Abstract:
In one disclosed embodiment, a collector is deposited and a base is grown on the collector, for example, by epitaxially depositing either silicon or silicon-germanium. An emitter is fabricated on the base followed by implant doping an extrinsic base region. For example, the extrinsic base region can be implant doped using boron. The extrinsic base region doping diffuses out during subsequent thermal processing steps in chip fabrication, creating an out diffusion region in the device, which can adversely affect various operating characteristics, such as parasitic capacitance and linearity. The out diffusion is controlled by counter doping the out diffusion region. For example, the counter doped region can be implant doped using arsenic or phosphorous. Also, for example, the counter doped region can be formed using tilt implanting or, alternatively, by implant doping the counter doped region and forming a spacer on the base prior to implanting the extrinsic base region.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to the field of fabrication of semiconductor devices. More specifically, the invention relates to the fabrication of silicon-germanium semiconductor devices.  
           [0003]    2. Related Art  
           [0004]    In a heterojunction bipolar transistor (“HBT”), a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer. The silicon-germanium HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. Speed and frequency response can be compared by the cutoff frequency which, simply stated, is the frequency where the gain of a transistor is drastically reduced. Cutoff frequencies in excess of 100 GHz have been achieved for the HBT, which are comparable to the more expensive GaAs. Previously, silicon-only devices have not been competitive for use where very high speed and frequency response are required.  
           [0005]    The higher gain, speeds, and frequency response of the HBT have been achieved as a result of certain advantages of silicon-germanium not available with pure silicon, for example, narrower band gap, and reduced resistivity. In addition, silicon-germanium may be epitaxially grown on silicon wafers using conventional silicon processing and tools, and allows one to engineer device properties such as the band gap, energy band structure, and mobilities. For example, it is known in the art that grading the concentration of germanium in the silicon-germanium base builds into the HBT device an electric field, which accelerates the carriers across the base, thereby increasing the speed of the HBT device compared to a silicon-only device. One method for fabricating silicon and silicon-germanium devices is by chemical vapor deposition (“CVD”). A reduced pressure chemical vapor deposition technique, or RPCVD, used to fabricate the HBT device allows for a controlled grading of germanium concentration across the base layer.  
           [0006]    Because the benefits of a high gain and high speed silicon-germanium HBT device can be either partially or completely negated by high base contact resistance, it is important that the resistance of the base contact be kept low. In addition to the contact resistance, the geometry of the base regions may also affect the base resistance. The geometry of the base region may necessitate providing a low resistance electrical pathway through a portion of the base itself between the base contact and the base-emitter junction, referred to as the extrinsic base region. The extrinsic base region is heavily doped by implantation (also called extrinsic doping) in order to provide reduced resistance from the base contact to the base-emitter junction.  
           [0007]    During the manufacture of an integrated circuit chip there are many processing steps which involve heating the wafer in which the integrated circuit chip is included. It is normal for dopants to diffuse out from where they have been implanted into surrounding areas of the chip during these heating processes. Typically, the out diffusion of dopants is accounted for in the design of a circuit device such as the HBT. Unwanted out diffusion can have disadvantageous effects, however, especially under certain circumstances. For example, there is drive in current technology to operate the HBT at lower voltages and comparatively higher collector currents for those low voltages. When the HBT is operated in a range of low voltage and high collector current, the effects of an energy barrier at the metallurgical transition from silicon-germanium to silicon near the base-collector junction can become more pronounced. Such an operating range can be characterized, for example, by a collector-emitter voltage in the range of approximately 1.0 to 4.0 volts and a collector current in the range of approximately 0 to 3.0 milliamperes (“mA”). Under these operating conditions, an energy barrier at the metallurgical transition from silicon-germanium to silicon near the base-collector junction has an effect of limiting current flow through the collector.  
           [0008]    Out diffusion of dopants from the heavily doped extrinsic base region acts to further restrict collector current flow under these conditions and there are other deleterious effects on the operating characteristics and device parameters of the HBT device. For example, the operating range over which the HBT can operate linearly as class A amplifier is reduced. Briefly stated, a device operates as a class A amplifier if output current flows for all values of the input, as opposed to, for example, class B operation in which output current flows for one-half the cycle of the input waveform. Linear operation, simply stated, is the amplification of an input signal without distortion. A wider operating range for linear class A operation, i.e. one in which the maximum and minimum voltages and currents of the device are spaced further apart, is desirable because design flexibility and reliability are increased. As another example, power output in class A operation can be reduced because the reduced collector current directly reduces power which, simply stated is the product of current times voltage.  
