Patent Publication Number: US-2007102729-A1

Title: Method and system for providing a heterojunction bipolar transistor having SiGe extensions

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      The present application is related to co-pending U.S. Patent Application Serial No. ______ entitled Method and System for Controlled Oxygen Incorporation in Compound Semiconductor Films for Device Performance Enhancement ( 3506 P) filed on even date herewith and assigned to the assignee of the present application, and U.S. Patent Application Serial No. ______ entitled Bandgap Engineered Mono-Crystalline Silicon Cap Layers for SiGe HBT Performance Enhancement ( 3508 P) filed on even date herewith and assigned to the assignee of the present application, and U.S. Patent Application Serial No. ______ entitled Bandgap and Recombination Engineered Emitter Layers for SiGe HBT Performance Optimization ( 3509 P) filed on even date herewith and assigned to the assignee of the present application. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to semiconductor processing, and more particularly to a method and system for dopant profiles providing improved performance of heterostructure devices such as heterojunction bipolar transistor (HBT) devices.  
     BACKGROUND OF THE INVENTION  
      The conventional SiGe HBT has significant advantages over a silicon bipolar junction transistor (BJT) in gain, frequency response, noise parameters and retaining the ability to be readily integrated with CMOS at relatively low cost. Cutoff frequencies (F t ) of conventional SiGe HBT devices have been reported to exceed 300 GHz, which is favorable as compared to GaAs devices. Moreover, GaAs devices are relatively high in cost and cannot achieve the level of integration of technologies such as BiCMOS. The silicon compatible conventional SiGe HBT provides a low cost, high speed, low power solution that is quickly replacing other compound semiconductor devices.  
       FIG. 1  depicts the filmstack of a conventional heterojunction bipolar transistor (HBT) device  10  formed on a substrate  11 . The conventional HBT device  10  includes a conventional collector region  12 , a conventional compound base region  16 , and a conventional emitter region  20 . The conventional HBT device  10  may also include a conventional spacer (or seed) layer  14  and a conventional capping layer  18 .  
      In a conventional HBT  10 , the conventional spacer layer  14  is typically an elemental semiconductor, such as silicon. The conventional base region  16  is typically formed from a compound semiconductor, such as SiGe or SiGeC (SiGe doped with C) (collectively hereinafter SiGe/SiGeC). The conventional capping layer  18  is typically an elemental semiconductor, such as silicon. The conventional emitter layer  18  is typically polysilicon. One of ordinary in the art will recognize that other materials of the poly-, mono-, and/or amorphous construction will also work well for the emitter layer, such as poly-SiGe or amorphous silicon, to name a few.  
      The conventional HBT  10  may either be npn or pnp, depending on the device application. For instance, with an npn SiGe/SiGeC HBT, the conventional collector region  12  is doped with n-type dopants such as arsenic and/or phosphorus. The process dopant gases are usually arsine (AsH 3 ) and/or phosphine (PH 3 ) respectively. The collector region  12  may be formed in an epitaxial reactor at temperatures in the 900° C. to 1000° C. range. The collector region  12  may be doped in-situ during epitaxial film growth or by ion implantation or diffusion sources after film growth. Silane (SiH 4 ) is the typical silicon source gas. Temperatures below 900° C. and greater than 1000° C. may also be used. The conventional spacer  14 , SiGe/SiGeC base layer  16 , and the conventional cap layer  18  are typically formed together in the same process. The silicon source gas for all layers is typically silane (SiH 4 ). Growth temperatures usually range between 500° C. and 900° C.; growth pressures typically ranges between one and one hundred torr. The conventional spacer region  14  may be either undoped or doped n-type with either arsenic or phosphorus, by the use of arsine (AsH 3 ) and/or phosphine (PH 3 ) gases, respectively. The conventional SiGe and/or SiGeC layer is typically grown at temperatures ranging from 600° C. to 700° C., although temperatures less than 600° C. and greater than 700° C. may be utilized. Germane (GeH 4 ) is the typical source of germanium. The epitaxial SiGe and/or SiGeC growth usually takes place in an LBCVD (low pressure chemical vapor deposition) reactor. However, other methods including UHVCVD (ultrahigh vacuum CVD) and MBE (molecular beam epitaxy) may be utilized. The conventional capping layer  18  is typically grown at temperatures in the range of 700° C. to 900° C. and may be either doped or undoped. If doped, the n-type species is usually arsenic and/or phosphorus, with arsine (AsH 3 ) and/or phosphine (PH 3 ) respectively.  
