I 1111111111111111 11111 111111111111111 111111111111111 IIIII lll111111111111111 US007902635B2 c12) United States Patent (IO) Patent No.: US 7,902,635 B2 Ma et al. (45) Date of Patent: Mar.8,2011
(54) HIGH-POWER-GAIN, BIPOLAR (56) References Cited TRANSISTOR AMPLIFIER U.S. PATENT DOCUMENTS (75) Inventors: Zhenqiang Ma, Middleton, WI (US); 5,304,816 A * 4/1994 Grinberg et al...... 257/26 Ningyue Jiang, Madison, WI (US) 5,329,144 A * 7/1994 Luryi ...... 257/197 2003/0048138 Al * 3/2003 Van De Westerlo et al. . 330/311 (73) Assignee: Wisconsin Alumni Research 2005/0139921 Al* 6/2005 Kang et al ...... 257 /347 Foundation, Madison, WI (US) OTHER PUBLICATIONS ( *) Notice: Subject to any disclaimer, the term ofthis N.L. Wang, eta!., Ultrahigh Power EfficiencyOperationofConunon patent is extended or adjusted under 35 Emitter andConunon-Base HBT's at 10 GHz, IEEE Transactions on U.S.C. 154(b) by 1010 days. Microwave Theory and Techniques, vol. 38, No. 10, Oct. 1990. (21) Appl. No.: 11/178,644 * cited by examiner
(22) Filed: Jul. 11, 2005 Primary Examiner - A. Sefer (65) Prior Publication Data (74) Attorney, Agent, or Firm - Boyle Fredrickson, S.C.
US 2007 /0007 626 Al Jan. 11, 2007 (57) ABSTRACT (51) Int. Cl. Improved radio frequency gain in a silicon-based bipolar HOJL 271082 (2006.01) transistor may be provided by adoption of a common-base (52) U.S. Cl. .... 257/592; 257/591; 257/574; 257/E29.04; configuration, preferably together with excess doping of the 257/E29.256; 257/E29.27 base to provide extremely low base resistances boosting per (58) Field of Classification Search ...... 257/491-492, formance over similar common-emitter designs. 257/574, 591-592 See application file for complete search history. 6 Claims, 3 Drawing Sheets
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~10 20' 18~ ~20 ~ 16a---.. 14 12 -- ..:c...- ..I..- - - .1 .1 -- 7( t -- (~ (~ 16b 18 20 20' FIG. 8 US 7,902,635 B2 1 2 HIGH-POWER-GAIN, BIPOLAR across the collector and base of the transistor so that the TRANSISTOR AMPLIFIER transistor may be operated in common-base configuration. It is thus one object of at least one embodiment of the STATEMENT REGARDING FEDERALLY invention to optimize a common-base transistor amplifier SPONSORED RESEARCH OR DEVELOPMENT 5 through a specially doped transistor providing a very low base resistance. This invention was made with United States government The silicon bipolar transistor may be a silicon bipolar junc support awarded by the following agency: NSF 0323 717. The tion transistor or a silicon-germanium heterojunction bipolar United States has certain rights in this invention. transistor. 10 Thus it is another object of at least one embodiment of the CROSS-REFERENCE TO RELATED invention to provide an amplifier that can be used with a APPLICATIONS variety of silicon transistor types. The amplifier may include at least one field-effect transis- BACKGROUND OF THE INVENTION 15 tor driving the input terminals, for example, with the drain This invention relates generally to high frequency transis (source as ground) of the field-effect transistor driving the tor amplifiers and in particular to a silicon transistor and emitter (base as AC ground) of the silicon bipolar transistor. circuit suitable for high gain, high frequency amplification. It is thus another object of at least one embodiment of the High frequency electrical amplifiers, such as those useful invention to provide an amplifier cell that may effectively for radio frequency (RF) and microwave power amplification, 20 combine the high breakdown voltage of the common-base frequently use transistors fabricated from gallium arsenide or transistor with a low breakdown voltage but high frequency other colunm III-V elements instead of silicon. Transistors operation of the field-effect transistor. using III-V semiconductors, however, are generally incom It is another object of at least one embodiment of the patible with large-scale integrated circuit techniques, such as invention to provide a power cell that eliminates the need for the complementary metal oxide semiconductor (CMOS) pro 25 ballast resistors by using multiple FETs to promote the shar cess, used in the fabrication oflogic circuitry. For this reason, ing of current among the bipolar transistors. It is another integrated circuits providing both logic circuitry and high