Abstract:
A circuit protection component including a crowbar device for protecting electronic devices from transients is generally disclosed. The circuit protection component may include a steering diode bridge and a crowbar device electrically connected to the steering diode bridge. The crowbar device may have a base and an emitter formed on a first layer, the first layer defining a breakdown voltage, which when exceeded allows current to pass under the emitter and out the device through a hole formed in the emitter.

Description:
RELATED APPLICATION 
       [0001]    This application claims priority to U.S. provisional application Ser. No. 61/668,326 filed Jul. 5, 2012. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    This disclosure relates generally to the field of circuit protection devices and more particularly to protecting circuit components from voltage transients, such as, may be generated by lightning strikes or power circuits. 
       BACKGROUND OF THE DISCLOSURE 
       [0003]    Unwanted changes in voltage or current within a circuit are generally referred to as a “transient.” Transients may be caused by a variety of sources. For example, lightning strikes may cause unwanted transients within a circuit. Additionally, transients may be manifested in a variety of ways. More specifically, transients may cause a number of different changes to the steady-state voltage or current conditions within a circuit. Furthermore, the current and voltage waveforms and magnitudes associated with transients also vary. As transients often cause unwanted spikes in voltage and/or current within a circuit, damage to circuit components can occur. 
         [0004]    Protecting circuits from transients often involves placing some type of protection component (or components) between the source of potential transients and the circuit to be protected. The protection component acts to restrict the current and voltage in the circuit to safe levels. Some protection components use semiconductor devices, due to the fact that semiconductor devices often have well defined clamping voltage and leakage current characteristics. 
         [0005]    In practice, protection components that utilize semiconductor devices may be configured to quickly limit any voltage transients to predefined levels. The capacitance of a semiconductor junction, however, changes with voltage. As such, protecting the circuit from transients may come at the expense of increased capacitance of the protection component. Some circuits, such as, for example, very high speed subscriber telephone lines (VDSL) may experience data loss due to increased capacitance levels. Accordingly, some protection components may cause data loss within the circuit being protected due to an increase in capacitance resulting from mitigating the transient. 
         [0006]    Furthermore, the resistance of some semiconductor devices also changes with voltage. As such, protecting the circuit from transients may also come at the expense of varying resistance, which may adversely affect the current within the circuit. Additionally, some semiconductor devices may become damaged by too many cycles of power dissipation, resulting in premature failure of the protection component. 
         [0007]    Therefore, there is a need to for protection components that are able to quickly clamp voltage of a transient to reasonable levels, sustain repeated exposure to high currents, and not substantially change the capacitance of the circuit being protected. 
       SUMMARY 
       [0008]    In accordance with the present disclosure, a circuit protection component for protecting an electronic device from a transient is provided. The circuit protection component may include a steering diode bridge and a crowbar device electrically connected to the steering diode bridge. The crowbar device may have a base and an emitter formed on a first layer, the first layer defining a breakdown voltage, which when exceeded allows current to pass under the emitter and out the device through a hole formed in the emitter. 
         [0009]    In some embodiments, the crowbar device for the circuit protection component may include a diffusion layer formed in a lower region of a substrate, a base formed on the substrate, and an emitter having a hole therein formed on the base, the crowbar device having a breakdown voltage, which when exceeded allows current to pass under the emitter and out the device through a hole formed in the emitter. 
         [0010]    Additional embodiments of the crowbar device may include an epitaxial layer formed on a substrate, a base formed on the epitaxial layer, and an emitter having a hole therein formed on the base, the crowbar device having a breakdown voltage, which when exceeded allows current to pass under the emitter and out the device through a hole formed in the emitter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    By way of example, specific embodiments of the disclosed device will now be described, with reference to the accompanying drawings, in which: 
           [0012]      FIG. 1  is a block diagram of a circuit connecting an external transmission line to a data line; 
           [0013]      FIG. 2  is a block diagram of a conventional circuit protection component; 
           [0014]      FIG. 3  is a block diagram of an example circuit protection component; 
           [0015]      FIG. 4  is a block diagram of an example crowbar protection device; 
           [0016]      FIGS. 5A-5C  are block diagrams of another example crowbar protection device; 
           [0017]      FIG. 6  is a block diagram of an example voltage response of a crowbar protection device; and 
           [0018]      FIGS. 7-9  are block diagrams of example doping profiles of a crowbar protection device, all arranged in accordance with at least some embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout. 
