Abstract:
The upper epitaxial layer of a bipolar transistor has a silicon germanium layer and an overlying cap layer. The upper epitaxial layer includes an intrinsic emitter region and a base region. The silicon germanium layer is spaced apart from the intrinsic emitter region, and lies outside of the depletion region associated with the junction between the intrinsic emitter region and the base region.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a bipolar transistor and, more particularly, to a bipolar transistor with a box-type germanium profile that lies outside of the emitter-base depletion region. 
     2. Description of the Related Art 
     A bipolar transistor is a three-terminal device that can, when properly biased, controllably vary the magnitude of the current that flows between two of the terminals. The three terminals include a base terminal, a collector terminal, and an emitter terminal. The charge carriers, which form the current, flow from the emitter terminal to the collector terminal, while variations in the voltage on the base terminal cause the magnitude of the current to vary. 
     FIG. 1 shows a cross-sectional diagram that illustrates a portion of a prior-art bipolar transistor  100 . As shown in FIG. 1, transistor  100  includes an n- epitaxial layer  110  that functions as the collector, shallow trench isolation (STI) regions  112  that are formed in layer  110 , and an epitaxial layer  114  that is formed on layer  110  and STI regions  112 . 
     Epitaxial layer  114  includes a p-type doped region  116  that has a lightly-doped region  116 A, and a heavily-doped region  116 B that is formed below lightly-doped region  116 A. Lightly-doped region  116 A has a top surface and a bottom region, while heavily-doped region  116 B has a top region and a bottom surface. The bottom region of lightly-doped region  116 A and the top region of heavily-doped region  116 B form a transition region where the dopant concentration increases from the concentration of lightly-doped region  116 A to the concentration of heavily-doped region  116 B. 
     Epitaxial layer  114  also includes an n-type doped region  118  that is formed in p-type doped region  116 . When unbiased, the physical junction between p-type doped region  116  and n-type doped region  118  forms a depletion region  120  where a junction region of doped region  116 A is free of holes and a junction region of doped region  118  is free of electrons. When doped region  116  and doped region  118  are forward biased, a smaller depletion region  120 A is formed. (A larger depletion region is formed when the doped region  116  and doped region  118  are reverse biased.) 
     As further shown in FIG. 1, epitaxial layer  114  includes a silicon germanium (SiGe) layer  114 A with a top surface that is positioned to lie within the smaller forward biased depletion region  120 A. In addition, the peripheral regions of SiGe layer  114 A have a p+ dopant concentration. Epitaxial layer  114  further includes a cap layer  114 B that is formed on SiGe layer  114 A to have substantially no germanium. 
     Transistor  100  also includes an n+ polysilicon (poly) region  122  that is formed on cap layer  114 B, and an oxide layer  124  that contacts cap layer  114 B and poly region  122 . Doped region  118  and poly region  122  function as the emitter of transistor  100 , while doped region  116  functions as the base. 
     Transistor  100  additionally includes a nitride layer  126  that contacts oxide layer  124  and poly emitter  122 , and a metalization plug  128  that is formed on poly emitter  122 . In addition, a base contact  130  is formed on SiGe layer  114 A to contact the peripheral p+ region of SiGe layer  114 A. 
     FIG. 2 shows a graph that illustrates a vertical view of epitaxial layer  114  under the central portion of poly region  122 . As shown in FIG. 2, n-type doped region  118  of epitaxial layer  114  extends down from the top surface of epitaxial layer  114  to a depth D 1 . Doped region  118  has an n+ dopant concentration at the surface that decreases to substantially zero at depth D 1 . 
     In addition, p-type doped region  116  of epitaxial layer  114  extends down from depth D 1  to a depth D 2 . (Epitaxial layer  114  also includes n-type dopants that outdiffused from the collector that extend from a depth D 3  to the bottom surface.) Doped region  116  includes lightly-doped region  116 A that lies adjacent to doped region  118 , and heavily-doped region  116 B that is formed under lightly-doped region  116 A. 
