Abstract:
Current protection in integrated circuit having multiple pads. Different types of current protection structures may be associated with different pads. A common current discharge or charge path may be used to provide current to or draw current from various of these heterogenic current protection structures. Since a common current discharge or charge path is used, the metallization used to formulate a discharge solution is significant simplified. Additionally, the protection structures may be provided with selectively conductive regions that are approximately radially symmetrical around the circumference of the pad. Accordingly, if the protection structures are slightly off center with respect to the bond pad (due to, for example, mask alignment error), the error in the amount of active region around the circumference of the pad is at least partially averaged out.

Description:
BACKGROUND 
       [0001]    Electronic circuitry provides complex functionality that is proving ever more useful. In one common form, circuitry is formed on a semiconductor or other substrate using micro-fabrication processing technology. Typically, circuits with small feature dimension sizes are not designed to carry large amounts of current. So long as the voltage range at any given node does not extend outside of its designed range, these currents remain relatively low and the circuitry will typically operate as designed. However, if the voltage range at any given node extends out of its designed range, a condition of Electrical OverStress (EOS) may occur. 
         [0002]    For example, most common semiconductor fabrication processes use substrate or bulk semiconductor with different dopants implanted into certain regions of the substrate. These implant regions define unique electrical characteristics that are important or essential for circuit functionality. Thus, EOS experienced at any of the implant regions may adversely impact circuit performance. Another area where EOS may adversely affect performance is in the interlayer dielectrics, which have voltage limitations as well. Driving a circuit outside of its normal operating range can often temporarily disable performance of the circuit, reduce the operational lifetime of the circuit, or even immediately destroy the circuit. EOS can take many forms, but commonly takes the form of Electro Static Discharge (ESD) events. 
         [0003]    Many current protection structures have been designed that are suitable for dissipating current to or from corresponding critical circuit nodes in order to provide protection to corresponding circuitry. Conventionally, a more likely source of excess current is on the pads of integrated circuits, where externally generated voltages and currents are applied to the integrated circuit. To deal with the potential of EOS events occurring at a given pad, conventional circuits often have current protection structures at or near each pad. 
         [0004]    Conventionally, each current protection structure discharges current to a power domain that is local to its respective core circuitry. This is true regardless of whether the protection structure is uni-directional (i.e., a protection structure that triggers current discharge only of one of the positive or negative excessive voltage condition) or bi-directional (i.e., a protection structure that triggers current discharge of both positive and negative excessive voltage conditions). This network of EOS protection structures results in complex metallization schemes with large pad structures with multiple circuit-wide interconnection busses. The complexity of such metallization requires significant space on the integrated circuit. To exacerbate the problem, the complex metallization causes significant voltage drops across the metallization, which is often countered by using strategically placed voltage clamps distributed throughout the circuit. Such distributed voltage clamps, of course, require additional circuit space. 
         [0005]    Currently, a circuit may operate in multiple voltage domains. For instance, mixed signal integrated circuits are in widespread use. Such mixed signal integrated circuits operate using digital voltage and current signals (thus operating in the digital voltage domain) as well as analog voltage and current signals (thus operating in the analog voltage domain). Furthermore, there are often components of a circuit that operate using different useful voltages. For instance, there may be high voltage components that high voltages may be acceptably applied to (thus operating in a high voltage domain), whereas other components may be lower voltage components for which such high voltages may represent a definitive EOS condition (thus operating in a lower voltage domain). The use of mixed signal integrated circuits complicates the metallization complexity with the need for signal isolation based on voltage range or signal type. Typical circuitry has pads for a particular voltage domain in one portion of the circuit to provide signal isolation. These complex networks of protection structures and bussing increase the risk for signal corruption, excessive pad loading, leakage current, and decreased durability when exposed to EOS events. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    Embodiments of the present invention relate to an integrated circuit that includes multiple pads. The pads may take any form including bond pads, contact pads, or any other mechanism for applying electrical signals to the integrated circuit. Different types of current protection structures may be associated with different sets of one or more of the pads. For instance, as an example only, there might be analog protection structures to protect analog pads, and digital protection structures to protect digital pads. Furthermore, although not required, the current protection structures may have different trigger points, or may simply just have different circuits. A common current discharge or charge path may be used to provide current to or draw current from various of these heterogenic current protection structures. Since a common current discharge or charge path is used across multiple current protection structures, the metallization used to formulate a discharge solution is significantly simplified, even if the current protection structures themselves are quite different. 
