( 526 ,  528 , and  530 , respectively) composed of doped material. 
     Referring to  FIG. 5B , the volumes  520 - 530  can each have a surface area on the surface  532  of the device. The HV n-well  500 A has a surface area  534 . In the drain region  524 , the N-LD  522  has a surface area  536  located within the surface area of the HV n-well  500 A. The n+ region  510  has a surface area  538  located within the surface area  536  of the N-LD. In the source region  506 , the P-body  518  has surface area  540  located within the surface area  534 . The n+ region  514  and the p+ region  516  have a surface area  542  and  544 , respectively, that is located within the surface area  540  of the P-body. 
       FIG. 6  illustrates a process  600  of fabricating a semiconductor device, including an LDMOS transistor, a PMOS transistor with floating operation capability (i.e., the source of the transistor is not grounded), and an NMOS transistor with floating operation capability, that is compatible with a sub-micron CMOS process. Conventional CMOS transistors can also be fabricated through process  600 . 
     The process  600  begins with forming a substrate (step  602 ). The substrate can be a p-type substrate or an n-type substrate. Referring to the example of  FIG. 7A , a semiconductor layer consisting of a p-type substrate  502  is formed. An HV n-well  500 A-B for the LDMOS transistor, the PMOS transistor with floating operation capability, and NMOS transistor with floating operation capability, is implanted into the substrate (step  604 ). As shown in  FIG. 7B , a separate HV n-well  500 A can be implanted for the LDMOS transistor. A CMOS n-well  106  for a conventional PMOS transistor and a CMOS p-well  122  for a conventional NMOS transistor are implanted into the substrate (step  606 ) ( FIG. 7C ). A non-self aligned P-body  518  for the drain region of the LDMOS transistor is implanted (step  608 ). As shown in  FIG. 7D , the P-body  518  is implanted into the HV n-well  500 A. During step  706 , a P-body can also be implanted for the NMOS transistor with floating operation capability. Referring again to  FIG. 7D , a P-body  700  for the NMOS transistor with floating operation capability is implanted into the HV n-well  500 B. 
     In one implementation, the non-self aligned P-body  518  is implanted into the HV n-well  500 A in two separate steps to allow for a better control of vertical depth and amount of lateral side diffusion of the P-body. Referring to  FIG. 8A , a first P-body implant  802  into the HV n-well  500 A limits the vertical depth of the P-body. The vertical depth of the first P-body implant  802  controls the vertical doping profile underneath the source region of the LDMOS transistor, and therefore determines the source-to-substrate punch-through breakdown voltage BV s     —     sub  of the LDMOS transistor. The first P-body implant can be a high energy implant. In one implementation, the first P-body implant  802  is implanted using a large-angle tilt (LAT) implant process. A normal angle implant tilt is typically 7 degrees. A LAT is typically larger than 7 degrees. As shown in  FIG. 8B , a second P-body implant  804  is implanted over the first P-body implant  802 . The second P-body implant  804  controls the channel length. The second P-body implant  804  also sets the surface concentration of the P-body to control the threshold voltage (V t ) of the LDMOS transistor. A subsequent P-body drive-in and annealing process that limits the amount of the lateral side diffusion  806  of the P-body (for further channel length control) is shown in  FIG. 8C . In one implementation, the subsequent annealing process is a rapid thermal anneal (RTA) process. 