object of at least one embodiment of the invention to provide powered radio frequency amplification are not easily manu an amplifier cell with reduced sensitivity to heating by factured. employing field-effect transistors with reduced sensitivity to Recently, the use of silicon transistors in high frequency 30 heating. amplification has become increasingly practical with smaller These particular objects and advantages may apply to only line width devices possible with advanced fabrication tech some embodiments falling within the claims and thus do not niques. Nevertheless, improved power gain at radio frequen define the scope of the invention. cies would be desirable. 35 BRIEF DESCRIPTION OF THE DRAWINGS BRIEF SUMMARY OF THE INVENTION
The present invention provides improved power gain at FIG. 1 is a schematic representation of an amplifier cell radio frequencies, including microwave frequencies using using a combined field-effect transistor and bipolar junction silicon transistors, by using a common-base configuration 40 transistor (FDBT), the latter in a common-base configuration based amplifier design. The power gain of silicon transistors and preferably doped per the present invention; is further improved together with a transistor having a doping FIG. 2 is a set of three linked graphs, the right graph profile optimized for that amplifier design. The transistor showing power gain in decibels as a function of the logarithm doping varies from the normal high to low doping concentra of frequency for a common-emitter bipolar transistor and a tion from emitter to base by significantly increasing the base 45 common-base bipolar transistor with two different doping doping. This doping significantly decreases the base resis schemes shown in left graphs, each showing doping concen tance relative to the total emitter resistance (including the tration as a function of spatial location within the emitter base emitter resistance and parasitic emitter resistance) substan and collector regions, the upper graph showing a conven tially improving the power gain over that obtainable with a tional doping pattern and the lower graph showing a doping similar common-emitter design or a common-base design 50 pattern to reduce base resistance; using a conventionally doped transistor. FIG. 3 is a schematic representation of a generic common The common-base design further provides improved emitter design (to the left) and the hybrid-n model for the breakdown voltage over the common-emitter design useful in transistor in that design (to the right) as defines variables applications where a high degree of ruggedness against over referenced in the present specification; voltage ( or a large safe operation area) is required. An ampli- 55 FIG. 4 is a figure similar to that of FIG. 3 showing a generic fier cell combining the amplifier of the present invention with common-base amplifier design and the T model for the tran a field-effect transistor (FET) front end termed: "FET disci sistor of that design; plined bipolar transistors" or FDBT can provide optimized, FIG. 5 is a figure similar to that of FIG. 1 showing a single concurrent high-power and high-frequency operation with FET feeding multiple bipolar transistors for increased fan improved performance against thermal effects. 60 out; Specifically then, the present invention provides an ampli FIG. 6 is a figure similar to that of FIG. 5 showing multiple fier having a silicon bipolar transistor with a predetermined field-effect transistors providing current to a single bipolar relative doping concentration in the base and emitter regions transistor in a fan-in situation; or to multiple bipolar transis such that the base resistance r6 is less than or equal to the sum tors for optimized fan-in and fan-out; of the emitter resistance re and the parasitic emitter resistance 65 FIG. 7 is a block diagram of an integrated circuit providing rex· Input terminals provide an input across the emitter and for both CMOS logic circuitry and power cells per the present base of the transistor and output terminals provide an output invention; and US 7,902,635 B2 3 4 FIG. 8 is a figure similar to that ofFIG.1 showing a cascade man-base gain curve 40 results from a decrease in the effec connection of the bipolar junction transistors to provide tive base resistance of the bipolar transistor 