         [0007]    Additionally, in accordance with another embodiment of the invention, the current protection structures may be provided with active current dissipation regions that are approximately radially symmetric around the circumference of the pad. Accordingly, if the protection structures are slightly off center with respect to the bond pad (due to, for example, mask alignment error), any error in spacing between active regions around the circumference of the pad is at least partially averaged out. 
         [0008]    These and other features of the embodiments of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    To further clarify the above and other advantages and features of embodiments of the present invention, a more particular description of the embodiments of the invention will be rendered by reference to the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The embodiments will be described and explained with additional specificity using the accompanying drawings in which: 
           [0010]      FIG. 1  illustrates a system that includes a circuit with multiple pads, and with multiple different current protection structures that share a common current discharge and charge path; 
           [0011]      FIG. 2  schematically illustrates an electrical relation between the pads, the current protection structures, and the common current discharge or charge path; 
           [0012]      FIG. 3A  illustrates a designed layout of a pad, an associated current protection structure that is radially symmetric with the pad, and a portion of the current discharge or charge path; 
           [0013]      FIG. 3B  illustrates the layout of  FIG. 3A  if subjected to some misalignment error between the pad and the current protection structure; 
           [0014]      FIG. 4  illustrates a combined cross-sectional view and schematic diagram of a current protection structure that may be used as one of the current protection structures of  FIGS. 1 ,  2 ,  3 A or  3 B; 
           [0015]      FIG. 5  illustrates a combined cross-sectional view and schematic diagram of a current protection structure that may be used as one of the current protection structures of  FIGS. 1 ,  2 ,  3 A or  3 B and that may be used with multiple current dissipation conduction paths; 
           [0016]      FIG. 6  illustrates a series connection of alternating P-type and N-type regions in a PNPNP configuration used to describe the operation of the current protection structure of  FIG. 4 ; and 
           [0017]      FIG. 7  illustrates a current protection structure of  FIGS. 4 and 6  expressed in the form of interconnected bipolar transistors. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Embodiments of the present invention relate to the use of a common discharge or charge path to serve multiple pad-oriented heterogenic current protection structures, even though those protection structures are structured differently or perhaps even serve different voltage domains. Accordingly, the common discharge or charge path may provide a discharge path for analog and/or digital pads, high voltage pads and/or normal voltage pads, or other heterogenic voltage domain situations. The integrated circuit may be included within a larger system, such as, for example, a computing system or an automobile to provide significant performance enhancements to that larger system. 
         [0019]      FIG. 1 , for example, illustrates a system  10  that includes an integrated circuit  100 . The integrated circuit  100  includes a number of pads  101 A through  101 L (referred to sometimes collectively as “pads  101 ”). In the illustrated embodiment, the integrated circuit  100  has 12 pads, although there is no significance attached whatsoever to this particular number of pads. The principles of the present invention may just as well apply to integrated circuits with any multiple number of pads. Furthermore, although the pads are illustrated as being squares in  FIG. 1  taking on perhaps the appearance of a bond pad, the principles of the present invention are not limited to the particular form of the pad. In this description and in the claims, a “pad” of a circuit is simply any electrical connection that has the capability to interface with components external to the circuit. An appropriate selection of a pad type is a design choice that will depend on the configuration of the circuit package. 
