Patent Publication Number: US-9899502-B2

Title: Bipolar junction transistor layout structure

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a bipolar junction transistor (hereinafter abbreviated as BJT) layout structure, and more particularly, to an emitter-central BJT layout structure. 
     2. Description of the Prior Art 
     As is well known to those of skill in the art, a BJT device is a three-terminal device that essentially including an emitter, a base and a collector. In normal operation, the emitter-base junction will be forward biased while the collector-base junction reversed biased by externally applied voltages, and the device is driven in a forward active mode. Furthermore, BJT device can be manufactured using complementary metal-oxide-semiconductor (CMOS) process, and therefore plays an important role in band-gap voltage reference circuits. Accordingly, BJT device often serves as switching device and is often used in high-voltage, high-frequency, and/or high-power applications. 
     A typical npn-BJT device includes an n-collector which is an n-well formed in a p-substrate, a p-base which is a p-well formed in the n-well, and an n-emitter which is an n-doped region formed in the p-well. And a typical pnp-BJT device includes electric characteristic complementary to the npn-BJT device. In order to enhance emitter injection efficiency, it has been developed to form the emitter enclosed by the base pick-up and the collector. In this approach, a relatively small base current (that is the input current, I B ) controls a relatively large collector current (that is the output current, I C ). And a ratio of the collector current I C  over the base current I D  is referred to current-gain β or beta-gain β:
 
β≡ I   C   /I   B  
 
     The current-gain β increases in proportion to the collector current I C  and is inversely proportional to the base current I B . That is, the current-gain β is increased if the collector current I C  is increased and/or if the base current I B  is decreased. More important, it is found that BJT performance is improved with current-gain β increase. Furthermore, the current gain β varies as a function of the base area or the collector area. It is found that the current gain β is increased when the base area is reduced or when the collector area is increased. However, such increase is limited because the base area cannot be increased without violating a design rule. 
     In view of the foregoing, it is desirable to provide a BJT layout structure that can efficaciously increase the current gain β without violating the design rule. 
     SUMMARY OF THE INVENTION 
     According to the claimed invention, a BJT layout structure is provided. The BJT layout structure includes a first emitter including a pair of first sides and a pair of second sides, a pair of collectors disposed at the first sides of the first emitter, and a pair of bases disposed at the second sides of the first emitter. The first sides of the first emitter are perpendicular to the second sides of the first emitter. The first emitter is disposed in between the pair of collectors and also in between the pair of bases. 
     According to the claimed invention, another BJT layout structure is provided. The BJT layout structure includes a first emitter extended along a first direction, at least a collector extended along the first direction and immediately adjacent to the first emitter along a second direction, and at least a first base immediately adjacent to the first emitter along the first direction. The second direction is perpendicular to the first direction. 
     According to the present invention, an emitter-central BJT layout structure is provided with the bases and the collectors being disposed at different pair sides of the emitter. In other words, a collector-base arrangement is provided at two adjacent sides of the emitter. Therefore, the base area can be efficaciously reduced and thus the current-gain β is increased. As a result, BJT performance is improved. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are schematic drawings illustrating a BJT layout structure provided by a first preferred embodiment of the present invention. 
         FIGS. 3 and 4  are schematic drawings illustrating a BJT layout structure provided by a second preferred embodiment of the present invention. 
         FIG. 5  is a schematic drawing illustrating a modification to the second preferred embodiment provided by the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIGS. 1 and 2 , which are schematic drawings illustrating a BJT layout structure provided by a first preferred embodiment of the present invention. As shown in  FIG. 1 , the BJT layout structure  100  provided by the preferred embodiment includes a substrate  102 , and a well region  104  such as an n-well region is formed in the substrate  102 . According to embodiments of the present invention, the well region  104  includes a conductivity type that is the same with the base of the BJT to be formed. Accordingly, when the BJT layout structure  100  is a pnp-BJT layout structure, the well region  104  includes the n type. Alternatively, when the BJT layout structure  100  is an npn-BJT layout structure, the well region  104  includes a p type. A first active region  110 E, a pair of second active regions  110 C and a pair of third active regions  110 B are formed on the substrate  102 . As shown in  FIG. 1 , the first active region  110 E is extended along a first direction D 1 , and the second active regions  110 C are also extended along the first direction D 1 . In other words, an extending direction of the first active region  110 E is parallel with an extending direction of the second active regions  110 C. The second active regions  110 C are disposed at two opposite sides of the first active region  110 E along the second direction D 2  while the third active regions  110 B are disposed at another two opposite sides of the first active region  110 E along the first direction D 1 . In accordance with the preferred embodiment, the first direction D 1  is perpendicular to the second direction D 2 . 
