Abstract:
The safe-operating area (SOA) in a heterojunction bipolar transistor (HBT) is improved by providing a collector region in the transistor having a graded (uniformly or stepped) doping between the base region and the underlying subcollector region with the collector doping being lower near the base and higher near the subcollector and with the collector doping being less than the doping of the subcollector. The non-uniformly doped collector reduces Kirk effect induced breakdown when collector current increases.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to heterojunction bipolar transistors (HBT), and more particularly the invention relates to improving the safe-operating area (SOA) of such a transistor. 
   Heterojunction bipolar transistors (e.g. III–V compound semiconductor) are used in amplifier circuits for telecommunications applications. A major concern lies in operating the transistors in safe-operating areas (SOA) to prevent overdrive and failure of the devices. As shown in  FIG. 1 , the SOA is defined by two boundaries. The first boundary, SOA Boundary I, is limited by the open-emitter base-collector junction breakdown voltage, BVcbo of the transistor. This boundary sets the operating limit of the transistor at low current densities. The second boundary, SOA Boundary II, is related to the collector breakdown when substantial injected current carriers are present in the collector. This boundary is important at medium to high current levels. If one attempts to operate a HBT beyond the SOA boundaries in the non-safe operating areas as shown in the figure, the device will catastrophically fail. The conventional way to increase the collector breakdown voltage is to increase the thickness and to decrease the doping concentration of the collector. Using the approach conventional HBT&#39;s have been produced with a BVcbo of around 70 volts by using a collector with a thickness of 3 μm and a dopant concentration of 6e 15 cm −3 . However, although a larger BVcbo moves SOA Boundary I to a higher Vce, the SOA Boundary II does not necessarily move to a higher collector current, Ic. In fact, breakdown always happens at a voltage smaller than BVcbo when there is large current flowing through the transistor. This is a result of the Kirk effect. 
   The Kirk effect results when the collector current increases to a high enough level and the number of injected electrons compensates the space charge in the collector and changes the electric field distribution. The effect happens when the effective injected charge density exceeds the background doping concentration in the collector, and the space charge changes sign and the location of the high field region moves from the base-collector junction to the collector-subcollector junction. The breakdown then is no longer controlled by the doping density in the collector alone, but also by the collector current. As Ic increases, the effective negative space charge density increases, and this causes the electric field to increase at the collector-subcollector junction, and results in a reduction of breakdown voltage. Further, decreasing of the collector doping will only improve the low current breakdown voltage but will not improve the medium and high current breakdown voltage. 
   BRIEF SUMMARY OF THE INVENTION 
   The standard heterojunction bipolar transistor has a uniformly doped collector. In accordance with the present invention, the collector of a heterojunction bipolar transistor has a non-uniform doping with the doping near the base region being more lightly doped than the subcollector side of the collector. The doping profile can have two or more distinctly doped layers or a continuous grading of the collector doping. 
   The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a graph of collector current, Ic, versus collector-emitter voltage, Vce, for a HBT which illustrates safe operation boundaries. 
       FIG. 2  is a section view of a HBT which is modified in accordance with an embodiment of the invention. 
       FIG. 3  illustrates SOA boundaries in plots of Ic versus Vbc for a transistor with a standard uniformly doped collector and for three transistors with non-uniform doping in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  is a section view of a heterojunction bipolar transistor which is modified in accordance with one embodiment of the invention. The transistor comprises a GaAs substrate  10  on which is formed an N +  doped GaAs subcollector region  12  and a N doped GaAs collector  14  which includes an N doped layer  14 ′ and N −  doped layer  14 ″ which abuts a P +  GaAs base  16 . An N doped InGaAs emitter  18  is formed on base  16  with an N +  cap layer  20  formed on emitter  18 . Cap layer  20  can comprise an N +  doped GaAs layer with an N +  InGaAs layer thereon. Contacts  22 ,  24 , and  26  are formed on the emitter, base, and collector, respectively. 
   Since the Kirk effect induced breakdown happens near the collector-subcollector junction, the provision of non-uniform doping as illustrated in  FIG. 2  with increased doping concentration in the collector near the subcollector layer will mitigate the effect. However, to have an optimum design, one has to be careful not to make the more heavily doped collector layer too thick or use a doping concentration close to that in the heavily doped subcollector layer. Otherwise, BVcbo and therefore the SOA boundary I will suffer. 
   Table I illustrates four collector structures and the respective calculated breakdown voltage. Collectors made of GaAs are assumed in the calculation. The standard structure has a uniformly doped collector, which one would normally use to have a high breakdown voltage. The other three collector structures, A, B, and C, all have non-uniform collector doping profiles, and each has a more heavily doped layer inserted in the subcollector side of the collector layer. The differences among the three structures, A, B, and C, are in the thickness of the low and high doped layers and the doping concentration in the high doped layer. All four structures have the same total collector thickness of 3 μm. The same emitter size of 24 μm 2  is used in the calculation. A constant breakdown field is assumed, and when the electric field reaches its value, the device fails because the collector breakdown and the SOA boundaries are closely related to each other. The BVcbo decrease when a more heavily doped layer is included in the collector near the subcollector region. However, if the layer is kept thin relative to the total collector thickness, and its doping level remains low relative to the subcollector doping which is typically on the order of 10 18  ions cm −3 , the decrease in BVcbo is minimal since a large portion of the collector close to the base remains at low doping level. The breakdown induced by the Kirk effect, however, changes drastically with changes in the collector structure. At Ic=10 mA, for example, one can see that the breakdown voltage can be increased by more than a factor of two if a proper structure is used. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Standard Structure 
               Structure A 
               Structure B 
               Structure C 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               Collector 
               3.0 μm, 7e15 cm −3   
               2.5 μm, 7e15 cm −3  + 
               2.5 μm, 7e15 cm −3  + 
               2.0 μm, 7e15 cm −3  + 
             
