Abstract:
Improved radio frequency gain in a silicon-based bipolar transistor may be provided by adoption of a common-base configuration, preferably together with excess doping of the base to provide extremely low base resistances boosting performance over similar common-emitter designs.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with United States government support awarded by the following agency: NSF 0323717. The United States has certain rights in this invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to high frequency transistor amplifiers and in particular to a silicon transistor and circuit suitable for high gain, high frequency amplification. 
     High frequency electrical amplifiers, such as those useful for radio frequency (RF) and microwave power amplification, frequently use transistors fabricated from gallium arsenide or other column III-V elements instead of silicon. Transistors using III-V semiconductors, however, are generally incompatible with large-scale integrated circuit techniques, such as the complementary metal oxide semiconductor (CMOS) process, used in the fabrication of logic circuitry. For this reason, integrated circuits providing both logic circuitry and high-powered radio frequency amplification are not easily manufactured. 
     Recently, the use of silicon transistors in high frequency amplification has become increasingly practical with smaller line width devices possible with advanced fabrication techniques. Nevertheless, improved power gain at radio frequencies would be desirable. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides improved power gain at radio frequencies, including microwave frequencies using silicon transistors, by using a common-base configuration-based amplifier design. The power gain of silicon transistors is further improved together with a transistor having a doping profile optimized for that amplifier design. The transistor doping varies from the normal high to low doping concentration from emitter to base by significantly increasing the base doping. This doping significantly decreases the base resistance relative to the total emitter resistance (including the emitter resistance and parasitic emitter resistance) substantially improving the power gain over that obtainable with a similar common-emitter design or a common-base design using a conventionally doped transistor. 
     The common-base design further provides improved breakdown voltage over the common-emitter design useful in applications where a high degree of ruggedness against over-voltage (or a large safe operation area) is required. An amplifier cell combining the amplifier of the present invention with a field-effect transistor (FET) front end termed: “FET disciplined bipolar transistors” or FDBT can provide optimized, concurrent high-power and high-frequency operation with improved performance against thermal effects. 
     Specifically then, the present invention provides an amplifier having a silicon bipolar transistor with a predetermined relative doping concentration in the base and emitter regions such that the base resistance r b  is less than or equal to the sum of the emitter resistance r e  and the parasitic emitter resistance r ex . Input terminals provide an input across the emitter and base of the transistor and output terminals provide an output across the collector and base of the transistor so that the transistor may be operated in common-base configuration. 
     It is thus one object of at least one embodiment of the invention to optimize a common-base transistor amplifier through a specially doped transistor providing a very low base resistance. 
     The silicon bipolar transistor may be a silicon bipolar junction transistor or a silicon-germanium heterojunction bipolar transistor. 
     Thus it is another object of at least one embodiment of the invention to provide an amplifier that can be used with a variety of silicon transistor types. 
     The amplifier may include at least one field-effect transistor driving the input terminals, for example, with the drain (source as ground) of the field-effect transistor driving the emitter (base as AC ground) of the silicon bipolar transistor. 
     It is thus another object of at least one embodiment of the invention to provide an amplifier cell that may effectively combine the high breakdown voltage of the common-base transistor with a low breakdown voltage but high frequency operation of the field-effect transistor. 
     It is another object of at least one embodiment of the invention to provide a power cell that eliminates the need for ballast resistors by using multiple FETs to promote the sharing of current among the bipolar transistors. It is another object of at least one embodiment of the invention to provide an amplifier cell with reduced sensitivity to heating by employing field-effect transistors with reduced sensitivity to heating. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of an amplifier cell using a combined field-effect transistor and bipolar junction transistor (FDBT), the latter in a common-base configuration and preferably doped per the present invention; 
         FIG. 2  is a set of three linked graphs, the right graph showing power gain in decibels as a function of the logarithm of frequency for a common-emitter bipolar transistor and a common-base bipolar transistor with two different doping schemes shown in left graphs, each showing doping concentration as a function of spatial location within the emitter base and collector regions, the upper graph showing a conventional doping pattern and the lower graph showing a doping pattern to reduce base resistance; 
         FIG. 3  is a schematic representation of a generic common-emitter design (to the left) and the hybrid-n model for the transistor in that design (to the right) as defines variables referenced in the present specification; 
         FIG. 4  is a figure similar to that of  FIG. 3  showing a generic common-base amplifier design and the T model for the transistor of that design; 
         FIG. 5  is a figure similar to that of  FIG. 1  showing a single FET feeding multiple bipolar transistors for increased fan out; 
         FIG. 6  is a figure similar to that of  FIG. 5  showing multiple field-effect transistors providing current to a single bipolar transistor in a fan-in situation; or to multiple bipolar transistors for optimized fan-in and fan-out; 
         FIG. 7  is a block diagram of an integrated circuit providing for both CMOS logic circuitry and power cells per the present invention; and 
         FIG. 8  is a figure similar to that of  FIG. 1  showing a cascode connection of the bipolar junction transistors to provide improved breakdown voltage. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , the present invention may be used, in one embodiment, to create an amplifier cell  10  having input terminals  12  for receiving a signal to be amplified and output terminals  14  providing an output signal, such as a radio frequency signal to an antenna. 
     The amplifier cell  10  may employ multiple amplifier rows  16  (only two of which are shown for clarity), providing parallel current paths. Each row  16  includes at least one series connected of a field-effect transistor  18  and a bipolar transistor  20 . The latter bipolar transistor  20  may be a bipolar junction transistor (BJT) or heterojunction bipolar transistor (HBT). 