     The gate oxide for each of the LDMOS transistor, the PMOS transistor with floating operation capability, and the NMOS transistor with floating operation capability, and the conventional CMOS transistors, is formed (step  610 ). The gate oxide for the LDMOS transistor can be formed at the same time as a gate oxide of the conventional CMOS transistors. The LDMOS transistor can, therefore, have a similar threshold voltage and gate oxide thickness and as the conventional CMOS transistors, and can be driven directly by conventional CMOS logic circuits. Alternatively, the gate oxide of the LDMOS transistor can formed at a different time than the gate oxide of the conventional CMOS transistors to allow the LDMOS transistor to be implemented with a dedicated thick gate oxide. When implemented with a thick gate oxide, the LDMOS transistor allows for higher gate drive in applications where a lower voltage power supply may not be readily available. This flexibility allows for optimization of the LDMOS transistor depending on specific requirements of a power delivery application, such as efficiency targets at a particular frequency of operation. Referring to the example of  FIG. 7E , the LDMOS gate oxide  508  is formed on a surface  702  of the substrate over an inner edge  704  of the P-body  518 . The gate oxide  524  of the PMOS transistor (with floating operation capability) is formed on the surface of the substrate on the HV n-well  500 B. The gate oxide  706  of the NMOS transistor (with floating operation capability) is also formed on the surface of the substrate on the HV n-well  500 B. The gate oxide  118  of the conventional PMOS transistor is formed on the surface of the substrate on the CMOS n-well  106 . The gate oxide  134  of the conventional NMOS transistor is formed on the surface of the substrate on the CMOS p-well  122 . A polysilicon layer is deposited over the gate oxide (step  510 ). As shown in  FIG. 7F , a polysilicon layer  708 A-C is deposited over the LDMOS gate oxide  508 , the PMOS gate oxide  524 , the NMOS gate oxide  706 , respectively. A polysilicon layer  120  is deposited over the conventional PMOS gate oxide  118 , and a polysilicon layer  136  is deposited over the conventional NMOS gate oxide  134 . 
     A shallow drain is implanted and diffused into the drain of the LDMOS transistor (step  614 ). The shallow drain can be implanted before or after the LDMOS gate is formed—i.e., the shallow drain can be non-self aligned or self aligned with respect to the LDMOS gate. The shallow drain can be implanted through a LAT implant or a normal angle tilt implant. In the example of  FIG. 7G , the shallow drain is the n-doped shallow drain N-LD  512 . The shallow drain implant N-LD  512  has a spacing  707  from the P-body implant that is controlled by masked gate dimensions. The spacing  707  can be sized such that that the N-LD  512  implant extends a predetermined distance d from the LDMOS gate as shown in  FIG. 9 . The predetermined distance d can be controlled by mask dimensions. In one implementation, the N-LD implant shares the same mask as the HV n-well to reduce the mask count. Such an approach is possible if the doping concentration of N-LD is lighter than the P-body so that the extra N-LD implant into the source of the LDMOS transistor does not affect the channel characteristics. 
     The n+ regions and p+ regions of the LDMOS transistor, the PMOS transistor with floating operation capability, and the NMOS transistor with floating operation capability, and the conventional CMOS transistors, are implanted (step  616 ). As shown in  FIG. 7H , the p+ regions  526  and  528  are implanted at the drain and source, respectively, of the PMOS transistor with floating operation capability. A p+ region  516  is also implanted at the source of the LDMOS transistor. The LDMOS transistor also include an n+ region  510  implanted at the drain and an n+ region  514  implanted at the source. The n+ regions  710  and  712  are implanted at the drain and source, respectively, of the NMOS transistor with floating operation capability. P+ regions  112 ,  114 , are implanted at the source and drain, respectively, of the conventional PMOS transistor. N+ regions  128 ,  130  are implanted at the source and drain regions, respectively, of the conventional NMOS transistor. P+ regions  526 ,  528 ,  516 ,  112 ,  114  and n+ regions  510 ,  514 ,  710 ,  712 ,  128 ,  130  can be formed through a 3 step process as described above in connection with a submicron CMOS process. 
     The process  600  provides several potential advantages. First, the P-body of the LDMOS transistor is implanted and diffused prior to formation of the gate oxide of the conventional CMOS transistors. The thermal cycle associated with the P-body implant therefore does not substantially affect the fixed thermal budget associated with sub-micron CMOS process steps (e.g., process step  206 ). Second, any channel length variation due to misalignment of the P-body  518  and n+ region  514  can be mitigated by a greater critical dimension (CD) control of the process  600 . 
     Also, PMOS transistors are typically formed on a conventional CMOS n-well. In applications where a shift in threshold voltages of CMOS transistors is tolerable, a PMOS transistor can be directly implemented in an HV n-well, such as the PMOS transistor with floating operation capability in the example of  FIG. 7H . Implementing a PMOS transistor directly in an HV n-well has the advantage of allowing the process  600  to skip a conventional CMOS n-well implant and masking step (while maintaining its thermal cycle), thereby potentially lowering the overall process manufacturing cost. 