             
               Structure 
                 
               0.5 μm, 2e16 cm −3     
               0.5 μm, 4e16 cm −3     
               1.0 μm, 4e16 cm −3     
             
             
               BVcbo 
                 80 V 
                 78 V 
               75.6 V 
                 67 V 
             
             
               BV at 10 mA 
               21.5 V 
               29.5 V 
               42.7 V 
               62.7 V 
             
             
                 
             
             
               BV = Breakdown Voltage 
             
           
        
       
     
   
     FIG. 3  shows the SOA (breakdown voltage as a function of IC) of the four devices. A great improvement in SOA Boundary II is obtained by using the invention illustrated in these embodiments. An added advantage for these structures is the reduced on resistance when the devices are in saturation because of the higher doping in the collector region near the subcollector layer. 
   While a two-step, low high collector doping profile is used in these embodiments, other embodiments can realize the non-uniform collector doping profile for the improvement of SOA Boundary II. For example, one can use multiple layers in the collector instead of two doping layers. The layer with the lowest doping concentration is near the base, and that with the highest doping concentration is near the subcollector which has the highest doping level. 
   Alternatively, a continuous grading in the collector doping profile can be used to improve SOA Boundary II. The key is to have the more heavily doped collector region near the subcollector layer and the more lightly doped region near the base, and the heaviest doping concentration in the collector layer remains lower than that in the subcollector layer. 
   The invention can be applied to all heterojunction bipolar transistors, including for example, AlGaAs/GaAs, InGaP/GaAs, InP/InGaAs, InAlAs/InGaAs, and InAlGaAs/InGaAs single and double heterojunction bipolar transistors with GaAs, InGaAs, InP, AlGaAs, InGaP, InAlAs, or a combination thereof as the collector material. The invention can be also applied to Si based bipolar transistors including Si/SiGe heterojunction bipolar transistors. 
   While the invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.