           [0009]    As a further example, out diffusion of dopants from the heavily doped extrinsic base region may increase a parasitic capacitance between the base and collector. Briefly, capacitance in an electric circuit relates to an effective flow of current due to the storage of electric charge between two otherwise electrically separated conductors. Parasitic capacitance between the base and collector effectively represents a near-short circuit in the HBT for a high frequency signal being amplified and is, thus, undesirable.  
           [0010]    Also, for example, out diffusion of dopants from the heavily doped extrinsic base region can reduce a breakdown voltage of the HBT device. Briefly, the presence of a voltage, greater than the breakdown voltage, between the base region and a conductive region below the collector can cause the intervening material, which physically and electrically separates the two, to start to conduct electricity, known as “breakdown” of the intervening material. When breakdown occurs the HBT device no longer functions as intended, and can be permanently damaged. Thus, it is undesirable for breakdown voltage to be reduced.  
           [0011]    Moreover, the effects of out diffusion on a device can limit the scalability of the device. Scalability, simply stated, refers to preserving the relative proportions of the various features of a device in such a way that the device still functions when the overall size of the entire device is reduced. As feature sizes of bipolar devices are reduced, it is important to achieve accurate control over the size of the various features in order to keep feature sizes in proportion. So for example, if out diffusion is not properly controlled the relative size of the out diffusion regions increases as the size of the entire bipolar device is reduced. An increase in the relative size of the out diffusion regions exacerbates the problems and disadvantages described above. Thus, the effects of out diffusion on the device can limit the scalability of the bipolar device. Furthermore, as feature size of CMOS devices is reduced it is important to achieve a concomitant reduction of feature size in bipolar devices on the same chip as CMOS devices.  
           [0012]    One approach to the problem has been to use carbon in conjunction with the implant doping of the extrinsic base regions as a “suppressant” to control the amount of subsequent out diffusion of dopants from the extrinsic base regions. In general, the use of carbon for control of diffusion is complicated to implement from a technological viewpoint, is not generally available in the industry, and can require expensive tooling or retooling of the fabrication facility.  
           [0013]    Thus, there is a need in the art to control out diffusion of dopants in bipolar devices. There is also need in the art for technologically simple, relatively low cost, readily available control of out diffusion of dopants in bipolar devices. There is a further need in the art for fabrication of bipolar devices which is scalable as the size of MOS and CMOS devices decreases.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention is directed to a high performance bipolar transistor. The invention is used to control out diffusion of dopants in bipolar devices. The invention overcomes the need in the art for technologically simple, relatively low cost, readily available control of out diffusion of dopants in bipolar devices. The invention also provides fabrication of bipolar devices which is scalable to the size of MOS and CMOS devices as the size of MOS and CMOS devices decreases.  
           [0015]    In one aspect of the invention a collector is deposited and a base is grown on the collector. For example, the base can be grown by epitaxially depositing either silicon or silicon-germanium on the collector. An emitter is then fabricated on the base followed by implant doping an extrinsic base region outside the emitter. For example, the extrinsic base region can be implant doped using boron with an implant dose of approximately 10 15  atoms per square centimeter. The extrinsic base region doping diffuses out during subsequent thermal processing steps in chip fabrication, creating an out diffusion region in the device. The out diffusion region can adversely affect various operating characteristics of the device, such as parasitic capacitance and linearity. The out diffusion is controlled by counter doping the out diffusion region. For example, the counter doped region can be implant doped using arsenic or phosphorous with an implant dose of approximately 10 13  atoms per square centimeter. Also, for example, the counter doped region can be formed using tilt implanting or, alternatively, by implant doping the counter doped region and then forming a spacer on the base prior to implanting the extrinsic base region.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 illustrates a cross sectional view of some of the features of an NPN HBT.  
