Fu, Jun

Schippel, Christian

Schwierz, Frank

Digitale Bibliothek Thüringen

  PROCEEDINGS 11-15 September 2006      FACULTY OF ELECTRICAL ENGINEERING AND INFORMATION SCIENCE    INFORMATION TECHNOLOGY AND ELECTRICAL ENGINEERING - DEVICES AND SYSTEMS, MATERIALS AND TECHNOLOGIES FOR THE FUTURE      Startseite / Index: http://www.db-thueringen.de/servlets/DocumentServlet?id=12391 51. IWK Internationales Wissenschaftliches Kolloquium International Scientific Colloquium Impressum  Herausgeber: Der Rektor der Technischen Universität llmenau  Univ.-Prof. Dr. rer. nat. habil. Peter Scharff  Redaktion: Referat Marketing und Studentische Angelegenheiten  Andrea Schneider   Fakultät für Elektrotechnik und Informationstechnik  Susanne Jakob  Dipl.-Ing. Helge Drumm  Redaktionsschluss: 07. Juli 2006  Technische Realisierung (CD-Rom-Ausgabe):  Institut für Medientechnik an der TU Ilmenau  Dipl.-Ing. Christian Weigel  Dipl.-Ing. Marco Albrecht  Dipl.-Ing. Helge Drumm  Technische Realisierung (Online-Ausgabe):  Universitätsbibliothek Ilmenau    Postfach 10 05 65  98684 Ilmenau  Verlag:   Verlag ISLE, Betriebsstätte des ISLE e.V.  Werner-von-Siemens-Str. 16  98693 llrnenau   © Technische Universität llmenau (Thür.) 2006  Diese Publikationen und alle in ihr enthaltenen Beiträge und Abbildungen sind urheberrechtlich geschützt. Mit Ausnahme der gesetzlich zugelassenen Fälle ist eine Verwertung ohne Einwilligung der Redaktion strafbar.   ISBN (Druckausgabe): 3-938843-15-2 ISBN (CD-Rom-Ausgabe): 3-938843-16-0  Startseite / Index: http://www.db-thueringen.de/servlets/DocumentServlet?id=12391  51. Internationales Wissenschaftliches KolloquiumTechnische Universita¨t IlmenauSeptember 11 - 15, 2006C. Schippel / F. Schwierz/ J. FuInvestigations on the Non-Ideality of the BaseCurrent of RF Bipolar TransistorsAbstractThe non-ideal characteristics of the base current of an experimental SiGe-HBThas been investigated through 2D device simulation. To determine the originof the device’s imperfect DC behaviour, several mechanisms of charge carrier re-combination within the transistor has been investigated. Since such recombina-tion mechanisms influence the characteristics of the device differently, the originof non-idealities can be identified by comparision of measurement and simulation.1 Introduction and MotivationWhen an established manufacturing process for semiconductor devices is modified,many uncertainties are involved. Subsequently, the devices fabricated in the first runsof the modified process do not perform as expected. At this point device simulation isable to provide insights into the origins of the these far-from-perfect device characteris-tics through applying appropriate physical assumptions. Thereby the technologists canbe supported to get the new processes from an experimental to a ready for productionstate with minimum effort in time and costs.This work is based on 2D device and technology simulations of experimental Silicon-Germanium heterojunction bipolar transistors (SiGe HBTs) in a BiCMOS process flow(see Fig. 1). In our study the simulators ATLAS [1] and ATHENA [2] have beenused. Fig. 2 shows the Gummel plot of imperfect devices having an unusually highideality factor of the base current ηIB . From the circuit designer’s point of view thisis a highly unpleasant device characteristic since the current gain β is degraded(Fig. 1(b)). Such a device behaviour can be modelled by assuming a low lifetimeof the majority charge carriers in the base (see again Fig. 2 for the agreement be-tween measurement and simulation). However, this approach does not provide ameaningful physical explanation and it is useless for the optimisation of the tech-nology. Therefore, in