       FIGS. 10A and 10B  shows a graph of current conductance as a function of voltage difference between the drain and source of a PMOS transistor implemented in an HV n-well and a conventional CMOS n-well, respectively. 
     As a PMOS transistor can be directly implemented in the HV n-well, an NMOS transistor can similarly be implemented within a P-body implant, such as the NMOS transistor with floating operation capability in the example of  FIG. 7H . A conventional sub-micron CMOS process can therefore skip a conventional CMOS P-well implant and masking step (while maintaining its thermal cycle) to lower the overall process manufacture cost. 
       FIGS. 11A and 11B  shows experimental data of a 3.3V NMOS transistor fabricated in a P-body implant and a 3.3V NMOS transistor fabricated in a conventional P-well, respectively. 
       FIG. 12  illustrates an alternative process  1200  of fabricating an LDMOS transistor that is compatible with a typical sub-micron CMOS process. 
     The process  1200  begins with forming a substrate (step  1202 ). The substrate can be a p-type substrate or an n-type substrate. Referring to the example of  FIG. 13A , a semiconductor layer consisting of a p-type substrate  1302  is formed. An HV n-well for the LDMOS transistor is implanted into the substrate (step  1204 ). The implanted well can be an HV (high voltage) n-well, such as HV n-well  1304  ( FIG. 13B ). A CMOS n-well  106  for a conventional PMOS transistor and a CMOS p-well  122  for a conventional NMOS transistor are implanted into the substrate (step  1206 ) ( FIG. 13C ). An LDMOS gate oxide and polysilicon is formed for the LDMOS transistor (step  1208 ) The LDMOS gate oxide and polysilicon is distinct from the gate oxide and polysilicon of the conventional CMOS transistors (step  1208 )—i.e., the gate of the LDMOS transistor is formed separate from and prior to the formation of the gate of the conventional CMOS transistors being fabricated at the same time. Referring to the example of  FIG. 13D , the LDMOS gate oxide  1306  is formed on the surface  1308  of the substrate on the HV n-well  1304 , and a polysilicon layer  1310  is deposited over the LDMOS gate oxide. 
     A self aligned P-body  1312  (with respect to the gate of the LDMOS transistor) for the drain region of the LDMOS transistor is implanted (step  1210 ). As shown in  FIG. 13E , the P-body  1312  is implanted into the HV n-well  1304 . The self aligned P-body  1312  can be implanted into the HV n-well in two steps, as discussed above, to allow for a better control of the vertical depth and the amount of lateral side diffusion of the P-body. The P-body drive-in and annealing process can occur prior to, for example, formation of the gate oxide of the conventional CMOS transistors such that a redesign of the thermal cycle allocated to sub-micron CMOS processes (e.g., process step  206 ) is not required. 
     The gate of the conventional CMOS transistors is formed (step  1212 ). Referring to  FIG. 13F , the gate oxide  118  of the conventional PMOS transistor is formed on the surface of the substrate on the CMOS n-well  106 , and the gate oxide  134  of the conventional NMOS transistor is formed on the surface of the substrate on the CMOS p-well  122 . A polysilicon layer  120  is deposited over the conventional PMOS gate oxide  118 , and a polysilicon layer  136  is deposited over the conventional NMOS gate oxide  134 . A shallow drain is implanted and diffused into the drain of the LDMOS transistor (step  1214 ). The shallow drain can be non-self aligned or self aligned. In the example of  FIG. 13G , the shallow drain is the n-doped shallow drain N-LD  1314 . The N-LD implant can share the same mask as the HV n-well to reduce the mask count. The n+ regions and p+ regions of the LDMOS transistor are implanted (step  1216 ). In one implementation, during this step, n+ and p+ regions associated with the CMOS transistors are also implanted. As shown in  FIG. 13H , a p+ region  1416  and an n+ region  1418  are implanted at the source of the LDMOS transistor. An n+ region  1420  is also implanted at the drain of the LDMOS transistor. Further, p+ regions  112 ,  114 , are implanted at the source and drain, respectively, of the conventional PMOS transistor, and n+ regions  128 ,  130  are implanted at the source and drain regions, respectively, of the conventional NMOS transistor. As in process  600 , formation of the p+ regions and the n+ regions can occur through a 3 step process as described above in connection with a sub-micron CMOS process. 
