Abstract:
A material made by arranging layers of gallium-arsenide-antimonide (GaAs x Sb 1−x , 0.0≦x≦1.0) and/or indium-gallium-arsenic-nitride (In y Ga 1−y As z N 1−z , 0.0≦y, z≦1.0) in a specific order is used to form the transistor base of a heterojunction bipolar transistor. By controlling the compositions of the materials indium-gallium-arsenic-nitride and gallium-arsenide-antimonide, and by changing the thickness and order of the layers, the new material would possess a specific energy gap, which in turn determines the base-emitter turn-on voltage of the heterojunction bipolar transistor.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the heterojunction bipolar transistors, and in particular to a structure of the heterojunction bipolar transistors allowing a better control of the base-emitter turn-on voltage.  
         [0003]     2. The Prior Arts  
         [0004]     The operation principle of the heterojunction bipolar transistor (HBT) lies in that the base-emitter turn-on voltage (Vbe) is correlated to the energy gap of the material forming the transistor base of the HBT. That is, the larger the energy gap is, the higher the base-emitter turn-on voltage gets. Therefore, prior arts generally achieve a specific base-emitter turn-on voltage by selecting a base material with a specific energy gap.  
         [0005]      FIG. 1  shows a structure of a typical HBT according to a prior art. As shown in  FIG. 1 , the epitaxy structure  1  of the HBT contains a subcollector layer  11 , a collector layer  12 , a base layer  13 , an emitter layer  14 , and an emitter cap layer  15 , sequentially stacked in this order on a gallium-arsenide substrate  10 . The material composition of the base layer  13  is the key factor affecting the base-emitter turn-on voltage of the HBT. The base layer  13  is usually formed using materials such as gallium-arsenide (GaAs), indium-gallium-arsenic-nitride (In x Ga 1−x As y N 1−y ), or gallium-arsenide-antimonide (GaAs x Sb 1−x ). Under a specific doping concentration, altering the x and y values of the above molecular formulas can control the energy gap of the transistor base. A specific base-emitter turn-on voltage is thereby achieved.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a new structure for HBTs. According to the present invention, the transistor base of a HBT contains multiple layers of gallium-arsenide-antimonide (GaAs x Sb 1−x , 0.0≦x≦1.0) and/or indium-gallium-arsenic-nitride (In y Ga 1−y As z N 1−z , 0.0≦y, z≦1.0) arranged in a specific order.  
         [0007]     The energy gap of the HBT base according to the present invention is determined, on one hand, by the material composition (namely, the x, y, z values) of the multiple layers of gallium-arsenide-antimonide (GaAs x Sb 1−x , 0.0≦x≦1.0) and/or indium-gallium-arsenic-nitride (In y Ga 1−y As z N 1−z , 0.0≦y, z≦1.0) forming the HBT base. On the other hand, the energy gap of the HBT base can be further controlled by varying the thickness and arranging the order of the multiple layers of gallium-arsenide-antimonide (GaAs x Sb 1−x , 0.0≦x≦1.0) and/or indium-gallium-arsenic-nitride (In y Ga 1−y As z N 1−z , 0.0≦y, z≦1.0) forming the HBT base. This allows the manufacturer another dimension of control over the energy gap of the transistor base, which is directly related to the base-emitter turn-on voltage of the HBT.  
         [0008]     The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  shows a structure of a typical HBT according to a prior art.  
         [0010]      FIG. 2  shows a structure of a HBT according to a first embodiment of the present invention.  
         [0011]      FIG. 3  shows a base layer structure of the HBT according to the first embodiment of the present invention.  
         [0012]     FIGS.  4 ( a ) through  4 ( l ) show various variations of the base layer structure of the HBT according to the first embodiment of the present invention.  
         [0013]     FIGS.  5 ( a ) through  5 ( r ) show various variations of the base layer structure of a HBT according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     With reference to the drawings and in particular to  FIG. 2 , which shows the structure of a HBT according to a first embodiment of the present invention. As shown in  FIG. 2 , the epitaxy structure  2  of the HBT contains a subcollector layer  21 , a collector layer  22 , a base layer  23 , an emitter layer  24 , and an emitter cap layer  25 , sequentially stacked in this order from bottom to top on a gallium-arsenide substrate  20 .  
