Abstract:
A ballasting region is placed between the base region and the collector contact of a bipolar junction transistor to relocate a hot spot away from the collector contact of the transistor. Relocating the hot spot away from the collector contact prevents the collector contact from melting during an electrostatic discharge (ESD) pulse.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to BJTs and, more particularly, to a BJT with ESD self protection. 
   2. Description of the Related Art 
   A bipolar junction transistor (BJT) is a well-known element that is utilized in a variety of circuits. BJTs are commonly formed by sandwiching a region of a first conductivity type, known as a base, between two regions of a second conductivity type, known as an emitter and a collector. 
     FIG. 1  shows a cross-sectional view that illustrates a prior-art BJT  100 . As shown in  FIG. 1 , BJT  100  includes a p− substrate  110 , and a buried layer  112  that is formed in p− substrate  110 . Buried layer  112  includes an inner n+ layer  112 A and an outer diffused n layer  112 B that extends out from inner n+ layer  112 A. 
   Further, BJT  100  includes an n− epitaxial layer  114  that is formed on buried layer  112 . BJT  100  is a high-voltage device which, when compared to a conventional low-voltage bipolar device, has a substantially thicker epitaxial layer. For example, n− epitaxial layer  114  can be approximately 15–17 um thick. 
   In addition, BJT  100  includes a p− base region  120  that is formed in n− epitaxial layer  114 , an n+ emitter region  122  that is formed in p− base region  120 , and a sinker down region  124  that is formed in n− epitaxial layer  114 . Sinker down region  124  includes an inner n+ region  124 A and an outer diffused n region  124 B that surrounds inner n+ region  124 A. 
   Sinker down region  124 , along with n-type buried layer  112  and n− epitaxial layer  114 , function as the collector. (N+ sinker down region  124 A can alternately extend down to contact n+ buried layer  112 A, be combined with an n+ sinker up region that extends up from n+ buried layer  112 A, or be implemented in any conventional manner.) 
   As further shown in  FIG. 1 , BJT  100  also includes a layer of isolation material  130  that is formed on the surface of n− epitaxial layer  114 , and a metal base contact  132  that is formed through isolation layer  130  to make an electrical connection with p− base region  120 . BJT  100  additionally includes a metal emitter contact  134  that is formed through isolation layer  130  to make an electrical connection with n+ emitter region  122 , and a metal collector contact  136  that is formed through isolation layer  130  to make an electrical connection with sinker down region  124 . Further, p− base region  120  is separated from collector contact  136  by a separation distance SD. 
   For normal operation, n+ emitter region  122  is commonly connected to ground, while n+ collector region  124  is connected to a positive voltage. Under these biasing conditions, BJT  100  is turned off when ground is placed on p− base region  120 . In this case, the voltage on p− base region  120  is equal to the voltage on n+ emitter region  122 , and less than the voltage on n+ collector region  124 , thereby reverse biasing the base-collector junction. 
   On the other hand, when the voltage on p− base region  120  rises to approximately 0.7V, BJT  100  turns on. In this case, the voltage on p− base region  120  forward biases the base-emitter junction. When the base-emitter junction becomes forward biased, p− base region  120  begins injecting holes into emitter region  122 , while n+ emitter region  122  begins injecting electrons into base region  120 . The electrons injected into p− base region  120  diffuse through the lightly-doped base region  120 , and are swept into n− epitaxial layer  114  by the electric field across the reverse-biased, base-collector junction. 
   Once swept into n− epitaxial layer  114 , the electrons follow the lowest resistance path to n+ collector region  124 . In this example, the lowest resistance path is illustrated by a current path P that moves vertically down, horizontally through n+ buried layer  112 A, and vertically up to sinker down region  124 . Normal operation continues as long as holes can continue to be supplied to p− base region  120  (for injection into n+ emitter region  122 ) via an external base current that flows into base region  120 . 
   In addition to normal operation, BJT  100  can also be utilized to provide the pads of a semiconductor device with electrostatic discharge (ESD) protection from voltage spikes. For example, n+ collector region  124  can be connected to an I/O pad to protect the I/O pad from voltage spikes. 
   During an ESD event, the voltage on n+ sinker down region  124  rises quickly, which causes the voltage on n-type buried layer  112  and n− epitaxial layer  114  to rise with respect to the voltage on p− base region  120 , thereby reverse biasing the pn junction between n− epitaxial layer  114  and p− base region  120 . 