     In addition, as further shown in FIG. 2, SiGe layer  114 A has a substantially uniform concentration of germanium (approximately 0.15 mole fraction) throughout layer  114 A. A SiGe layer with a substantially constant concentration of germanium is known as having a box-type germanium profile. 
     A box-type germanium profile has a down ramp  132  that defines a thin top surface layer  134  of SiGe layer  114 A that has a decreasing germanium concentration. (The germanium concentration can not go to zero instantly.) Thus, down ramp  132  is substantially linear with a very large slope. 
     One important measure of a transistor is the amount of base to collector current amplification, known, as beta, provided by the transistor. The germanium in epitaxial layer  114  increases the beta when compared to a standard silicon base layer that has no germanium. This is because SiGe layer  114 A has a narrower band gap than the silicon of doped region  118 . The difference in band gaps enhances the efficiency of carrier injection from doped region  118  into doped region  116 . 
     To obtain the increased beta, surface layer  134  of SiGe layer  114 A must lie within the smaller depletion region  120 A when transistor  100  is forward biased. When surface layer  134  of SiGe layer  114 A does not lie within depletion region  120 A when forward biased, there is substantially no increase in the beta of transistor  100  over what can be obtained with a comparable transistor that does not utilize a silicon germanium base. Thus, if cap layer  114 B is too thick and surface layer  134  of SiGe layer  114 A does not lie within junction depletion region  120 A when forward biased, SiGe layer  114 A provides no appreciable increase in beta. 
     Another important measure of a bipolar transistor is the time required for a minority carrier to pass through the base region, known as the base transit time. This is an important measure for gigahertz frequency devices because the base transit time is one of the major components of the total transit time which, in turn, limits the maximum frequency of the signal. 
     One component of the base transit time is the boron concentration in doped region  116 . As noted above, doped region  116  (the base) has a lightly doped region  116 A adjacent to doped region  118  (the emitter) and a heavily doped region  116 B adjacent to lightly doped region  116 A. 
     As a result, the boron concentration decreases from a peak value at a depth D 4  in heavily doped base region  116 B when moving towards doped region  118  (the emitter). This decrease in boron concentration sets up a retarding field that slows electrons injected by the emitter. This results in increased base transit time and, as a result, a decreased fT peak. 
     One approach to reducing the base transit time is to utilize a SiGe layer with a triangular-type germanium profile. FIG. 3 shows a graph that illustrates a transistor  300  with a triangular-type germanium profile. Transistor  300  is similar to transistor  100  and, as a result, utilizes the same reference numerals to designate the structures that are common to both transistors. 
     As shown in FIG. 3, transistor  300  differs from transistor  100  in that transistor  300  has a triangular-type germanium profile with a down ramp  310  that extends from the top surface of SiGe layer  314 A at depth D 1  to a point at depth D 5  that corresponds with the location of the peak germanium concentration. As a result, the concentration of germanium linearly increases from zero at the top surface of SiGe layer  314 A to approximately 0.15 mole fraction at depth D 5  near the bottom surface of SiGe layer  314 A with a slope that is much less than the slope of down ramp  132 . 
     In operation, the decreasing-towards-the-emitter slope of the triangular-type germanium profile sets up a uniform field across SiGe layer  314 A that is opposite to the retarding field set up by the decreasing-towards-the-emitter slope of the boron dopant concentration. As a result, electrons injected by the emitter (doped region  118 ) experience a reduced retarding field and, therefore, have a reduced base transit time. However, transistors with triangular-type germanium profiles have a number of disadvantages including being difficult and expensive to fabricate. 
     In addition to box-type and triangular-type germanium profiles, other types of profiles have also been used. For example, a silicon germanium block with a lower concentration pedestal formed on the block is described by Deixler et al., “Explorations for High Performance SiGe-Heterojunction Bipolar Transistor Integration,” IEEE Proc. BCTM 1.3, pp. 30-33, 2001. 
