The base current of AlGaAs/GaAs heterojunction bipolar transistor subjected to a long burn-in test often exhibits an abnormal characteristic with an ideality factor of about 3, rather than a normal ideality factor between 1 and 2, in the midvoltage range, We develope an analytical model to investigate the physical mechanisms underlying such a characteristic. Consistent with the finding of an experimental work reported recently, our model calculations show that the recombination current in the base has an ideality factor of about 3 in the midvoltage range and that such a current is responsible for the observed abnormal base current in heterojunction bipolar transistor after a long burn-in test. Post-burn-in data measured from two different heterojunction bipolar transistors are also included in support of the model

Huang, C. I.

Liou, J. J.

Sheu, S.

Williamson, D. C.

English

University of Central Florida (UCF): STARS (Showcase of Text, Archives, Research & Scholarship)

University of Central Florida STARS Faculty Bibliography 1990s Faculty Bibliography 1-1-1996 Modeling The Post-Burn-In Abnormal Base Current In Algaas/Gaas Heterojunction Bipolar Transistors S. Sheu University of Central Florida J. J. Liou University of Central Florida C. I. Huang D. C. Williamson Find similar works at: https://stars.library.ucf.edu/facultybib1990 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 1990s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Sheu, S.; Liou, J. J.; Huang, C. I.; and Williamson, D. C., "Modeling The Post-Burn-In Abnormal Base Current In Algaas/Gaas Heterojunction Bipolar Transistors" (1996). Faculty Bibliography 1990s. 1754. https://stars.library.ucf.edu/facultybib1990/1754 Journal of Applied Physics 79, 7348 (1996); https://doi.org/10.1063/1.361451 79, 7348© 1996 American Institute of Physics.Modeling the post-burn-in abnormal basecurrent in AlGaAs/GaAs heterojunctionbipolar transistorsCite as: Journal of Applied Physics 79, 7348 (1996); https://doi.org/10.1063/1.361451Submitted: 24 July 1995 . Accepted: 24 January 1996 . Published Online: 04 June 1998S. Sheu, J. J. Liou, C. I. Huang, and D. C. WilliamsonModeling the post-burn-in abnormal base current in AlGaAs/GaAsheterojunction bipolar transistorsS. Sheu and J. J. LiouElectrical and Computer Engineering Department, University of Central Florida, Orlando, Florida 32816C. I. HuangDevice Technology Division, Wright Laboratory, Wright-Patterson AFB, Ohio 45433D. C. WilliamsonElectromagnetic and Reliability Directorate, Rome Laboratory, Griffiss AFB, New York 13422~Received 24 July 1995; accepted for publication 24 January 1996!The base current of AlGaAs/GaAs heterojunction bipolar transistor subjected to a long burn-in testoften exhibits an abnormal characteristic with an ideality factor of about 3, rather than a normalideality factor between 1 and 2, in the midvoltage range. We develope an analytical model toinvestigate the physical mechanisms underlying such a characteristic. Consistent with the finding ofan experimental work reported recently, our model calculations show that the recombination currentin the base has an ideality factor of about 3 in the midvoltage range and that such a current isresponsible for the observed abnormal base current in heterojunction bipolar transistor after a longburn-in test. Post-burn-in data measured from two different heterojunction bipolar transistors arealso included in support of the model. © 1996 American Institute of Physics.@S0021-8979~96!07609-7#I. INTRODUCTIONBurn-in tests carried out in a thermal and/or electricalstress condition are useful in determining the long-term per-formance of AlGaAs/GaAs heterojunction bipolar transistors~HBTs!.1–3 Experimental results often show that the burn-intest increases considerably the base current IB but does notalter notably the collector current IC . Furthermore, an abnor-mal base current with an ideality factor n'3 in the midvolt-age range is often observed in the Gummel plot of a HBTsubjected to a relatively long-hour burn-in test.1–3 An at-tempt has been made earlier to model the HBT post-burn-inbehavior.4 The analysis was based on the theory that thedefects at the base surface may migrate to the heterointerfaceduring the high thermal/electrical stress condition ~i.e.,recombination/thermal enhanced defect diffusion5!. Whilesuch a model can successfully describe IB and IC in HBTssubjected to a relative short burn-in test ~IB and IC after 144h stress shown in Fig. 1!, it fails to predict IB with n'3characteristics observed in the HBT after a long-hour