         [0020]    Each of the pads  101  has an associated current protection structure. With reference to  FIG. 1 , each of the pads  101 A through  101 L has a corresponding current protection structure  111 A through  111 L, respectively. The current protection structures  111 A through  111 L will sometimes be referred to herein collectively as “current protection structures  111 ”. The current protection structures  111  serve to provide appropriate current protection for their corresponding pads. If, at any of the corresponding pads, one or more conditions occur that are indicative of an Electrical OverStress (EOS) event, the associated current protection structure activates to discharge current from or charge current to the associated pad. This positive or negative current dissipation protects the core circuitry from excessive currents. 
         [0021]    The current protection structures  111  may be partially or fully heterogenic. The current protection structures  111  may be “partially heterogenic” in that at least some of the current protection structures  111  may have different structures. The current protection structures  111  may even be “fully heterogenic” in that the principles of the present invention are still applicable even if all of the current protection structures  111  have different structures. “Different structures” as the term is used herein when applied to describe the current protection structures  111  is more than mere processing variations that might result from the imperfect manufacture of the same structural design of a current protection structure. Processing technology inherently results in some structural differences even if starting from the same circuit design, even when that design is repeated on the same wafer or die. Instead, “different structures” as the term is used herein are structures that are formulated from different designs. 
         [0022]    One reason for having different structures for the current protection structures  111  is that each pad  101  may have a distinct voltage domain. Proper current protection for a particular voltage domain may require different designs to assign proper current protection given each voltage domain&#39;s anticipated normal operation. For example purposes only, a dashed box  121 A is illustrated as surrounding the pad  101 A, symbolizing that the pad  101 A serves a particular voltage domain (also referred to herein as “voltage domain  121 A”). Some pads may be of a common voltage domain. For instance, a dotted box  121 B surrounds the pads  101 B through  101 E, symbolizing that the pads  101 B through  101 E each serve a common voltage domain (also referred to herein as “voltage domain  121 B”). Likewise, as symbolically illustrated, pads  101 F and  101 G are associated with a common voltage domain  121 C, pad  101 H is associated with voltage domain  121 D, pad  101 I is associated with voltage domain  121 E, and pads  101 J through  101 L are associated with a common voltage domain  121 F. The voltage domains  121 A through  121 F may be referred to hereinafter collectively as “voltage domains  121 ”. 
         [0023]    A “voltage domain” is defined by its expected normal voltage operation. As a corollary to that statement, “different voltage domains” are domains in which the expected normal operational voltages are different. For instance, pads that correspond to a digital voltage domain are expected to have digital voltages (but not analog voltages) applied during normal operation. On the other hand, pads that correspond to an analog voltage domain are expected to have analog voltages (but not digital voltages) applied during normal operation. Also, a pad may correspond to a mixed signal voltage domain if the pad normally operates with both digital and analog signals. The analog voltage domain, the digital voltage domain, and the mixed signal voltage domains are each different voltage domains. 
         [0024]    A pad may even have multiple voltage domains. For instance, one pad might be a high voltage digital pad, thereby permitting normal operation at high voltages. On the other hand, a normal voltage digital pad might be expected to experience lower voltages during normal operation. A digital voltage domain might also be defined by the expected voltage levels representing binary one and zero signals. A digital voltage domain might also be defined by the number of acceptable digital levels. For instance, a digital pin sometimes might have a high and a low discrete level, as well as one (or more) intermediate discrete voltage level defining one (or more) intermediate digital values. 
         [0025]    As another example of multiple voltage domains, an analog pin may have a particular expected voltage range, while another analog pin may have a different voltage range. Regardless of whether one voltage range overlaps (partially or fully) or not with the other voltage range, these analog pins will have different voltage domains due to their difference in acceptable voltage ranges. As the termed is used herein, pins have “different voltage domains” so long as any one of the voltage domains of one pad is different than any of the one or more voltage domains of the other pad. Thus, even pads that belong to the same voltage domain (e.g., the analog voltage domain) may have different voltage domains, just as people have different characteristics even though they are in some ways the same. 