20 as will now be improved breakdown voltage. explained. Referring to FIG. 3, a bipolar transistor 35 in a common- DETAILED DESCRIPTION OF THE PREFERRED 5 emitter configuration 41 may be modeled according to a small EMBODIMENT signal hybrid it model 42 in which the operating characteris tics of the transistor 35 are represented by equivalent capaci Referring now to FIG. 1, the present invention may be used, tors, resistors and current sources. Hybrid it model 42 is in one embodiment, to create an amplifier cell 10 having input augmented with elements representing parasitic emitter resis terminals 12 for receiving a signal to be amplified and output 10 tance rex and the collector resistance re These two resistances terminals 14 providing an output signal, such as a radio fre rex and re are normally ignored because of their negligible quency signal to an antenna. value in comparison to the total base resistance r for doping The amplifier cell 10 may employ multiple amplifier rows 6 profile 22, however, for higher doping concentrations in the 16 (only two of which are shown for clarity), providing par 15 base region 26 of doping profile 36, r can be reduced to a allel current paths. Each row 16 includes at least one series 6 much smaller value and becomes comparable to rex and re. connected of a field-effect transistor 18 and a bipolar transis tor 20. The latter bipolar transistor 20 may be a bipolar junc In this small-signal hybrid it model 42 of a bipolar transis tion transistor (BJT) or heterojunction bipolar transistor tor 35 in a common-emitter configuration 41, the base termi nal of the bipolar transistor 35 connects through a resistance (HBT). 20 The bipolar transistor 20 is arranged in common-base con r6 to a junction with a capacitor Cµ (also named as CBc), C,, figuration in which the base of the bipolar transistor 20 and resistor r,,. The latter two elements, C,, and r,,, are con (marked by the letter "B") is referenced to signal AC ground. nected in parallel between resistor r 6 and a junction of resistor The collector (marked by the letter "C") is attached to the rex' a current source 44 equal to gm vBE where vBE is the voltage across resistor r,,, and a resistor r • The remaining end output terminal 14 and the emitter (marked by the letter "E") 25 0 is connected to the drain (marked by the letter "D") of the of resistor rex connects to the emitter terminal of the device. field-effect transistor 18. The remaining end of Cµ connects to the remaining termi The source of the field-effect transistor 18 (marked by the nals of the current source 44, resistor r0 and resistor re the letter "S") is connected to ground and the gate (marked by the latter of which leads to the collector terminal. letter "G") is connected to the input terminal 12. Generally 30 Referring to FIG. 4, similarly a small-signal T-model 46 therefore, a signal at the gate of field-effect transistor 18 controls the current from drain to source of the field-effect (equivalent to hybrid it model 42) may be created for the transistor 18 in tum controlling the current from collector to common-base configuration 47 of bipolar transistor 35 or 20. emitter of the bipolar transistor 20. The small signal T-model 46 provides for a resistor rexjoining
Referring now to FIG. 2, the doping profile 22 of a standard 35 the emitter to the common junction of a resistor r 0 and the bipolar transistor 35 intended for a common-emitter amplifier parallel combination of resistor re and capacitor C,,. The design provides for a decreasing concentration of dopant as remaining terminals of resistor re and capacitor C,, in tum one moves from the emitter region 24 to the base region 26 connect to a junction of a base resistance r6 , a current equal to and to the collector region 28. Such a bipolar transistor 35, gm vBE where vBE is the voltage across resistor re, and a capaci- when used in a common-emitter configuration, produces a 40 tor Cµ- common-emitter gain curve 30 providing generally decreas The remaining end ofr0 connects to the remaining termi ing power gain as frequency increases, where power gain may nals of the parallel connected current source 44 and capacitor be measured as maximum available gain (MAG), and maxi Cµ, and to resistor re, the latter of which leads to the collector mum stable gain (MSG) in decibels. terminal. Alternatively, when