     The bipolar transistor  20  is arranged in common-base configuration in which the base of the bipolar transistor  20  (marked by the letter “B”) is referenced to signal AC ground. The collector (marked by the letter “C”) is attached to the output terminal  14  and the emitter (marked by the letter “E”) is connected to the drain (marked by the letter “D”) of the field-effect transistor  18 . 
     The source of the field-effect transistor  18  (marked by the letter “S”) is connected to ground and the gate (marked by the letter “G”) is connected to the input terminal  12 . Generally therefore, a signal at the gate of field-effect transistor  18  controls the current from drain to source of the field-effect transistor  18  in turn controlling the current from collector to emitter of the bipolar transistor  20 . 
     Referring now to  FIG. 2 , the doping profile  22  of a standard bipolar transistor  35  intended for a common-emitter amplifier design provides for a decreasing concentration of dopant as one moves from the emitter region  24  to the base region  26  and to the collector region  28 . Such a bipolar transistor  35 , when used in a common-emitter configuration, produces a common-emitter gain curve  30  providing generally decreasing power gain as frequency increases, where power gain may be measured as maximum available gain (MAG), and maximum stable gain (MSG) in decibels. 
     Alternatively, when a bipolar transistor  35  of this type is used in a common-base configuration, a common-base gain curve  32  is produced, again providing power gain as a function of frequency, typically having less power gain than the common-emitter design for the majority of a generally useful frequency range  34 . 
     In the present invention, a doping profile  22  can be employed with the bipolar transistor  20  of  FIG. 1 . With the configuration of  FIG. 1 , a high breakdown voltage from the base collector junction of bipolar transistor  20  is available. The use of ballast resistors generally for maintaining thermally stable operation of multiple parallel bipolar transistor  20  is thus eliminated. Power performance, including output RF power, power gain, power-added efficiency and over-voltage, is improved. 
     In another embodiment of the present invention, a doping profile  36  is employed with the bipolar transistor  20  of  FIG. 1  in which the doping in the base region  26  is substantially increased with respect to the doping in emitter region  24  and collector region  28 . This change in doping profile  36  results in an enhanced common-base gain curve  40  outperforming the common-emitter gain curve  30  for a widened useful frequency range  37 . This improvement in the enhanced common-base gain curve  40  results from a decrease in the effective base resistance of the bipolar transistor  20  as will now be explained. 
     Referring to  FIG. 3 , a bipolar transistor  35  in a common-emitter configuration  41  may be modeled according to a small signal hybrid π model  42  in which the operating characteristics of the transistor  35  are represented by equivalent capacitors, resistors and current sources. Hybrid π model  42  is augmented with elements representing parasitic emitter resistance r ex  and the collector resistance r c . These two resistances r ex  and r c  are normally ignored because of their negligible value in comparison to the total base resistance r b  for doping profile  22 , however, for higher doping concentrations in the base region  26  of doping profile  36 , r b  can be reduced to a much smaller value and becomes comparable to r ex  and r c . 
     In this small-signal hybrid π model  42  of a bipolar transistor  35  in a common-emitter configuration  41 , the base terminal of the bipolar transistor  35  connects through a resistance r b  to a junction with a capacitor C μ  (also named as C BC ), C π  and resistor r π . The latter two elements, C π  and r π , are connected in parallel between resistor r b  and a junction of resistor r ex , a current source  44  equal to g m v BE  where v BE  is the voltage across resistor r π , and a resistor r o . The remaining end of resistor r ex  connects to the emitter terminal of the device. 
     The remaining end of C μ  connects to the remaining terminals of the current source  44 , resistor r o  and resistor r c  the latter of which leads to the collector terminal. 
     Referring to  FIG. 4 , similarly a small-signal T-model  46  (equivalent to hybrid π model  42 ) may be created for the common-base configuration  47  of bipolar transistor  35  or  20 . The small signal T-model  46  provides for a resistor r ex  joining the emitter to the common junction of a resistor r o  and the parallel combination of resistor r e  and capacitor C π . The remaining terminals of resistor r e  and capacitor C π  in turn connect to a junction of a base resistance r b , a current equal to g m v BE  where v BE  is the voltage across resistor r e , and a capacitor C μ . 
     The remaining end of r o  connects to the remaining terminals of the parallel connected current source  44  and capacitor C μ , and to resistor r c , the latter of which leads to the collector terminal. 
     The H-parameters of the small signal hybrid π model  42  representing the common-emitter configuration  41  are derived as the following where subscript symbols: i stands for input port; o stands for output port; r stands for reverse transmission; f stands for forward transmission; e stands for common-emitter; and b stands for common-base, according to well known convention: 
                     h   ie     =       r   b     +         (       1     j   ⁢           ⁢   ω   ⁢           ⁢     C   μ         +     r   c       )     ⁡     [       Z   1     +       r   ex     ⁡     (     1   +       g   m     ⁢     Z   1         )         ]           1     j   ⁢           ⁢   ω   ⁢           ⁢     C   μ         +     Z   1     +       (       r   ex     +     r   c       )     ⁢     (     1   +       g   m     ⁢     Z   1         )                     (   1   )                 h   re     =           r   ex     ⁡     (     1   +       g   m     ⁢     Z   1         )       +     Z   1           1     j   ⁢           ⁢   ω   ⁢           ⁢     C   μ         +     Z   1     +       (       r   ex     +     r   c       )     ⁢     (     1   +       g   m     ⁢     Z   1         )                   (   2   )                 h   fe     =             g   m     ⁢     Z   1         j   ⁢           ⁢   ω   ⁢           ⁢     C   μ         -     Z   1     -       r   ex     ⁡     (     1   +       g   m     ⁢     Z   1         )             1     j   ⁢           ⁢   ω   ⁢           ⁢     C   μ         +     Z   1     +       (       r   ex     +     r   c       )     ⁢     (     1   +       g   m     ⁢     Z   1         )                   (   3   )                 h   oe     =         1   +       g   m     ⁢     Z   1             1     j   ⁢           ⁢   ω   ⁢           ⁢     C   μ         +     Z   1     +       (       r   ex     +     r   c       )     ⁢     (     1   +       g   m     ⁢     Z   1         )           +     1     r   o                 (   4   )               
where Z 1  is given by
 