         [0026]    Voltage domains may also be defined by whether the corresponding current protection structure has a positive or negative trigger voltage, or both. In this description and in the claims, a “positive trigger voltage” is a voltage level that if experienced at the pad will cause significant current to be drawn by the current protection structure from the pad, thereby resulting in a sharp voltage drop at the pad. On the other hand, a “negative trigger voltage” is a voltage level that if experienced at the pad will cause significant current to be provided by the current protection structure to the pad, thereby causing a sharp increase in the voltage at the pad towards ground. 
         [0027]    Current protection structures that have both a positive and negative trigger voltage are bi-directional. However, the current protection structure need not have both a positive and a negative trigger voltage to be “bi-directional”, as the term is used herein. For example, a bi-directional current protection structure may just have a positive trigger voltage, but when a negative voltage is encountered, the current protection structure may behave more as a diode in the negative direction. On the other hand, a bi-directional current protection structure may just have a negative trigger voltage, but when a positive voltage is encountered, the current protection structure might simply behave as a diode in the positive direction. A voltage domain may also be defined by the level of the positive and/or negative trigger voltage of a current protection structure corresponding to a pad. 
         [0028]    Referring to  FIG. 1 , although the circuit  100  is shown as including six voltage domains  121 A through  121 F, the principles of this particular embodiment of the present invention may apply to a circuit that has any configuration of voltage domains and any correlation of such voltage domains to pads. 
         [0029]    In any case, a current protection structure is activated upon the detection of one or more conditions at an associated pad, or at a node that is close to the pad. The one or more conditions will depend on the voltage domain of the pad. For instance, current protection structure  111 A may be said to be of a first voltage domain  121 A if the current protection structure  111 A serves to discharge current from or charge current to any of its associated pad  101 A when a first set of one or more voltage conditions is present at the pad  101 A. On the other hand, current protection structures  111 B through  111 E may be said to be of a second voltage domain  121 B if the current protection structures  111 B through  111 E each serve to discharge current from or charge current to its associated pads (in the illustrated case, pads  101 B through  101 E, respectively) when a second set of one or more voltage conditions is present at the associated pad  101 B through  101 E. Similarly, current protection structures  111 F and  111 G may be said to be of a third voltage domain  121 C if the current protection structures  111 F and  111 G each serve to discharge current from or charge current to its associated pad (in the illustrated case, pads  101 F and  101 G) when a third set of one or more voltage conditions is present at the associated pad  101 F and  101 G. The same might be said for voltage domains  121 D through  121 F as well. 
         [0030]    A current discharge or charge path  120  serves at least two of the current protection structures  111 , but possibly more than two or even all of the current protection structures  111 . Accordingly, if any of the current protection structures  111  connected to the path  120  were to trigger, the trigger current protection structure may then use the path  120  to shunt current. By sharing the current discharge or charge path across multiple current protection structures, the amount of space occupied to discharge or charge current is greatly reduced. Furthermore, since the current protection structures  111  are situated between the respective pads  101  and the current charge or discharge path  120 , the voltage on the current charge or discharge path  120  need not be carefully regulated in many applications. Thus, there may often be no particular need for distributed voltage clamps on the current charge or discharge path  120 . 
         [0031]    Moreover, when an EOS event occurs across any two pads coupled to the common charge or discharge path  120 , the common charge or discharge path  120  serves to provide a low impedance route for the current to shunt through. For instance, suppose that an ElectroStatic Discharge (ESD) event occurs between pads  101 B and  101 I of circuit  100  of  FIG. 1 , each of the current protection structures  111 B and  111 I may trigger thereby providing a low voltage drop across the current protection structures  111 B and  111 I. Since the common charge or discharge path  120  may be low impendance since due to it being shared, a low impedance path is created from the pad  101 B, through the current protection structure  111 B, through the common charge or discharge path  120 , through the current protection structure  111 I and to pad  101 I. The dangerous ESD current is thus taken safely off the circuit  100  through this shunting operation. 
