Abstract:
The components of a silicon controlled rectifier, which are a p-doped anode, an n-well middle region, a p-well middle region, and an n-doped cathode, are formed along sidewalls and a bottom surface of a shallow trench isolation structure. The p-doped anode and the n-doped cathode are formed directly underneath a top surface of a silicon substrate. A trigger mechanism that provides an instantaneous turn-on current to latch the silicon controlled rectifier to an on-state is also provided. The trigger mechanism provides a temporary surge in the voltage of the p-doped middle region, causing the instantaneous turn-on current to flow from the p-doped middle region to the n-doped cathode. Combined with the proximity of the p-doped anode to the n-doped cathode, the trigger mechanism provides a fast turn on and a short low resistance current path for the electrostatic discharge protection circuit.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a semiconductor structures, and particularly to semiconductor structures for an electrostatic discharge protection circuit that employs a lateral junction breakdown triggered silicon controlled rectifier. 
       BACKGROUND OF THE INVENTION 
       [0002]    An electrostatic discharge (ESD) event can occur in a semiconductor chip when a charged conductor (including the human body) discharges through the semiconductor chip. Three models for ESD events are widely accepted in the semiconductor industry: Human Body Model (HBM), Machine Model (MM), and Charged Device Model (CDM). 
         [0003]    An electrostatic charge may accumulate on a human body, for example, when he walks on a carpet. Contact of a body part, e.g., a finger, to a device containing a semiconductor chip causes the body to discharge, possibly causing damage to the semiconductor device. The model used to simulate this event is the Human Body Model (HBM). A similar discharge may occur from a charged conductive object, such as a metallic tool. The model used to characterize this event is known as the Machine Model (MM). Static charge may accumulate on a semiconductor chip through handling or contact with packaging materials or work surfaces. The model used to simulate the transfer of charge from a charged device to another object is referred to as the Charged Device Model (CDM). The CDM employs an energy source that can discharge through its lead into another device of different potential. Often, the rise time is less than 0.5 ns, and the duration can be less than 2 ns. 
         [0004]    As electronic devices scale down in each successive generation of semiconductor technology, the thickness of gate oxide decreases, which results in reduction of gate oxide breakdown voltage and makes semiconductor devices more vulnerable to electrostatic discharge events. Further, the cross-sectional area of metal wiring lines decreases, resulting in an increase in power bus resistance. 
         [0005]    The reduction in the current carrying capabilities is more prominent in a positive power supply network, or the Vdd network. A diode based electrostatic discharge protection circuit, which utilizes the positive power supply network, becomes less effective in protecting semiconductor devices with the scaling of semiconductor devices as the current carrying capabilities of an electrostatic discharge protection circuit are limited by the increased power bus resistance. Typically, a ground network has more robust wiring having a lower bus resistance than the positive power supply network. 
         [0006]    Silicon controlled rectifier (SCR) based electrostatic discharge protection circuit employs only the ground network to conduct electrostatic discharge current. Since the positive power supply network is not used to conduct an electrostatic discharge current, SCR based electrostatic discharge protection circuit offers an advantage over diode based electrostatic discharge protection circuits in terms of the capability to conduct a large amount of current. 
         [0007]      FIG. 1A  shows a physical structure of a silicon controlled rectifier (SCR), which comprises a stack of a p-doped anode  10 , an n-doped middle region  20 , a p-doped middle region  30 , and an n-doped cathode  40 . An anode lead wire  12  is connected to the p-doped anode  10 , a cathode lead wire  42  is connected to the n-doped cathode  42 , a first gate lead wire  32  may optionally be connected to the p-doped middle region  30 , and a second gate lead wire  33  may optionally be connected to the n-doped middle region  20 .  FIG. 1B  shows an equivalent circuit schematic for the SCR in  FIG. 1A , wherein the physical structure of the SCR in  FIG. 1A  is represented by a PNP bipolar transistor and an NPN bipolar transistor. 
         [0008]    Referring to  FIG. 2 , a prior art SCR based electrostatic discharge protection circuit formed in a silicon substrate  8  comprises a p-doped anode  10 , an n-well middle region  20 ′, a heavily n-doped middle region  20 ″, a p-well middle region  30 ′, an n-doped cathode  40 , and a gate electrode  36  on a gate dielectric  34 , which is located on the p-well middle region  30 ′. The n-well middle region  20 ′, and the heavily n-doped middle region  20 ″ collectively comprise an n-doped middle region  20 . The p-well middle region  30 ′ is also a p-doped middle region  30 . A p-doped middle region contact  31  comprises a heavily p-doped silicon material. The portion of the silicon substrate  8  that is located beneath the n-well middle region  20 ′ and the p-well middle region  30 ′ comprises the substrate layer  2 , which has the original doping level of the silicon substrate  8 . Shallow trench isolation (STI)  50  separates the p-doped anode  10  and the heavily n-doped middle region  20 ″. The STI  50  also separates the n-doped cathode  40  and the p-doped middle region contact  31 . 