     It is noteworthy that fabrication of the conventional BJT can be integrated with fabrication of the conventional planar metal-oxide-semiconductor field-effect transistor (MOSFET). In some embodiments of the present invention, for example but not limited to, the collector of the BJT device is formed in the substrate, and the base of the BJT device is formed in the collector utilizing step(s) for forming a lateral well. And the emitter of the BJT device is formed in the base utilizing step(s) for forming lateral source/drain. However, the conventional BJT device fabricated by the conventional planar MOSFET process flow has poor performance and is thus not suitable for high performance applications. One cause of the poor performance is that the emitter is necessarily much smaller than the base. As such, the conventional BJT device has high emitter series resistance and low current conduction capability. Another cause of the poor performance is that the base-emitter junction of the convention BJT device is not well defined. As such, the conventional BJT device suffers high base leakage current. Therefore, the preferred embodiment provides an approach of fin-based BJT device. According to the approach, the first active region  110 E includes at least a fin structure, the second active regions  110 C respectively include at least a fin structure, and the third active regions  110 B respectively include at least a fin structure. Amounts of the fin structures in the first active region  110 E, the second active regions  110 C and the third active regions  110 B can be obtained by performing any well-known planar-fin conversion method. Furthermore, the fin structures can be formed by any known method and thus those details are omitted herein in the interest of brevity. Additionally, epitaxial layers can be formed on the fin structures in the first active region  110 E, the second active regions  110 C and the third active regions  110 B in order to lower the contact resistance. 
     Please still refer to  FIG. 1 . According to the preferred embodiment, the BJT layout structure  100  further includes a plurality of first gate electrodes  120 E. The first gate electrodes  120 E are extended along the second direction D 2  and arranged along the first direction D 1 . More important, the first gate electrodes  120 E overlap the first active region  110 E as shown in  FIG. 1 . The BJT layout structure  100  further includes a plurality of second gate electrodes  120 C and a plurality of third gate electrodes  120 B. The second gate electrodes  120 C are extended along the second direction D 2  and arranged along the first direction D 1 . And the second gate electrodes  120 C overlap the second active regions  110 C as shown in  FIG. 1 . The third gate electrodes  120 B of the BJT layout structure  100  are extended along the second direction D 2  and arranged along the first direction D 1 . And the third gate electrodes  120 B overlap the third active regions  110 B as shown in  FIG. 1 . Furthermore, the first gate electrodes  120 E are parallel with the third gate electrodes  120 B. The first gate electrodes  120 E, the second gate electrodes  120 C and the third gate electrodes  120 B respectively include a gate dielectric layer (not shown) and a gate conductive layer (not shown). In some embodiments of the present invention, the gate conductive layer includes polysilicon, and the polysilicon is preferred to be doped, and a voltage can be applied thereon to change the characteristics of the resulting BJT. 
     Please refer to  FIGS. 1 and 2 . The first active region  110 E and the first gate electrodes  120 E construct an emitter E (shown in  FIG. 2 ), and the emitter E is extended along the first direction D 1 . The second active regions  110 C and the second gate electrodes  120 C construct collector pick-ups, and are taken as a pair of collectors C (shown in  FIG. 2 ) in accordance with the preferred embodiment. Accordingly, the collectors C are also extended along the first direction D 1 . The third active regions  110 B and third gate electrodes  120 B construct base pick-ups, and are taken as a pair of bases B (shown in  FIG. 2 ) in accordance with the preferred embodiment. However, those skilled in the art would easily realize that the bases B include not only the third active regions  110 B and the third gate electrodes  120 B, which serve as base pick-ups as mentioned above, but also the well region  104 . Furthermore, a plurality of contacts plugs can be provided to construct electrical connections for the emitter E, the collectors C and the bases B. As shown in  FIG. 2 , the BJT layout structure  100  provided by the preferred embodiment includes the emitter E, and the emitter E includes a pair of first sides  130  and a pair of second sides  132 . And the first sides  130  are perpendicular to the second sides  132 . Additionally, a length of the first sides  130  is preferably longer than a length of the second sides  132 , but not limited to this. The BJT layout structure  100  further includes the pair of collectors C disposed at the first sides  130  of the emitter E. Specifically, the emitter E is disposed in between the pair of collectors C. The BJT layout structure  100  further includes the pair of bases B disposed at the second sides  132  of the emitter E. Specifically, the emitter E is disposed in between the pair of bases B. As shown in  FIG. 2 , the well region  104  is disposed under the emitter E and the pair of bases B. 