     The Deixler germanium profile has a number of advantages over a standard box-type germanium profile including improved base current linearity, decreased emitter base capacitance, and increased fT at low current levels. However, the Deixler profile has one significant drawback, that being that the Deixler profile also has a decreasing-towards-the-emitter boron concentration. 
     As a result, electrons injected by the emitter into the base also experience the retarding field caused by the decreasing-towards-the-emitter boron concentration. This results in increased base transit time and, as a result, a decreased fT peak. Thus, there is a need for alternate structures that can reduce the base transit time of a bipolar transistor. 
     SUMMARY OF THE INVENTION 
     The present invention provides a bipolar transistor that has a boxed germanium profile with a reduced transit time. The bipolar transistor of the present invention includes a first epitaxial layer of a first conductivity type, and a second epitaxial layer that contacts the first epitaxial layer. 
     The second epitaxial layer has a first doped region of the first conductivity type, and a second doped region of a second conductivity type that is formed below the first doped region. The second doped region has a lightly-doped region adjacent to the first doped region, and a heavily-doped region adjacent to the lightly-doped region. The lightly-doped region has a top surface, and the heavily-doped region has a peak dopant concentration at a peak position within the heavily-doped region. 
     The second epitaxial layer also includes a depletion region between the first doped region and the second doped region, and a silicon germanium (SiGe) region that has a top surface. The top surface of the SiGe region is spaced apart from the depletion region. Further, the transistor of the present invention has a conductive region that is formed to contact the first doped region. 
     In addition, the top surface of the SiGe region can lie within a range from below the top surface of the lightly-doped region, and to above the peak position. For example, the top surface of the SiGe region can lie in a transition region between the lightly-doped region and the heavily-doped region at a point where the dopant concentration has the largest gradient. 
    
    
     A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional diagram illustrating a portion of a prior art bipolar transistor  100 . 
     FIG. 2 is a graph illustrating a vertical view of epitaxial layer  114  under the central portion of poly region  122 . 
     FIG. 3 is a graph illustrating a transistor  300  with a triangular-type germanium profile. 
     FIG. 4 is a cross-sectional diagram illustrating a portion of a bipolar transistor  400  in accordance with the present invention. 
     FIG. 5 is a graph further illustrating epitaxial layer  414  in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 4 shows a cross-sectional diagram that illustrates a portion of a bipolar transistor  400  in accordance with the present invention. Transistor  400  is similar to transistor  100  and, as a result, utilizes the same reference numerals to designate the structures that are common to both transistors. 
     Transistor  400  differs from transistor  100  in that transistor  400  includes an epitaxial layer  414  that has a silicon germanium (SiGe) layer  414 A, and a cap layer  414 B that is formed on SiGe layer  414 A. In accordance with the present invention, SiGe layer  414 A is formed outside of junction depletion region  120 . (Since layer  414 A lies outside of region  120  when no bias is applied, layer  414 A must lie outside of region  120 A when a forward bias is applied.) 
     As further shown in FIG. 4, the top surface of SiGe region  414 A lies in a region R that ranges from a position below the top surface of lightly-doped region  116 A, to a position above the peak dopant concentration in heavily-doped region  116 B. For example, the top surface of SiGe region  414 A can be located at an intermediate position between the top surface of the lightly-doped surface and the bottom region of lightly-doped region  116 A. The intermediate position can be any intermediate position, such as approximately ⅓ or ⅔ of the distance from the top surface of lightly-doped region  116 A to the bottom region of lightly-doped region  116 A. 
     Better performance can be obtained when the top surface of SiGe region  414 A is positioned in the transition region between lightly-doped region  116 A and the heavily-doped region  116 B. Best performance can be obtained when the top surface is positioned in the transition region approximately where the dopant concentration has the largest gradient (rate of increase). 