         [0032]    The low impedance property of this shunt path means that the current will likely follow the shunt path, rather than flow through undesirable paths in the protected circuitry to other pads thereby causing harm to the protected circuitry. Since the ESD current has little drive to follow such undesirable paths through protected circuitry, pads of very different voltage domains may be placed closer to each other or even interleaved with greater assurance that the shunt path will be used in case of an ESD event. For instance, a high voltage pad may be placed proximate to low voltage pads without particular concern. Thus, the principles of the present invention may provide considerable design flexibility in placement of different pad types on an integrated circuit. 
         [0033]      FIG. 2  illustrates a schematic  200  of the electrical connections of the circuit  100  of  FIG. 1 . For each of the pads  101 , the associated current protection structure  111  intervenes to provide current to or draw current from the associated pad  101 . The voltage domains  121  are also identified by bracketing the corresponding pads  101  and current protection structures  111 . 
         [0034]    Referring to pad  101 A, for example, if the current protection structure  111 A is not activated, then signals applied to the pad  101 A are provided to the protected circuit  102 A. On the other hand, if excessive current or voltage is applied to the pad  101 A due to, for example, electrostatic discharge, the current protection structure  111 A activates and draws current from the pad  101 A into the discharge/charge path  120 . Conversely, if excessive current is drawn from the pad  101 A due to, for example, negative electrostatic discharge, the current protection structure  111 A may potentially activate and provide current from the current discharge/charge path  120  to the pad  101 A. In any of these cases, the protected circuitry  102 A does not experience physical damage or degradation. 
         [0035]      FIG. 3A  illustrates a specific layout  300 A of a pad  301 A in conjunction with a current protection structure  303 A and a current discharge/charge path  305 A. As previously noted, the current protection structure  303 A is associated with a single pad  301 A. Although the pad  301 A is illustrated as being square-shaped, the pad  301 A may take any form. The pad  301 A may be provided at one terminal of the current protection structure. The region  303 A represents a selectively conductive region of the current protection structure. At the outer perimeter of the current protection structure  303 A lies a conductive material  302 A (also referred to herein as the “perimeter terminal  302 A”) that serves as the other terminal of the current protection structure  303 A. The perimeter terminal  302 A of the current protection structure is electrically connected (as represented by connection  304 A) to the current discharge/charge path  305 A that is shared amongst multiple pads and current protection structures. 
         [0036]    In operation, if the current protection structure is activated, current passes through the selectively conductive region  303 A in an appropriate direction between the pad  301 A and the perimeter terminal  302 A. The perimeter terminal  302 A is electrically coupled to the common current discharge/charge path  305 A through the connection  304 A thereby allowing the path  305 A to serve as a current source or sink for the current protection structure  303 A. The precise nature of the connection between the perimeter terminal  302 A and the current discharge/charge path  305 A is not important to the principles of the present invention. In one embodiment, one of the sides of the perimeter terminal may simply be the common discharge/charge path that is connected to one or more other current protection structures. In another embodiment, the path  305 A may underly or overly much or all of the pad  301 , but at a different metal layer. 
         [0037]    In the specific example of  FIG. 3A , the selectively conductive region  303 A of the current protection structure is designed to be radially symmetrical around the pad  301 A. In other words, given any radial line extending outwards from the center of the pad  301 A, the ratio of the distance from the center to where the line intersects the outer edge of the pad  301 A to the distance from the center to where the line intersects the perimeter terminal  302 A will be approximately constant as the radial line is rotated in a circle about the center of the pad  301 A. 
         [0038]    This design is desirable in that breakdown activation of the current protection structure  303 A will occur throughout the entire area of the current protection structure  303 A. However, given the intricate and sometimes imprecise nature of semiconductor processing technology, it is difficult to precisely align one circuit structure with another. 