a bipolar transistor 35 of this type is 45 The H-parameters of the small signal hybrid it model 42 used in a common-base configuration, a common-base gain representing the common-emitter configuration 41 are curve 32 is produced, again providing power gain as a func derived as the following where subscript symbols: i stands for tion of frequency, typically having less power gain than the input port; o stands for output port; r stands for reverse trans- common-emitter design for the majority of a generally useful 50 mission; f stands for forward transmission; e stands for com frequency range 34. mon-emitter; and b stands for common-base, according to In the present invention, a doping profile 22 can be well known convention: employed with the bipolar transistor 20 of FIG. 1. With the configuration of FIG. 1, a high breakdown voltage from the base collector junction of bipolar transistor 20 is available. (1) The use of ballast resistors generally for maintaining ther- 55 mally stable operation of multiple parallel bipolar transistor 20 is thus eliminated. Power performance, including output RF power, power gain, power-added efficiency and over voltage, is improved. r,x(l + gmZiJ + Z1 (2) hre = - ~------In another embodiment of the present invention, a doping 60 1 -.- + Z1 + (r + r,)(l + gmZ1) profile 36 is employed with the bipolar transistor 20 of FIG. 1 JWCµ 0 in which the doping in the base region 26 is substantially gmZ1 (3) increased with respect to the doping in emitter region 24 and -:---C -Z1 - ro(l + gmZ1) JW µ collector region 28. This change in doping profile 36 results in h1, = -1-~------an enhanced common-base gain curve 40 outperforming the 65 -.- + Z1 + (r0 + r,)(l + gmZiJ JWCµ common-emitter gain curve 30 for a widened useful fre quency range 37. This improvement in the enhanced com- US 7,902,635 B2 5 6 -continued gmZ1 gmZ1 (10) (4) ~ - Z1 - r,x(l + gmZiJ jwCµ MSG,= _JW_µ______------1 r0 (1 +gmZ1)+Z1 r0 (1 + gmZ1) + Z1
where Z1 is given by Under the assumption MSG>>l and lgmZ 1 1,.,~>>l in the intermediate frequency range and using approximation 10
CY 1 a::::: 1, re= - ;::::: -. gm gm where kT=26 meV. 15 Eq. 10 can be simplified as:
These four H-parameters, h,e, hre' hfe and h 0 e are equivalent to h11, h 12, h21 andh22 in the general two-port network format, respectively. gmZ1 (11) Similarly, the H-parameters for the small signal T model 46 jwCµ 20 MSG,"' ru(l + gmZiJ + Z1 "' representing the common-base configuration 47 can be derived as:
For the common-base configuration 47, since the value of 5 ( ) 25 r0 is fairly large, Eq. 6 can thus be simplified as:
1 - gmZ2 . ) Z2 (6) rb ( _Z_2_+_ro- + JWCµ + -Z2_+_r_o h,b = __J_·w_r_bc~"-- (12) hrb = 1 2 1 + jwCµ(rb + r,) 1 + (-_-_gm_Z_ + jwCµ)(rb + r,) Z2 + ro 30
(7) From Eqs. 7 and 12, MSG for the common-base configu- ration is:
(8) hob=------35 (13)
Since MSG>> 1 and lgmZ 1""a, in the intermediate fre- where Z is given 2 2 4o quency range, Eq. 13 can be further simplified as,
(14)
45
It is noted, fromEqs. 11 and 14, thatbothMSG6 andMSGe In order to calculate the difference/ratio of power gain follow a -10 dB/decade degradation trend, which is com between two configurations, approximations, justified by monly observed for MSG versus frequency. The ratio of actual values, may be made to simplify the derived H-param 50 MSG's between the common-base and common-emitter con- eters and the power gain expressions in different frequency figurations, using ranges. Since most of the transistors 35, 20 are operated in the intermediate frequency range within the fmax of the devices
(for RF and microwave power amplification) and the devices CY re= - ;::::- are potentially unstable in this frequency range, it is impera- 55 gm gm tive to specifically consider the maximum stable power gain (MSG) in this useful frequency range. MSG can be expressed again, is: in terms ofH-parameters as shown in Eq. 9:
60 (15) (9) MSG= ltl