   When the rising voltage on n− epitaxial layer  114  (the collector) exceeds a breakdown voltage of the pn junction, avalanche multiplication causes large numbers of holes to be injected into p− base region  120 , and large numbers of electrons to be injected into n− epitaxial layer  114 . Ideally, the electrons injected into n− epitaxial layer  114  follow the same low-resistance path P to sinker down region  124  as described above. 
   On the other hand, the holes injected into p− base region  120  flow out of p− base region  120  into a circuit which causes the potential on p− base region  120  to rise and forward bias the base-emitter junction. For example, when a BJT is utilized as an ESD protection device, the base of the BJT can be connected to ground via a resistor. In this case, when the hole current from p− base region  120  flows to ground via the resistor, the resistor causes the voltage on p− base region  120  to rise and forward bias the base-emitter junction. 
   When the base-emitter junction becomes forward biased, p− base region  120  begins injecting holes into n+ emitter region  122 , while n+ emitter  122  begins injecting electrons into p− base region  120 . The electrons injected into p− base region  120  from n+ emitter region  122  diffuse to the base-collector junction, where the electrons are swept into n− epitaxial layer  114  by the electric field across the reverse-biased junction. The electrons from n+ emitter region  122  join the avalanche-generated electrons flowing to sinker down region  124 , thereby significantly increasing the current sunk by BJT  100 . 
   As noted above, the electrons injected into n− epitaxial layer  114  ideally follow the low-resistance current path P to sinker down region  124 . However, due to the high electric field that is present during an ESD event, and the large number of electrons that are injected into the lightly-doped epitaxial layer  114 , the electrons can flow laterally just below the surface of epitaxial layer  114  from p− base  120  to sinker down region  124 . 
   As illustrated in  FIG. 1 , one problem with a significant lateral electron flow at the surface of n− epitaxial layer  114  is that the electron flow causes the lattice temperature to rise significantly, and can cause a localized hot spot  140  to develop next to the interface between sinker down region  124  and collector contact  136 . Hot spot  140  can cause collector contact  136  to melt which, in turn, can lead to the electrical inoperability of BJT  100 . Thus, there is a need for a BJT which can be used as an ESD protection device without being melted by an ESD event. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view illustrating a prior-art bipolar junction transistor (BJT)  100 . 
       FIG. 2  is a plan view illustrating an example of a bipolar junction transistor (BJT)  200  in accordance with the present invention. 
       FIG. 3A  is a cross-sectional view illustrating an example of an embodiment  300  of BJT  200  in accordance with the present invention. 
       FIG. 3B  is a cross-sectional view illustrating an example of a method of forming embodiment  300  of BJT  200  in accordance with the present invention. 
       FIG. 4A  is a cross-sectional view illustrating an example of an embodiment  400  of BJT  200  in accordance with the present invention. 
       FIG. 4B  is a cross-sectional view illustrating an example of a method of forming embodiment  400  of BJT  200  in accordance with the present invention. 
       FIG. 5  is a plan view illustrating an example of an embodiment  500  of BJT  200  in accordance with the present invention. 
       FIG. 6  is a plan view illustrating an example of an embodiment  600  of BJT  200  in accordance with the present invention. 
       FIG. 7  is a schematic diagram illustrating an example of a circuit  700  in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  shows a plan view that illustrates an example of a bipolar junction transistor (BJT)  200  in accordance with the present invention. As described in greater detail below, BJT  200  relocates a hot spot away from the collector contact, thereby allowing BJT  200  to be used in electro-static discharge (ESD) applications without melting the collector contact. 
   BJT  200  is similar to BJT  100  and, as a result, utilizes the same reference numerals to designate the structures which are common to both BJTs. As shown in  FIG. 2 , BJT  200  differs from BJT  100  in that BJT  200  includes a ballasting region  210  that contacts the surface of n− epitaxial layer  114  and lies between p− base region  120  and sinker down region  124 . In operation, ballasting region  210  relocates the hot spot away from collector contact  136 , thereby allowing BJT  200  to be used in ESD applications without destroying the collector contact. 
     FIG. 3A  shows a cross-sectional view that illustrates an example of an embodiment  300  of BJT  200  in accordance with the present invention. As shown in  FIG. 3A , ballasting region  210  of embodiment  300  includes an n+ protection region  310  that contacts the surface of n− epitaxial layer  114 . 