         [0039]    For instance,  FIG. 3B  illustrates a similar structure as compared to  FIG. 3A , with elements  301 B through  305 B of  FIG. 3B  approximately correlating to elements  301 A through  305 A, respectively, of  FIG. 3A . However, one primary difference is that the pad  301 B is not perfectly aligned with respect to the current protection structure  303 B. This might cause breakdown to occur at region  306 , rather than more uniformly across the entire structure. While this misalignment may have been exaggerated, there will always be some finite amount of misalignment between two circuit elements due to mask alignment error. Since the current protection structure  303 B is designed to be radially symmetric around the pad  301 B, however, the alignment errors should remain relatively small regardless of the direction of misalignment. Without being radial symmetric, the direction of misalignment would be more significant since misalignment in one direction might cause a high activation voltage, whereas the same distance of misalignment in the other direction might cause a lower activation voltage. Thus, the radial symmetry in the design of the current protection structure  303 B and the pad  301 A permits for greater control over the activation voltage. 
         [0040]    Accordingly, embodiments have been described in which multiple heterogenic current protection structures may be served by a single common current discharge or charge path. Furthermore, an example of a current protection structure that is designed to be radially symmetric with respect to a pad has been described. The precise nature of the current protection structure is not critical to the invention so long as the current protection structure is able to provide a low impedance shunt path to and from a common charge/discharge path. In one example, the current protection structure provides normal operation during a moderate reverse voltage mode, while still providing current dissipation functions if excessive positive or negative currents are experienced. Such an example will now be described with respect to  FIGS. 4 through 7 . Although the specific example is described with respect to  FIGS. 4 through 7 , the principles of the present invention are not limited to any particular usage of a current protection structure. As such,  FIGS. 4 through 7  should not be construed in any way to restrict the broader principles of the present invention. 
         [0041]      FIG. 4  illustrates a current protection structure  400  manufactured on a semiconductor substrate that may be used to protect circuitry from EOS while permitting operation in a reverse voltage condition. The current protection structure is described in further detail in commonly-owned co-pending United States patent application number [SERIAL NUMBER NOT YET ASSIGNED] (Attorney Docket Number 14993.54) entitled “Single Well Excess Current Dissipation Circuit” filed on the same day herewith, which application is incorporated herein by reference in its entirety. 
         [0042]    For clarity, portions of the current protection structure  400  are illustrated in cross-section as they might be processed on a semiconductor substrate, while other portions are illustrated using simple circuit symbols. In addition to providing reverse voltage protection without triggering the current protection structure  400 , the current protection structure  400  may also be processed using a single-well technology in which all wells are manufactured of the same polarity (i.e., all n-type or all p-type). In the illustrated case of  FIG. 4 , all of the wells are n-type. 
         [0043]    In this description and in the claims, an “n-type” region or “n-region” of a semiconductor material is said to have an n-type polarity and is a region in which there are more n-type dopants than p-type dopants, if there are any p-type dopants at all. On the other hand, a “p-type” region or “p-region” of a semiconductor material is said to have a p-type polarity and is a region in which there are more p-type dopants than n-type dopants, if there are any n-type dopants. Generally, the p-type polarity is considered to be the opposite of the n-type polarity. 
         [0044]    The current protection structure  400  includes two autonomous n-well regions  411  and  412  within a p-type semiconductor substrate  405 . An “n-well” region is a well that is formed as an n-type region within a larger p-type region, as opposed to a “p-well” region which is formed as a p-type region within a larger n-type region. Techniques for forming n-well and p-well regions in a substrate are well known in the art and thus will not be discussed here. It will be understood that a p-type semiconductor region in contact with an n-type semiconductor region will cause a diode effect, with current being permitted to pass from the p-type region to the n-type region if the voltage at the p-type region is higher than the voltage at the n-type region. However, current is not permitted to flow from the n-type region to the p-type region absent a significantly high voltage at the n-type region with respect to the p-type region. This higher voltage is often referred to as a diode&#39;s “breakdown” voltage or “reverse breakdown” voltage. 