the present work a more fundamental approach is applied.Through making educated guesses on the origins of the imperfection, their influ-ence on the device characteristics can be simulated. Subsequently, by comparisionFigure 1: Investigated device struc-ture and its main regions: emitterlength 0.25µm, base width 70nm,base-subcollector separation (SICwidth) 700nm, lateral emitter-baseseparation 750nm10-1810-1610-1410-1210-1010-810-610-410-20 0.2 0.4 0.6 0.8 1 1.2I C, I B [A/µm] VBE [V](a)IB measuredIC measuredIB simulatedIC simulated-200204060801001200 0.2 0.4 0.6 0.8 1 1.2I C/I BVBE [V](b)β measuredβ simulatedFigure 2: Gummel plot (a) and current gainβ (b) of an experimental SiGe HBT and itssimulated counterpart at VCB = 0V . Theagreement was achieved by assuming a car-rier life time of the holes in the base region of4× 10−10sof measurement and simulation the different origin can be discarded or confirmed.2 BasicsIn general, the short lifetime of the charge carriers is caused by an increased recom-bination rate. The recombination takes place either at interface traps or at bulks traps[3]. In the case of the given HBT, the interfaces between monocrystalline and polycrys-talline emitter and between base region and the SiO2 spacer, as well as bulk traps inthe base region are relevant (see again Fig. 1).The Gummel plot of IB can be divided into low, medium and high current injection re-gions. While the last one is dominated by parasitics like emitter and base resistances,the other regions are heavily influenced by recombination processes. They can, there-fore, be used for investigations on the origins of the non-ideal characteristics.The base current IB is described, in logarithmical form, by the equationln(IB) = ln(IB0) +q · VBEηIB · k · T(1)with the parameters saturation base current IB0 and ideality factor ηIB . For bothmeasured and simulated Gummel plots, these parameters can be obtained by fitting.3 ResultsFor the investigation of the recombination mechanisms, devices with various interfaceand bulk trap densities in extremely wide ranges have been simulated. Afterwardsthe ideality factors ηIB of these devices have been determined and compared to theηIB=1.75 of the real device.• Influence of the Bulk Material of the Base: The density of acceptoric traps in thebase has been varied between 1012cm−3 and 1017cm−3. As can be seen from10-1610-1410-1210-1010-810-610-4 0.2  0.3  0.4  0.5  0.6  0.7  0.8  0.9  1I B [A/µm]VBE [V]IB measuredNT=1x1012cm-3NT=1x1013cm-3NT=1x1014cm-3NT=1x1015cm-3NT=1x1016cm-3NT=1x1017cm-3Figure 3: Gummel plot of IB of the sim-ulated HBT assuming various bulk trapdensities NT . 1 1.2 1.4 1.6 1.8 21012 1013 1014 1015 1016 1017η IBBulk trap density NT [cm-3]low current injectionhigh current injectionFigure 4: Ideality factor ηIB vs. bulk trapdensity NT .Figs. 3 and 4, at lower trap densities the ideality factors ηIB of the low and mediumcurrent injection regions are different. For trap densities of about 1017cm−3 the ηIBof the experimental HBT has been reproduced. However, such a trap concentra-tion is highly unrealistic. Therefore is cannot be used to explain the imperfectionof the investigated device.• Influence of the Mono/Poly Interface: The density of acceptoric traps NT at theinterface between the monocrystalline and polycrystalline parts of the emitter hasbeen varied between 105cm−2 and 1013cm−2. The correspondig Gummel plots are10-1610-1410-1210-1010-810-610-410-2 0.2  0.3  0.4  0.5  0.6  0.7  0.8  0.9  1I B [A/µm]VBE [V]IB measuredNT=1x105cm-2NT=1x109cm-2NT=1x1010cm-2NT=1x1011cm-2NT=1x1012cm-2NT=1x1013cm-2Figure 5: Gummel plot of IB of thesimulated HBT assuming different in-terface trap densities at the monocrys-talline/polycrystalline interface NT . 