         [0020]      FIG. 1  illustrates a block diagram of an example circuit  100 , arranged in accordance with various embodiments of the present disclosure. As depicted, the circuit  100  includes an external transmission line  110  and a data line  120  connected via capacitive voltage transformer circuit  130 . In describing the circuit  100  below, reference may be made to the transmission line side  102  and the data line side  104 . As depicted, the capacitive voltage transformer circuit  130  includes transformers  132 ,  134  and capacitors  136 ,  138 . In general, the capacitive voltage transformer circuit  130  operates to filter and/or transform the voltage signals from the external transmission line  110  to the data line  120 . For example, the data line  120  may be a VDSL and the external transmission line may connect the VDSL line to external locations (e.g., central transmission office, an Internet service provider, or the like.) 
         [0021]    During operation, the external transmission line  110  may be exposed to a wide range of electrical interferences, such as, for example, lightning strikes, power cross, power induction, or the like, all of which may result in voltage transients in the transmission line side  102 . In order to protect the circuit  100  from voltage transients, a circuit protection component is often connected to the transmission line side  102 . For example,  FIG. 1  depicts a gas discharge tube  140  connected to the external transmission line  110 . In general, the gas discharge tube  140  operates to dissipate voltage transients through a plasma gas contained within the gas discharge tube  140 . The gas discharge tube  140  has a maximum voltage, sometimes referred to as a “sparkover voltage.” When the sparkover voltage is exceeded, the gas within the gas discharge tube  140  becomes ionized, which causes the gas discharge tube  140  to conduct current thereby diverting the transient to ground. Various standards exist for ensuring that the external transmission line  110  and related circuitry can withstand voltage transients. For example, the Telecordia GR-1089 standard lists various voltage surge conditions that external transmission lines may be tested against. 
         [0022]    As will be appreciated, while the gas discharge tube  140  diverts any voltage transients to ground, changes to the steady state voltage and/or current within the transmission line side  102  will be communicated (via the capacitive voltage transformer circuit  130 ) to the data line side  104 . Furthermore, transients communicated to the data line side  104  will be of much shorter duration, yet much higher current. More particularly, as the cores of the transformers  132  and  134  saturate due to the voltage transient, the current in the transformers  132  and  134  will rise. As a result, the voltage transient communicated to the data line side  104  will have a higher current than the original voltage transient on the transmission line side  102 . Accordingly, voltage transients on the data line side  104  will have a different waveform and much higher current than those on the transmission line side  102 . For example, a typical voltage transient on the transmission line side  102  may have a pulse width of several hundred microseconds. The voltage transient communicated to the data line side  104  may have a pulse width of less than a microsecond, yet have several hundred amps of current. 
         [0023]    The data line side  104 , then, may also include a circuit protection component  150 . The circuit protection component  150  (described in greater detail below) may be configured to protect the data line side  104  from voltage transients communicated from the transmission line side  102 . As stated above, semiconductor devices are often used to form the circuit protection component  150  due to their well-defined clamping voltages and low leakage currents. However, the capacitance of a semiconductor junction changes with voltage as the width of the depletion layer increases with applied voltage. 
         [0024]      FIG. 2  illustrates a block diagram of a conventional circuit protection component  200 . As depicted, the circuit protection component  200  includes an avalanche diode  202  connected in series with a steering diode bridge  210 . The steering diode bridge  210  includes four diodes  212 ,  214 ,  216 , and  218 . In general, the circuit protection component  200  may be connected to a circuit to be protected via terminals  222  and  224 . Furthermore, the capacitance of the circuit protection component  200  may be adjusted based on an external bias, such as, may be applied at terminals  226  and  228 . Although the avalanche diode normally has high capacitance, the steering diode bridge  210  effectively lowers the capacitance of the circuit protection component  200 , as the capacitance presented to the data line  120  is that of the steering diode bridge  210  rather than the avalanche diode  202 . Furthermore, the capacitance of the circuit protection component  200  may be further reduced by reverse biasing the diodes  212 ,  214 ,  216  and  218 . 
         [0025]    Although the circuit protection component  200  may provide for fast voltage clamping and capacitance levels appropriate for the data line  120 , the circuit protection component  200  may not be able to handle the high current levels resulting from the voltage transients described above. More specifically, resistive drops in the avalanche diode  202  may cause the voltage across the avalanche diode  202  to rise significantly above the clamping voltage, which can cause damage to components in the data line side  104 . Additionally, the avalanche diode  202  may become damaged by too many cycles of power dissipation. More particularly, repetitive voltage transients (e.g., as may be caused by induced power) may damage the avalanche diode  202 . 