         [0045]    Occasionally, while describing the operation of the current protection structure  400  of  FIG. 4 , reference will be made to the PNPNP stack  600  of  FIG. 6  which illustrates the relationship of the p-type and n-type junctions of  FIG. 4 . Likewise,  FIG. 7  illustrates the relationship in the form of interconnected bipolar transistors  700 . 
         [0046]    Since  FIG. 6  is used to describe only the principles of operation, the size of the n-type and p-type regions of  FIG. 6  are not drawn to scale when compared to the corresponding components of  FIG. 4 . In  FIG. 6 , the n-region  602  corresponds to the n-well  411  of  FIG. 4 , and the n-region  604  corresponds to the n-well  412  of  FIG. 4 . The p-region  603  corresponds to the p-type substrate  405  of  FIG. 4 . Note that in  FIG. 4 , there may be an n-channel field  414  surrounding the n-well  411 . The thickness of this n-channel field  414  may be controlled at the time of circuit manufacture to thereby control the breakdown voltage between the diode defined by the n-well  411  and the p-type substrate  405 . Mechanisms for forming such an n-channel field of a specific width are known in the art and thus will not be described here. Although not shown, an n-tub of higher n-type dopant density than the n-well  411  may be used internal to the n-well  411  to provide a further adjustment to the breakdown voltage. 
         [0047]    Referring to  FIGS. 6 and 7 , the n-region  602  of  FIG. 6  corresponds to the n-type base terminal of the PNP bipolar transistor  701  and the n-type collector terminal of the NPN bipolar transistor  702 , which are shown coupled together in  FIG. 7  since the terminals are both formed using the same n-type region  602 . Also, the n-region  604  of  FIG. 6  corresponds to the n-type emitter terminal of the NPN bipolar transistor  702  and corresponds to the n-type base terminal of the PNP bipolar transistor  703 . Once again, these terminals are coupled together since they are formed of the same n-type region  604  The p-region  603  of  FIG. 6  corresponds to the p-type collector terminal of PNP bipolar transistor  701 , the p-type emitter terminal of PNP bipolar transistor  703 , and the p-type base terminal of NPN bipolar transistor  702 , which are shown coupled together. 
         [0048]    Referring back to  FIG. 4 , the n-well  411  is coupled to a first circuit node  401  through a first parallel combination of a p-type contact region  431  and an n-type contact region  432 . The net dopant density of each of the p-type contact region  431  and the n-type contact region  432  is greater than the net dopant density of the n-well  411 . This higher net dopant density is expressed in  FIG. 4  by the p-type contact region  431  being designated as “P+”, and the n-type contact region  432  being designated as “N+”. The “net dopant density” is the concentration per unit volume of dominant dopant species (n-type dopants if an n-type region, and p-type dopants if a p-type region) minus the concentration per unit volume of minority dopant species (p-type dopants if an n-type region, and n-type dopants if a p-type region). 
         [0049]    Referring to  FIGS. 4 and 6 , the p+ contact region  431  of  FIG. 4  corresponds to the p-region  601  of  FIG. 6 . The p-region  601  is coupled to one terminal  621  of the PNPNP stack  600 . The terminal  401  of  FIG. 4  corresponds to the terminal  621  of  FIG. 6 . The resistor  403  of  FIG. 4  corresponds to the resistor  611  of  FIG. 6  having resistance R. Referring to  FIGS. 4 and 7 , the p+ contact region  431  of  FIG. 4  corresponds to the p-type emitter terminal of the PNP bipolar transistor  701 . The terminal  401  of  FIG. 4  corresponds to terminal  721  of  FIG. 7 . The resistor  403  of  FIG. 4  corresponds to the resistor  711 A of  FIG. 7 . 