     More important, the collectors C and the emitter E are immediately adjacent to each other along the second direction D 2 . It is noteworthy that “immediately adjacent” is referred to that no conductive device and/or element, such as gate electrode and/or active region, is disposed in between the collectors C and the emitter E in accordance with the preferred embodiment. In the same concept, the bases B and the emitter E are immediately adjacent to each other along the first direction D 1 . And it is also referred to that no conductive device and/or element, such as gate electrode and/or active region, is disposed in between the bases B and the emitter E in accordance with the preferred embodiment. In other words, the preferred embodiment provides a “collector-base” arrangement at any two adjacent sides of the emitter E. And the collector-base arrangement includes an L shape, as depicted by the dotted line in  FIG. 2 . 
     Please still refer to  FIGS. 1 and 2 . According to the BJT layout structure  100  provided by the preferred embodiment, the emitter E includes the first active region  110 E and the first gate electrodes  120 E overlapped thereto, the collectors C include the second active regions  110 C and the second gate electrodes  120 C overlapped thereto, and the bases B include the third active regions  110 B and the third gate electrodes  120 B overlapped thereto. It is noteworthy that since no conductive device or element is disposed in between the collectors C and the emitter E, a spacing distance S between the emitter E and the collectors C can be reduced as much as possible. And thus current receiving is improved. More important, a width of the well region  104  can be reduced as much as possible as long as design rule is complied. According to the following equivalence (1): 
     
       
         
           
             
               
                 
                   
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     It is concluded that the collector current I C  is in inverse proportion to the width W B  of the base B. Therefore, by reducing the width W B  of the base B, the collector current I C  is increased and thus the current-gain β of the BJT layout structure  100  is enhanced. 
     Please refer to  FIGS. 3 and 4 , which are schematic drawings illustrating a BJT layout structure provided by a second preferred embodiment of the present invention. As shown in  FIG. 3 , the BJT layout structure  200  provided by the preferred embodiment includes a substrate  202 , and a well region  204  such as an n-well region is formed in the substrate  202 . As mentioned above, the well region  204  includes a conductivity type that is the same with the base of the BJT to be formed. Therefore, when the BJT layout structure  200  is a pnp-BJT layout structure, the well region  204  includes the n type. Alternatively, when the BJT layout structure  200  is an npn-BJT layout structure, the well region  204  includes a p type. A plurality of first active regions  210 E, a pair of second active regions  210 C, a pair of third active regions  210 B, and a middle active region  212 B are formed on the substrate  202  as shown in  FIG. 3 . In the preferred embodiment, the BJT layout structure  200  preferably includes two first active regions  210 E, but not limited this. The first active regions  210 E, the second active regions  210 C and the middle active region  212 B are all extended along a first direction D 1 . In other words, an extending direction of the first active regions  210 E is parallel with an extending direction of the second active regions  210 C and an extending direction of the middle active region  212 B. The second active regions  210 C are disposed at two opposite sides of the first active regions  210 E along a second direction D 2 . Specifically, all of the first active regions  210 E are disposed in between the pair of second active regions  210 C as shown in  FIG. 3 . The third active regions  210 B are disposed at another two opposite sides of the first active regions  210 E along the first direction D 1 . Specifically, all of the first active regions  210 E are disposed in between the pair of third active regions  210 B as shown in  FIG. 3 . In accordance with the preferred embodiment, the first direction D 1  is perpendicular to the second direction D 2 . Furthermore, the preferred embodiment further provides the middle active region  212 B disposed in between the two first active regions  210 E. Additionally, the first active regions  210 E respectively include at least a fin structure, the second active regions  210 C respectively include at least a fin structure, the third active regions  210 B respectively include at least a fin structure, and the middle active region  212 B includes at least a fin structure in accordance with the preferred embodiment, but not limited to this. 