         [0009]    An input/output pad (I/O pad)  100 , to which electrostatic discharge protection is provided, is connected to the p-doped cathode  10 . The n-doped cathode  40  is connected to ground. The p-doped middle region contact  31  and the gate electrode  36  are also connected to the ground. During an electrostatic discharge event, the voltage of the heavily n-doped middle region  20 ″ rises instantaneously, causing an instantaneous current to flow through the device formed by the heavily n-doped middle region  20 ″, the p-well middle region  30 ′, and the n-doped cathode  40 . The instantaneous current causes the SCR to latch on to the on-state. The SCR remains turned on until the current decreased to a low-current dropout level. Thus, the prior art SCR based electrostatic discharge (ESD) protection circuit passes current during an electrostatic discharge event. 
         [0010]    The efficiency of the prior art SCR based ESD protection circuit depends on the dimension of the physical components of the SCR, especially on the dimension between the p-doped anode  10  and the n-doped cathode  40 . Referring to  FIG. 3 , a graph showing the dependence of the turn-on time, i.e., the time needed for the prior art SCR circuit to turn on after an ESD event, is plotted as a function of the spacing S ac  between the p-doped anode  10  and the n-doped cathode  40 . For an SCR based ESD protection circuit to turn on with minimal delay from an ESD event, the dimension between the p-doped anode  10  and the n-doped cathode  40  needs to be minimized. 
         [0011]    Therefore, there exists a need for an effective SCR based ESD protection circuit having a small turn-on delay time and low critical rate of voltage rise. 
         [0012]    Further, there exists a need for an SCR based ESD protection circuit having a small dimension between a p-doped anode and an n-doped cathode. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention addresses the needs described above by providing silicon controlled rectifier based electrostatic discharge protection structures having a small dimension between a p-doped anode and an n-doped cathode. 
         [0014]    Specifically, the components of a silicon controlled rectifier, which are a p-doped anode, an n-well middle region, a p-well middle region, and an n-doped cathode, are formed along sidewalls and a bottom surface of a shallow trench isolation structure. The p-doped anode and the n-doped cathode are formed directly underneath a top surface of a silicon substrate. A trigger mechanism that provides an instantaneous turn-on current to latch the silicon controlled rectifier to an on-state is also provided. The trigger mechanism provides a temporary surge in the voltage of the p-doped middle region, causing the instantaneous turn-on current to flow from the p-doped middle region to the n-doped cathode. Combined with the proximity of the p-doped anode to the n-doped cathode, the trigger mechanism provides a fast turn on and a short low resistance current path for the electrostatic discharge protection circuit. 
         [0015]    According to a first embodiment of the present invention, a semiconductor structure comprises: 
         [0016]    a shallow trench isolation structure containing a bottom surface having a width, a first sidewall, and a second sidewall and located in a semiconductor substrate, wherein the bottom surface adjoins the first sidewall and the second sidewall; 
         [0017]    a p-doped anode located directly beneath a top surface of the semiconductor substrate and abutting the first sidewall; 
         [0018]    an n-well middle region abutting the p-doped anode, the first sidewall, and the bottom surface; 
         [0019]    a p-well middle region abutting the n-well middle region, the bottom surface, and the second sidewall; 
         [0020]    an n-doped cathode located directly beneath the top surface of the semiconductor substrate and abutting the p-well middle region and the second sidewall, wherein the p-doped anode and the n-doped cathode are separated by a distance that is substantially the same as the width of the bottom surface; and 
         [0021]    a heavily n-doped middle region disjoined from the first sidewall, the second sidewall, the p-doped anode, and the n-doped cathode and abutting the n-well middle region and the p-well middle region, wherein the p-doped anode is wired to an input/output pad by a first metal wiring and the n-doped cathode is wired to ground by a second metal wiring. 
         [0022]    According to a second embodiment of the present invention, a semiconductor structure comprises: 
         [0023]    a shallow trench isolation structure containing a bottom surface having a width, a first sidewall, and a second sidewall and located in a semiconductor substrate, wherein the bottom surface adjoins the first sidewall and the second sidewall; 
         [0024]    a p-doped anode located directly beneath a top surface of the semiconductor substrate and abutting the first sidewall; 
         [0025]    an n-well middle region abutting the p-doped anode, the first sidewall, and the bottom surface;
       a p-well middle region abutting the n-well middle region, the bottom surface, and the second sidewall;       
 
         [0027]    an n-doped cathode located directly beneath the top surface of the semiconductor substrate and abutting the p-well middle region and the second sidewall, wherein the p-doped anode and the n-doped cathode are separated by a distance that is substantially the same as the width of the bottom surface; and 
         [0028]    a heavily n-doped electrostatic discharge (ESD) trigger region located within the semiconductor substrate, electrically connected to the p-well middle region, and disjoined from the first sidewall, the second sidewall, the p-doped anode, the n-doped cathode, and the n-well middle region, wherein the p-doped anode and the heavily n-doped ESD trigger region are wired to an input/output pad by a first metal wiring and the n-doped cathode is wired to ground by a second metal wiring. 