stresstest, as evidenced by the results of IB measured after 300 hstress given in Fig. 1. Sugahara et al.3 have suggested thatsuch an abnormal current can be attributed to a significantincrease in the number of defects in the strained base ~i.e.,stress-induced defects! during the long stress hours. Also,they have demonstrated that the post-burn-in IB can begreatly reduced if the base lattice strain is relaxed.This article presents a comprehensive theoretical studyon the abnormal base current in the post-burn-in HBT. Basedon the Shockley–Read–Hall ~SRH! recombination statistics,a model for the recombination current in the base region isdeveloped. Our model calculations show that such a currenthas an ideality factor of about 3 in the midvoltage range andthus is responsible for the observed abnormal base current inHBT after a long burn-in test. With the aid of the model andmeasurement data, physical mechanisms underlying the ob-served abnormal base current in the post-burn-in HBT arealso discussed.II. MODEL DEVELOPMENTA. Pre-burn-in HBTWe focus on the base current of a mesa-etched N/p1/nHBT. There are two major components for the base currentof pre-burn-in HBT,IB5IBL1IBN , ~1!where IBL is the base leakage current and IBN is the normalbase current. For the bias condition of applied base-collectorvoltage VCB50 and base-emitter voltage VBE.0 ~i.e.,forward-active mode!, the base leakage current is originatedfrom the leakage of electron from the base to emitter throughthe emitter-base periphery and is the dominate current com-ponent for IB at relatively small VBE .6 This current is givenby6IBL5PEJBL8 @12exp~2VBEFL /VT!# , ~2!where PE is the emitter perimeter length, JBL8 is the fullyactivated ~i.e., VBE@VT! base leakage current density, andFL is an empirical parameter determining the shape of thebase leakage current.The normal base current in general consists of:~1! the recombination current ISCRE in the emitter side of theheterojunction space-charge region;~2! the recombination current ISCRB in the base side of theheterojunction space-charge region;~3! the surface recombination current IRS at the emitter sidewalls and extrinsic base surface;~4! the recombination current IQNB in the QNB; and~5! the injection current IRE from the base into emitter.7348 J. Appl. Phys. 79 (9), 1 May 1996 0021-8979/96/79(9)/7348/5/$10.00 © 1996 American Institute of PhysicsThe details of these current components can be been found inthe literature.7 For pre-burn-in HBTs, IQNB is negligible be-cause the number of defects in the QNB is small and the baseis very thin. In addition, ISCRB is neglected due to the factthat the majority of the space-charge region ~SCR! resides inthe emitter because of the very high base doping density.Thus,IBN5ISCRE1IRS1IRE . ~3!The ideality factor of this current ranging from 1 to 2.B. Post-burn-in HBTAfter a long burn-in test, the number of defects in thebase will be increased significantly due to the strained latticeduring the stress test.3 As a result, substantial electron–holerecombination occurs in both the base side of the SCR andthe QNB, and the conventional thin QNB and thin SCR ap-proximations are no longer valid. Thus, for a HBT after along burn-in test,IBN5IBASE1ISCRE1IRS1IRE , ~4!where IBASE5ISCRB1IQNB , andIBASE5AqE0X2USRH~x !dx1AqEX2XBUSRH~x !dx . ~5!Here A is the emitter area, x50 and X2 are the boundaries ofbase-side SCR, x5X2 and XB are the boundaries of theQNB, and USRH is the total SRH recombination rate sum-ming the recombination rates at each trapping state ETi ~i51,2,.. . ,N , N is the total number of trapping states!,USRH5(i51NUiSRH, ~6!and7UiSRH5~pn2ni2!~11G!~NTis iv th!$p1n12ni3cosh@~ETi2Ei!/kT#%21. ~7!p and n are hole and electron concentrations in the QNB, niis the intrinsic free-carrier concentration, G is the trap-assisted tunneling factor, NTi is the trapping density at ETi ,si ~'10214 cm22! is the capture cross section at NTi , v th~'107 cm/s! is the electron thermal velocity, and Ei is theintrinsic Fermi energy. The trap-assisted tunneling is impor-tant for the high-field region, such as the emitter-base SCR,where electrons can tunnel through the energy band via trapsand subsequently recombine with holes.8 In a low-field re-gion, such as the QNB, G approaches zero. This factor isgiven by8G5S DEkT D E01 expS uDEkT 2K8u1.5D du . ~8!Here DE is the energy between the conduction-band edgeand the trapping state energy since electrons in these ener-gies are tunneling possible, and K8 is a parameter inverselyproportional to the local electric field j,K85~4/3!