     Please refer to  FIG. 3 . According to the preferred embodiment, the BJT layout structure  200  further includes a plurality of first gate electrodes  220 E. The first gate electrodes  220 E are extended along the second direction D 2  and arranged along the first direction D 1 . More important, the first gate electrodes  220 E overlap the two first active regions  210 E as shown in  FIG. 3 . In the same concept, the BJT layout structure  200  further includes a plurality of second gate electrodes  220 C and a plurality of third gate electrodes  220 B. The second gate electrodes  220 C are extended along the second direction D 2  and arranged along the first direction D 1 . And the second gate electrodes  220 C overlap the second active regions  210 C as shown in  FIG. 3 . The third gate electrodes  220 B of the BJT layout structure  200  are extended along the second direction D 2  and arranged along the first direction D 1 . And the third gate electrodes  220 B overlap the third active regions  210 B as shown in  FIG. 3 . Furthermore, the first gate electrodes  220 E are parallel with the third gate electrodes  220 B. More important, the first gate electrodes  220 C cross the middle active region  212 B which is disposed in between the two first active regions  210 E, and thus a portion of each first gate electrode  220 C respectively overlaps the middle active region  212 B as shown in  FIG. 3 . The first gate electrodes  220 E, the second gate electrodes  220 C and the third gate electrodes  220 B respectively include a gate dielectric layer (not shown) and a gate conductive layer (not shown). In some embodiments of the present invention, the gate conductive layer includes polysilicon, and the polysilicon is preferred to be doped, and a voltage can be applied thereon to change the characteristics of the resulting BJT. 
     Please refer to  FIGS. 3 and 4 . The two first active regions  210 E and the first gate electrodes  220 E construct a first emitter E 1  and a second emitter E 2  (shown in  FIG. 4 ), and the first emitter E 1  is parallel with the second emitter E 2 . The second active regions  210 C and the second gate electrodes  220 C construct collector pick-ups, and are taken as a pair of collectors C (shown in  FIG. 4 ) in accordance with the preferred embodiment. The third active regions  210 B and the third gate electrodes  220 B construct base pick-ups, and are taken as a pair of bases B (shown in  FIG. 4 ) in accordance with the preferred embodiment. It is noteworthy that in the preferred embodiment, the middle active region  212 B, which is disposed in between the two first active regions  210 E, and the portion of the first gate electrodes  220 E overlapped thereto construct a middle base pick-up, and is taken as a middle base Bm (shown in  FIG. 4 ). The middle base Bm is disposed in between the first emitter E 1  and the second emitter E 2 . Furthermore, the middle active region  212 B and the pair of third active regions  210 B include the same conductivity type. It is noteworthy that the middle base Bm is electrically connected to the bases B. Consequently, the middle base Bm and the pair of bases B include an H shape, as depicted by the dotted line L 1  shown in  FIG. 4 . However, those skilled in the art would easily realize that the bases B include not only the third active regions  210 B, the third gate electrodes  220 B and the middle base pick-up (that is the middle base Bm), but also the well region  204 . Furthermore, a plurality of contacts plugs can be provided to construct electrical connections for the emitter E, the collectors C and the base B. As shown in  FIG. 4 , the BJT layout structure  200  provided by the preferred embodiment includes the first emitter E 1  and the second emitter E 2 , and the first emitter E 1  and the second emitter E 2  respectively include a pair of first sides  230  and a pair of second sides  232 . And the first sides  230  are perpendicular to the second sides  232 . Additionally, a length of the first sides  230  is preferably longer than a length of the second sides  232 , but not limited to this. The BJT layout structure  200  further includes the pair of collectors C disposed at the first sides  230  of the first emitter E 1  and the second emitter E 2 . Specifically, the first emitter E 1  and the second emitter E 2  are disposed in between the pair of collectors C. The BJT layout structure  200  further includes the pair of bases B disposed at the second sides  232  of the first emitter E 1  and the second emitter E 2 . Specifically, the first emitter E 1  and the second emitter E 2  are disposed in between the pair of bases B. As shown in  FIG. 4 , the well region  204  is disposed under the first emitter E 1 , the second emitter E 2 , the pair of bases B, and the middle base Bm. 