         [0015]     The base layer  23  further contains at least an intermediate layer  230 . The intermediate layer  230  includes a first base layer  230   a  and a second base layer  230   b  stacked upon the first base layer  230   a  in the direction toward the emitter layer. The base layer  23  can have multiple intermediate layers  230 , sequentially stacked on the collector layer  22 . On the topmost intermediate layer  230  and beneath the emitter layer  24 , the base layer  23  can further contain an optional first base layer  230   a  as shown in  FIG. 2 .  
         [0016]     In the following explanation to the various variations of the embodiments of the present invention, only the base layer  23  structure is depicted in the rest of the accompany drawings, as illustrated in  FIG. 3  which shows the same base layer structure of the HBT of  FIG. 2 .  
         [0017]     The first base layer  230   a  can be made of gallium-arsenide-antimonide (GaAs x Sb 1−x , 0.0≦x≦1.0) or indium-gallium-arsenic-nitride (In y Ga 1−y As z N 1−z , 0.0≦y, z≦1.0). The first base layer  230   a  can also be made of gallium-arsenide (GaAs) when x=1.0 in the molecular formula GaAs x Sb 1−x , or when y=0.0 and z=1.0 in the molecular formula In y Ga 1−y As z N 1−z . The first base layer  230   a  can have a thickness between 1-300 Å. On the other hand, the second base layer  230   b  can also be made of gallium-arsenide-antimonide (GaAs p Sb 1−p , 0.0≦p≦1.0) or indium-gallium-arsenic-nitride (In q Ga 1−q As r N 1−r , 0.0≦q, r≦1.0). The second base layer  230   b  can also be made of gallium-arsenide (GaAs) when p=1.0 in the molecular formula GaAs p Sb 1−p  or when q=0.0 and r=1.0 in the molecular formula In q Ga 1−q As r N 1−r . Please note that, if the first and second base layers  230   a  and  230   b  are made of the same type of material such as GaAs x Sb 1−x  and GaAs p Sb 1−p , their material composition must be different (that is, x≠p in the previous molecular formulas). The second base layer  230   b  can also have a thickness between 1-300 Å. In the following description and in the accompany drawings, gallium-arsenide-antimonide (GaAs x Sb 1−x , 0.0≦x≦1.0) of a specific composition is referred to as material A, gallium-arsenide-antimonide (GaAs p Sb 1−p , 0.0≦p≦1.0) of another specific composition different from that of material A is referred to as material B, and indium-gallium-arsenic-nitride (In m Ga 1−m As n N 1−n , 0.0≦m, n≦1.0) is referred to as material C.  
         [0018]     By controlling the thickness of each of the base layers  230   a  and  230   b,  the composition of materials A, B, and C, the number of intermediate layers  230  interposed between the collector layer  22  and the emitter layer  24 , the base layer  23  can have a specific base-emitter turn-on voltage. Besides, the choice of materials for the first and second base layers  230   a  and  230   b  (thereby establishing a specific interleaving arrangement of materials A, B, and C) would also affect the HBT&#39;s base-emitter turn-on voltage.  
         [0019]     FIGS.  4 ( a ) through  4 ( l ) show various variations of the base layer structure according to the first embodiment of the present invention. As shown in FIGS.  4 ( a ) and  4 ( c ), the first base layer  230   a  is made of the material A and the second base layer  230   b  is made of the material B. The base layer depicted in structure  FIG. 4 ( c ) contains at least an intermediate layer  230  and an additional first base layer  230   a  (of material A) right next to the emitter layer  24 .  FIG. 4 ( c ) has a structure identical to that of  FIG. 4 ( a ) except that there is no additional first base layer  230   a  (of material A) in  FIG. 4 ( c ). FIGS.  4 ( b ) and  4 ( d ) are exactly like FIGS.  4 ( a ) and  4 ( c ) except that, in FIGS.  4 ( b ) and  4 ( d ), the first base layer  230   a  is made of the material B and the second base layer  230   b  is made of the material A. FIGS.  4 ( e ) through  4 ( h ) are exactly like FIGS.  4 ( a ) through  4 ( d ) except that the former use the materials A and C instead of the materials A and B. Similarly, FIGS.  4 ( i ) through  4 ( l ) are exactly like FIGS.  4 ( a ) through  4 ( d ) except that the former use the materials B and C instead of the materials A and B.  