         [0017]    [0017]FIG. 2 illustrates a cross sectional view of some of the features of an NPN HBT fabricated in accordance with one embodiment of the present invention.  
         [0018]    [0018]FIG. 3 illustrates in greater detail a portion of the cross sectional view of FIG. 2, and shows a cross sectional view of some of the features of an NPN HBT fabricated in accordance with one embodiment of the present invention.  
         [0019]    [0019]FIG. 4 illustrates a cross sectional view, at a level of detail similar to that of FIG. 3, of some of the features of an NPN HBT fabricated in accordance with another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    The present invention is directed to a high performance bipolar transistor. The following description contains specific information pertaining to the implementation of the present invention. One skilled in the art will recognize that the present invention may be implemented in a manner different from that specifically discussed in the present application. Moreover, some of the specific details of the invention are not discussed in order to not obscure the invention. The specific details not described in the present application are within the knowledge of a person of ordinary skill in the art.  
         [0021]    The drawings in the present application and their accompanying detailed description are directed to merely example embodiments of the invention. To maintain brevity, other embodiments of the invention which use the principles of the present invention are not specifically described in the present application and are not specifically illustrated by the present drawings.  
         [0022]    By way of background, FIG. 1 shows a cross sectional view of various features and components of structure  100  which includes various features and components of an NPN heterojunction bipolar transistor (“HBT”). Structure  100  includes collector  104 , base  120 , and emitter  130 . Collector  104  is N type single crystal silicon which can be deposited epitaxially using an RPCVD process in a manner known in the art. Base  120  is P-type silicon-germanium single crystal deposited epitaxially, for example, in a “nonselective” RPCVD process. As seen in FIG. 1, base  120  is situated on top of, and forms a junction with, collector  104 . Base contacts  121  are polycrystalline silicon-germanium deposited epitaxially, for example, in a “nonselective” RPCVD process. Emitter  130 , which is situated above and forms a junction with base  120 , is comprised of N-type polycrystalline silicon. Collector  104 , base  120 , and emitter  130  thus form a heterojunction bipolar transistor, or HBT, which is generally referred to by numeral  150  in FIG. 1.  
         [0023]    As seen in FIG. 1, buried layer  102 , which is composed of N+ type material—meaning that it is relatively heavily doped N-type material—is formed in silicon substrate  101  in a manner known in the art. Collector sinker  106 , also composed of N+ type material, is formed by diffusion of heavily concentrated dopants in N-type epitaxial silicon  105  from the surface of collector sinker  106  down to buried layer  102 . Buried layer  102 , along with collector sinker  106 , provide a low resistance electrical pathway from collector  104  through buried layer  102  and collector sinker  106  to a collector contact (the collector contact is not shown in any of the Figures). Deep trench  108  and field oxide  110  isolation structures composed of silicon oxide (SiO 2 ) material are formed in a manner known in the art. Deep trench  108  and field oxide  110  isolation structures provide electrical isolation from other devices on silicon substrate  101  in a manner known in the art.  
         [0024]    Also as seen in FIG. 1, emitter  130  also comprises “out-diffusion region”  132  formed by the out-diffusion of N+ dopants from the polycrystalline silicon of emitter  130  into the single crystal layer below emitter  130 . As seen in FIG. 1, polycrystalline silicon emitter  130  is situated above N+ out-diffusion region  132 . Dielectric segments  140 , which can be composed of silicon oxide, provide electrical isolation to emitter  130  from base  120 .  
         [0025]    Base  120  includes intrinsic base region  122 . Single crystal N+ out-diffusion region  132  is situated above single crystal intrinsic base region  122 . The base-emitter junction is formed within the single crystal layer at the boundary of N+ out-diffusion region  132  and intrinsic base region  122 . Intrinsic base region  122  and base contacts  121  are electrically connected with each other through extrinsic base regions  124 . Extrinsic base regions  124  and intrinsic base region  122  comprise base  120 .  