         [0029]    The semiconductor structure may further comprise a p-doped middle region contact containing a heavily p-doped semiconductor and wired to the ground by the second metal wiring. 
         [0030]    The heavily n-doped ESD trigger region may abut the p-well middle region. Alternatively, the semiconductor structure may further comprise a p-doped electrostatic discharge (ESD) implantation region which abuts the heavily n-doped ESD trigger region and the p-well middle region, and is disjoined from the n-well middle region and the n-doped cathode. 
         [0031]    According to a third embodiment of the present invention, a semiconductor structure comprises:
       a shallow trench isolation structure containing a bottom surface having a width, a first sidewall, and a second sidewall and located in a semiconductor substrate, wherein the bottom surface adjoins the first sidewall and the second sidewall;       
 
         [0033]    a p-doped anode located directly beneath a top surface of the semiconductor substrate and abutting the first sidewall;
       an n-well middle region abutting the p-doped anode, the first sidewall, and the bottom surface;       
 
         [0035]    a p-well middle region abutting the n-well middle region, the bottom surface, the second sidewall, and the top surface of the semiconductor substrate; 
         [0036]    an n-doped cathode located directly beneath the top surface of the semiconductor substrate and abutting the p-well middle region and the second sidewall, wherein the p-doped anode and the n-doped cathode are separated by a distance that is substantially the same as the width of the bottom surface;
       a heavily n-doped electrostatic discharge (ESD) trigger region located within the semiconductor substrate, abutting the p-well middle region, and disjoined from the first sidewall, the second sidewall, the p-doped anode, the n-doped cathode, and the n-well middle region; and       
 
         [0038]    a gate located on the top surface of the semiconductor substrate, abutting the p-well middle region, and containing a gate dielectric and a gate conductor, wherein the p-doped anode and the heavily n-doped ESD trigger region are wired to an input/output pad by a first metal wiring and the n-doped cathode is wired to ground by a second metal wiring. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]      FIG. 1A  is a schematic diagram of a prior art physical structure of a silicon controlled rectifier (SCR).  FIG. 1B  is an equivalent circuit schematic for the SCR in  FIG. 1A . 
           [0040]      FIG. 2  is a prior art electrostatic discharge (ESD) protection structure employing an SCR. 
           [0041]      FIG. 3  is a graph showing the dependence of turn-on time on the spacing S ac  between an anode and a cathode of an SCR. 
           [0042]      FIGS. 4A-4E  show a first exemplary structure according to a first embodiment of the present invention.  FIG. 4A  is a top-down view,  FIGS. 4B and 4C  are vertical cross-sectional views along the planes B-B′ and C-C′, respectively, and  FIGS. 4D , and  4 E are horizontal cross-sectional views along the planes D-D′ and E-E′. 
           [0043]      FIG. 5  is a horizontal cross-sectional views along the planes E-E′ of the first exemplary structure with annotations for geometrical aspects of the first exemplary structure and exemplary equipotential lines at an initial phase of an ESD event. 
           [0044]      FIGS. 6A-6F  show a second exemplary structure according to a first embodiment of the present invention.  FIG. 6A  is a top-down view,  FIGS. 6B ,  6 C, and  6 F are vertical cross-sectional views along the planes B-B′, C-C′, and F-F′, respectively, and  FIGS. 6D and 6E  are horizontal cross-sectional views along the planes D-D′ and E-E′. 
           [0045]      FIG. 7  is a top down views of the second exemplary structure showing a set of vertical planes PQ, QR, and RS that are used to generate a composite vertical cross-sectional view of the second exemplary structure in  FIG. 8 . 
           [0046]      FIG. 8  is a composite vertical cross-sectional view of the second exemplary structure along the set of planes PQ, QR, and RS in  FIG. 7 . 
           [0047]      FIG. 9  is a composite vertical cross-sectional view of the second exemplary structure along the set of planes PQ, QR, and RS in  FIG. 7  with an alternate wiring of terminals. 
           [0048]      FIGS. 10-13  are vertical cross-sectional views of a third, fourth, fifth, and sixth exemplary structures, respectively, according to a second embodiment of the present invention. 