         [0020]     In all the afore-mentioned structures depicted in FIGS.  4 ( a ) through  4 ( l ), if required, a spacer layer (not shown in FIGS.  4 ( a ) through  4 ( l )) can be optionally interposed between any pair of immediately adjacent first and second base layers  230   a  and  230   b,  regardless of whether the adjacent first and second base layers  230   a  and  230   b  are within the same intermediate layer or not. The spacer layer is made of gallium-arsenide-antimonide (GaAs a Sb 1−a , 0.0≦a≦1.0) or indium-gallium-arsenic-nitride (In b Ga 1−b As c N 1−c , 0.0≦b, c≦1.0) having a graded composition that is different from the materials used to make the first and second base layers  230   a  and  230   b.  Specifically speaking, the a, b, and c parameters in the molecular formulas of gallium-arsenide-antimonide (GaAs a Sb 1−a , 0.0≦a≦1.0) or indium-gallium-arsenic-nitride (In b Ga 1−b As c N 1−c , 0.0≦b, c≦1.0) forming the spacer layer changes gradually from low to high or from high to low monotonously along the direction from the collector layer  22  to the emitter layer  24 . For example, a spacer layer made of GaAs a Sb 1−a  having a thickness of 30 Å is interposed between a first base layer  230   a  made of GaAs and a second base layer  230   b  made of GaAs 0.9 Sb 0.1 . Within the 30 Å thickness, the spacer layer has a parameter in its composition GaAs a Sb 1−a  gradually varies from 1.0 to 0.9.  
         [0021]     A similar but different approach is that the spacer layer further contains multiple sub-spacer layers. Each of the sub-spacer layers is made of gallium-arsenide-antimonide (GaAs a Sb 1−a , 0.0≦a≦1.0) or indium-gallium-arsenic-nitride (In b Ga 1−b As c N 1−c , 0.0≦b, c≦1.0) with a specific a, b, and c values that are different from the materials used to make the adjacent sub-spacer layers, the first and second base layers  230   a  and  230   b.  For example, three sub-spacer layers are interposed between a first base layer  230   a  made of GaAs and a second base layer  230   b  made of GaAs 0.9 Sb 0.1  within an intermediate layer  23 . The three sub-spacer layers are made of GaAs 0.97 Sb 0.03  (a=0.97), GaAs 0.95 Sb 0.05  (a=0.95), and GaAs 0.92 Sb 0.08  (a=0.92) and have a thickness of 40 Å, 35 Å, and 30 Å respectively. Each sub-space layer can have a thickness between 1-300 Å.  
         [0022]     FIGS.  5 ( a ) through  5 ( r ) show various variations of the base layer structure according to the second embodiment of the present invention. Using  FIG. 5 ( a ) as an example, the base layer  33  between the collector layer  32  and the emitter layer  34  contains at least an intermediate layer  330 . The base layer  33  further contains at least an intermediate layer  330 . The intermediate layer  330  consists of a first base layer  330   a,  a second base layer  330   c,  and a third based layer  330   b  interposed between the first and second base layers  330   a  and  330   c.  The three base layers  330   a,    330   b,  and  330   c  are stacked in the direction toward the emitter layer  34 . The base layer  33  can have multiple intermediate layers  330 , sequentially stacked on the collector layer  32 . On the topmost intermediate layer  330  and beneath the emitter layer  34 , the base layer  33  can further contain an optional first base layer  330   a  or third base layer  330   b.    