      Use of the conventional SiGe/SiGeC layer for the conventional compound base region  16  results in a base-emitter heterojunction. Because SiGe has a lower energy bandgap than silicon, the base-emitter heterojunction results in a bandgap offset between the conventional compound base  16  and the conventional emitter  20 . This bandgap reduction is translated into both the conduction and valence bands in such a way as to improve device performance.  
       FIG. 2  depicts the energy band structure  30  of an npn HBT device in a forward active mode, in which the base-collector junction is reverse biased and the base-emitter junction is forward biased. The lowering of the conduction band lowers the barrier against electron injection from emitter to base and thereby results in an increase in current density for a give base-emitter voltage bias. Elevating the valence band energy provides a barrier against hole-diffusion from the conventional compound base  16  to the conventional emitter  20 . An increase in electron injection, combined with reduced hole current provides higher gains and higher F t  than can be realized by a similarly doped and structured silicon BJT. Thus, a silicon BJT with the same dimensions, layer thickness, and doping levels in collector, base, and emitter as a conventional SiGe/SiGe HBT regions will not operate as efficiently as the conventional SiGe/SiGeC HBT.  
      Therefore, advantages of SiGe/SiGeC may be realized by a bandgap reduction that creates an energy band offset at the base-emitter SiGe heterojunction of the HBT. As a result, increased current density and current gain for a given base-emitter bias may be achieved. The bandgap offset is typically generated by the incorporation of germanium (Ge) with the silicon lattice. Stated differently, a diamond crystalline structure including a blend of silicon and germanium results in a bandgap that is less than that of silicon only. Furthermore, a lower resistivity is possible with addition of Ge to a Si lattice. In addition, boron diffusivity is greatly reduced with the addition of Ge. A silicon interstitial and boron pairing primarily enhance boron diffusivity. However, Ge increases the vacancy population, or diminishes the interstitial population, which acts to reduce boron diffusion. Therefore, advantages of SiGe may include: 
          1. Bandgap engineering flexibility     2. Reduction in dopant diffusivity, esp. boron     3. Ease of integration with standard silicon technologies        

       FIG. 3  depicts a film stack  40  of a conventional SiGe/SiGeC HBT device having a trapezoidal silicon germanium region with a ramped profile on the side of the base-emitter heterojunction. The bandgap offset, defined where the metallurgical junction aligns with the base-emitter heterojunction (ΔEG( 0 )), and/or the bandgap grading across the neutral base region (ΔEG(grade)) are key components to the HBT device performance. Each of these bandgap effects is induced by the incorporation of Ge into the silicon lattice. Current density (J c ) is exponentially dependent on the bandgap offset at the base-emitter heterojunction (ΔEG( 0 )) and linearly dependent on the Ge grade or (ΔEG(grade)). 
 J c αexp[ΔEG( 0 )]*(ΔEG(grade))  
 The higher current densities and lower base resistance values allow improved unity gain cutoff frequencies and maximum oscillation frequencies than comparable silicon BJTs, and are comparable to other compound devices such as GaAs. 