           [0049]      FIGS. 14-17  are vertical cross-sectional views of a seventh, eighth, ninth, and tenth exemplary structures, respectively, according to a third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0050]    As stated above, the present invention relates to semiconductor structures for an electrostatic discharge protection circuit that employs a lateral junction breakdown triggered silicon controlled rectifier, which are now described in detail with accompanying figures. It is noted that like and corresponding elements are referred to by like reference numerals. 
         [0051]    Referring to  FIGS. 4A-4E , a first exemplary structure according to a first embodiment of the present invention comprises shallow trench isolation  50 , a p-doped anode  10 , an n-well middle region  20 ′, a p-well middle region  30 ′, an n-doped cathode  40 , a heavily n-doped middle region  20 ″, and a substrate layer  2 , each of which is located in a semiconductor substrate  8 . Preferably, the first exemplary structure further comprises a p-doped middle region contact  31  as well. Each of the p-doped anode  10 , the heavily n-doped middle region  20 ″, the n-doped cathode  40 , and the p-doped middle region contact  31  comprises a doped semiconductor material and is located on a top surface  9  of the semiconductor substrate  8 . 
         [0052]    A portion S 1  of the shallow trench isolation  50  is located between the p-doped anode  10  ad the n-doped cathode  40 . The portion S 1  of the shallow trench isolation  50  has a bottom surface  53  having a width w, a first sidewall  51 , and a second sidewall  52  and located in the semiconductor substrate  8 . The first sidewall  51  and the second sidewall  52  are substantially vertical. The bottom surface  53  adjoins the first sidewall  51  and the second sidewall  52 . 
         [0053]    The p-doped anode  10  is located directly beneath the top surface  9  of the semiconductor substrate  8  and abuts the first sidewall  51 . The n-well middle region  20 ′ abuts the p-doped anode  10 , the first sidewall  51 , and the bottom surface  53  of the portion S 1  of the shallow trench isolation  50 . The p-well middle region  30 ′ abuts the n-well middle region  20 ′, the bottom surface  53 , and the second sidewall  52 . The n-doped cathode  40  is located directly beneath the top surface  9  of the semiconductor substrate  8  and abuts the p-well middle region  30 ′ and the second sidewall  52 . The p-doped anode  10  and the n-doped cathode  40  are separated by a distance that is substantially the same as the width of w of the bottom surface  53 . The heavily n-doped middle region  20 ″ is disjoined from the first sidewall  51 , the second sidewall  52 , the p-doped anode  10 , and the n-doped cathode  40 . The heavily n-doped middle region  20 ″ abuts the n-well middle region  20 ′ and the p-well middle region  30 ′. The n-well middle region  20 ′ and the p-well middle region  30 ′ abut the substrate layer  2 . 
         [0054]    A silicon controlled rectifier (SCR) comprises the p-doped anode  10 , the n-well middle region  20 ′, the p-well middle region  30 ′, and the n-doped cathode  40 , all of which are laid out along the surfaces of the portion S 1  of the shallow trench isolation  50 , specifically, along the set of surfaces containing the first sidewall  51 , the bottom surface  53 , and the second sidewall  52 . The distance that an electrostatic discharge (ESD) current needs to flow through the SCR is substantially equivalent to the sum of the height of the first sidewall  51 , the width w of the bottom surface  53 , and the height of the second sidewall  52 . The height of the first or second sidewall ( 51  or  52 ) is substantially the same as the depth of the shallow trench isolation  50 , which is typically in the range from about 150 nm to about 400 nm, and may scale down with scaling of devices. The width w of the bottom surface  53  is in the range from about 100 nm and 300 nm, and may scale down with scaling of devices. 
         [0055]    Optionally but preferably, the first exemplary semiconductor structure comprises a p-doped electrostatic discharge (ESD) implantation region  33  that abuts the heavily n-doped middle region  20 ″ and the p-well middle region  30 ′. For example, a top surface of the p-doped ESD implantation region  33  may abut the heavily n-doped middle region  20 ″ and side surfaces and a bottom surface of the p-doped ESD implantation region  33  may abut the p-well middle region  30 ′. The p-doped ESD implantation region  33  is disjoined from the n-well middle region  20 ′ and the n-doped cathode  40 . The p-doped ESD implantation region  33  is optional, that is, the present invention may be practiced with, or without, the p-doped ESD implantation region  33 . 