         [0023]     All three base layers  330   a,    330   b,  and  330   c  can have a thickness between 1-300 Å. The first base layer  330   a  can be made of gallium-arsenide-antimonide (GaAs x Sb 1−x , 0.0≦x≦1.0) or indium-gallium-arsenic-nitride (In y Ga 1−y As z N 1−z , 0.0≦y, z≦1.0). The first base layer  330   a  can also be made of gallium-arsenide (GaAs) when x=1.0 in the molecular formula GaAs x Sb 1−x  or when y=0.0 and z=1.0 in the molecular formula In y Ga 1−y As z N 1−z . The second base layer  330   c  can also be made of gallium-arsenide-antimonide (GaAs p Sb 1−p , 0.0≦p≦1.0) or indium-gallium-arsenic-nitride (In q Ga 1−q As r N 1−r , 0.0≦q, r≦1.0). The second base layer  330   c  can also be made of gallium-arsenide (GaAs) when p=1.0 in the molecular formula GaAs p Sb 1−p  or when q=0.0 and r=1.0 in the molecular formula In q Ga 1−q As r N 1−r . The third base layer  330   b  can also be made of gallium-arsenide-antimonide (GaAs t Sb 1−t , 0.0≦t≦1.0) or indium-gallium-arsenic-nitride (In u Ga 1−u As v N 1−v , 0.0≦u, v≦1.0). The third base layer  330   b  can be made of gallium-arsenide (GaAs) when t=1.0 in the molecular formula GaAs t Sb 1−t  or when u=0.0 and v=1.0 in the molecular formula In u Ga 1−u As v N 1−v . Please note that, if all three base layers  330   a,    330   b,  and  330   c  are made of the same type of material such as GaAs x Sb 1−x , GaAs p Sb 1−p , and GaAs t Sb 1−t , their material composition must be different (that is, x≠p≠t in the previous molecular formulas).  
         [0024]     Exactly like the first embodiment of the present invention, if required, a spacer layers (not shown in FIGS.  5 ( a ) through  5 ( r )) can be optionally interposed between any pair of immediately adjacent first, second, and third base layers  330   a,    330   b,  and  330   c,  regardless of whether the adjacent first, second, and third base layers  330   a,    330   b,  and  330   c  are within the same intermediate layer or not. Each of the spacer layer can have a thickness between 1-300 Å and can be made of gallium-arsenide-antimonide (GaAs a Sb 1−a , 0.0≦a≦1.0) or indium-gallium-arsenic-nitride (In b Ga 1−b As c N 1−c , 0.0≦b, c≦1.0) that is different from the materials used for the base layers at its sides. If only one spacer layer is used, the spacer layer can have a graded composition as described in the previous embodiment of the present invention. Besides using a graded composition, another approach for the spacer layer is to contain multiple sub-spacer layers. Each of the sub-spacer layers is made of gallium-arsenide-antimonide (GaAs a Sb 1−a , 0.0≦a≦1.0) or indium-gallium-arsenic-nitride (In b Ga 1−b As c N 1−c , 0.0≦b, c≦1.0) with specific a, b, and c values that are different from the materials used to make the adjacent sub-spacer layers, the first, second, and second base layers  330   a,    330   b,  and  330   c.  For example, a spacer layer made of GaAs a Sb 1−a  having a thickness of 30 Å is interposed between a first base layer  330   a  made of GaAs and a third base layer  330   b  made of GaAs 0.9 Sb 0.1 . Within the 30 Å thickness, the spacer layer has the a parameter in its composition GaAs a Sb 1−a  gradually varies from 1.0 to 0.9. For another example, three sub-spacer layers are interposed between a first base layer  330   a  made of GaAs and a third base layer  330   b  made of GaAs 0.9 Sb 0.1 . The three spacer layers are made of GaAs 0.97 Sb 0.03  (a=0.97), GaAs 0.95 Sb 0.05  (a=0.95), and GaAs 0.92 Sb 0.08  (a=0.92) respectively and have a thickness of 40 Å, 35 Å, and 30 Å respectively.  
         [0025]     In FIGS.  5 ( a ) through  5 ( c ), the first, third, and second base layers  330   a,    330   b,    330   c  are made of material A, B, and C respectively. As shown in  FIG. 5 ( a ), an optional fist base layer  330   a  is on the topmost intermediate layer  330  and beneath the emitter layer  34 . On the other hand, as shown in  FIG. 5 ( b ), an optional third base layer  330   b  is on the topmost intermediate layer  330  and beneath the emitter layer  34 . The FIGS.  5 ( d ) through  5 ( r ) are exactly like the FIGS.  5 ( a ) through  5 ( c ) except that different materials are used for the first, third, and second base layers respectively.  
         [0026]     Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art (such as arranging gallium-arsenide-antimonide (GaAs x Sb 1−x , 0.0≦x≦1.0), or indium-gallium-arsenic-nitride (In y Ga 1−y As z N 1−z , 0.0≦y, z≦1.0) in a specific order as the transistor base of a HBT). Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.