 
      The base-emitter bandgap offset, (ΔEG( 0 )), is determined by the relative position of the metallurgical junction with respect to the germanium profile near the base-emitter heterojunction as depicted in  FIG. 3 . The metallurgical junction is approximately located by the pn junction formed inside the silicon germanium layer, where the n-type dopants from the emitter and/or cap layer intersect the p-type dopant from the conventional SiGe/SiGeC base layer  16 . This also defines the front edge, or base-emitter edge, of the neutral base region; this is the point where x=0. Stated differently, the neutral base region of the conventional compound base layer  16  is located to the right of x=0 in  FIG. 3 . The base-emitter edge for the conventional compound base  16  is at x=0. The intersection is typically formed during thermal anneals, which occur after the formation or growth of the film stacks making up the conventional HBT  10 .  FIG. 3  depicts a trapezoidal silicon germanium region with a ramped profile on the side of the base-emitter heterojunction. One of ordinary skill in the art will, however, recognize that other profiles are also possible. These profiles include, but are not limited to box profiles, triangular profiles, and profiles that include a curvature of shape.  
      Variations in processing, such as thermal depositions and thermal anneals that occur either during or following the NPN HBT  10  filmstack formation, can cause the metallurgical junction to vary its position relative to the base-emitter heterojunction and relative to the dopant concentrations at their point of intersection. For instance, as depicted in  FIG. 3 , sliding the metallurgical junction backwards and forwards (to simulate thermal processing effects) will result in an up and down movement of the base-emitter bandgap offset, ΔEG( 0 ). In addition, the approximate physical location of the metallurgical junction may be defined as the point at which both dopant profiles are equal. Therefore, variations to the magnitudes of dopant concentration at this point also may have implications to device characteristics. Such device parameters related to depletion region formation will be impacted to include emitter-base junction capacitance, C je . The variation of ΔEG( 0 ) will equate to variation in electron injection from emitter to base, and hence variation in current density, J c , which will cause variations in electron-current dependent device parameters such as current gain (β), unity gain cutoff frequency (F t ), and maximum oscillation frequency (F max ), and other parameters. One of ordinary skill in the art will recognize that even though  FIG. 3  depicts a trapezoidal profile; other profiles are still susceptible to this type of variation. These profiles include, but are not limited to box profiles, triangular profiles, and profiles that include a curvature of shape.  
      In order to fabricate the conventional HBT device  10 , the SiGe or SiGeC is grown on the conventional spacer layer  14 . The SiGe/SiGeC layer of the conventional base region  16  is typically pseudomorphically grown to match the lattice of the silicon in the conventional spacer  14 . Consequently, the SiGe/SiGeC is in a compressively strained state. The conventional capping layer  18  is grown on the SiGe/SiGeC. The conventional capping layer  18  helps to maintain the SiGe/SiGeC for the conventional base  16  in a strained state during thermal treatments to help reduce or prevent crystalline defects.  
      Portions of the conventional HBT device  10  are also doped during fabrication. The conventional capping layer  18  may be doped as discussed above. The addition of dopants such as C (e.g. in SiGeC) or O (oxygen) may further reduce the diffusion rate of boron in the conventional base region  16  and allow for engineering of minority carrier lifetimes and/or recombination current for added design flexibility to achieve critical performance objectives, such as, for instance current gains and breakdown voltages. The conventional emitter  20  and the conventional collector  12  are also typically doped to form an NPN or a PNP conventional HBT  10 .  
      A base-emitter metallurgical junction results from fabrication of the conventional HBT device  10 . The site of the base-emitter metallurgical junction can be approximated by the location at which the base dopant and emitter dopant are equal. The base-emitter metallurgical junction is desired to be within the conventional base region  16  in order to take advantage of the bandgap offset due to the heterojunction. The BE bandgap offset is a significant component in determining the collector current, the base current, the current gain, and F t  and F max  figures a conventional SiBe/SiBeCHBT device  10 .  
       FIG. 4  is a graph  50  depicting the dopant profiles for the conventional HBT  10 . Thus, the conventional graph  50  includes profiles illustrating the positions of the As dopant  52  for the conventional emitter region  20 , Ge dopant  56  for the SiGe/SiGeC layer of the conventional base region  16 , and boron dopant  54  for the conventional base region. Note that the specific shapes and locations of the profiles  52 ,  54 , and  56  for explanatory purposes and not necessarily meant to represent a particular real-world device. The graph  50  is a conventional scissor profile, named so because of the shapes of the profiles  52  and  54  where the profiles  52  and  54  meet. The metallurgical junction  60  is also shown in the graph  50 . The bandgap offset is denoted as ΔEG. At the metallurgical junction the bandgap offset is ΔEG (x=0) because x=0 is defined by the metallurgical junction.  