   In the  FIG. 3A  example, n+ protection region  310  has a depth, measured along a line normal to the surface of n− epitaxial layer  114 , which is significantly shallower than p− base region  120  and n+ sinker down region  124 A. In addition, n+ protection region  310  extends laterally from n+ sinker down region  124 A towards p− base region  120 , but remains spaced apart from p− base region  120 . Further, to accommodate n+ protection region  310 , a separation distance SD between p− base region  120  and collector contact  136  is increased. 
   In operation, in response to an ESD event (e.g., in response to 100 nS of a 2 kV human body model (HBM) stress), embodiment  300  of BJT  200  operates the same as BJT  100  except that n+ protection region  310  forces the hot spot that results from the lateral current flow away from the collector contact region. 
   As shown in  FIG. 3A , n+ protection region  310  causes a localized hot spot  312  to develop at an end E of n+ protection region  310  that lies closest to p− base region  120 . Although the peak temperature lies at the end, substantially elevated temperatures also extend towards p− base region  120  and n+ sinker down region  124 A. 
   As a result, the distances between the elements must be adjusted to insure that the temperature on the metal contacts is insufficient to melt the contacts. Thus, the use of n+ protection region  310  relocates the hot spot away from collector contact  136 , thereby allowing embodiment  300  of BJT  200  to be used in ESD applications without destroying the collector contact. 
     FIG. 3B  shows a cross-sectional view that illustrates an example of a method of forming embodiment  300  of BJT  200  in accordance with the present invention. As shown in  FIG. 3B , a semiconductor device  350  is conventionally formed to have p− base region  120  formed in n− epitaxial layer  114 . 
   Following this, as further shown in  FIG. 3B , a mask  352  is formed and patterned to expose a portion of p− base  120 , and a portion of n− epitaxial layer  114  that is spaced apart from p− base region  120 . Next, the exposed regions are implanted with an n-type dopant to form n+ emitter region  122  and n+ protection region  310 . Mask  352  is then removed. 
   After this, the method continues with conventional steps. In the  FIG. 3B  example, no additional masking steps are required to form n+protection region  310  because n+ protection region  310  is formed at the same time as n+ emitter region  122 . Further, n-type sinker down region  124  can be formed before or after regions  122  and  310  are formed. Alternately, n+ protection region  310  can have a different depth or dopant concentration by utilizing separate masking and implant steps to form n+ emitter region  122  and n+ protection region  310 . 
     FIG. 4A  shows a cross-sectional view that illustrates an example of an embodiment  400  of BJT  200  in accordance with the present invention. As shown in  FIG. 4A , ballasting region  210  of embodiment  400  includes an electrically-floating p− protection region  410  that contacts the surface of n− epitaxial layer  114 . Further, to accommodate p− protection region  410 , a separation distance SD between p− base region  120  and collector contact  136  is increased. 
   In operation, in response to an ESD event (e.g., in response to 100 nS of a 2 kV HBM stress), embodiment  400  of BJT  200  operates the same as BJT  100  except that p− protection region  410  forces the electron flow vertically down and away from the surface of epitaxial layer  114  substantially along the current path P, thereby eliminating or substantially reducing the lateral surface flow of electrons. 
   As shown in  FIG. 4A , p− protection region  410  causes a localized hot spot  412  to develop at the interface between buried layer  112  and n− epitaxial layer  114 . Thus, the use of p−region  410  relocates the hot spot away from collector contact  136 . As a result, embodiment  400  of BJT  200  can be used in ESD applications without destroying the collector contact. 
     FIG. 4B  shows a cross-sectional view that illustrates an example of a method of forming embodiment  400  of BJT  200  in accordance with the present invention. As shown in  FIG. 4B , the method utilizes a semiconductor device  450  that has been conventionally formed to have an n− epitaxial layer  114 . 
   Following this, as further shown in  FIG. 4B , a mask  452  is formed and patterned to expose spaced-apart portions of n− epitaxial layer  114 . Next, the exposed regions are implanted with a p-type dopant to form p− base region  120  and p− protection region  410 . Mask  452  is then removed. 
   After this, the method continues with conventional masking and implanting steps to form n+ emitter region  122  in p− base region  120  and n-type sinker down region  124  in epitaxial layer  114  so that p− protection region  410  lies between p− base region and n+ sinker down region  124 A. 