     LDMOS Transistor Performance 
     The three-way performance tradeoff between the on-resistance R dson , the drain-to-substrate breakdown voltage BV d     —     s , and the source-to-substrate punch-through breakdown voltage BV s     —     sub  of an LDMOS transistor can be improved by using a triple diffusion (N+/N-LD/HV n-well) drain structure that can be fabricated through a process compatible with a typical sub-micron CMOS process. 
     LDMOS transistors can be fabricated on a common HV n-well. A main design requirement of the common HV n-well is to provide an optimized vertical doping profile to achieve the highest drain-to-substrate breakdown voltage BV d     —     sub  and source-to-substrate punch-through breakdown voltage BV s     —     sub  as required among all LDMOS transistors being fabricated. For a high voltage LDMOS transistor—e.g., greater than 30V—the HV n-well is generally deeper and lighter doped than a regular (conventional) n-well for the CMOS transistor. Since the HV n-well is implanted at the beginning of the processes  600 ,  1200 , its formation has no impact on fixed thermal budgets (that have been optimized for the mass production of sub-micron CMOS devices) allocated to sub-micron CMOS processes. An extra drive-in for the HV n-well can be accommodated if a co-drive-in with a CMOS n-well is not sufficient. Generally, a deep HV n-well with retrograded vertical doping profile offers the best drain-to-substrate breakdown voltage BV d     —     sub  and source-to-substrate punch-through breakdown voltage BV s     —     sub  performances. 
     The shallow self aligned diffused drain implant and diffusion (N-LD  512 ) has a spacing from the P-body implant that is controlled by masked gate dimensions. A main design requirement of the N-LD is to achieve an optimized lateral doping profile to achieve the best performance tradeoff between the on-resistance R dson  and the drain-to-substrate breakdown voltage BV d     —     sub  of the LDMOS transistor. Since the N-LD is a shallow diffusion, it has little impact on the vertical doping profile of the LDMOS transistor, and therefore, has little impact on the drain-to-substrate breakdown voltage BV d     —     sub  and source-to-substrate breakdown voltage BV s     —     sub  characteristics of the transistor. The spacing of the N-LD implant from the P-body allows for a better control of the drain-to-substrate breakdown voltage BV d     —     sub  by lowering the doping levels at the boundary of the HV n-well/P-body junction. Moreover, such a spacing results in improved hot carrier injection (HCI) stability of the LDMOS transistor. Generally, a graded lateral doping profile in the drain region of the LDMOS transistor (e.g., as shown in  FIGS. 7H and 9 ) offers a better performance tradeoff between the on-resistance R dson  and the drain-to-substrate breakdown voltage BV d     —     sub  than a uniform lateral doping profile. A graded lateral doping profile can be achieved by using a large-angel tilt (LAT) N-LD implant. Furthermore, since a deep drive-in is not required for the N-LD implant, the N-LD can be self aligned to the gate—i.e., implanted after formation of the LDMOS gate, including gates of the CMOS transistors. Therefore, the addition of the N-LD implant has substantially no impact on fixed thermal budgets associated with CMOS process steps (e.g., process step  206 ). 
     The above description describes LDMOS transistors having varied drain-to-substrate breakdown voltage BV d     —     sub  ratings that can be fabricated in processes compatible with a typical sub-micron CMOS process. 
     The following description describes alternative examples of LDMOS transistors that can be fabricated through processes, such as processes  600 ,  1200 , that are compatible with a sub-micron CMOS process. 