         [0026]      FIG. 3  illustrates a block diagram of a circuit protection component  300 , arranged according to various embodiments of the present disclosure. As depicted, a crowbar device  302  is connected in series with a steering diode bridge  310 . The steering diode bridge  310  includes diodes  312 ,  314 ,  316 , and  318 . Various examples of the crowbar device  302  are described later, particularly with relation to  FIGS. 4-5 . In general, the circuit protection component  300  may be configured as the circuit protection component  150  shown in  FIG. 1 . For example, the circuit protection component  300  may be connected to the data line  120  via terminals  322  and  324 . Furthermore, the capacitance of the circuit protection component  300  may be adjusted based on an external bias, such as, may be applied at terminals  326  and  328 . As such, when transients are communicated from the transmission line side  102  to the data line side  104 , the circuit protection component  300 , and particularly, the crowbar device  302 , may suppress them. 
         [0027]    In some examples, the diodes  312 ,  314 ,  316 , and  318  may be fabricated using a high resistivity epitaxial layer (e.g., between 50 and 200 Ohm-centimeter, or the like) on a heavily doped N-type substrate. A selective anode region is diffused into the epitaxial layer to form the diode. As the epitaxial layer is of low doping, this region rapidly depletes with applied voltage. Furthermore, as will be appreciated, the capacitance of the diodes may be determined by the thickness of the epitaxial layer. In some embodiments, the thickness of the epitaxial layer may be chosen to give the diodes  312 ,  314 ,  316 , and  318  desired capacitance and voltage overshoot characteristics. The low doped region between the heavily doped anode and cathode may, in some example, be approximately between 5 and 15 microns in width. In some examples, the polarity of the diode regions described above for the diodes  312 ,  314 ,  316 , and  318  may be reversed. 
         [0028]      FIG. 4  illustrates a block diagram of an example crowbar protection device  400 , arranged in accordance with various embodiments of the present disclosure. In some examples, the crowbar protection device  400  may be implemented in the circuit protection component  300  as the crowbar device  302 . As depicted, the crowbar protection device  400  is fabricated on a substrate  410 , such as, for example, silicon. With some embodiments, the substrate  410  is an N-type substrate having a resistivity of approximately 25 Ohm-centimeter. As depicted, the substrate  410  includes a deep diffusion area  412 , located near the bottom of the substrate  410 . The deep diffusion area  412  forms the anode of the crowbar protection device  400 . In some examples, the deep diffusion area  412  may be a P+ type diffusion area. 
         [0029]    A base  414  and an emitter  416  are additionally depicted. In some examples, the base  414  may be a P-type base and the emitter  416  may be an N+ type emitter. The base  414  and emitter  416  may be formed into the front of the substrate  410  using conventional semiconductor manufacturing techniques, which will be apparent to those of ordinary skill in the art. As depicted, the crowbar protection device  400  is an NPNP type semiconductor devices. In some examples, the emitter  416  overlaps the edge of the base  414 , to form a low voltage breakdown region. Additionally, in some examples, the crowbar protection device  400  may additionally have oxide layers and metal layers (not shown) formed on the top and/or bottom of the device to form an anode and cathode for connecting the device to a circuit (e.g., the circuit protection component  300 , or the like.) 
         [0030]    During operation, when a voltage transient appears across the crowbar protection device  400 , current flows along a trigger path  420  (shown as dotted line.) Current initially flows through a portion of the emitter  416  as illustrated. Due to the high resistivity of the substrate  410 , high voltage drops may be developed as current passes through the region along trigger path  420 . This may be particularly true during periods of fast and/or large current transients. The current then passes through a breakdown region  422  along the path  426 . The breakdown region  422  may define the triggering voltage. The trigger voltage may be selected to suit desired device characteristics, and may, in some examples, be in the range of 8-50 Volts. When the trigger voltage is exceeded, current passes under the emitter  416  (as shown) and out through the region  424 , where a hole is formed in the emitter  416 . With some examples, when the voltage at point  428  reaches approximately 0.7 Volts, the emitter  416  becomes forward biased and the crowbar protection device  400  begins to regenerate and enter the conducting state. In some examples, the emitter  416  may be formed having a number of holes in the region  424 . However, a single hole is shown in  FIG. 4  for clarity. 