      Although the conventional HBT device  10  functions, one of ordinary skill in the art will readily recognize that the conventional HBT device  10  may be subject to significant variations in parameters such as base and collector currents as well as current gain. In particular, ΔEG (x=0) occurs in the ramp section,  56 A, of the Ge profile  56 . Consequently, the concentration of Ge dopant may vary as the position of the metallurgical junction (x=0) changes. The position of the metallurgical junction may change because of variations in thermal cycles in downstream processing as well as variations in the process of forming the metallurgical junction. Consequently, the parameters such as base current, collector current, and current gain for the conventional HBT device  10  may be unstable.  
      Accordingly, what is needed is a method and system for improving manufacturability and performance of the conventional HBT device  10 . The present invention addresses such a need.  
     BRIEF SUMMARY OF THE INVENTION  
      The present invention provides a method and system for providing a bipolar transistor. The method and system comprise providing a compound base region, providing an emitter region and providing a collector region. The emitter region is coupled with the base region. The compound base region is coupled with the collector region and includes a compound box extension. The compound box extension resides substantially between the emitter and the compound base region.  
      According to the method and system disclosed herein, the present invention allows diffusion and strain limiting impurities such as oxygen and/or carbon to be provided in a controlled manner that allows for improved performance of the bipolar transistor. 
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       FIG. 1  is a diagram of a conventional heterojunction bipolar transistor device.  
       FIG. 2  depicts the energy band structure of a conventional npn HBT device in a forward active mode. %  
       FIG. 3  depicts a film stack of conventional SiGe/SiGeC HBT device having a trapezoidal silicon germanium region with a ramped profile on the side of the base-emitter heterojunction.  
       FIG. 4  depicts the dopant profile for a conventional heterojunction bipolar transistor device.  
       FIG. 5  is a diagram of a film stack of one embodiment of a heterojunction bipolar transistor device in accordance with the present invention.  
       FIG. 6  depicts dopant profiles for one embodiment of a heterojunction bipolar transistor device in accordance with the present invention.  
       FIG. 7  depicts dopant profiles for one embodiment of a heterojunction bipolar transistor device in accordance with the present invention.  
       FIG. 8  is a flow chart depicting one embodiment of a method in accordance with the present invention for providing a heterogeneous bipolar transistor device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention relates to semiconductor devices. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein.  
      The present invention provides a method and system for providing a bipolar transistor. The method and system include providing a compound base region including includes a compound box extension, providing an emitter region, and providing a collector region. The emitter region is coupled with the base region. The SiGe base region is coupled with the collector region and includes a SiGe box extension. The box extension resides substantially between the emitter and the heterogeneous base region.  
      The present invention will be described in terms of a particular HBT device. However, one of ordinary skill in the art will readily recognize that the method and system may be applicable to other device(s) having other, additional, and/or different components, dopants, and/or positions not inconsistent with the present invention. The present invention is also described in the context of particular methods. One of ordinary skill in the art will, however, recognize that the method could have other and/or additional steps. Moreover, although the methods are described in the context of providing a single HBT device, one of ordinary skill in the art will readily recognize that multiple devices may be provided in parallel and/or series. The present invention is also described in the context of particular dopant profiles. However, one of ordinary skill in the art will readily recognize that the shapes, locations, and other features of the profiles may vary. The method is also described in the context of particular methods. However, one of ordinary skill in the art will recognize that the methods may omit or combine steps for ease of explanation. In addition, many industries allied with the semiconductor industry could make use of the hetero extension technique. For example, a thin-film head (TFH) process in the data storage industry or an active matrix liquid crystal display (AMLCD) in the flat panel display industry, or in manufacturing of laser emitting diodes (LED), and the micro-electromechanical (MEM) industry could readily make use of the processes and techniques described herein. Similarly, the method and system may be used in vertical thin film transistor (VTFTs) and strained field effect transistor (FET) devices such as strained Si, strained SiGe, and strained Ge channel devices. The method and system may also be used with other strained layers in other compounds. Examples of such devices may include but are not limited to devices using GaAs, InP, and AlGaAs. One of ordinary skill in the art will readily recognize that the present invention may be used in conjunction with such devices. Thus, the terms used herein, including but not limited to the term semiconductor, may thus include the aforementioned and related industries.  