         [0056]    The p-doped anode  10 , the p-well middle region  30 ′, the p-doped middle region contact  31 , and p-doped ESD implantation region  33  are doped with p-type dopants. The n-doped cathode  40 , the n-well middle region  20 ′, and the heavily n-doped middle region  20 ″ are doped with n-type dopants. The dopant concentration of the p-doped anode  10 , the n-doped cathode  40 , and the p-doped middle region contact  31  may be in the range from about 1.0×10 18 /cm 3  to about 5.0×10 21 /cm 3 , and typically in the range from about 5.0×10 19 /cm 3  to about 5.0×10 20 /cm 3 . The dopant concentration of the n-well middle region  20 ′ and the p-well middle region  30 ′ may be in the range from about 1.0×10 16 /cm 3  to about 5.0×10 19 /cm 3 , and typically in the range from about 1.0×10 17 /cm 3  to about 5.0×10 18 /cm 3 . The dopant concentration of the p-doped ESD implantation region  33  may be in the range from about 3.0×10 16 /cm 3  to about 30×10 20 /cm 3 , and typically in the range from about 3×10 17 /cm 3  to about 3.0×10 19 /cm 3 . 
         [0057]    The various views are also annotated with various geometrical features of the first exemplary structure. A p-doped ESD implantation region center X, or the center of the p-doped ESD implantation region  33 , is herein defined as the geometric center of the volume of the p-doped ESD implantation region  33 . The coordinate of the p-doped ESD implantation region center X is given by (X 1 , X 2 , X 3 ), wherein each X i  is given by 
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         [0000]    for i=1, 2, 3 and the volume integrals are defined over the volume of the p-doped ESD implantation region  33 . Similarly, an n-doped cathode center Y, or the center of the n-doped cathode  40 , is herein defined as the geometric center of the n-doped cathode  40 ; and a p-doped middle region contact center Z, or the center of the p-doped middle region contact  31 , is herein defined as the geometric center of the p-doped middle region contact  31 . The formula for the coordinates of Y and Z are given by 
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         [0000]    for i=1, 2, 3 and the volume integrals are defined over the volume of n-doped cathode  40  for Equation 2 and over the volume of the p-doped middle region contact  31  for Equation 3. 
         [0058]    A first vector K 1  herein connotes the vector from the p-doped ESD implantation region center X to the n-doped cathode center Y. A second vector K 2  herein connotes the vector from the p-doped ESD implantation region center X to the p-doped middle region contact center Z. Since the vectors K 1  and K 2  are not located within any plane of the various cross-sections in  FIGS. 6A-6E , their projections to the top surface  9  of the semiconductor substrate  8  are shown in  FIG. 4A  along with a projection of the p-doped ESD implantation region  33 , which is marked with a dotted rectangle. The geometrical placement of the p-doped ESD implantation region  33 , the n-doped cathode  40 , and the p-doped middle region contact  31  in the first exemplary structure is such that the projection P of the first vector K 1  onto the direction of the second vector K 2  has the same direction as the second vector K 2  and a smaller magnitude than the second vector K 2 . In other words, the p-doped middle region contact center Z is farther away than the n-doped cathode center Y from the p-doped ESD implantation region center X, and the angle between the two vectors K 1  and K 2  is less than 90 degrees. 
         [0059]    The benefit of such a geometrical placement of the components is seen in  FIG. 5 , which shows exemplary equipotential lines V 1  and V 2  at an initial phase of an ESD event in the same horizontal cross-sectional view as  FIG. 4E . The projections of the points X, Y, and Z onto the plane of the cross-section of  FIG. 5  are labeled points X′, Y′, and Z′, respectively. As an positive voltage is instantaneously applied to the heavily n-doped middle region  20 ″ either by a direct wiring to the source of the ESD event or through the p-doped anode  10 , a reverse diode breakdown occurs between the heavily n-doped middle region  20 ″ and the p-doped ESD implantation region  33  since the p-doped ESD implantation region  33  is in general doped with a higher doping concentration than the p-well middle region  30 ′. If the p-doped ESD implantation region  33  is not present in the structure, the breakdown occurs between the heavily n-doped middle region  20 ″ and the p-well middle region  30 ′. An increase in the breakdown voltage, or a turn-on voltage of the SCR, may occur relative to a structure with the p-doped ESD implantation region  33 . 
         [0060]    Once a breakdown occurs between the heavily n-doped middle region  20 ″ and one of the -doped ESD implantation region  33  the p-well middle region  30 ′, an initial current flows from the n-well middle region  20 ′, optionally through the p-doped ESD implantation region  33 , and to the p-well middle region  30 ′. Once the initial current flows into the p-well middle region  30 ′, another current path underneath the portion S 1  of the shallow trench isolation  50  is established in the SCR. This is because the initial current serves as a base current that momentarily turns on a PNP bipolar transistor comprising the p-doped anode  10 , the n-well middle region  20 ′, and the p-well middle region  30 ′. 
         [0061]    As the initial current flows, equipotential lines such as V 1  and V 2  are instantaneously formed in the p-well middle region  30 ′ from the center of the current source, which is the p-doped ESD implantation region  33 , during the initial phase of the ESD event. By placing the n-doped cathode center Y in closer proximity the p-doped ESD implantation region center X relative to the p-doped middle region contact center Z in  FIGS. 4A-4E , the voltage at the point Y′ becomes high relative to the voltage at the n-doped anode center Y, which is maintained at ground potential. 