                 Z   1     =           r   π       1   +     j   ⁢           ⁢   ω   ⁢           ⁢     r   π     ⁢     C   π           ⁢           ⁢   and   ⁢           ⁢     r   π       =     β     g   m           ,       g   m     =       qI   c     kT             
where kT=26 meV.
 
     These four H-parameters, h ie , h re , h fe  and h oe  are equivalent to h 11 , h 12 , h 21  and h 22  in the general two-port network format, respectively. 
     Similarly, the H-parameters for the small signal T model  46  representing the common-base configuration  47  can be derived as: 
                     h   ib     =           r   b     ⁡     (     1   -       g   m     ⁢     Z   2       +     j   ⁢           ⁢   ω   ⁢           ⁢     r   c     ⁢     C   μ         )         1   +     j   ⁢           ⁢   ω   ⁢           ⁢       C   μ     ⁡     (       r   b     +     r   c       )             +     r   ex     +     Z   2               (   5   )                 h   rb     =           r   b     ⁡     (         1   -       g   m     ⁢     Z   2             Z   2     +     r   o         +     j   ⁢           ⁢   ω   ⁢           ⁢     C   μ         )       +       Z   2         Z   2     +     r   o             1   +       (         1   -       g   m     ⁢     Z   2             Z   2     +     r   o         +     j   ⁢           ⁢   ω   ⁢           ⁢     C   μ         )     ⁢     (       r   b     +     r   c       )                   (   6   )                 h   fb     =     -           g   m     ⁢     Z   2       +     j   ⁢           ⁢   ω   ⁢           ⁢     r   b     ⁢     C   μ           1   +     j   ⁢           ⁢   ω   ⁢           ⁢       C   μ     ⁡     (       r   b     +     r   c       )                       (   7   )                 h   ob     =     1       r   b     +     r   c     +     1       jω   ⁢           ⁢     C   μ       +       1   -       g   m     ⁢     Z   2             Z   2     +     r   o                         (   8   )               
where Z 2  is given
 