     More important, the collectors C and the first emitter E 1 /the second emitter E 2  are immediately adjacent along the second direction D 2 , respectively. It is noteworthy that “immediately adjacent” is referred to that no conductive device and/or element, such as gate electrode and/or active region, is disposed in between one of the collectors C and the first emitter E 1  and between the other collector C and the second emitter E 2  in accordance with the preferred embodiment. In the same concept, the bases B and the first emitter E 1 /the second emitter E 2  are immediately adjacent along the first direction D 1 . And it is also referred to that no conductive device and/or element, such as gate electrode and/or active region, is disposed in between one of the bases B and the first emitter E 1  and between the other base B and the second emitter E 2  in accordance with the preferred embodiment. In other words, the preferred embodiment provides a “collector-base” arrangement at two adjacent sides of the first emitter E 1 , and a “collector-base” arrangement at two adjacent sides of the second emitter E 2 . And the “collector-base” arrangement includes an L shape, as depicted by the dotted line L 2  in  FIG. 4 . 
     Please still refer to  FIGS. 3 and 4 . According to the BJT layout structure  200  provided by the preferred embodiment, the first emitter E 1  and the second emitter E 2  respectively include the first active region  210 E and the first gate electrodes  220 E overlapped thereto, the collectors C include the second active regions  210 C and the second gate electrodes  220 C overlapped thereto, the bases B include the third active regions  210 B and the third gate electrodes  220 B overlapped thereto, and the middle base Bm includes the middle active region  212 B and the portion of the first gate electrodes  220 E overlapped thereto. It is noteworthy that, since no conductive device or element is disposed in between the collector C and the first emitter E 1  and between the collector C and the second emitter E 2 , a spacing distance S between the first/second emitters E 1 /E 2  and the collectors C can be reduced as much as possible. And thus current receiving is improved. More important, a width of the well region  204  can be reduced as much as possible as long as design rule is complied. That is, a width W B  of the bases B can be reduced as much as possible. As shown in the abovementioned equivalence (1), because the collector current I C  is in inverse proportion to the width W B  of the base B, the collector current I C  is increased by reducing the width W B  of the base B, and thus the current-gain β of the BJT layout structure  200  is enhanced. 
     Please refer to  FIG. 5 , which is a schematic drawing illustrating a modification to the second preferred embodiment provided by the present invention. It is noteworthy that elements the same in the instant modification and the second preferred embodiment are depicted by the same numerals and those details are omitted in the interest of brevity. The difference between the modification and the second preferred embodiment is: The middle base Bm is disposed in between the first emitter E 1  and the second emitter E 2  according to the second embodiment, however the middle base Bm is eliminated according to the modification. Furthermore, the first active regions  210 E of the first emitter E 1  and the first active regions  210 E of the second emitter E 2  are merged as shown in  FIG. 5 . It should be easily realized by those skilled in the art that the first active regions  210 E of the first emitter E 1  and the first active regions  210 E of the second emitter E 2  can be remained separate physically but electrically connected to each other by the gate electrodes  220 E. Consequently, a device having a length twice the length of the abovementioned BJT device is obtained. That is, the modification provides a BJT layout structure  200 ′ including an emitter area two times to the abovementioned BJT layout structure  100 / 200 . Additionally, by eliminating the middle base Bm, the planar-fin conversion is further simplified. 
     Briefly speaking, according to the present invention, an emitter-central BJT layout structure is provided with the bases and the collectors being disposed at different pair sides of the emitter. In other words, a collector-base arrangement is provided at least at two adjacent sides of the emitter. That is, the collector-base arrangement includes an L shape according to the present invention. Therefore, the base area can be efficaciously reduced and thus the current-gain β is increased without violating the design rule. As a result, BJT performance is improved. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.