     CMOS n-Well as HV n-Well 
     An interesting feature of conventional low voltage CMOS transistors—e.g., 3.3V to 5V—fabricated within a sub-micron CMOS process is that the sub-micron CMOS process typically includes implanting a CMOS n-well having a breakdown voltage around 30V. For LDMOS transistors designed for applications of a medium voltage range (e.g., 5V to 25V), these LDMOS transistors can be fabricated on a regular CMOS n-well, thus eliminating a separate HV n-well implant and masking step—i.e., steps  604 ,  1204  of processes  600 ,  1200 , respectively. The remaining steps of processes  600 ,  1200  can be unaltered. 
       FIG. 14  shows an example LDMOS transistor  1400  fabricated on a p-type substrate  1402  having a CMOS n-well implant  1404  for the LDMOS transistor. The LDMOS transistor  1400  includes a drain region  1406 , a source region  1408 , and a gate  1410 . The drain region  1406  includes an n-doped n+ region  1412  and an n-doped shallow drain (N-LD)  1414 . The source region  1408  includes an n-doped n+ region  1416 , a p-doped p+ region  1418 , and a p-doped P-body  1420 . 
     CMOS n-Well as N-LD 
     For LDMOS transistors designed for application in a high voltage range, the HV n-well will typically be much deeper than the regular CMOS n-well. It is therefore possible to substitute the CMOS n-well for the N-LD, thus eliminating the N-LD implant and masking step—i.e., steps  614 ,  1214  of processes  600 ,  1200 , respectively. Therefore, in processes  600 ,  1200  above, a CMOS n-well can be implanted before the gate of the LDMOS transistor is formed, and the CMOS n-well can serve as the shallow drain and would be non-self aligned with respect to the gate. The remaining steps of processes  600 ,  1200  can be unaltered. 
       FIG. 15  shows an example LDMOS transistor  1500  fabricated on a p-type substrate  1502  having a CMOS n-well  1504  as the shallow drain. The LDMOS transistor  1500  has an HV n-well implant  1506  for the transistor. The LDMOS transistor  1500  includes a drain region  1508 , a source region  1510 , and a gate  1512 . The drain region  1508  includes an n-doped n+ region  1514  and an n-doped shallow drain (CMOS n-well)  1504 . The source region  1510  includes an n-doped n+ region  1516 , a p-doped p+ region  1518 , and a p-doped P-body  1520 . 
     DDD as N-LD 
     In applications where the sub-micron CMOS process includes fabrication of a DDD (Double Doped Drain) HV-CMOS transistor module, the same DDD implant can be implemented as the shallow drain of the LDMOS transistor to modulate the resistance of the drain, thus eliminating the N-LD implant and masking steps  614 ,  1214  described above. The remaining steps of processes  600 ,  1200  can be unaltered. The DDD implant can be self aligned or non-self aligned with respect to the LDMOS gate. In addition, the DDD implant can have an offset from the P-body implant such that the DDD implant extends a predetermined distance d from the LDMOS gate. 
       FIG. 16  shows an example LDMOS transistor  1600  fabricated on a p-type substrate  1602  having a DDD implant  1604  as the shallow drain. The LDMOS transistor  1600  has a CMOS n-well implant  1606  for the transistor. The LDMOS transistor  1600  includes a drain region  1608 , a source region  1610 , and a gate  1612 . The drain region  1608  includes an n-doped n+ region  1614  and an n-doped shallow drain (CMOS n-well)  1604 . The source region  1610  includes an n-doped n+ region  1616 , a p-doped p+ region  1618 , and a p-doped P-body  1620 . 
     LDD as N-LD 
     In a conventional sub-micron CMOS process, a LDD (Lightly Doped Drain) implant and spacer formation step can be introduced to improve NMOS transistor ruggedness against hot electron degradation. In one implementation, the LDD implant can be used as the shallow drain for the LDMOS transistor, thus eliminating the N-LD implant and masking steps  614 ,  1214  of processes  600 ,  1200 , respectively. The remaining steps of processes  600 ,  1200  can be unaltered. 