     
       
         
           
             
               Z 
               2 
             
             = 
             
               
                 
                   
                     r 
                     e 
                   
                   
                     1 
                     + 
                     
                       j 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ω 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         r 
                         e 
                       
                       ⁢ 
                       
                         C 
                         π 
                       
                     
                   
                 
                 ⁢ 
                 and 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   r 
                   e 
                 
               
               = 
               
                 
                   1 
                   
                     
                       g 
                       m 
                     
                     + 
                     
                       1 
                       
                         r 
                         π 
                       
                     
                   
                 
                 = 
                 
                   
                     β 
                     
                       
                         g 
                         m 
                       
                       ⁡ 
                       
                         ( 
                         
                           1 
                           + 
                           β 
                         
                         ) 
                       
                     
                   
                   = 
                   
                     α 
                     
                       g 
                       m 
                     
                   
                 
               
             
           
         
       
     
     In order to calculate the difference/ratio of power gain between two configurations, approximations, justified by actual values, may be made to simplify the derived H-parameters and the power gain expressions in different frequency ranges. Since most of the transistors  35 ,  20  are operated in the intermediate frequency range within the f max  of the devices (for RF and microwave power amplification) and the devices are potentially unstable in this frequency range, it is imperative to specifically consider the maximum stable power gain (MSG) in this useful frequency range. MSG can be expressed in terms of H-parameters as shown in Eq. 9: 
     
       
         
           
             
               
                 
                   MSG 
                   = 
                   
                      
                     
                       
                         h 
                         f 
                       
                       
                         h 
                         r 
                       
                     
                      
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     From Eqs. 2 and 3, the maximum stable gain MSG for the common-emitter configuration  41  can be derived as the following: 
     
       
         
           
             
               
                 
                   
                     MSG 
                     e 
                   
                   = 
                   
                     
                        
                       
                         
                           
                             
                               
                                 g 
                                 m 
                               
                               ⁢ 
                               
                                 Z 
                                 1 
                               
                             
                             
                               j 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               ω 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 C 
                                 μ 
                               
                             
                           
                           - 
                           
                             Z 
                             1 
                           
                           - 
                           
                             
                               r 
                               ex 
                             
                             ⁡ 
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   
                                     g 
                                     m 
                                   
                                   ⁢ 
                                   
                                     Z 
                                     1 
                                   
                                 
                               
                               ) 
                             
                           
                         
                         
                           
                             
                               r 
                               ex 
                             
                             ⁡ 
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   
                                     g 
                                     m 
                                   
                                   ⁢ 
                                   
                                     Z 
                                     1 
                                   
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             Z 
                             1 
                           
                         
                       
                        
                     
                     = 
                     
                        
                       
                         
                           
                             
                               
                                 g 
                                 m 
                               
                               ⁢ 
                               
                                 Z 
                                 1 
                               
                             
                             
                               j 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               ω 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 C 
                                 μ 
                               
                             
                           
                           
                             
                               
                                 r 
                                 ex 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     
                                       g 
                                       m 
                                     