       FIG. 5  is a diagram of one embodiment of a filmstack of a heterojunction bipolar transistor device  100  in accordance with the present invention. The HBT device  100  is formed on a substrate  101 . The HBT device  100  includes a collector region  102 , a compound base region  106  having a box extension, described below, and an emitter region  110 . The HBT device  100  may also include a spacer layer  104  and a capping layer  108 . The spacer layer  104  and the capping layer  108  are analogous to the conventional spacer layer  14  and the conventional capping layer  18  described in  FIG. 1 .  
      Referring back to  FIG. 5 , the spacer layer  104  is typically undoped silicon. The compound base region  106  is typically formed from a layer that is typically SiGe or SiGeC. Consequently, a base-emitter heterojunction and a base-emitter metallurgical junction are present in the HBT  100 . The SiGe/SiGeC for the base layer  106  is preferably pseudomorphically grown on the spacer layer  104  to match the lattice of the silicon in the spacer layer  104 . Consequently, the SiGe/SiGeC is in a compressively strained state. The capping layer  108  is grown on the SiGe/SiGeC and helps to maintain the SiGe/SiGeC for the conventional base  16  in a strained state during subsequent thermal treatments to help reduce or prevent crystalline defects. The base region  106  formed in the SiGe/SiGeC layer is doped, generally using boron to provide a p-type base. The Ge in the SiGe/SiGeC of the base region  106  results in a lower diffusion rate for the boron dopant. The addition of other dopants such as C (e.g. in SiGeC) or O may further reduce the diffusion rate of boron in the base region  106 . Although the capping layer  108  is typically undoped silicon, the capping layer  108  might be doped, for example using arsenic or phosphorus. The emitter  110  and the collector  102  are also typically doped to form an NPN or a PNP HBT  100 .  
      As mentioned above, the compound base region  106  includes a box extension  160 / 160 ′ described in more detail below. For the HBT device  100 , the box extension  160 / 160 ′ includes at least a Ge dopant, and may include other dopants. Consequently, The box extension  160 / 160 ′ will be termed a compound box extension. The compound box extension  160 / 160 ′ may enhance the thermal stability of the HBTO device  100 , and may reduce the sensitivity of HBT  100  to variations in formation, may provide improved control over device parameters such as collector current, base current and current gain.  
       FIG. 6  is a graph  120  depicting one embodiment of dopant profiles in accordance with the present invention for one embodiment of the HBT device  100  in accordance with the present invention. The graph  120  depicts the compound box extension  160  among other features of the HBT device  100 . The graph  120  is for an HBT device  100  having an n-type emitter formed using As as a dopant and a p-type base using B as a dopant. However, the same principles apply for other dopants including dopants having other types. Thus, the As profile (emitter dopant profile)  130  for the emitter  110 , the B profile (base dopant profile)  140  for the compound base  106 , and the Ge profile  150  for the SiGe/SiGeC are depicted. Also shown are the Ge extension  152  and an emitter extension  132  for the As dopant. The Ge profile  150  includes a peak within the neutral base region of the compound base  110 .  