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7105 106 107 108 109 1010 1011 1012 1013η IBInterface trap density NT [cm-2]low current injectionhigh current injectionFigure 6: Ideality factor ηIB vs. in-terface trap density at the monocrys-talline/polycrystalline interface NT .depicted in Fig. 5. Fig. 6 shows the ηIB vs. NT characteristics obtained from theseresults. For low trap densities, different ideality factors for the low and mediumcurrent injection regions have been observed. However, the results indicate thateven for very high trap densities the ηIB of the measured device cannot be re-produced. Therefore it is most unlikely that the mono/poly interface causes theproblems in the manufactured device.• Influence of the Spacer/Base Interface: Variing the trap density in this regionbetween 1010cm−2 and 1013cm−2 results in values for ηIB between 1.18 and 1.93(see Fig. 8). The experimental value of 1.75 is well within this range and can be10-1310-1210-1110-1010-910-810-710-610-510-410-3 0.2  0.3  0.4  0.5  0.6  0.7  0.8  0.9  1I B [A/µm]VBE [V]IB measuredNT=2x1011cm-2NT=4x1011cm-2NT=7x1011cm-2NT=9x1011cm-2NT=1x1013cm-2Figure 7: Gummel plot of IB of thesimulated HBT assuming different in-terface trap densities at the monocrys-talline/polycrystalline interface NT . 1 1.2 1.4 1.6 1.8 21010 1011 1012 1013η IBInterface trap density NT [cm-2]Figure 8: Ideality factor ηIB vs. in-terface trap density at the monocrys-talline/polycrystalline interface NT .achieved by the assumption of a trap density of about 5× 1011cm−2, which seemsto be a realistic value. Furthermore, as shown in Fig. 9, the absolute values forFigure 9: Comparision of the mea-sured and simulated base current. Atrap density of 5 × 1011cm−2 at thespacer/base interface has been as-sumed. The disagreement in the highcurrent injection region is due to ex-ternal parasitic elements, which havebeen neglected.10-1210-1110-1010-910-810-710-610-510-410-3 0.2  0.3  0.4  0.5  0.6  0.7  0.8  0.9  1I B [A/µm]VBE [V]measuredsimulated with NT=5x1011cm-2the measured and simulated base current match perfectly in the low and mediumcurrent injection regions. This is an additional indication for the appropriatenessof our assumption.4 ConclusionsA method to support the technology development through device simulationhas been elaborated and demonstrated for a problem regarding the DC char-acteristics of RF HBTs. A similar procedure can be used in many othercases where the behaviour of real devices differs from the expected one,for example to explain degradation effects, and for reliability investigations.AcknowledgementsThe authors would like to thank Sebastian Schmidt and Wolfgang Einbrodt of the X-FAB Semiconductor Foundries AG. This work has been supported by the TMWTA un-der contract number B509-03006.References[1] Silvaco International, Santa Clara, CA 95054, USA Device Simulation Software, 2005[2] Silvaco International, Santa Clara, CA 95054, USA 2D Process Simulation Software, 2005[3] W. Liu, Handbook of III-V Heterojunction Bipolar Transistors, John Wiley & Sons, Inc., ISBN 0-471-24904-1Authors:Dipl.-Ing. Christian SchippelPD Dr. habil. Frank SchwierzTechnical University IlmenauDepartment of Solid State ElectronicsPF 100565, 98684 IlmenauTel: +49 3677 69 3225Fax: +49 3677 69 3132E-mail: christian.schippel@tu-ilmenau.deDr. Jun FuX-FAB Semiconductor Foundries AGHaarbergstrasse 6199097 Erfurt

Investigations on the non-ideality of the base current of RF bipolar transistors

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Investigations on the non-ideality of the base current of RF bipolar transistors

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