     FIG. 5 shows a graph that further illustrates epitaxial layer  414  in accordance with the present invention. As shown in FIG. 5, SiGe layer  414 A has a box-type germanium profile with a number of down ramps  432 A,  432 B,  432 C,  432 D, and  432 E. Each of the down ramps  432 A,  432 B,  432 C,  432 D, and  432 E illustrate the different fT peak values that can be obtained by positioning the top surface of SiGe layer  414 A at different depths within range R that correspond with different dopant concentrations. 
     In operation, when a positive bias, such as 1.2V, is placed on collector  110 , a positive bias, such as 0.7V is placed on p-type doped region  116 , and ground is placed on n-type doped region  118 , the base-emitter junction is forward biased. 
     As a result, n-type doped region  118  injects electrons into base region  116 . As the injected electrons transition from lightly-doped region  116 A to heavily-doped region  116 B, the increased p-type dopant concentration sets up an electric field that retards or slows down the injected electrons. 
     However, in accordance with the present invention, when the top surface of SiGe layer  414 A is positioned within range R below the top surface of lightly-doped region  116 A and above the position that corresponds with the peak dopant concentration of heavily-doped region  116 B, SiGe layer  414 A counteracts the electric field set up by the increasing boron dopant concentration. As a result, the base transit time is reduced, and the fT value (unity gain bandwidth) is increased. 
     For example, as illustrated with down ramp  432 A, when SiGe layer  414 A is formed so that the box-type down ramp intersects a point A where the dopant concentration of the second conductivity type corresponds with a position approximately ⅓ of the way below the top surface of lightly-doped region  116 A, simulated results indicate that an if peak of 44.8 GHz can be obtained. 
     The dopant concentration at point A is substantially greater (an order of magnitude in this example) than the dopant concentration of p-type doped region  116  at depth D 1 , and substantially less (an order of magnitude in this example) than the peak dopant concentration of heavily-doped region  116 B at depth D 4 . 
     As illustrated with down ramp  432 B, when SiGe layer  414 A is formed so that the box-type down ramp intersects a point B where the dopant concentration of the second conductivity type corresponds with a position approximately ⅔ of the way below the top surface of lightly-doped region  116 A, simulated results indicate that an fT peak of 49.0 GHz can be obtained. 
     As further illustrated with down ramp  432 C, when SiGe layer  414 A is formed so that the box-type down ramp can intersect a point C in the transition region between lightly-doped region  116 A and heavily-doped region  116 B, where the dopant concentration of the second conductivity type has the largest gradient (rate of increase), simulated results indicate that an fT peak of 51.8 GHz can be obtained. 
     Similarly, down ramp  432 D shows that when the box-type down ramp intersects a point D where the dopant concentration of the second conductivity type corresponds with a position between the transition region and the peak dopant concentration of heavily-doped region  116 B, simulated results indicate that an fT peak of 49.3 GHz can be obtained. 
     In addition, down ramp  432 E shows that when the box-type down ramp intersects a point E where the dopant concentration of the second conductivity type corresponds with the position of the peak dopant concentration of heavily-doped region  116 B, simulated results indicate that an fT peak of 41.5 GHz can be obtained. 
     Thus, by positioning SiGe layer  414 A so that the top surface of layer  414  is outside of the junction depletion region, and along a line of increasing dopant concentration between the top surface of lightly-doped region  116 A and the position that corresponds with the peak concentration in heavily-doped region  116 B, the retarding field of the increasing dopant concentration can be counteracted. Thus, the present invention provides the benefits of a triangular-sized germanium profile with the ease of fabricating a box-sized germanium profile. 
     It should be understood that the above description is an example of the-present invention, and that various alternatives to the invention described herein may be employed in practicing the invention. For example, although the method is described with respect to npn transistors, the method applies equally well to pnp transistors where the conductivity types are reversed. Thus, it is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.