      The compound box extension  160  includes the extensions  152  and  132  for the Ge profile  150  of the SiGe/SiGeC and the As  130  for the emitter  130 , respectively. In another embodiment, the B profile  140  may include a box extension (not shown) analogous to the extension  132  in lieu of the extension  132 . The Ge extension  152  has a concentration that is relatively flat and has a value that is less than the peak concentration  141  of the profile  140  for the base dopant. In a preferred embodiment, the Ge extension  152  for the compound box extension  160  has a concentration of at least one percent and not more than twenty-five percent Ge. Also in a preferred embodiment, the Ge extension  152  has a length of 0.1 nm to fifteen nm. Although depicted with a particular Ge profile  150 , the Ge extension  152  may be used with virtually any profile, including but not limited to grade, triangle, and trapezoid profiles of the Ge. In addition, the compound box extension  160  includes an n-type dopant, here As extension  132 . The emitter extension  132  is also relatively flat and, in a preferred embodiment, has a concentration that is less than the maximum for the emitter dopant  130 . In a preferred embodiment, the emitter extension  132  has a concentration of at least 5×10 16  atoms/cm 3  and not more than 5×10 19  atoms/cm 3 . Furthermore, both the extensions  152  and  132  are both in the box extension  160 . Stated differently, the extensions  152  and  132  overlap. In addition, the base dopant B  140  overlaps the emitter dopant at the emitter extension  132 .  
      Because of the use of the compound box extension  160 , including extensions  152  and  132 , the HBT device  100  may have improved performance. In particular, the metallurgical junction, ΔE(x=0) is where the base dopant B  140  and the emitter dopant, the extension  132 , cross. This metallurgical junction is within the compound region  152  of the compound box extension  160 . Thus, the metallurgical junction occurs where the Ge concentration and the As concentration are relatively constant. Because the metallurgical junction occurs where the Ge concentration is substantially the same, the processing related variations in performance of the HBT device  100  are reduced. Stated differently, the variations in the exact position of the metallurgical junction may not significantly alter parameters of the HBT device  100 . In addition, the compound box extension  160  may improve the thermal stability of the metallurgical junction. Consequently, parameters such as collector and base currents and current gain may be more closely controlled.  
       FIG. 7  depicts another graph  120 ′ of embodiment of dopant profiles in accordance with the present invention for another embodiment of the HBT device  100 . The graph  120 ′ is analogous to the graph  120 . Consequently, analogous components are labeled similarly. The graph  120 ′ is for an HBT device  100  having an n-type emitter formed using As as a dopant and a p-type base using B as a dopant. However, the same principles apply for other dopants including dopants having other types. Thus, the As profile  130 ′ for the emitter  110 , the B profile  140 ′ for the base  106 , and the Ge profile  150 ′ for the SiGe/SiGeC are depicted. Also shown is the box extension  160 ′ that includes the Ge extension  152 ′, optional emitter extension  132 ′ for the As dopant, and the base extension  142 . The Ge profile  150 ′ includes a peak within the neutral base region of the compound base  110 ′.  
      The box extension  160 ′ includes the extensions  152 ′,  132 ′, and  142  for the Ge profile  150 ′ of the SiGe/SiGeC, the As profile  130 ′ for the emitter  110 , and the B profile  140 ′ for the base  106 , respectively. The Ge extension  152 ′ has a concentration that is relatively flat and has a value that is less than the peak concentration  141 ′ of the profile  140 ′ for the base dopant. In a preferred embodiment, the Ge extension  152 ′ for the box extension  160 ′ has a concentration of at least one percent and not more than twenty-five percent Ge. Also in a preferred embodiment, the Ge extension  152 ′ has a length of 0.1 nm to fifteen nm. Although depicted with a particular Ge profile  150 ′, the Ge extension  152 ′ may be used with virtually any profile, including but not limited to grade, triangle, and trapezoid profiles of the Ge. In addition, the box extension  160 ′ includes an n-type dopant as well as a p-type dopant, which are emitter extension  132 ′ and base extension  142 , respectively. The emitter extension  132 ′ is relatively flat and, in a preferred embodiment, has a concentration that is less than the maximum for the emitter dopant  130 ′. In a preferred embodiment, the emitter extension  132 ′ has a concentration of at least 5×10 16  atoms/cm 3  and not more than 5×10 19  atoms/cm 3 . The base extension  142 ′ is relatively flat and, in a preferred embodiment, has a concentration that is less than the maximum for the base dopant  140 ′. Furthermore, the extensions  132 ′,  142 , and  152 ′ are in the box extension  160 . Stated differently, the extensions  132 ,  142 , and  152 ′ overlap.  