       FIG. 17  shows an example of an LDMOS transistor  1700  fabricated on a p-type substrate  1702  having an LDD  1704 ,  1706  diffused into the source region  1708  and drain region  1710 , respectively of the LDMOS transistor. The LDMOS transistor  1700  has an HV n-well implant  1712  for the LDMOS transistor. The LDMOS transistor also includes a gate  1714 . The drain region  1710  further includes an n-doped n+ region  1716 . The source region  1708  also includes an n-doped n+ region  1718 , a p-doped p+ region  1720 , and a p-doped P-body  1722 . 
     N-LD Implant Defined by N+ Slit Mask 
     In one implementation, a graded shallow drain surface implant is achieved by utilizing a slit mask to create multiple standard n+ implants spaced apart relative to each other along the surface of the LDMOS transistor in the drain region, thus eliminating the N-LD implant and masking step—i.e., steps  614 ,  1214  described above. The multiple n+ implants in the drain region results in an overall lower doping through dopant-side diffusion. This implementation is particularly suited for LDMOS transistors with a high breakdown voltage specification. The remaining steps of processes  600 ,  1200  can be unaltered. 
       FIG. 18  illustrates an example of an LDMOS transistor  1800  fabricated on a p-type substrate  1802  having a graded shallow drain surface implant  1804 . The LDMOS transistor  1800  has an HV n-well implant  1806  for the transistor. The LDMOS transistor also includes a gate  1808 . The drain region  1810  further includes n-doped n+ regions  1812 . The source region  1814  includes an n-doped n+ region  1816 , a p-doped p+ region  1818 , and a p-doped P-body  1820 . 
     p-Type LDMOS Transistor 
     A p-type high voltage LDMOS transistor can be fabricated.  FIG. 19  shows an example a p-type LDMOS transistor  1900  fabricated on a p-type substrate  1902 . The LDMOS transistor  1900  has an HV n-well implant  1904  for the transistor. The LDMOS transistor also includes a gate  1906 . The drain region  1908  include a p-doped p+ region  1910  and a p-doped P-body  1912 . The source region  1914  includes a p-doped p+ region  1916 , and an n-doped n+ region  1918 . 
       FIG. 20  shows experimental data of such a p-type LDMOS transistor. As with the LDMOS transistor illustrated in  FIG. 5A , the p-type LDMOS transistor  1900  is fabricated with a non-self aligned P-body implant  1912 . More generally, a common feature of the LDMOS transistors illustrated in  FIGS. 14-19  is that the P-body implant is formed prior to gate formation of conventional CMOS transistors. This ensures that the LDMOS transistors can be fabricated in a process that is compatible with a sub-micron CMOS process having fixed parameters that have been optimized for the mass production of sub-micron CMOS devices. 
     The availability of complementary p-type LDMOS transistor simplifies the design of level shift circuits. The p-type LDMOS transistor, as with each of the LDMOS transistors described above, can be implemented with either a thick or thin gate oxide. Referring again to  FIG. 19 , the p-type LDMOS transistor  1900  is implemented with a thick gate oxide  1920 . For example, when an LDMOS transistor, such as LDMOS transistor  416  ( FIG. 5A ) is implemented with a high voltage gate—i.e., a gate with a thick gate oxide—a standard high-side p-type transistor (e.g., a PMOS transistor) can be implemented within a switching regulator circuit, thus obviating a need for high-side gate drive considerations. Such an approach results in a hybrid switching regulator, with a low-side LDMOS transistor and a high-side PMOS transistor that minimizes dynamic capacitive losses associated with a high-side PMOS pull-up transistor, as illustrated in the switching regulator  400  of  FIG. 4 . The low-side LDMOS transistor can have an optimized on-resistance R dson  (thin or thick gate oxide). The high-side PMOS transistor can be designed such that dynamic capacitive losses typically associated with high-side PMOS pull-up transistors is minimized. In typical DC-DC conversion applications, in which the conduction duty of the high-side switch is relatively low, the on-resistance R dson  of the high-side transistor is a secondary consideration. 