         [0050]    Referring back to the illustrated embodiment of  FIG. 4 , the n+ contact region  432  is coupled to the first circuit node  401  through a resistor circuit element  403 . In this description and in the claims, a “resistor circuit element” is a resistor that is specifically formed as a desired portion of a circuit pattern. The p+ contact region  431  is coupled to the first circuit node  401  without an intervening resistor circuit element in the illustrated embodiment. 
         [0051]    A second n-well  412  is coupled to the second circuit node  402  through a parallel combination of a p+ contact region  421  and an n+ contact region  422 . In the illustrated embodiment, the third and fourth contact regions  421  and  422  are coupled to the second circuit node  402  without an intervening resistor element. In one embodiment, the first circuit node  401  is an I/O pad in which input and/or output signals may be applied. The second circuit node  402  may be a substantially fixed voltage supply such as, for example, ground. The substrate  405  may also be connected to ground. The remaining circuit elements  423  serve to reduce the breakdown voltage of the diode defined by the interface between the n-well  411  to p-type substrate  405 . 
         [0052]    Referring to  FIGS. 4 and 6 , the p+ contact region  421  of  FIG. 4  corresponds to the p-region  605  of  FIG. 6 . The second circuit node  402  of  FIG. 4  corresponds to the circuit node  622  of  FIG. 6 . Since the n-well  412  is connected through the n+ region  422  to the circuit node  402  with some resistance,  FIG. 6  shows a small resistor  612  having resistance r 1  coupled between the n-region  604  and the second circuit node  622 . Furthermore, since p-type substrate  405  may well be grounded, and the second circuit node  402  is grounded, the p-region  603  is shown coupled to the second circuit node  622  through resistor  613  having resistance r 2 . The resistors r 1  and r 2  may be parasitic, as opposed to an expressed resistor circuit element in the design. However, the resistors may also be expressed design elements. 
         [0053]    Referring to  FIGS. 4 and 7 , the p+ contact region  421  of  FIG. 4  corresponds to the p-type collector terminal of PNP bipolar transistor  703  of  FIG. 7 . The second circuit node  402  of  FIG. 4  corresponds to the circuit node  722  of  FIG. 7 . Since the n-well  412  is connected through the n+ region  422  to the circuit node  402  with some resistance,  FIG. 7  shows a small resistor  712  having resistance r 1  coupled between the n-type base terminal of PNP bipolar transistor  703  and the second circuit node  722 . Furthermore, since p-type substrate  405  may well be grounded, and the second circuit node  402  may well be grounded, the p-type base terminal of NPN bipolar transistor  702  is shown coupled to the second circuit node  722  through resistor  713  having resistance r 2 . 
         [0054]    As will be apparent to those of ordinary skill in the art, the polarities of each of the regions of  FIG. 4 ,  6  and  7 , may be reversed. In other words, p-type regions may be replaced by n-type regions, and vice verse. 
         [0055]      FIG. 5  illustrates a dual reference mode form of the current protection structure  400  of  FIG. 4 . While the current protection structure  400  of  FIG. 4  uses a single reference node  402  as a current source or sink, the current protection structure  500  of  FIG. 5  includes two references nodes  502  and  504  to source current to or sink current from the circuit node  501 . The operation of the components  501 ,  502 ,  503 ,  505 ,  511 ,  512 ,  521 ,  522 ,  523 ,  531  and  532  of  FIG. 5  will operate much as described above for the components  411 ,  412 ,  421 ,  422 ,  423 ,  431  and  432  described with respect to  FIG. 4  in sourcing or sinking current to or sinking current from circuit node  501  using reference node  502 . However, the reference node  504  will operate using regions  541 ,  542  and  543  within n-well  513  much as described above for the reference node  402  operating using regions  421 ,  422  and  423  within n-well  412 . Accordingly, dual reference node current dissipation is achieved. 
         [0056]    Therefore, a current protection structure is further described that permits for proper and adjustable current dissipation while permitted normal reverse voltage operation. Furthermore, this is achieved by using single well technology thereby simplifying the fabrication of the current protection structure. 
         [0057]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.