         [0062]    Once current begins to flow between the p-well middle region  30 ′, and the n-doped cathode  40 , another current path from the p-doped anode  10  through the n-well middle region  20 ′ to the p-well middle region  30 ′ is formed, conducting even more current. This is because the instantaneous voltage bias between the point Y′ and the n-doped anode center Y initiates the base current of an NPN transistor comprising the heavily n-doped middle region  20 ″, the p-well middle region  30 ′, and the n-doped cathode  40 . Therefore, the silicon controlled rectifier (SCR) circuit comprising the p-doped anode  10 , n-well middle region  20 ′, the p-well middle region  30 ′, and the n-doped cathode  40  is subsequently latched to an on-state. The steady state current IS of the SCR during the steady on-state is shown in  FIG. 4B . 
         [0063]    Referring to  FIGS. 6A-6F , a second exemplary structure according to the first embodiment of the present invention is shown. In this example, the angle between the first vector K 1  and the second vector K 2  is minimized by placing the p-doped middle region contact  31  on the opposite side of the p-doped ESD implantation region  33  relative to the n-doped cathode  40 . The angle between the first vector K 1  and the second vector K 2  is less than 45 degrees, and preferably less than 15 degrees. The smaller angle between the two vectors K 1  and K 2  has the advantageous effect of applying a higher voltage to the area of the p-well middle region  30 ′ directly beneath the n-doped cathode  40  (corresponding to the point X′ in  FIG. 5 ) during the initial current flow, which lowers a trigger voltage for the turn-on of the SCR. 
         [0064]    Referring to  FIG. 7 , a set of vertical planes that are formed by connecting the points P, Q, R, and S is shown in the same view as  FIG. 6A . Plane PQ is a vertical plane bounded by the points P and Q; plane QR is the plane bounded by the points Q and R; and plane R and S is the plane bounded by the points R and S. 
         [0065]    Referring to  FIG. 8 , a vertical cross-sectional view along a composite plane formed by juxtaposition of the planes PQ, QR, and RS of the second exemplary structure is shown with a schematic diagram for wiring of the components of the SCR. Pivot axes Q′−Q″ and R′−R″ of the cross-sectional view, respectively corresponding to the points Q and R in  FIG. 7 , are also shown. The p-doped anode  10  is wired to an input/output pad  100  by a first metal wiring  91 . The input/output pad may be an input pad, an output pad, or a bidirectional pad and electrically connected to lead wires in a semiconductor package. The p-doped anode may also be connected to any internal circuitry that is vulnerable to electrostatic discharge. The n-doped cathode  40  and the p-doped middle region contact  31  are wired to ground by a second metal wiring  92 .  FIG. 8  also shows the path of an instantaneous current I i  during a turn-on phase of the SCR. The steady state current I s  of the SCR during the steady on-state is as shown in  FIGS. 4B and 6B . 
         [0066]    Referring to  FIG. 9 , a vertical cross-sectional view along the composite plane formed by juxtaposition of the planes PQ, QR, and RS of the second exemplary structure is shown with a schematic diagram for alternate wiring of the components of the SCR. Pivot axes Q′−Q″ and R′−-R″ of the cross-sectional view, respectively corresponding to the points Q and R in  FIG. 7 , are also shown. In addition to the wiring shown in  FIG. 8 , the heavily n-doped middle region  20 ″ is also wired to the input/output pad  100  by the first metal wiring  91 . Due to a decrease in the voltage drop across the p-doped anode  10  and the heavily n-doped middle region  20 ″, the alternate wiring provides a lower turn-on voltage for the SCR relative to the wiring shown in  FIG. 8 . The n-doped cathode  40  and the p-doped middle region contact  31  are wired to ground by a second metal wiring  92 . The path of an instantaneous current I i  during a turn-on phase of the SCR is also shown. 
         [0067]    Referring to  FIG. 10 , a vertical cross-sectional view of a third exemplary structure according to a second embodiment of the present invention is shown. The vertical cross-sectional view may be from a single cross-sectional plane or may comprise two cross-sectional planes adjoined at a pivot axis V-V′. In other words, all components in the cross-sectional view may be arranged in a linear alignment or may be located on a set of two planes adjoined at a pivot axis V-V′. 
         [0068]    Compared to the first exemplary structure, the third exemplary structure comprises all the components of the first exemplary structure except a heavily n-doped middle region  20 ″ and an optional p-doped electrostatic discharge (ESD) implantation region  33 . The structural characteristics of the common components between the third exemplary structure and the first exemplary structure are the same. See above for descriptions of the common components between the third and first exemplary structures. 