         [0031]    In some examples, the crowbar protection device  400  may be configured to protect a circuit against standard transient waveforms. However, as will be appreciated, the crowbar protection device  400  may also be subjected to other transients, such as, electrostatic discharge (ESD). The high static voltages of these other transients can rupture the oxide on the crowbar protection device  400 . In some examples, protection for such transients may be provided by setting the doping level in region  424  such that the depletion layer hits the diffusion area  412  at voltages lower than that which causes oxide damage but higher than the breakdown voltage. 
         [0032]      FIG. 5A  illustrates a block diagram of another example crowbar protection device  500 , arranged in accordance with various embodiments of the present disclosure. In some examples, the crowbar device  500  may be implemented in the circuit protection component  300  as the crowbar device  302 . As depicted, the crowbar device  500  is fabricated on a substrate  510 , such as, for example, silicon. With some embodiments, the substrate  510  is a P+ type substrate. An epitaxial layer  512  has been fabricated (e.g., grown, or the like) onto the substrate  510 . In some embodiments, the epitaxial layer  512  is an N-type epitaxial layer. 
         [0033]    A base  514  and an emitter  516  are additionally depicted. The emitter  516  is depicted having portions  516 A- 516 D. In some examples, the base  514  may be a P-type base and the emitter  516  may be an N+ type emitter. The base  514  and emitter  516  may be formed into the epitaxial layer  512  using conventional semiconductor manufacturing techniques, which will be apparent to those of ordinary skill in the art. With some examples, the substrate  510 , the epitaxial layer  512 , the base  514 , and the emitter  516  form an NPNP type semiconductor devices. With some examples, the emitter  516  overlaps the edge of the base  514 , to form a low voltage breakdown region. Additionally, in some examples, the crowbar device  500  may additionally have oxide layers and metal layers formed on the top and/or bottom of the device to form an anode and cathode for connecting the device to a circuit (e.g., the circuit protection component  300 , or the like.) 
         [0034]    With some embodiments, the epitaxial layer  512  may be doped between the substrate  510  and the base  514  (e.g., as highlighted by region  522 .) The doping characteristics of region  522  may be selected to give the crowbar device  500  desired switching characteristics. In general, the concentration of dopants near the emitter  516 A to base  514  junction (e.g., highlighted by region  524 ) may be greater than that of the region  522 . It is to be appreciated, that the doping described above with respect to region  522  may be selected to give the region  524  a higher breakdown voltage than the region  522 . Furthermore, it is to be appreciated, however, that other techniques to increase the breakdown voltage of region  522  may be implemented without departing from the spirit and scope of the disclosure. 
         [0035]    During operation, when a voltage transient appears across the crowbar device  500 , current flows along a trigger path  520  (shown as dotted line.) Current initially flows through the emitter  516 A and through the base  514  and under emitter  516 B and out through region  526 , where a hole in the emitter  516  is formed. In some examples, the emitter  516  may be formed having a number of holes in the region  526 . However, a single hole is shown in  FIG. 5  for clarity. The breakdown voltage (also referred to as trigger voltage) may be selected to suit desired device characteristics, and may, in some examples, be in the range of 8-50 Volts. When the breakdown voltage of region  524  is exceeded, current begins to flow along path  528 . In some examples, the concentration of dopants in region  522  may be greater than that of other regions within the epitaxial layer  512 . Furthermore, the concentration of dopants in region  522  may be selected to facilitate a uniform conduction of current along path  528 . 
         [0036]      FIG. 5B  illustrates a block diagram of another an example crowbar protection device  500 ′. As can be seen, the device  500 ′ is similar to the device  500 . More particularly, the device  500 ′ includes the substrate  510 , the epitaxial layer  512 , and the base  514  as described above with respect to  FIG. 5A  and the device  500 . The device  500 ′ includes an emitter  516 ′. As depicted, the emitter  516 ′ includes portions  516 A′ and  516 B′ formed above the base  514 . The emitter  516 ′ further includes a hole in region  526 , similar to that described above with respect to  FIG. 5A . 