From Eq. 11 and Eq. 14, one can see that MSGe is depen- From Eqs. 2 and 3, the maximum stable gain MSG for the 65 dent on rex+re and MSG6 is dependent on r6 . common-emitter configuration 41 can be derived as the fol For the common-base configuration 4 7, MSC 6 increases as lowing: r6 decreases. In the present invention, the value of r6 is US 7,902,635 B2 7 8 decreased to less than rex+re by the increased doping of the quarter of the cut-off frequency fr of a transistor having a base region described above to provide performance using a profile of22 under common-emitter configuration. common-base configuration that can be superior to the per One can also notice that neither MSGe nor MSG6 is depen formance from a common-emitter configuration. dent on the parasitic collector resistance re Although In the high frequency range, the devices are uncondition 5 increased re can increase the RC delay (via Cµ·rc) of the ally stable. MAG can be expressed as, transistors 20, 35, which in turn reduces the device cut-off frequency fr, there is no significant effect ofrcon small-signal power gain within the frequency range of concern. MAG~MSG(K-v'VK2-1) (16) Referring now to FIG. 5, a given row 16a of the amplifier where K is the Rollett's stability factor (the K-factor), 10 cell 10 may provide for multiple parallel bipolar transistor 20 limited to a number permitting effective current sharing between these bipolar transistors 20. These bipolar transistors 2Re(Zu)Re(Z22) - Re(Z12Z21) (17) K = ------ 20 may be driven by a single field-effect transistor 18 to IZ12Z2il provide for fan out of that control. 15 Conversely as shown in FIG. 6, each row 16 may provide Due to the complication of the K-factor in MAG, a simpli for multiple field-effect transistors 18 connected in parallel to fied expression of MAG for both CE and CB configurations is control the current through one bipolar transistor 20 for fan in of that control. These variations in amplifier cell 10 of FIG. 1 impossible to obtain. The relative size of MAGe and MAG6 is compared qualitatively. match the available power of the devices in a given fabrica After converting the K-factor into the H-parameter repre- 20 tion. 6, sentation and substituting Eq. 1-4 and Eq. 5-8 with appropri- Alternatively as shown in FIG. the multiple field-effect ate approximations, the K-factor for the CE and the CB con transistors 18 connected in parallel may control multiple par figurations can be derived, respectively, as, allel bipolar transistors 20 and 20' for optimized power deliv ery, connectivity. The number of parallel bipolar transistors 25 20 is limited by thermal effects that can be tolerated by these (18) transistors 20 in parallel. A large number of parallel bipolar transistors 20 without using ballast resistors is not recom (19) mended. Referring to FIG. 7, the amplifier cells 10 may be fabri- 30 cated on a substrate 50 together with conventional CMOS type logic circuitry 52, the latter which may be used to pro A simple direct comparison shows that ke>k . Conse 6 vide signal to the amplifier cells 10 per line 54 and to receive quently, the frequency point (fK~i cB) at which the K-factorof feedback or monitoring signals per line 56. The present inven the CB configuration reaches u~ity (break-point of MSG/ tion, by providing improved power gain in silicon devices, MAG) is larger than the corresponding frequency point of the 35 makes such bipolar CMOS integration valuable in a variety of CE configuration (fK~i cE), i.e., fK~i cB>fK~I cE· By compar applications including radio transmitters and the like where ing the slopes of MACt and MAG; versus frequency using extensive digital domain processing of signals may be desir d(MAGe(dB))/d(log w) and d(MAG (dB))/d(log w)), it can 6 able. be shown that MAG decreases faster with frequency than 6 Referring now to FIG. 8, in an alternative embodiment of MAGe. 40 the amplifier cell 10, each of the bipolar transistors 20 may be IfMSGb>MSGe (Eq. 15) for the case ofrb
MAG6 and MAGe. A straightforward comparison of these of field-effect transistors receiving the input at gates of the two ratios shows that kjk6>r6/(re+rex) regardless of relative field-effect transistors to produce driving signals through size of r6 and re+rex· When both MAG and frequency are 55 drains of the field-effect transistors a plurality of silicon bipo plotted in logarithmic scale, kjk6>r6/(re+rex), in light of the lar transistors receiving the driving signals across emitter and fact that MAG6 and MAGemerge together (with gain value of bases of the silicon bipolar transistors; an output connected unity) at the same fmax, indicates that MAG6 must be larger across collectors and bases of the plurality of silicon bipolar than MAGe in the frequency range of f