                                     ⁢ 
                                     
                                       Z 
                                       1 
                                     
                                   
                                 
                                 ) 
                               
                             
                             + 
                             
                               Z 
                               1 
                             
                           
                         
                         - 
                         1 
                       
                        
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     Under the assumption MSG&gt;&gt;1 and |g m Z 1 |≈β&gt;&gt;1 in the intermediate frequency range and using approximation 
     
       
         
           
             
               α 
               ≈ 
               1 
             
             , 
             
               
                 r 
                 e 
               
               = 
               
                 
                   α 
                   
                     g 
                     m 
                   
                 
                 ≈ 
                 
                   
                     1 
                     
                       g 
                       m 
                     
                   
                   . 
                 
               
             
           
         
       
     
     Eq. 10 can be simplified as: 
     
       
         
           
             
               
                 
                   
                     
                       MSG 
                       e 
                     
                     ≈ 
                     
                        
                       
                         
                           
                             
                               g 
                               m 
                             
                             ⁢ 
                             
                               Z 
                               1 
                             
                           
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             ω 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               C 
                               μ 
                             
                           
                         
                         
                           
                             
                               r 
                               ex 
                             
                             ⁡ 
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   
                                     g 
                                     m 
                                   
                                   ⁢ 
                                   
                                     Z 
                                     1 
                                   
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             Z 
                             1 
                           
                         
                       
                        
                     
                     ≈ 
                     
                        
                       
                         
                           
                             
                               g 
                               m 
                             
                             ⁢ 
                             
                               Z 
                               1 
                             
                           
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             ω 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               C 
                               μ 
                             
                           
                         
                         
                           
                             
                               r 
                               ex 
                             
                             ⁢ 
                             
                               g 
                               m 
                             
                             ⁢ 
                             
                               Z 
                               1 
                             
                           
                           + 
                           
                             Z 
                             1 
                           
                         
                       
                        
                     
                   
                   = 
                   
                     1 
                     
                       ω 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           C 
                           μ 
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               r 
                               ex 
                             
                             + 
                             
                               r 
                               e 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     For the common-base configuration  47 , since the value of r o  is fairly large, Eq. 6 can thus be simplified as: 
     
       
         
           
             
               
                 
                   
                     h 
                     rb 
                   
                   = 
                   
                     
                       j 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ω 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         r 
                         b 
                       
                       ⁢ 
                       
                         C 
                         μ 
                       
                     
                     
                       1 
                       + 
                       
                         j 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           
                             C 
                             μ 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 r 
                                 b 
                               
                               + 
                               
                                 r 
                                 c 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     From Eqs. 7 and 12, MSG for the common-base configuration is: 
     
       
         
           
             
               
                 
                   
                     MSG 
                     b 
                   
                   = 
                   
                     
                        
                       
                         
                           
                             
                               g 
                               m 
                             
                             ⁢ 
                             
                               Z 
                               2 
                             
                           
                           + 
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             ω 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               r 
                               b 
                             
                             ⁢ 
                             
                               C 
                               μ 
                             
                           
                         
                         
                           j 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           ω 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             r 
                             b 
                           
                           ⁢ 
                           
                             C 
                             μ 
                           
                         
                       
                        
                     
                     = 
                     
                        
                       
                         
                           
                             g 
                             m 
                           
                           ⁢ 
                           
                             Z 
                             2 
                           
                         
                         
                           j 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           ω 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             r 
                             b 
                           
                           ⁢ 
                           
                             C 
                             μ 
                           
                         
                       
                        
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     Since MSG&gt;&gt;1 and |g m Z 2 |≈α, in the intermediate frequency range, Eq. 13 can be further simplified as, 
     
       
         
           
             
               
                 
                   
                     MSG 
                     b 
                   
                   = 
                   
                     α 
                     
                       ω 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         r 
                         b 
                       
                       ⁢ 
                       
                         C 
                         μ 
                       
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
     It is noted, from Eqs. 11 and 14, that both MSG b  and MSG e  follow a —10 dB/decade degradation trend, which is commonly observed for MSG versus frequency. The ratio of MSG&#39;s between the common-base and common-emitter configurations, using 
               r   e     =       α     g   m       ≈     1     g   m               
again, is:
 