         [0037]    During operation, when a voltage transient appears across the crowbar device  500 ′, current flows along trigger path  520 ′ (shown as dotted line.) As depicted, current initially flows through the epitaxial layer  512  to the junction between the base  514  and the epitaxial layer  512  (e.g., region  524 ), and then through the base  514  under emitter  516 A′ and out through region  526 . In some examples, the emitter  516  may be formed having a number of holes in the region  526 . However, a single hole is shown in  FIG. 5  for clarity. As described above with respect to the device  500  and  FIG. 5A , when the breakdown voltage of region  524  is exceeded, current begins to flow along path  528 . In some examples, the concentration of dopants in region  522  may be greater than that of other regions within the epitaxial layer  512 . Furthermore, the concentration of dopants in region  522  may be selected to facilitate a uniform conduction of current along path  528 . 
         [0038]      FIG. 5C  illustrates a block diagram of another an example crowbar protection device  500 ″. As can be seen, the device  500 ″ is similar to the device  500 ′ described above. More particularly, the device  500 ″ includes the substrate  510 , the epitaxial layer  512 , the base  514  and the emitter  516 ′ as described above with respect to  FIG. 5B . In some examples, the diffusion region  522 ′ of the epitaxial layer  512  below the base may have multiple diffusion regions. For example, the device  500 ″ is shown having diffusion regions  522 A″- 522 D″. The diffusion level of the regions  522 A″- 522 D″ may be selected to provide the device  500 ″ with the desired breakdown voltage between the base  514  and region  522 ″ such that the breakdown voltage of this region is higher than that of the breakdown voltage defined by the junctions between the epitaxial layer  512  and the corner of the base  514  (e.g., region  524 ). The operation of the device  500 ″ is similar to that as described above with respect to the device  500 ′. More particularly, the initial current path  520 ′ is shown flowing through the region  524 . After voltage breakdown, the current will flow though the path  528 . 
         [0039]      FIG. 5D  illustrates a block diagram of another an example crowbar protection device  500 ′″. As depicted, the crowbar device  500 ′″ includes the substrate  510 , the epitaxial layer  512 , the base  514  and the emitter portions  516 A and  516 B. In some examples, the substrate  510  and the base  514  may be formed from P-type (e.g., P or P+) materials. In some examples, the epitaxial layer  512  and the emitter  516  may be formed from N-type (e.g., N or N+) materials. Additionally, although not shown, the epitaxial layer  514  may be doped (e.g., as at region  522  shown in  FIGS. 5A-5C .) 
         [0040]    NPN and PNP junctions that are formed between the layers  510 ,  512 ,  514 , and  516  of the crowbar protection device  500 ′″ are shown. As will be appreciated, the illustrated NPN and PNP junctions are also present (although not illustrated for clarity) in the crowbar protection devices  500 ,  500 ′, and  500 ″. As depicted, NPN junctions  530 A and  530 B are shown being formed from the emitter  516 A,  516 B, the base  514 , and the epitaxial layer  512 . Additionally, PNP junctions  540 A and  540 B are shown being formed from the base  514 , the epitaxial layer  512  and the substrate  510 . As described above with respect to  FIGS. 5A-5B , when the voltage drop across current path  520  exceeds the breakdown voltage (e.g., 0.7 Volts), the NPN junctions  530 A,  530 B and the PNP junctions  540 A,  540 B being to regenerate, which causes the crowbar protection device  500 ′″ to fully turn on and conduct along the conduction path  528 . 
         [0041]      FIG. 6  illustrates a block diagram of an example voltage transient waveform  610  (solid line) and corresponding example voltage response waveform  620  (dashed line) of a crowbar device, arranged in accordance with various embodiments of the present disclosure. In some examples, either of the crowbar devices  400  and/or  500  may have a voltage response substantially similar to that depicted in  FIG. 6 . As depicted, the block diagram defines the transient waveform  610  and voltage response waveform  620  by voltage magnitude, represented on the y-axis  602  and a voltage pulse duration, represented on the x-axis  604 . The voltage transient waveform  610  quickly increases in magnitude until the trigger point  612  (e.g., the voltage breakdown point of the crowbar device) is reached. As detailed above, due to the fact the crowbar device present a high resistance area to the current path, the voltage across the crowbar device will also increases in magnitude to the trigger point, similar to the transient response. As the crowbar device breaks down and enters the conducting state, the voltage across the crowbar falls to a small, steady state level  622  in time period  630 . In some examples, the time period  630  may be 0.5 microseconds, or less. More specifically, various crowbar devices according to the present disclosure, such as, the crowbar devices  400  and  500 , may have a voltage response similar to the voltage response waveform  620  with a corresponding time period  630  that is less than 0.5 microseconds. Conversely, conventional crowbar devices have voltage responses with substantially greater time periods, such as, for example, 1 microsecond or greater. As detailed above, voltage responses of a microsecond or greater may be insufficient to adequately protect the data line  120  from damage. 