         [0026]    Continuing with FIG. 1, N+ out-diffusion region  132  in the single crystal layer is formed by out-diffusion of heavy concentration of N type—for example, arsenic—dopants after ion implantation doping of polycrystalline silicon emitter  130 . The N+ doping renders emitter  130  an N-type emitter. Ion implantation of extrinsic base regions  124  has resulted in the heavily doped P+ implanted extrinsic base regions  124 . In one embodiment, the dopant used to form implanted extrinsic base regions  124  can be boron. The heavy doping in implanted extrinsic base regions  124  lowers the overall resistance of extrinsic base regions  124 . The overall base resistance of HBT  150  is thereby improved by lowering the contribution of extrinsic base regions  124  to the series resistance of the path from the base contacts, through the heavily doped extrinsic base regions  124  to intrinsic base region  122 . Out diffusion from heavily doped P+ implanted extrinsic base regions  124  into N-epitaxial silicon  105  forms P+ out diffusion regions  126 .  
         [0027]    As seen in FIG. 1, P+ out diffusion regions  126  surround collector  104 , degrading the performance of HBT  150  under certain conditions, as described above. For example, P+ out diffusion regions  126  effectively decrease distance W Cx    155 , which is the distance from the highly conductive region of buried layer  102  to the highly conductive region of extrinsic base regions  124 . Decreasing distance W Cx    155  tends to increase base-collector parasitic capacitance and decrease breakdown voltage of HBT  150 , as described above. Thus, FIG. 1 illustrates an example of the some of the effects of dopant out diffusion on a silicon-germanium HBT.  
         [0028]    [0028]FIG. 2 shows a cross sectional view of various features and components of structure  200  which includes various features and components of an NPN HBT fabricated in accordance with one embodiment. Structure  200  includes features and components which are analogous in form and function to corresponding features and components of structure  100  in FIG. 1. The corresponding features and components are numbered in a manner consistent with FIG. 1. In particular, portions of selected features and components of structure  100  of FIG. 1 corresponding to collector  104 , base  120 , and emitter  130  are shown, respectively, as collector  204 , base  220 , and emitter  230 .  
         [0029]    In addition, silicon substrate  101 , buried layer  102 , epitaxial silicon  105 , collector sinker  106 , deep trench  108  and field oxide  110  isolation structures, base contacts  121 , intrinsic base region  122 , extrinsic base regions  124 , out diffusion regions  126 , out-diffusion region  132 , dielectric segments  140 , and distance W Cx    155  are shown, respectively, as silicon substrate  201 , buried layer  202 , epitaxial silicon  205 , collector sinker  206 , deep trench  208  and field oxide  210  isolation structures, base contacts  221 , intrinsic base region  222 , extrinsic base regions  224 , out diffusion regions  226 , out-diffusion region  232 , dielectric segments  240 , and distance W Cx    255 . Thus, collector  204 , base  220 , and emitter  230  form NPN HBT  250  which is analogous to NPN HBT  150  in FIG. 1. The region enclosed by dashed line  260  corresponds to structures  360  of FIG.  3  and  460  of FIG. 4, which show the area enclosed by dashed line  160  in greater detail.  
         [0030]    Structure  200  includes collector  204 , base  220 , and emitter  230 . Collector  204  is N type single crystal silicon which can be deposited epitaxially using an RPCVD process in a manner known in the art. Base  220  is P-type silicon-germanium single crystal which can be deposited epitaxially in a “nonselective” RPCVD process according to one embodiment. As seen in FIG. 2, base  220  is situated on top of, and forms a junction with, collector  204 . Base contacts  221  are polycrystalline silicon-germanium which can be deposited epitaxially in a “nonselective” RPCVD process according to one embodiment. Emitter  230 , which is situated above and forms a junction with base  220 , is comprised of N-type polycrystalline silicon. Collector  204 , base  220 , and emitter  230  form NPN HBT  250  analogous to NPN HBT  150  in FIG. 1.  