     
       
         
           
             
               
                 
                   
                     
                       MSG 
                       b 
                     
                     
                       MSG 
                       e 
                     
                   
                   = 
                   
                     
                       
                         α 
                         ⁡ 
                         
                           ( 
                           
                             
                               r 
                               ex 
                             
                             + 
                             
                               1 
                               / 
                               
                                 g 
                                 m 
                               
                             
                           
                           ) 
                         
                       
                       
                         r 
                         b 
                       
                     
                     ≈ 
                     
                       
                         
                           r 
                           ex 
                         
                         + 
                         
                           r 
                           e 
                         
                       
                       
                         r 
                         b 
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     From Eq. 11 and Eq. 14, one can see that MSG e  is dependent on r ex +r e  and MSG b  is dependent on r b . 
     For the common-base configuration  47 , MSC b  increases as r b  decreases. In the present invention, the value of r b  is decreased to less than r ex +r e  by the increased doping of the base region described above to provide performance using a common-base configuration that can be superior to the performance from a common-emitter configuration. 
     In the high frequency range, the devices are unconditionally stable. MAG can be expressed as,
 
MAG=MSG( K−√ {square root over (K 2 −1)})   (16)
 
where K is the Rollett&#39;s stability factor (the K-factor),
 
     
       
         
           
             
               
                 
                   K 
                   = 
                   
                     
                       
                         2 
                         ⁢ 
                         
                           Re 
                           ( 
                           
                             Z 
                             11 
                           
                           ) 
                         
                         ⁢ 
                         
                           Re 
                           ⁡ 
                           
                             ( 
                             
                               Z 
                               22 
                             
                             ) 
                           
                         
                       
                       - 
                       
                         Re 
                         ⁡ 
                         
                           ( 
                           
                             
                               Z 
                               12 
                             
                             ⁢ 
                             
                               Z 
                               21 
                             
                           
                           ) 
                         
                       
                     
                     
                        
                       
                         
                           Z 
                           12 
                         
                         ⁢ 
                         
                           Z 
                           21 
                         
                       
                        
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
     Due to the complication of the K-factor in MAG, a simplified expression of MAG for both CE and CB configurations is impossible to obtain. The relative size of MAG e  and MAG b  is compared qualitatively. 
     After converting the K-factor into the H-parameter representation and substituting Eq. 1-4 and Eq. 5-8 with appropriate approximations, the K-factor for the CE and the CB configurations can be derived, respectively, as, 
     
       
         
           
             
               
                 
                   
                     
                       K 
                       CB 
                     
                     ≈ 
                     
                       
                         [ 
                         
                           
                             
                               ( 
                               
                                 
                                   r 
                                   b 
                                 
                                 + 
                                 
                                   2 
                                   ⁢ 
                                   
                                     r 
                                     ex 
                                   
                                 
                               
                               ) 
                             
                             ⁢ 
                             
                               C 
                               μ 
                             
                           
                           + 
                           
                             
                               r 
                               e 
                             
                             ⁢ 
                             
                               C 
                               π 
                             
                           
                         
                         ] 
                       
                       ⁢ 
                       ω 
                     
                   
                   = 
                   
                     
                       k 
                       b 
                     
                     ⁢ 
                     ω 
                   
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       K 
                       CE 
                     
                     ≈ 
                     
                       
                         [ 
                         
                           
                             
                               ( 
                               
                                 
                                   r 
                                   ex 
                                 
                                 + 
                                 
                                   2 
                                   ⁢ 
                                   
                                     r 
                                     b 
                                   
                                 
                               
                               ) 
                             
                             ⁢ 
                             
                               C 
                               μ 
                             
                           
                           + 
                           
                             
                               r 
                               e 
                             
                             ⁢ 
                             
                               
                                 C 
                                 π 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     
                                       2 
                                       ⁢ 
                                       
                                         r 
                                         b 
                                       
                                     
                                     
                                       r 
                                       ex 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                         
                         ] 
                       
                       ⁢ 
                       ω 
                     
                   
                   = 
                   
                     
                       k 
                       e 
                     
                     ⁢ 
                     ω 
                   
                 
               
               
                 
                   ( 
                   19 
                   ) 
                 
               
             
           
         