         [0042]      FIG. 7  illustrates a block diagram of an example doping profile of a crowbar device having a corresponding electric field distribution  700  (dotted line). The magnitude of the electric field is shown on the y-axis  702 . As depicted, the crowbar device has four layers,  710 ,  720 ,  730 , and  740 . In some examples, the crowbar device may be an NPNP crowbar device, such as, the crowbar devices  400  and  500  described above. Furthermore, the layers  710  and  730  may be N and N+ type layers respectively, while the layers  720  and  740  may be P and P+ type layers respectively. As depicted, the epitaxial doping is uniform between layers  720  and  740 . The doping concentration and width may be selected such that region between layers  720  and  740  (e.g., regions  424  and  522  depicted above) is fully depleted at the required trigger voltage. As an example, the resistivity the region between layers  720  and  740  may be approximately 1.0 Ohm-centimeter and have a thickness of approximately 2 microns to have a trigger voltage of about 20 Volts. More specifically, the region may be fully depleted when the applied voltage reaches about 20 Volts. As depicted, once the region is fully depleted, the electric field at the junction between layers  730  and  740  is heavily forward biased and the crowbar device may rapidly being to conduct. 
         [0043]    As will be appreciated by those of ordinary skill in the art, the switching speed of a transistor may be inversely proportional to the square of the transistor&#39;s base width. In some embodiments, this width (e.g., the width of the substrate  410  or the width of the epitaxial layer  512 ) may be between 50 and 100 microns. Thus, the switching speed may be substantially reduced as compared to conventional crowbar devices, making the crowbar devices now suitable for transient protection and particularly to transient protection of VDSL devices. Additionally, as will be appreciated, the reverse bias used to moderate the capacitance variation of the steering diode bridge (e.g.,  310 ) will exist across the crowbar device. This will effectively reduce the base width of the transistor, further improving the switching speed. 
         [0044]    If a requirement is to maximize the switching speed of the crowbar device, the doping profile may be adjusted so that the main volume of the region (e.g., the region  424  or  522 ) is depleted at low bias voltages to give the narrowest base width over the full range of bias conditions. This is more fully illustrated in  FIG. 8 , which depicts a block diagram of an example doping profile of a crowbar device having a corresponding electric field distribution  800  (dotted line). The magnitude of the electric field is shown on the y-axis  802 . As depicted, the crowbar device has five layers,  810 ,  820 ,  830 ,  840 , and  850 . In some examples, the crowbar device may be an NPNP crowbar device, such as, the crowbar devices  400  and  500  described above. Furthermore, the layers  810 ,  830 , and  840  may be N+, N, and N+ type layers respectively, while the layers  820  and  840  may be P and P+ type layers respectively. 
         [0045]    As depicted, the layer  830  is very lightly doped, so that it will be depleted at low bias voltages by the adjacent adjoining layer  840 , thus preventing the layer  850  from being depleted. As the corresponding electric field distribution  800  is more rectangular, a narrower region (e.g., the regions  424  or  522 ) may be required to provide for a desired trigger voltage, which may in turn further enhance the switching speed as well as provide lower capacitance. 
         [0046]      FIG. 9  illustrates a block diagram of an example doping profile of a crowbar device having a corresponding electric field distribution  900  (dotted line). The magnitude of the electric field is shown on the y-axis  902 . As depicted, the crowbar device has five layers,  910 ,  920 ,  930 ,  940 , and  950 . In some examples, the crowbar device may be an NPNP crowbar device, such as, the crowbar devices  400  and  500  described above. Furthermore the layers  910 ,  930 , and  940  may be N+, N+, and N type layers respectively, while the layers  820  and  840  may be P and P+ type layers respectively. 
         [0047]    As depicted, a higher doping of the layer  930  near the layer  920  is evident by the electric field distribution  900 . More particularly, the depletion layer is constrained in the region of the junction between layers  920  and  930  until the layer  930  is fully depleted. Once the layer  930  is fully depleted, the lightly doped region rapidly depletes and junction between layers  940  and  950  becomes heavily forward biased. An advantage of using this doping profile is that the transistor gain may be keep relatively low and constant. This may allow problems like excessive leakage current at the expense of switching speed to be negated.