         [0031]    As seen in FIG. 2, buried layer  202 , composed of N+ type material, is formed in silicon substrate  201 . Collector sinker  206 , also composed of N+ type material, is formed by diffusion of heavily concentrated dopants in N-type epitaxial silicon  205  from the surface of collector sinker  206  down to buried layer  202 . Buried layer  202 , along with collector sinker  206 , provide a low resistance electrical pathway from collector  204  through buried layer  202  and collector sinker  206  to a collector contact (the collector contact is not shown in any of the Figures). Deep trench  208  and field oxide  210  isolation structures composed of silicon oxide, which can be formed in a manner known in the art, provide electrical isolation from other devices on silicon substrate  201 , as known in the art. Also, emitter  230  comprises out-diffusion region  232  formed by the out-diffusion of N+ dopants from the polycrystalline silicon of emitter  230  into the single crystal layer below emitter  230 . Emitter  230  is situated above N+ out-diffusion region  232 . Dielectric segments  240 , which can be composed of silicon oxide, provide electrical isolation to emitter  230  from base  220 . Base  220  includes intrinsic base region  222 . Single crystal N+ out-diffusion region  232  is situated above single crystal intrinsic base region  222 . The base-emitter junction is formed within the single crystal layer at the boundary of N+ out-diffusion region  232  and intrinsic base region  222 . Intrinsic base region  222  and base contacts  221  are electrically connected with each other through extrinsic base regions  224 . Extrinsic base regions  224  and intrinsic base region  222  comprise base  220 .  
         [0032]    Continuing with FIG. 2, N+ out-diffusion region  232  in the single crystal layer is formed by out-diffusion of heavy concentration of N type—for example, arsenic—dopants after ion implantation doping of polycrystalline silicon emitter  230 . The N+ doping renders emitter  230  an N-type emitter. Ion implantation of extrinsic base regions  224  has resulted in the heavily doped P+ implanted extrinsic base regions  224 . In one embodiment, the dopant used to form implanted extrinsic base regions  224  can be boron. The heavy doping in implanted extrinsic base regions  224  lowers the overall resistance of extrinsic base regions  224 . The overall base resistance of HBT  250  is thereby improved by lowering the contribution of extrinsic base regions  224  to the series resistance of the path from the base contacts, through the heavily doped extrinsic base regions  224  to intrinsic base region  222 . Out diffusion from heavily doped P+ implanted extrinsic base regions  224  into N-epitaxial silicon  205  forms P+ out diffusion regions  226 .  
         [0033]    As seen in FIG. 2, P+ out diffusion regions  226  surround collector  204 , degrading the performance of HBT  250  under certain conditions, as described above. For example, P+ out diffusion regions  226  can effectively decrease distance W Cx    255 , which is the distance from the highly conductive region of buried layer  202  to the highly conductive region of extrinsic base regions  224 . Decreasing distance W Cx    255  tends to increase base-collector parasitic capacitance and decrease breakdown voltage of HBT  250 , as described above. Effects of dopant out diffusion can be suppressed or counteracted by forming counter-doped regions  228 . When HBT  250  is viewed as a 3-dimensional device, counter-doped regions  228  form a single connected region in the form of a halo around collector  204 , thus the process of forming counter-doped regions  228  is also referred to as “halo” doping.  
         [0034]    The process of counter doping is known as applied in the fabrication of metal oxide semiconductor (“MOS”) and complementary metal oxide semiconductor (“CMOS”) devices such as field effect transistors (“FET”). For example, an application of counter doping to FET devices using tilt implant doping is described in “A 0.1-μm CMOS Technology with Tilt Implanted Punchthrough Stopper (TIPS)” by Takahashi HORI, in  IEDM  94, pp 75-78, copy right 1994 by the Institute for Electrical and Electronics Engineers (“IEEE”). As another example, an application of counter doping to FET devices is described in “Source-to-Drain Nonuniformly Doped Channel (NUDC) MOSFET Structures for High Current Drivability and Threshold Voltage Controllability” by Yoshinori Okumura, et al., IEEE Transactions on Electron Devices, vol. 39, no. 11, pp 2541-52, November 1992. Counter doping in FET devices, as described in the two examples cited, is used for entirely different reasons compared to the reasons for its application in the present invention as described herein. Moreover, the results of counter doping in FET devices, such as described in the two cited examples, are entirely different from the results achieved by the invention as described in the present application.  