       
     
     A simple direct comparison shows that k e &gt;k b . Consequently, the frequency point (f K=1,CB ) at which the K-factor of the CB configuration reaches unity (break-point of MSG/MAG) is larger than the corresponding frequency point of the CE configuration (f K=1,CE ), i.e., f K=1,CB &gt;f K=1,CE . By comparing the slopes of MAG e  and MAG b  versus frequency using d(MAG e (dB))/d(log ω) and d(MAG b (dB))/d(log ω)), it can be shown that MAG b  decreases faster with frequency than MAG e . 
     If MSG b &gt;MSG e  (Eq. 15) for the case of r b &lt;r e +r ex , in light of the fact of f K=1,CB &gt;f K=1,CE , then it is always true that MAG b &gt;MAG e . If MSG b &lt;MSG e  (Eq. 15) for the case of r b &gt;r e +r ex , the value of k e /k b  (=f K=1,CB /f K=1,CE , as seen from Eq. 18 and Eq. 19. It can be roughly approximated as 
             1   +       2   ⁢     r   b         r   ex             
for simplicity) and that of r b /(r e +r ex )(=MSG e /MSG b , Eq. 15) need to be compared in order to compare the relative size of MAG b  and MAG e . A straightforward comparison of these two ratios shows that k e /k b &gt;r b /(r e +r ex ) regardless of relative size of r b  and r e +r ex . When both MAG and frequency are plotted in logarithmic scale, k e /k b &gt;r b /(r e +r ex ), in light of the fact that MAG b  and MAG e  merge together (with gain value of unity) at the same f max , indicates that MAG b  must be larger than MAG e  in the frequency range of f&lt;f max . As a result, in the high frequency range, the CB configuration always offers higher power gain (MAG b ) than the CE configuration (MAG e ) in spite of the ratio of MSG b /MSG e .
 
     In the present invention, common-base configuration in this high frequency range  38  of  FIG. 2  is used to provide performance that is superior to the performance from a common-emitter configuration of a transistor using a profile  22 . The dividing point between frequency range  34  and  38  is quarter of the cut-off frequency f T  of a transistor having a profile of  22  under common-emitter configuration. 
     One can also notice that neither MSG e  nor MSG b  is dependent on the parasitic collector resistance r c . Although increased r c  can increase the RC delay (via C μ ·r c ) of the transistors  20 ,  35 , which in turn reduces the device cut-off frequency f T , there is no significant effect of r c  on small-signal power gain within the frequency range of concern. 
     Referring now to  FIG. 5 , a given row  16   a  of the amplifier cell  10  may provide for multiple parallel bipolar transistor  20  limited to a number permitting effective current sharing between these bipolar transistors  20 . These bipolar transistors  20  may be driven by a single field-effect transistor  18  to provide for fan out of that control. 
     Conversely as shown in  FIG. 6 , each row  16  may provide for multiple field-effect transistors  18  connected in parallel to control the current through one bipolar transistor  20  for fan in of that control. These variations in amplifier cell  10  of  FIG. 1  match the available power of the devices in a given fabrication. 
     Alternatively as shown in  FIG. 6 , the multiple field-effect transistors  18  connected in parallel may control multiple parallel bipolar transistors  20  and  20 ′ for optimized power delivery, connectivity. The number of parallel bipolar transistors  20  is limited by thermal effects that can be tolerated by these transistors  20  in parallel. A large number of parallel bipolar transistors  20  without using ballast resistors is not recommended. 
     Referring to  FIG. 7 , the amplifier cells  10  may be fabricated on a substrate  50  together with conventional CMOS-type logic circuitry  52 , the latter which may be used to provide signal to the amplifier cells  10  per line  54  and to receive feedback or monitoring signals per line  56 . The present invention, by providing improved power gain in silicon devices, makes such bipolar CMOS integration valuable in a variety of applications including radio transmitters and the like where extensive digital domain processing of signals may be desirable. 
     Referring now to  FIG. 8 , in an alternative embodiment of the amplifier cell  10 , each of the bipolar transistors  20  may be replaced by two or three series connected bipolar transistors  20  and  20 ′ to form a cascode stage that improves breakdown voltage of the series connected transistors as may be particularly important for small or high speed devices. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.