       FIG. 21  illustrates a non-hybrid switching regulator  2100  having a switching circuit  2102  that includes a high-side LDMOS transistor  2104  and a low-side LDMOS transistor  2106 . The LDMOS transistors  2104 ,  2106  can be fabricated through process  600  or  1200 . The switching regulator  2100  operates in similar fashion to the switching regulator  400  ( FIG. 4 ). However, the switching regulator  2100  includes an LDMOS driver  2108  to drive the high-side LDMOS transistor  2104 . Generally, the LDMOS driver  2108  cannot be fabricated using conventional CMOS transistors. However, using through processes  600 ,  1200 , the LDMOS driver  2108  can be fabricated using PMOS transistors with floating operation capability and NMOS transistors with floating operation capability. LDMOS driver  428  can be fabricated using conventional CMOS transistors, or using PMOS transistors with floating operation capability and NMOS transistors with floating operation capability. Controller  424  is typically fabricated using conventional CMOS transistors. 
     Other Device Structures 
     NPN Transistor 
     Generally, only PNP transistors can be fabricated in a typical sub-micron CMOS process. However, process  600  can be modified to allow fabrication of an NPN transistor.  FIG. 22  shows a cross-sectional view of an example NPN transistor  2200  that can be fabricated through a process compatible with a sub-micron CMOS process. 
       FIG. 23  illustrates a process  2300  for fabricating an PNP transistor, such as PNP transistor  2200 . The process  2300  begins with forming a substrate (step  2302 ), such as p-type substrate  2202  ( FIG. 22 ). A well for the NPN transistor is implanted into the substrate (step  2304 ). The implanted well can be an HV (high voltage) n-well  2204 , as shown in the example of  FIG. 22 . A non-self aligned P-body is implanted into the surface of the transistor (step  2306 ), which is illustrated as P-body  2206  in  FIG. 22 . The n+ regions and p+ regions of the PNP transistor are implanted (step  2308 ), such as n+ regions  2208  and  2210 , and p+ region  2212  ( FIG. 22 ). 
       FIG. 24  shows experimental I-V characteristics of such a PNP transistor. The availability of complementary NPN and PNP transistors enhances high performance analog circuit design. 
     CMOS Transistors with Floating Operation Capability 
     An NMOS transistor with floating operation capability (i.e., the source of the NMOS transistor is not grounded) can be implemented through processes  600 ,  1200 , as described above. Such an NMOS transistor, together with a PMOS transistor fabricated in an HV n-well, allows for the implementation of high-side drive (HSD) circuits (e.g., LDMOS driver  2208 ) with CMOS transistor logic as shown in  FIGS. 25A and 25B . 
       FIGS. 26A and 26B  show experimental data of such CMOS transistors with floating operation capability. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, although some of the LDMOS transistor structures described above do not have LOCOS field oxide (FOX)  2702  on the drain region of the devices. The processes described above also apply to LDMOS transistor structures with LOCOS on the drain region of the devices such as LDMOS transistor  2700  shown in  FIG. 27 . The devices described above can be implemented in general half-bridge or full-bridge circuits, and also in other power electronics systems. 
     A common feature of the LDMOS transistors described above is that the P-body implant is formed prior to gate oxide formation of conventional CMOS transistors to ensure that the LDMOS transistors can be fabricated in a process that is compatible with a sub-micron CMOS process. As discussed above, in one implementation, the P-body can implanted in two steps using a first high energy implant and a second implant, followed by a RTA process. The first high energy implant can be implanted using a LAT implant.  FIG. 28  shows a process  2800  for implanting the P-body without substantially disturbing the CMOS process thermal cycle. The second implant (step  2806 ), or both the high energy implant (step  2802 ) and second implant, can occur after gate formation of CMOS transistors (step  2804 ). The second implant is followed by a RTA process (step  2808 ). The RTA process is implemented with a short duration of time and at temperatures such that thermal cycles allocated to fabricating sub-micron CMOS transistors are substantially unaffected. As discussed above, an LDMOS transistor can be fabricated on an n-type substrate. In such an implementation, an SOI (silicon-on-insulator) insulation layer can be deposited (or grown) on the n-type substrate. A p-well for the LDMOS transistor and CMOS transistors can then be implanted. The process steps following formation of the substrate in processes  600 ,  1200  can then occur. 
     Accordingly, other implementations are within the scope of the following claims.