         [0035]    Continuing with FIG. 2, counter-doped regions  228  composed of N+ type material can be formed, for example, by implant doping of arsenic or phosphorous. The overlapping concentrations of N type dopants from counter-doped regions  228  with the concentrations of P type dopants from out diffusion regions  226  forms cross doped regions  227 . Thus, gradients are formed in the concentration of dopants. In other words, the concentration of dopants varies within out diffusion regions  226 , cross doped regions  227 , and counter-doped regions  228 . For example, the concentration of (P type) boron in out diffusion regions  226  and cross doped regions  227 , in one embodiment, can be approximately 10 20  atoms per cubic centimeter, which can be produced using an implant dose of approximately 10 15  atoms per square centimeter. The concentration of (N type) arsenic or phosphorous in cross doped regions  227  and counter-doped regions  228 , for example, can range from approximately 5*10 17  to approximately 10 19  atoms per cubic centimeter, which can be produced using an implant dose of approximately 10 13  atoms per square centimeter. Also, for example, the concentration of N type dopants can form a gradient from the relatively high concentrations of approximately 10 19  atoms per cubic centimeter in counter-doped regions  228  to a lower concentration of approximately 10 18  atoms per cubic centimeter in collector  204 , to a lower concentration of approximately 10 16  atoms per cubic centimeter in epitaxial silicon  205 . Thus, out diffusion regions  226 , cross doped regions  227 , and counter-doped regions  228  are well-defined but without sharp boundaries. Thus, FIG. 2 illustrates an example of counter doping to suppress out diffusion of dopants and counteract degrading effects of dopant out diffusion on the operating characteristics of a silicon-germanium HBT.  
         [0036]    [0036]FIG. 3 shows a more detailed cross sectional view of selected features and components of structure  200  of FIG. 2. FIG. 3 shows structure  360  comprising a region of NPN HBT  250  fabricated in accordance with one embodiment that corresponds to the region of NPN HBT  250  enclosed by dashed line  260  of FIG. 2. Structure  360  includes features and components which are analogous in form and function to corresponding features and components of structure  200  in FIG. 2. The corresponding features and components are numbered in a manner consistent with FIG. 2. In particular, portions of selected features and components of structure  200  of FIG. 2 corresponding to emitter  230 , dielectric segments  240 , out-diffusion region  232 , base  220  including intrinsic base region  222  and extrinsic base regions  224 , collector  204 , epitaxial silicon  205 , out diffusion regions  226 , cross doped regions  227 , and counter-doped regions  228  enclosed by dashed line  260  in FIG. 2 are shown, respectively, as emitter  330 , dielectric segments  340 , out-diffusion region  332 , base  320  including intrinsic base region  322  and extrinsic base regions  324 , collector  304 , epitaxial silicon  305 , out diffusion regions  326 , cross doped regions  327 , and counter-doped regions  328  in structure  360  in FIG. 3.  
         [0037]    As FIG. 3 shows, in one embodiment, extrinsic base regions  324  can be P type doped using implant doping. The implanting can be applied at an angle, also referred to as “tilt implanting” or “angle implanting” or applied in a non-angled manner, i.e. conventionally. In one embodiment, extrinsic base regions  324  can be conventionally implant doped by performing P type implant doping  344 , represented by non-angled arrows  344  in FIG. 3. For example, extrinsic base regions  324  can be implant doped using conventional implanting with boron, using an implant dose of approximately 10 15  atoms per square centimeter, to produce P+ doped extrinsic base regions  324  with a dopant concentration of approximately 10 20  atoms per cubic centimeter, as noted above in connection with FIG. 2. P type implant doping  344  can be either followed or preceded by N type implant doping of counter-doped regions  328 . N type counter-doped regions  328  can be formed using tilt implanting  342 , represented by angled arrows  342  in FIG. 3. For example, counter-doped regions  328  can be implant doped using tilt implanting  342  with arsenic or phosphorous, using an implant dose of approximately 10 13  atoms per square centimeter, to produce N+ doped counter-doped regions  328  with a dopant concentration in the range of approximately 5*10 17  to approximately 10 19  atoms per cubic centimeter, as noted above in connection with FIG. 2.  
         [0038]    Thus, FIG. 3 illustrates an example, using tilt implanting, of implementing counter doping to suppress out diffusion of dopants and counteract degrading effects of dopant out diffusion on the operating characteristics of a silicon-germanium HBT by sequentially implant doping N and P type dopants.  
         [0039]    [0039]FIG. 4 shows a more detailed cross sectional view of selected features and components of structure  200  of FIG. 2. FIG. 4 shows structure  460  comprising a region of NPN HBT  250  fabricated in accordance with one embodiment that corresponds to the region of NPN HBT  250  enclosed by dashed line  260  of FIG. 2. Structure  460  includes features and components which are analogous in form and function to corresponding features and components of structure  200  in FIG. 2. The corresponding features and components are numbered in a manner consistent with FIG. 2. In particular, portions of selected features and components of structure  200  of FIG. 2 corresponding to emitter  230 , dielectric segments  240 , out-diffusion region  232 , base  220  including intrinsic base region  222  and extrinsic base regions  224 , collector  204 , epitaxial silicon  205 , out diffusion regions  226 , cross doped regions  227 , and counter-doped regions  228  enclosed by dashed line  260  in FIG. 2 are shown, respectively, as emitter  430 , dielectric segments  440 , out-diffusion region  432 , base  420  including intrinsic base region  422  and extrinsic base regions  424 , collector  404 , epitaxial silicon  405 , out diffusion regions  426 , cross doped regions  427 , and counter-doped regions  428  in structure  460  in FIG. 4.  
         [0040]    As FIG. 4 shows, in one embodiment, N type counter-doped regions  428  can be formed using conventional, i.e. non-angled, implant doping. Counter-doped regions  428  can be formed using N type implant doping  446 , represented by non-angled arrows in FIG. 4. For example, counter-doped regions  428  can be implant doped with arsenic or phosphorous, using an implant dose of approximately 10 13  atoms per square centimeter, to produce N+ doped counter-doped regions  428  with a dopant concentration in the range of approximately 5*10 17  to approximately 10 19  atoms per cubic centimeter, as noted above in connection with FIG. 2.  
         [0041]    Implant doping  446  is followed by the formation of spacers  441 . Spacers  441  can be formed, for example, by depositing a conformal layer of dielectric, such as silicon oxide, over emitter  430  and then etching back the conformal layer. Formation of spacers  441  is followed by P type doping  448  of extrinsic base regions  424  using conventional, i.e. non-angled, implanting. In one embodiment, extrinsic base regions  424  can be conventionally implant doped using P type doping  448 , represented by non-angled arrows in FIG. 4. For example, extrinsic base regions  424  can be implant doped using non-angled implanting with boron, using an implant dose of approximately 10 15  atoms per square centimeter, to produce P+ doped extrinsic base regions  424  with a dopant concentration of approximately 10 20  atoms per cubic centimeter, as noted above in connection with FIG. 2.  
         [0042]    Thus, FIG. 4 illustrates an example, using conventional implanting, of implementing counter doping to suppress out diffusion of dopants and counteract degrading effects of dopant out diffusion on the operating characteristics of a silicon-germanium HBT by sequentially implant doping N and P type dopants.  
         [0043]    It is appreciated by the above detailed description that the invention provides a method for controlling out diffusion of dopants in the fabrication of bipolar transistors. The method eliminates problems associated with out diffusion of dopants from the heavily doped extrinsic base regions near the base-collector junction of a bipolar transistor. Using the invention, out diffusion of dopants can be controlled to improve the linearity, power output, base-collector parasitic capacitance and breakdown voltage of an HBT or conventional bipolar transistor. Further, using the invention, the scalability of the HBT can be improved where reduced feature size is needed. Although the invention is described as applied to the construction of a heterojunction bipolar transistor, it will be readily apparent to a person of ordinary skill in the art how to apply the invention in similar situations where control of dopant out diffusion for improved operating characteristics of a bipolar device is needed.  
         [0044]    From the above description of the invention it is manifest that various techniques can be used for implementing the concepts of the present invention without departing from its scope. For example, although the particular embodiment of the present invention described here is applied to silicon-germanium bipolar HBT device, the invention is also applicable, for example, to silicon or silicon-germanium bipolar or BiCMOS devices. Moreover, while the invention has been described with specific reference to certain embodiments, a person of ordinary skill in the art would recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. The described embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that the invention is not limited to the particular embodiments described herein, but is capable of many rearrangements, modifications, and substitutions without departing from the scope of the invention.  
         [0045]    Thus, a high performance bipolar transistor has been described.