         [0069]    However, the third exemplary structure has an additional component, which is a heavily n-doped electrostatic discharge (ESD) trigger region  60  located within the semiconductor substrate  8 , electrically connected to the p-well middle region  30 ′, and disjoined from the first sidewall  51 , the second sidewall  52 , the p-doped anode  10 , the n-doped cathode  40 , and the n-well middle region  20 ′. The heavily n-doped ESD trigger region  60  comprises a heavily n-doped semiconductor material, and has a doping concentration in the range from about 1.0×10 18 /cm 3  to about 5.0×10 21 /cm 3 , and typically in the range from about 5.0×10 19 /cm 3  to about 5.0×10 20 /cm 3 , i.e., about the same level of doping concentration as the n-doped cathode  40 . The p-doped anode  10  and the heavily n-doped ESD trigger region  60  are wired to an input/output pad  100  by a first metal wiring  91  and the n-doped cathode  40  is wired to ground by a second metal wiring  92 . The p-doped middle region contact  31  contains a heavily p-doped semiconductor material and wired to ground by the second metal wiring  92 . To prevent metallization of exposed portion of the p-well middle region  30 ′ between the heavily n-doped ESD trigger region  60  and the n-doped cathode  40 , a patterned dielectric layer  88  comprising a dielectric material such as silicon nitride or silicon oxide may be formed and provide electrical isolation between the heavily n-doped ESD trigger region  60  and the n-doped cathode  40 . 
         [0070]    The heavily n-doped ESD trigger region  60  abuts the p-well middle region  30 ′. Also, the p-well middle region  50  abuts a top surface  9  of the semiconductor substrate  8  between the n-doped cathode  40  and the heavily n-doped ESD trigger region  60 . 
         [0071]    Preferably, the heavily n-doped ESD trigger region  60  is farther away from the p-doped anode  10  than the n-doped cathode  50  is from the p-doped anode  10 . This is because the intended ESD current conduction path is through the SCR, i.e., through the p-doped anode  10 , the n-well middle region  20 ′, the p-well middle region  30 ′, and the n-doped cathode  40 . By keeping the p-doped anode  10  and the n-doped cathode  40  close together, the SCR remains a main current conduction path so that the characteristics of the ESD protection device maintain the electrical characteristics of the SCR. The direction of an instantaneous current I i  during the turn-on phase of the SCR and the direction of a steady state current I s  during the steady on-state are indicated by labeled arrows. 
         [0072]    Referring to  FIG. 11 , a fourth exemplary structure according to the second embodiment of the present invention comprises all the components of the third exemplary structure above with the following modifications. The p-well middle region  50  does not abut a top surface  9  of the semiconductor substrate  8  between the n-doped cathode  40  and the heavily n-doped ESD trigger region  60 . Instead, another portion of the shallow trench isolation  50  occupies the volume between the n-doped cathode  40  and the heavily n-doped ESD trigger region  60 . The another portion of the shallow trench isolation  50  has another bottom surface  56  having another width w′, a third sidewall  54 , and a fourth sidewall  55  and located in the semiconductor substrate  8 . The another bottom surface  56  adjoins the third sidewall  54  and the fourth sidewall  55 . The n-doped cathode  40  abuts the third sidewall  54 , the heavily n-doped ESD trigger region  60  abuts the fourth sidewall  55 , and the p-well middle region  30 ′ abuts the third sidewall  54 , the fourth sidewall  55 , and the another bottom surface  56 . The n-doped cathode  40  and the heavily n-doped ESD trigger region  60  are separated by another distance that is substantially the same as the another width w′ of the another bottom surface  56 . 
         [0073]    Due to the presence of the another portion of the shallow trench isolation  50  in the fourth exemplary structure relative to the third exemplary structure, the fourth exemplary structure may have a higher turn-on voltage for the SCR. Preferably, the shortest distance between the heavily n-doped ESD trigger region  60  and the p-doped anode  10  is greater than the shortest distance between the n-doped cathode  40  and the p-doped anode  10 . The direction of an instantaneous current I i  during the turn-on phase of the SCR and the direction of a steady state current I s  during the steady on-state are indicated by labeled arrows. 