      Because of the use of the box extension  160 ′, including extensions  132 ′,  142  and  152 ′, the HBT device  100  may have improved performance. In particular, the metallurgical junction, ΔE(x=0) is approximately located in the region where the base dopant in the base extension  142  and the emitter dopant in the emitter extension  132 ′ cross. This metallurgical junction is substantially within the compound box region  152 ′ of the box extension  160 ′. Thus, the approximate “center” of the metallurgical junction occurs where the Ge concentration, the B concentration, and the As concentration are relatively equivalent. Because the metallurgical junction occurs where the Ge concentration is substantially the same, the processing related variations in performance of the HBT device  100 ′ are reduced. Stated differently, the variations in the exact position of the metallurgical junction may not significantly alter parameters of the HBT device  100 ′. In addition, the box extension  160 ′ may improve the thermal stability of the metallurgical junction. Consequently, parameters such as collector and base currents and current gain may be more closely controlled. Thus, the HBT device may have improved characteristics.  
       FIG. 8  is a flow chart depicting another embodiment of a method  200  in accordance with the present invention for providing a heterogeneous bipolar transistor device. The method  200  is described in the context of the HBT device  100 . However, one of ordinary skill in the art will readily recognize that the method  200  could be used with other HBT devices in accordance with the present invention. One of ordinary skill in the art will also recognize that for ease of explanation, steps may be omitted, combined, or performed in a different order. A collector region  102  is provided, via step  202 . Step  202  preferably includes doping the collector region  102  with the desired n-type or p-type dopant(s) depending on the transistor type, npn or pnp. In general, n-type dopants such as As or P are used, while boron is used as a p-type dopant. A SiGe spacer layer  104  may optionally be provided, via step  204 . A compound base region  106  is provided, via step  206 . The compound base region  106  is coupled with the collector region  102 . In some embodiments, the compound base region  106  is provided such that a seed layer and the spacer layer  104  may reside between the compound base region  106  and the collector region  102 . Step  206  may include growing a seed layer and the collector spacer layer  104  as well as the SiGe/SiGeC base layer  106 . The compound base region  106  is preferably grown with the desired dopant(s), such as B added in situ. A compound box extension  160 / 160 ′ is provided, via step  208  in the same process chamber as  206  and immediately after  206 . In addition, an emitter spacer layer and the capping layer  108  may be provided between the emitter  110  and the base  106  in the same process chamber immediately after the SiGe box extension  160 / 160 ′ is provided. The emitter  110  may be provided, via step  210 .  
      The SiGe box extension  160 / 160 ′ is provided in step  208 . Step  208  includes doping the SiGe box extension  160 / 160 ′ with some combination of dopants used for the SiGe/SiGeC base  106  and the emitter  110 . In a preferred embodiment, this includes using some or all of the impurities C,  0 , P, As, and B as dopants in some combination. In one embodiment, the stack comprised of layers  104 ,  106 , and  108  may be grown using a chemical vapor deposition process with deposition temperatures ranging between 500° C. and 900° C. Arsine (AsH 3 ), Phosphine (PH 3 ), and Diobrane (B 2 H 6 ) may be used as precursors for n and p type dopants. If C and/or O are used in growing this stack, then methyl silane (CH 3 SiH 3 ) may be used s the carbon source and heliox or other gases containing oxygen may be used as the oxygen source. Some dopants in this stack may also be implanted. C or O may be used throughout the box extension or only part of the box extension  160 .  
      Thus, using the method  200 , the HBT device  100  may be formed. Thus, the method  200  may provide an HBT device that has reduced processing related variations, including improved the thermal stability of the metallurgical junction. This is due to the metallurgical emitter-base junction residing in a region of constant Ge concentration, resulting in better control over device parameters such as collector current and current gain.  
      A method and system for providing a heterojunction bipolar transistor has been disclosed. The present invention has been described in accordance with the embodiments shown, and one of ordinary skill in the art will readily recognize that there could be variations to the embodiments, and any variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.