   In the  FIG. 4B  example, no additional masking steps are required to form p− protection region  410  because p− protection region  410  is formed at the same time as p− base region  120 . Alternately, p− protection region  410  can have a different depth or dopant concentration by utilizing separate masking and implant steps to form p− base region  120  and p− protection region  410 . 
     FIG. 5  shows a plan view that illustrates an example of an embodiment  500  of BJT  200  in accordance with the present invention. As shown in  FIG. 5 , ballasting region  210  of embodiment  500  includes an n+ sinker down extension  510  that has a finger shape. Further, to accommodate n+ sinker down extension  510 , a separation distance SD between p− base region  120  and collector contact  136  is increased. 
   In operation, in response to an ESD event (e.g., in response to 100 nS of a 2 kV human body model (HBM) stress), embodiment  500  of BJT  200  operates the same as BJT  100  except that n+ sinker down extension  510  forces the hot spot that results from the lateral current flow away from the collector contact region in a manner similar to embodiment  300 . As a result, embodiment  500  of BJT  200  can be used in ESD applications without destroying the collector contact. 
   Embodiment  500  can be formed in the same manner as embodiment  300 , except that mask  352  illustrated in  FIG. 3B  must be modified to have a finger shaped pattern as illustrated by n+ ballasting region  510 . Thus, n+ ballasting region  510  can be formed at the same time that n+ emitter  122  is formed. 
     FIG. 6  shows a plan view that illustrates an example of an embodiment  600  of BJT  200  in accordance with the present invention. As shown in  FIG. 6 , ballasting region  210  of embodiment  600  includes a significantly larger, e.g., 2×, separation distance SD between p− base region  120  and collector contact  136  than would be found in a standard BJT, such as BJT  100 . 
   In operation, in response to an ESD event (e.g., in response to 100 nS of a 2 kV HBM stress), embodiment  600  of BJT  200  operates the same as BJT  100  except that the larger separation distance SD forces the electron flow vertically down and away from the surface of n− epitaxial layer  114  substantially along the current path P, thereby eliminating or substantially reducing the lateral surface flow of electrons. 
   The significantly larger separation distance SD causes a localized hot spot to develop at the interface between buried layer  112  and n− epitaxial layer  114 . Thus, the use of a significantly larger separation distance SD relocates the hot spot away from collector contact  136 . As a result, embodiment  600  of BJT  200  can be used in ESD applications without destroying the collector contact. 
     FIG. 7  shows a schematic diagram that illustrates an example of a circuit  700  in accordance with the present invention. As shown in  FIG. 7 , circuit  700  includes a pad  710 , and an ESD BJT  712  that is connected between pad  710  and ground. In addition, circuit  700  includes a resistor R that is connected between ESD BJT  712  and ground, and a circuit BJT  714  that is connected between pad  710  and ground. 
   ESD BJT  712  can be implemented with embodiments  300 – 600  of BJT  200 , while circuit BJT  714  can be implemented with a conventional BJT, such as BJT  100 , that can be damaged by an ESD strike. In each case, the separation distance SD between p− base region  120  and collector contact  136  of BJT  712  is greater than the separation distance SD of BJT  714 . Further, when embodiment  600  is utilized, the separation distance SD of BJT  712  is substantially greater, e.g., 2×, than the separation distance SD of BJT  714 . In operation, when an ESD event occurs, ESD BJT  712  shunts the voltage strike to ground, thereby protecting circuit BJT  714  from damage. 
   Thus, the present invention provides a BJT that can be utilized as an ESD protection device without melting the collector contact. One of the advantages of the present invention is that ESD BJT  712  can be modeled or simulated in cases where other devices, such as a silicon controlled rectifier (SCR) structures, can not be modeled or simulated. 
   Further, the present invention provides a BJT that can function as both a conventional bipolar device (with greater resistance), and as an ESD protection device. Thus, in the present invention, circuit BJT  714  can optionally be eliminated (if the base of circuit BJT  714  is connected to a circuit which can forward bias the base-emitter junction during an ESD event) because ESD BJT  712  can function as a conventional bipolar device (with greater resistance) during normal circuit operation, and as an ESD protection device should an ESD event occur. As a result, the BJT of the present invention provides ESD self protection. 
   It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. For example, elements of the above embodiments can be combined together. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.