         [0074]    Referring to  FIG. 12 , a fifth exemplary structure according to the second embodiment of the present invention comprises all the components of the third exemplary structure above. The fifth exemplary structure further comprises a p-doped ESD implantation region  33  which abuts the heavily n-doped ESD trigger region  60  and the p-well middle region  30 ′. The p-doped ESD implantation region  33  in the fifth exemplary structure has the same doping as the p-doped ESD implantation region  33  in the first exemplary structure. The p-doped ESD implantation region  33  is disjoined from the n-well middle region  20 ′ and the n-doped cathode  40 . The p-doped ESD implantation region  33  may, or may not, adjoin the shallow trench isolation  50 . The direction of an instantaneous current I i  during the turn-on phase of the SCR and the direction of a steady state current I s  during the steady on-state are indicated by labeled arrows. To prevent metallization of exposed portion of the p-well middle region  30 ′ between the heavily n-doped ESD trigger region  60  and the n-doped cathode  40 , a patterned dielectric layer  88  comprising a dielectric material such as silicon nitride or silicon oxide may be formed and provide electrical isolation between the heavily n-doped ESD trigger region  60  and the n-doped cathode  40 . 
         [0075]    Referring to  FIG. 13 , a sixth exemplary structure according to the second embodiment of the present invention comprises all the components of the fourth exemplary structure above. The sixth exemplary structure further comprises a p-doped ESD implantation region  33  which abuts the heavily n-doped ESD trigger region  60  and the p-well middle region  30 ′. The p-doped ESD implantation region  33  in the fifth exemplary structure has the same doping as the p-doped ESD implantation region  33  in the first exemplary structure. The p-doped ESD implantation region  33  is disjoined from the n-well middle region  20 ′ and the n-doped cathode  40 . The p-doped ESD implantation region  33  may, or may not, adjoin the shallow trench isolation  50 . The direction of an instantaneous current I i  during the turn-on phase of the SCR and the direction of a steady state current I s  during the steady on-state are indicated by labeled arrows. 
         [0076]    Referring to  FIG. 14 , a seventh exemplary structure according to a third embodiment of the present invention comprises all the components of the third exemplary structure above. The seventh exemplary structure further comprises a gate located on the top surface  9  of the semiconductor substrate  8 , abutting p-well middle region  30 ′, and containing a gate dielectric  34  and a gate conductor  36 . In other words, the seventh exemplary structure contains an n-type field effect transistor (NFET) in which p-well middle region  30 ′ functions as a body, the n-doped cathode  40  functions as a source, the heavily n-doped ESD trigger region  60  functions as a drain, and a gate is located between the n-doped cathode  40  and the heavily n-doped ESD trigger region  60 . A threshold voltage adjustment implantation or a halo implantation may be performed into the p-well middle region  30 ′ to adjust the threshold voltage of the NFET. 
         [0077]    The p-doped anode  40  and the heavily n-doped ESD trigger region  60  are wired to an input/output pad  100  by a first metal wiring  91 . The n-doped cathode  40 , the p-doped middle region contact  31 , and the gate conductor  26  are wired to ground by a second metal wiring  92 . The NFET is a grounded gate NFET (GGNFET) that provides an instantaneous current during the turn-on phase of the SCR, which comprises the p-doped anode  10 , the n-well middle region  20 ′, the p-well middle region  30 ′, and the n-doped cathode  40 . The direction of an instantaneous current I i  during the turn-on phase of the SCR and the direction of a steady state current I s  during the steady on-state are indicated by labeled arrows. 
         [0078]    Referring to  FIG. 15 , an eighth exemplary structure according to the third embodiment of the present invention comprises all the components of the seventh exemplary structure above. The eighth exemplary structure further comprises a p-doped ESD implantation region  33  which abuts the heavily n-doped ESD trigger region  60  and the p-well middle region  30 ′. The p-doped ESD implantation region  33  in the eighth exemplary structure has the same doping as the p-doped ESD implantation region  33  in the first exemplary structure. Due to the higher doping concentration in the p-doped ESD implantation region  33  relative to the p-well middle region  30 ′, an instantaneous current during the turn-on phase of the SCR is initiated with a lower trigger voltage on the input/output pad  100  compared to the seventh exemplary structure. The gate is grounded as in the seventh exemplary structure. The direction of an instantaneous current I i  during the turn-on phase of the SCR and the direction of a steady state current I s  during the steady on-state are indicated by labeled arrows. 
         [0079]    Referring to  FIG. 16 , a ninth exemplary structure according to the third embodiment of the present invention comprises all the elements of the seventh exemplary structure. However, the gate is not grounded. Instead, the gate is controlled independently to control the turn-on voltage of the SCR. The direction of an instantaneous current I i  during the turn-on phase of the SCR and the direction of a steady state current I s  during the steady on-state are indicated by labeled arrows. 
         [0080]    Referring to  FIG. 17 , a tenth exemplary structure according to the third embodiment of the present invention comprises all the elements of the eighth exemplary structure. As in the ninth exemplary structure, the gate is controlled independently to control the turn-on voltage of the SCR. The direction of an instantaneous current I i  during the turn-on phase of the SCR and the direction of a steady state current I s  during the steady on-state are indicated by labeled arrows. 
         [0081]    While the invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims.