Abstract:
The present invention provides a “subcollector-less” silicon-on-insulator (SOI) bipolar junction transistor (BJT) that has no impurity-doped subcollector. Instead, the inventive vertical SOI BJT uses a back gate-induced, majority carrier accumulation layer as the subcollector when it operates. The SOI substrate is biased such that the accumulation layer is formed at the bottom of the first semiconductor layer. The advantage of such a device is its CMOS-like process. Therefore, the integration scheme can be simplified and the manufacturing cost can be significantly reduced. The present invention also provides a method of fabricating BJTs on selected areas of a very thin BOX using a conventional SOI starting wafer with a thick BOX. The reduced BOX thickness underneath the bipolar devices allows for a significantly reduced substrate bias compatible with the CMOS to be applied while maintaining the advantages of a thick BOX underneath the CMOS. A back-gated CMOS device is also provided.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. application Ser. No. 10/931,855, filed Sep. 1, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor device, and more particularly a vertical bipolar transistor that is formed using silicon-on-insulator (SOI) integrated bipolar transistor and complementary metal oxide semiconductor (hereinafter BiCMOS) technology.  
       BACKGROUND OF THE INVENTION  
       [0003]     The semiconductor industry has been seeking more cost effective solutions for manufacturing BiCMOS devices for mass applications of radio frequency (RF)/analog and wireless/fiber-based telecommunications for decades. Si/SiGe BiCMOS technology is widely used and has been quite successful. However, as complementary metal oxide semiconductor (CMOS) adopts thin silicon-on-insulator (SOI) substrates for lower power and higher speed (due to device scaling), the thick subcollector of conventional bipolar junction transistors (BJTs) becomes incompatible with the integration of high-performance SOI CMOS devices.  
         [0004]     In order to facilitate integration with SOI CMOS, lateral SOI BJTs have been proposed and studied. See, for example, S. Parke, et al. “A versatile, SOI CMOS technology with complementary lateral BJT&#39;s”, IEDM,  1992 , Technical Digest, 13-16 Dec. 1992, page(s) 453-456; V. M. C. Chen, “A low thermal budget, fully self-aligned lateral BJT on thin film SOI substrate for lower power BiCMOS applications”, VLSI Technology, 1995. Digest of Technical Papers. 1995 Symposium on VLSI Technology, 6-8 Jun. 1995, page(s) 133-134; T. Shino, et al. “A 31 GHz finax lateral BJT on SOI using self-aligned external base formation technology”, Electron Devices Meeting, 1998. IEDM &#39;98 Technical Digest, International, 6-9 Dec. 1998, page(s) 953-956; T. Yamada, et al. “A novel high-performance lateral BJT on SOI with metal-backed single-silicon external base for low-power/low-cost RF applications”, Bipolar/BiCMOS Circuits and Technology Meeting, 1999. Proceedings of the 1999, 1999, page(s) 129-132; and T. Shino, et al. “Analysis on High-Frequency Characteristics of SOI Lateral BJTs with Self-Aligned External Base for 2-GHz RF Applications”, IEEE, TED, vol. 49, No. 3, pp. 414, 2002.  
         [0005]     Even though lateral SOI BJT devices are easier to integrate with SOI CMOS, the performance of such devices is quite limited. This is because the base width in the lateral SOI BJTs is determined by lithography. Hence, it cannot be scaled down (less than 30 nm) readily without more advanced and more expensive lithography technologies such as e-beam lithography.  
         [0006]     Another type of SOI BJT, which is a vertical SOI SiGe bipolar device, has also been proposed and demonstrated to offer higher base-collector breakdown voltage, higher early voltage and better BVCEO-fT tradeoff. This type of SOI BJT is described, for example, in J. Cai, et al., “Vertical SiGe-Base Bipolar Transistors on CMOS-Compatible SOI Substrate”, 2003 IEEE Bipolar/BiCMOS Circuits and Technology Meeting. This SOI BJT device uses a fully depleted SOI layer as the collector at zero substrate bias. The application of a substrate bias to this SOI BJT device allows for significant improvement in overall device performance by reducing collector space-charge region transit time and collector resistance through the formation of an accumulation layer.  
         [0007]     A problem with the SOI BJT device described above is that the buried oxide (BOX) layer in high performance CMOS SOI substrates is typically 100-200 nm thick. As a result, the substrate bias needed for significant performance improvement is unacceptably large (greater than about 20 V). In order for these devices to be practical for SOI BiCMOS applications, the substrate bias must be held at or below the voltage applied to the CMOS, typically less than 3 V.  
         [0008]     In view of the above, there is a need for providing a SOI BJT structure that overcomes the drawbacks mentioned in the prior art SOI BJTs.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention provides a vertical SOI BJT which uses a SOI layer with a back gate-induced majority carrier accumulation layer as a subcollector located on regions of a second buried insulating region having a second thickness using a standard SOI starting wafer with a first buried insulating region having a first thickness and the method thereof. In accordance with the present invention, the first thickness of the first buried insulating region is greater than the second thickness of the second buried insulating region. The reduced thickness of the second buried insulating region underneath the bipolar devices allows for a significantly reduced substrate bias that is CMOS compatible, while maintaining the advantages of the thick first buried insulating region underneath the CMOS.  
         [0010]     The accumulation layer can then be formed to reduce collector resistance and transit time by applying a back-bias that will not compromise the quality and reliability of the CMOS.  
         [0011]     A method of forming a bipolar transistor including a localized thin buried insulating region (second buried insulating region) is provided. In broad terms, the method of the present invention includes the steps of:  
         [0012]     providing a silicon-on-insulator (SOI) substrate comprising a first semiconductor layer containing a first conductivity type dopant located over a first buried insulating layer, wherein a portion of the first buried insulating layer beneath said first semiconductor layer is removed providing an undercut region;  
         [0013]     forming a second buried insulating layer on exposed surfaces of said first semiconductor layer, wherein said second buried insulating layer is thinner than said first buried insulating layer;  
         [0014]     filling the undercut region and the removed portion of the first semiconductor layer with a conductive back electrode material;  
         [0015]     forming a base comprising a second semiconductor layer containing a second conductivity type dopant that is different than the first conductivity type dopant on said substrate;  
         [0016]     forming an emitter comprising a third semiconductor layer including said first conductivity type dopant over a portion of said base; and  
         [0017]     biasing the conductive back electrode material to form an accumulation layer at an interface between the first semiconductor layer and the second buried insulating layer.  
         [0018]     The first semiconductor layer includes an intrinsic collector and an extrinsic collector. The base may include a single crystal portion atop semiconductor material, and a polycrystalline portion atop insulating material.  
         [0019]     In accordance with the present invention, a vertical NPN or PNP SOI BJT can be formed. The NPN transistor is formed when the first and third semiconductor layers contain an n-type dopant, while the second semiconductor layer comprises a p-type dopant. A PNP transistor is formed when the first and third semiconductor layers contain a p-type dopant and the second semiconductor layer contains an n-type dopant.  
         [0020]     Specifically, a trench is first etched through the first semiconductor layer of an SOI substrate exposing the first buried insulating layer which normally has a thickness from about 100 to about 500 nm. A portion of the first buried insulating layer is then removed using an isotropic etch process that undercuts the first semiconductor layer. A thin insulating layer (less than about 15 nm) is then grown to form the second buried insulating layer. The trench and area where the first buried insulating layer was removed is filled in with a conductive material such as in-situ doped polysilicon. The conductive-fill can then be used to apply a substrate bias. These processing steps provide a structure that includes a conductive back electrode that contains a second buried insulating region of a second thickness located on a surface thereof and a first buried insulating region of a first thickness that is greater than the second thickness that is located abutting the region containing the conductive back electrode and the overlayer first insulating layer. In accordance with the present invention, a bipolar device can be formed atop this structure such that it is located above the second buried insulating layer.  
         [0021]     With such a reduced buried insulating layer thickness underneath the bipolar device, a significantly reduced substrate bias (less than 3 V) compatible with the CMOS is able to create a strong enough vertical electric field to form an accumulation layer which forms the subcollector of the inventive device, while maintaining the advantages of a thick first buried insulating layer underneath the CMOS.  
         [0022]     There are no known alternative solutions to this problem. One possible alternative is to use a patterning process to form regions of thin and thick buried insulating regions on the SOI wafer during a SIMOX (separation by implantation of oxygen) process. However, by using an oxygen implant, it is difficult to make a buried insulating region having a thickness of less than 10 nm. Moreover, it is difficult to control the thickness of the buried insulating region formed by conventional SIMOX processes. Also, this method purposed above would require costly additional lithography and implant steps to produce the SOI wafers.  
         [0023]     In addition to the method described above, the present invention also contemplates the bipolar transistor that is formed utilizing the above method. Specifically, and in broad terms, the bipolar transistor of the present invention comprises:  
         [0024]     a conductive back electrode for receiving a bias voltage;  
         [0025]     a second buried insulating layer located over said conductive back electrode having a second thickness;  
         [0026]     a first buried insulating layer located adjacent to said second buried insulating layer and said conductive back electrode, said first buried insulating layer having a first thickness that is greater than the second thickness;  
         [0027]     a first semiconductor layer located predominately over said second buried insulating layer, said first semiconductor layer including a first conductivity type dopant, wherein said conductive back electrode is biased to form an accumulation layer in said first semiconductor layer at an interface between said first semiconductor layer and said second buried insulating layer;  
         [0028]     a base located atop at least said first semiconductor layer, said base comprising a second semiconductor layer having a second conductivity type dopant that differs from the first conductivity type dopant; and  
         [0029]     an emitter comprising a third semiconductor layer of the first conductivity type dopant located over a portion of said base.  
         [0030]     In addition to SOI BJT&#39;s the present invention, in particularly the substrate including different regions of buried insulating thickness could be used as a substrate for forming a back-gated complementary metal oxide semiconductor (CMOS) device. The back-gated CMOS device could be formed alone on the substrate or it could be formed with a bipolar transistor, including the SOI HBT described above, in BiCMOS applications. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]      FIGS. 1A-1B  are pictorial representations (through different views) illustrating a single-finger emitter device of the present invention along two directions that are perpendicular to each other.  
         [0032]      FIGS. 2A-2E  illustrate the process flow for making the substrate that is employed in the present invention which includes a second buried insulating region that is thin and an adjoining first buried insulating region that has a thickness that is greater than the second buried insulating region. The second buried insulating region is located atop a conductive back electrode.  
         [0033]      FIGS. 3A-3B  show cross sections of an SOI wafer that underwent the process illustrated in  FIGS. 2A-2E . In the drawings, the first buried insulating layer was undercut by 0.3 microns. An 8 nm thick thermal oxide, e.g., the second buried insulating layer, was then grown followed by LPCVD polysilicon fill to form the conductive back electrode.  
         [0034]      FIG. 4  is a cross sectional view illustrating an expanded view of the structure shown in  FIG. 2E .  
         [0035]      FIGS. 5A and 5B  are pictorial representations (through different views) illustrating a back-gated CMOS device of the present invention along two directions that are perpendicular to each other. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     The present invention, which provides a vertical SOI BJT which uses a SOI layer with a back gate-induced accumulation layer as the subcollector located on regions of a second buried insulating region having a second thickness using a standard SOI starting wafer with a first buried insulating region having a first thickness and the method thereof, will now be described in greater detail by referring to the drawings that accompany the present application. The drawings are provided for illustrative purposes and thus they are not drawn to scale. Moreover, in the drawings like and/or corresponding elements are referred to by like reference numerals.  
         [0037]     The present invention provides a bipolar transistor structure that includes a conductive back electrode for receiving a bias voltage, a second buried insulating layer located over the conductive back electrode, and a first semiconductor layer, which comprises an SOI layer of a SOI substrate, located over the second buried insulating layer. The first semiconductor layer includes a collector region containing a first conductive type dopant. The collector region includes an intrinsic collector and an extrinsic collector. The extrinsic collector and the intrinsic collector, which are of the first conductivity type, have different dopant concentration; the extrinsic collector having a higher dopant concentration that the intrinsic collector.  
         [0038]     In accordance with the present invention, a base comprising a second semiconductor layer containing a second conductivity type dopant is located atop the first semiconductor layer. The inventive bipolar transistor also includes an emitter comprising a third semiconductor layer containing the first conductivity type dopant located over a portion of the base, e.g., the second semiconductor layer. During operation, the conductive back electrode is biased to form an accumulation layer in the SOI layer at an interface between the SOI layer and the second buried insulating layer. The configuration of the inventive bipolar transistor structure will become more apparent by referring to  FIGS. 1A-1B .  
         [0039]     One possible device layout of the inventive bipolar transistor is shown in  FIGS. 1A-1B  wherein a single-finger emitter device is shown. By “finger”, it is meant that the emitter has at least one portion that extends outward from a common emitter region. Although the drawings show a one-finger emitter device, the present invention is not limited to only that device layout. Instead, the present invention contemplates device layouts that include a number of emitter-fingers. Multi-finger configurations are preferred over the singe-finger device layout since they typically reduce the emitter resistance for achieving high ƒ max . i.e., the maximum oscillation frequency at which the unilateral power gain becomes unity.  
         [0040]     The cross sectional views of the single-finger emitter device layout is shown in  FIGS. 1A and 1B .  FIG. 1A  is the cross sectional view along an axis B-B′, while  FIG. 1C  is the cross sectional view along an axis C—C′; the two axis are perpendicular to each other. Specifically, the cross sectional views shown in  FIG. 1A  and  FIG. 1B  depict a vertical bipolar transistor  10  of the present invention. The vertical bipolar transistor  10  includes a Si-containing substrate layer  14 , a first buried insulating layer  16  having a first thickness, a second buried insulating layer  22  having a second thickness that is less than the first thickness of the first buried insulating layer  16 . As shown, the first buried insulating layer  16  is located on an upper surface of the Si-containing substrate  14  and the second buried insulating layer  22  is located around the conductive back electrode  24 . The second buried insulating layer  22  thus includes an upper portion  22   u  located atop the conductive back electrode  24  and a lower portion  221  located atop the Si-containing substrate  14 . The upper portion  22   u  of the second buried insulating layer  22  is the region in which the accumulation layer will form thereon.  
         [0041]     The vertical bipolar transistor  10  shown in  FIGS. 1A-1B  further includes trench isolation regions  28  that are located, as shown in  FIG. 1A , atop the first buried insulating layer  16 , as well as atop the conductive back electrode  24 , as shown in  FIG. 1B . Hence, the trench isolation regions  28  surround the active device region of the structure. The structure also includes a first semiconductor layer  18  (hereinafter referred to as the SOI layer) which is located on the upper portion  22   u  of the second buried insulating layer  22  as well as a portion of the first buried insulating layer  16 . The first semiconductor layer  18  is the original SOI layer of the initial substrate employed in the present invention.  
         [0042]     In accordance with the present invention, the first semiconductor layer  18  is the collector region of the inventive structure that is doped with a first conductivity type dopant, either an n- or p-type dopant. The collector region includes an extrinsic collector  41  and an extrinsic collector  43  that has a greater dopant density, i.e., concentration, as compared to intrinsic collector  41 . As shown, the intrinsic collector  41  is located between two extrinsic collectors  43 .  
         [0043]     A base (or base region)  100  is located atop the SOI layer  18  and the trench isolation region  28 . The base  100  comprises a second semiconductor layer of a second conductivity type dopant that differs in terms of its conductivity from the first conductivity type dopant. The base  100  comprises a polycrystalline portion  100   b  and a single crystalline portion  100   a . As shown, the polycrystalline portion  100   b  is located predominately atop isolation regions, while the single crystal portion  100   a  is located atop the SOI layer  18 .  
         [0044]     Atop of the base  100  is an emitter  52  which is comprised of a third semiconductor layer. The third semiconductor layer forming emitter  52  may be comprised of the same or different material as the base  100  or the SOI layer  18 . The emitter  52  is heavily doped with the first conductivity type dopant. First insulator  30  and second insulator  36  are located in the structure as well. The first insulator  30  is located atop the structure including the SOI layer  18  and it has an opening therein in which the intrinsic portion  100   a  of the base  100  is in contact with the SOI layer  18 . The second insulator  36  is located atop portions of the base  100  and it also has an opening therein that allows the emitter  52  to be in contact with the intrinsic portion  100   a  of the base  100 .  
         [0045]     Although not shown, the exposed portions of the emitter  52 , the SOI layer  18 , the polycrystalline region  100   b  and conductive back electrode  24  may include a metal silicide. The metal silicide located atop the exposed surfaces of the conductive back electrode  24  is the region in which biasing of the substrate can take place. During biasing, a portion of the SOI layer  18  that is located atop the upper portion  22   u  of the second buried insulating region  22  is converted into an accumulation layer  62 . The accumulation layer  62  is a majority carrier layer that serves as the subcollector of the inventive bipolar transistor. This is unlike prior art bipolar transistor in which the subcollector is comprised of an impurity-doped region.  
         [0046]     The process flow for making a substrate that includes the buried insulating regions of different thicknesses is illustrated in  FIGS. 2A-2E . It is noted that the process shown and described in making the substrate shown is similar to the process disclosed in co-pending and co-assigned U.S. patent application Ser. No. 10/787,002, filed Feb. 25, 2004, the entire content of which is incorporated herein by reference. In the following description of the first and second buried insulating layers (or regions)  16  and  22 , respectively, are referred to as buried oxide (BOX) regions. Although BOX regions are depicted and described as oxides, the present invention works equally well when the regions  16  and  22  are other insulating materials, i.e., nitrides or oxynitrides.  
         [0047]      FIG. 2A  shows the cross-section of a typical SOI substrate  12  used for a high-performance CMOS application that can be employed in the present invention. The initial SOI substrate  12  comprises a Si-containing substrate layer  14 , a first buried insulating layer  16  of a first thickness (herein after thick BOX)  16 , and a top Si-containing layer  18  (which is, in accordance with the nomenclature of the present invention, the first semiconductor layer or the SOI layer  18 ). The term “Si-containing” is used herein to denote any semiconductor material that includes silicon therein.  
         [0048]     Illustrative examples of such Si-containing materials include but are not limited to: Si, SiGe, SiGeC, SiC, Si/Si, Si/SiGe, preformed SOI wafers, silicon germanium-on-insulators (SGOI) and other like semiconductor materials.  
         [0049]     The SOI layer  18  of the initial SOI substrate  12  is typically a doped layer, which may contain an n- or p-type dopant. Doping can be introduced into the SOI layer  18  prior to, or after formation of the SOI substrate  12 . The doped SOI layer  18  comprises the collector region of the inventive bipolar transistor  10 . The dopant concentration within the SOI layer  18  is typically from about 1E17 to about 1E19 atoms/cm 3 .  
         [0050]     The Si-containing layer  18  of the SOI substrate  12  may have a variable thickness, which is dependent on the technique that is used in forming the SOI substrate  12 . Typically, however, the Si-containing layer  18  of the SOI substrate  12  has a thickness from about 10 to about 1000 nm, with a thickness from about 50 to about 500 nm being more typical. The thickness of the thick BOX  16  may also vary depending upon the technique used in fabricating the SOI substrate  12 . Typically, however, the thick BOX  16  of the present invention has a thickness from about 100 to about 1000 nm, with a BOX thickness from about 120 to about 200 nm being more typical. The thickness of the Si-containing substrate layer  14  of the SOI substrate  12  is inconsequential to the present invention.  
         [0051]     The initial SOI substrate  12  can be formed using a layer transfer process such as, a bonding process. Alternatively, a technique referred to as separation by implanted oxygen (SIMOX) wherein ions, typically oxygen, are implanted into a bulk Si-containing substrate and then the substrate containing the implanted ions is annealed under conditions that are capable of forming a buried insulating layer, i.e., thick BOX  16 , can be employed.  
         [0052]     Next, and as shown in  FIG. 2B , at least one trench  26  that extends to the upper surface of the Si-containing substrate layer  14  is formed by lithography and etching. The lithography step includes applying a photoresist to the surface of the SOI substrate  12 , exposing the photoresist and developing the exposed photoresist using a conventional resist developer. The etching step used in forming the trench  26  includes any standard Si directional reactive ion etch process. Other dry etching processes such as plasma etching, ion beam etching and laser ablation, are also contemplated herein. The etch can be stopped on the top of the thick BOX  16  (not shown), or on the Si-containing substrate  14  underneath the thick BOX  16 , as shown in  FIG. 2B . As shown, portions of the SOI layer  18  and the thick BOX  16  that are protected by the patterned photoresist are not removed during etching. After etching, the patterned photoresist is removed utilizing a conventional resist stripping process.  
         [0053]     An isotropic oxide etch selective to silicon (such as a timed hydrofluoric acid based etch or similar etch chemistry) is then used to remove portions of the thick BOX  16  underneath the SOI layer  18  where the vertical bipolar device will be fabricated (See  FIG. 2C ). The isotropic etch forms an undercut  20  beneath the SOI layer  18  that will be subsequently filled with a conductive back electrode material. The SOI layer  18  is supported by portions of the thick BOX  16  that are not removed by this etch. Before this etching step, all pad layers should be removed from atop the SOI layer otherwise bending of the SOI layer occurs.  
         [0054]     A thermal process such as a wet and/or dry oxidation, nitridation or oxynitridation, is then used to grow the second buried insulating layer  22 , i.e., thin BOX, on the exposed surfaces of the SOI layer  18 , see  FIG. 2D . Note that the second buried insulating layer (hereinafter thin BOX)  22  forms on the exposed horizontal and vertical surfaces of the SOI layer  18  as well as the exposed surface of the Si-containing substrate layer  14 . The thin BOX  22  formed on the SOI layer  18  is given the reference numeral  22   u , while the BOX formed on the Si-containing substrate layer  12  is given the reference numeral  221 . In accordance with the present invention, the thin BOX  22  has a second thickness that is less than the first thickness of the first buried insulating layer, i.e., thick BOX  16 . Typically, the thin BOX  22  has a thickness from about 1 to about 15 nm. Deposited oxides such as a low-temperature oxide (LTO) or a high-density oxide (HTO) can also be employed. When deposited oxides are used, the oxide would also be present on the sidewalls of the opened structure as well. Note that the oxide also grows, although to a lesser extent, on oxide surfaces as well. The growth of oxide on an oxide surface is not, however, differentiated in the drawings of the present application.  
         [0055]     At this point of the present invention, a conductive back gate electrode material (which becomes the conductive back electrode  24 ) such as, for example, doped polysilicon, a silicide or a conductive metal is deposited to fill in the area previously occupied by the removed thick BOX  16 . The deposition is performed using a conventional deposition process such a chemical vapor deposition, plasma-assisted chemical vapor deposition, chemical solution deposition, evaporation and the like. In one embodiment, doped polysilicon is used as the conductive back electrode material and it is deposited at a temperature from about 400° to about 700° C. using a low-pressure chemical vapor deposition (LPCVD) process. Doping of the polysilicon layer may occur in-situ or after deposition using an ion implantation process. The structure can then be planarized, if needed, by chemical mechanical polishing or by a dry etch of the polysilicon selective to oxide. The resultant structure that is formed after performing the above steps is shown, for example, in  FIG. 2E .  
         [0056]      FIG. 3A  and  FIG. 3B  show an SEM cross section of an SOI wafer that underwent the process described above. The BOX was undercut by 0.3 microns. An 8 nm thick thermal oxide was then grown followed by LPCVD polysilicon fill.  
         [0057]      FIG. 4  shows an expanded cross sectional view of the structure depicted in  FIG. 2E . Region  102  denotes the active device area in which a bipolar transistor can be formed. The active area  102  includes an upper thin BOX  22   u  located atop the conductive back electrode  24 . The conductive electrode  24 , in turn, is located on the lower thin BOX  221 , which is located atop the Si-containing substrate layer  14 .  
         [0058]     After providing the structure shown in  FIG. 2E  (or  FIG. 4 ), a bipolar device such as shown in  FIG. 1B  is formed atop the structure utilizing conventional BiCMOS processing techniques that are well known to those skilled in the art. Specifically, the following process can be used in forming the bipolar transistor atop the structure shown in  FIG. 2E  (or  FIG. 4 ).  
         [0059]     First, trench isolation regions  28  are formed into the structure shown in  FIG. 2E  (or  FIG. 4 ) utilizing conventional processes well known to those skilled in the art. For example, the trench isolation regions  28  can be formed by trench definition and etching, optionally lining the trench with a liner material and then filling the trench with a trench dielectric material such as, for example, tetraethylorthosilicate (TEOS) or a high-density oxide. The trench dielectric material can be densified after the filling of the trench and, if needed, a planarization process, such as chemical mechanical polishing, can be employed.  
         [0060]     Next, the SOI layer  18  can be further doped at this point of the present invention with a first conductivity type dopant (n- or p-type) using various masked implantation schemes to provide an extrinsic collector and/or intrinsic collector within the SOI layer  18 . A first insulator  30 , such as an oxide, nitride, oxynitride or multilayers thereof, is then formed on the surface of the structure by a thermal process or by deposition, such as chemical vapor deposition. The thickness of the first insulator  30  can vary depending on the technique used in forming the same. Typically, the first insulator  30  has a thickness from about 10 to about 100 nm.  
         [0061]     After forming the first insulator  30  on the surface of the structure shown in  FIG. 2E  (or  FIG. 4 ), the first insulator  30  is patterned to provide an opening that exposes a surface of the SOI layer  18 . The at least one opening in the first insulator  30  is formed by lithography and etching.  
         [0062]     Next, the base  100  is formed by utilizing a low-temperature epitaxial growth process that is typically performed at a temperature from about 450° C. to about 800° C. In accordance with the present invention, the base  100  is comprised of a second semiconductor layer that can include, for example, Si, SiGe or combinations thereof. The low-temperature epitaxial process forms a base  100  that comprises an intrinsic portion  100   a  that is typically monocrystalline, and an extrinsic portion  100   b  that is typically polycrystalline. The area in which the material of base  100  changes from monocrystalline to polycrystalline is referred to as a facet region.  
         [0063]     The base  100  can be doped during the epitaxial growth process or it can be doped after utilizing ion implantation. An annealing step can be used to activate the dopants within the base layer. The base  100  is doped with a second conductivity type dopant that differs in conductivity type from that of the SOI layer  18 , which is comprised of a first semiconductor material.  
         [0064]     The base  100  has a variable thickness in which the extrinsic portion  100   b  is thickener than the intrinsic portion  100   a . On average, the base  100  typically has a thickness from about 10 to about 150 nm.  
         [0065]     A second insulator  36  that may comprise the same or different insulator as the first insulator  30  is then formed on top of the base  100 . The second insulator  36  is formed by a conventional deposition process such as chemical vapor deposition. The thickness of the second insulator  36  may vary depending on the process used in forming the same. Typically, the second insulator  36  has a thickness from about 50 to about 150 nm.  
         [0066]     The second insulator  36  is then patterned by lithography and etching to provide at least one opening that exposes the surface of the underlying intrinsic portion  100   a  of the base  100 .  
         [0067]     Next, the emitter  52  comprising a third semiconductor layer, such as Si, SiGe or a combination thereof, is formed on the second insulator  36  as well as the exposed surface of the intrinsic portion  100   a  of the base  100 . In accordance with the present invention, the emitter  52  includes the same conductivity type dopant as the SOI layer  18 . The doping of the emitter  52  may occur in-situ or post deposition utilizing an ion implantation process. The emitter  52  formed typically has a thickness from about 50 to about 200 nm.  
         [0068]     The emitter  52  and portions of the second insulating layer  36  are then patterned by lithography and etched providing the structure shown in  FIGS. 1B and 1C . After patterning, the exposed surfaces containing Si, i.e., extrinsic base portion  100   b , conductive back electrode  24  and emitter  52 , are then subjected to a conventional silicidation process in which a silicide metal such as Ti, Ni, Co, W, Re or Pt (singularly or alloys thereof) is first deposited and then annealed to cause interaction of the metal and Si and subsequent formation of a silicide on each region including metal and Si. Alloys of the above mentioned metals are also contemplated herein. Any remaining metal, not silicided, is typically removed after the silicide process using a conventional wet etching process. It is noted that the silicides formed in the extrinsic portion  100   b  of the 100 are self-aligned to the base emitter  52 . The silicidation process is not shown in the drawings the present invention.  
         [0069]     At this point of the present invention, an optional barrier material such as a nitride can be formed atop the structure. The optional barrier material is not shown in the drawings of the present invention.  
         [0070]     An interconnect dielectric such as, for example, boron phosphorus doped silicate glass, an oxide, an organic polymer or an inorganic polymer is then deposited using a conventional deposited process such as chemical vapor deposition, plasma-assisted chemical vapor deposition, evaporation, spin-on coating, chemical solution deposition and the like. The interconnect dielectric has a thickness after deposition that is on the order of about 500 to about 1000 nm. After deposition of the interconnect dielectric, the interconnect dielectric is planarized by chemical mechanical polishing or other like planarization process so as to have a thickness after planarization from about 300 to about 600 nm and thereafter a contact opening that extends to the surface of each silicide is formed by lithography and etching. Each of the contact openings is then filled with a metal contact such as W, Cu, Al, Pt, Au, Rh, Ru and alloys thereof. The formation of the interconnect structure is not shown in the drawings.  
         [0071]     The structure can now be biased by applying an external voltage to the conductive back electrode  24  through the contacts produced above. The biasing causes an accumulation layer  62  to be formed in a portion of the base SOI layer  18  that is located above the thin BOX  22   u . The amount of voltage applied in forming the accumulation layer  62  is typically 5 V or less. The accumulation layer  62  serves as the subcollector of the inventive structure.  
         [0072]     The method described above can be used to form a plurality of vertical bipolar transistors on the active area of the SOI substrate. The methods described above can also be used in conjunction with a conventional CMOS process flow which is capable of forming CMOS devices such as field effect transistors, in areas adjacent to the areas containing the vertical bipolar transistors of the present invention, to form BiCMOS for RF or mixed-signal applications. In the prior art, the CMOS devices are typically formed prior to the bipolar devices with CMOS areas usually protected during fabrication of the bipolar transistors. The drawback of this method is that the MOS device performance often becomes degraded to the excessive thermal budget that CMOS devices experience during the fabrication of the dipolar devices, such as dopant activation anneal after implants. An advantage of this invention over prior art processes is that the inventive method utilizes the typical CMOS process to form a bipolar device hence the CMOS and bipolar devices can be fabricated interactively and share the same activation anneal.  
         [0073]     It is noted that the processing steps described above in forming the substrate shown in  FIGS. 2A-2E  and  FIG. 4  can be used in conjunction with conventional back-gate processes to form a back-gated CMOS device. The CMOS device can be provided separately from the SOI BJT device depicted in  FIG. 1A  and  FIG. 1B , or it can be formed together with the SOI BJT device or any other bipolar transistor in a BiCMOS application.  
         [0074]     Specifically,  FIG. 5A  and  FIG. 5B  shows a back-gated CMOS device that can be formed on the substrate shown in  FIG. 2A  or  FIG. 4 . The back-gated device  300  includes Si-containing substrate layer  14 , a first buried insulating layer  16  having a first thickness located atop a portion of the Si-containing substrate  14 . Other portions of the Si-containing substrate include the lower portion  221  of the second buried insulating layer  22  having a second thickness that is less than the first thickness. A back-gate electrode  24  is located atop the lower portion  221  of the second buried insulating layer  22 . An upper portion  22   u  of the second buried insulating layer  22  is located atop the conductive back electrode  24  and portions of first buried insulating layer  16 . SOI layer  18  including source/drain diffusion regions  310  and extension regions  320  is located atop the upper portion  22   u  of the second buried insulating layer. Trench isolation regions  28  which extend to the first buried insulating layer  16  are located abutting the SOI layer  18 . The front-gated device also includes, a gate dielectric  300  located atop a portion of the SOI layer  18  and a gate conductor  350  located atop the gate dielectric  350 . An optional, but preferred, reoxidation liner  340  is present on at least the sidewalls of the gate conductor  350 . Insulating spacers  330  are located in the structure as well. Gate dielectric  300  and gate conductor  350  are components of a field effect transistor.  
         [0075]     The back-gated device  300  is fabricated by first utilizing the methodology used in forming the substrate shown in  FIGS. 2E and 4 , and then by utilizing conventional CMOS processes steps that are well known in the art. The gate dielectric  300 , the gate conductor  350  and the insulating spacers  330  are composed of conventional materials well known to those skilled in the art. For example, the gate dielectric  300  can be an oxide, nitride, oxynitride, a dielectric material having a dielectric constant greater than 4.0, preferably greater than 7.0, or a stack thereof; the gate conductor  350  can be composed of polySi, polySiGe, a metal, a metal silicide, a metal nitride or any combination, including multilayers thereof, and the insulating spacers  330  are composed of an oxide, nitride, oxynitride or combinations, including multilayers thereof.  
         [0076]     While the present invention has been particularly shown and described with respect to preferred embodiments, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.