~2m*DE3!0.5/~q\j!. ~9!m* is the effective electron mass and \ is the reduced Planckconstant. When j is large, K8 is small, and G becomes large.For the QNB, the minority-carrier lifetime tB is relatedto the electron concentration as9tB5~n2n0!/USRH5Dn/USRH, ~10!where n0 is the equilibrium electron concentration and Dn isthe excess electron concentration. For a base with an arbi-trary length,7Dn5Dn~X2!sinh@~XB2x !/Ln#/sinh@~XB2X2!/Ln# . ~11!Here Ln5(DntB)0.5 is the electron diffusion length in theQNB and, using the thermionic and tunneling mechanisms atheterointerface and Boltzmann statistics in the QNB,10Dn~X2!5qvngnNE exp~2VB1 /VT!/z , ~12!z5qDn /~XB2X21Dn /vs!1qvngn3exp@~VB22DEC /q !/VT# , ~13!where vn is the electron thermal velocity, gn is the electrontunneling coefficient, NE is the emitter doping concentration,and VB1 and VB2 are the barrier potentials on the emitter andbase sides of the junction, respectively. Since tB and Dn arerelated to each other, a numerical procedure is needed tocalculate USRH , and thus IQNB , iteratively, provided the pa-rameters associated with the SRH process ~i.e., ETi , NTi ,and N! are specified.For the SCR, n , p , and j distributions in the base side ofSCR needed in Eqs. ~7! and ~8! are given byn~x !5n~X2!exp@2Vi~x !/VT# , ~14!p~x !5p~X2!exp@Vi~x !/VT# , ~15!j~x !52~qNB /eB!~X22x ! ~16!FIG. 1. Base and collector current characteristics of a post-burn-in ~sub-jected to 240 °C temperature and 104 A/cm2 current density stress! AlGaAs/GaAs HBT calculated from a previously developed model Ref. 4 and ob-tained from measurements. The model of Ref. 4 gives an accurate predictionfor the HBT behavior after a relatively short stress test ~i.e., 144 h!, but failsto describe the base current ~i.e., with an ideality factor of about 3 betweenVBE50.3 and 1.2 V! of the HBT subjected to a long stress test ~i.e., 300 h!.7349J. Appl. Phys., Vol. 79, No. 9, 1 May 1996 Sheu et al.where Vi is the electrostatic potential @i.e., Vi(X2)50 is cho-sen as the reference potential#, NB is the base doping con-centration, and eB is the dielectric permittivity in base. Theposition-dependent Vi in the base side of SCR can be ex-pressed as7Vi~x !520.5~qNB /eB!~X22x !2. ~17!As is shown later, IBASE has an ideality factor of about 3in the midvoltage range and thus is the current componentcontributing to the abnormal base current observed in thepost-burn-in HBT.III. RESULTS AND DISCUSSIONSWe first investigate the effects of ETi and N on the re-combination current in the base. The device considered has atypical makeup of 531017 cm23 emitter doping concentra-tion, 0.15 mm emitter layer thickness, 1019 cm23 base dopingconcentration, and 0.1 mm base layer thickness. Also, theconduction-band edge EC has been chosen as the referencefor ETi ~i.e., ETi50 if located at EC!. Three different ETi of0.7, 1.1, and 1.4 eV will be considered to represent varioustrapping state locations in the band gap ~i.e., deep-,intermediate-, and shallow-level trapping states!. Further-more, only ETi below Ei are considered because only thesetypes of ETi are important to trap-assisted tunneling in thebase side of the SCR.8 As is shown later, this is a majormechanism contributing to the abnormal base current.Figure 2 shows IBASE calculated from the model usingfixed NTi51019 cm23 and a single trap with ETi50.7 eV, asingle trap with ETi51.4 eV, and multiple traps withETi50.7, 1.1, and 1.4 eV ~i.e., N53!. The results suggestthat IBASE is relatively insensitive to ETi , but depends moreon the number of trapping state N , particularly at small VBE .Furthermore, all three currents exhibit an n'3 characteristic.Intuitively, one expects IBASE increases with increasingETi and increasing N because USRH is inversely and directlyproportional cosh(ETi/kT) and N @see Eqs. ~6! and ~7!#, re-spectively. This is true for small VBE ~i.e., VBE,0.8 V!,where the electric field in the SCR is high, and recombina-tion via trap-assisted tunneling in the SCR is the dominantprocess. For high VBE , however, the electric field in the SCRis small, and the SRH recombination in the QNB is moreFIG. 2. Recombination current in the base vs VBE calculated from the modelfor three different cases of NTi and N .FIG. 3. Recombination current in the base calculated with and without thetrap-assisted tunneling mechanism.FIG. 4. SRH recombination rates vs base position calculated for three dif-ferent VBE .FIG. 5. Recombination current in the base vs VBE calculated from the modelfor three different NTi .7350 J. Appl. Phys., Vol. 79, No. 9, 1 May 1996 Sheu et al.significant. Since USRH in the QNB is a function of the elec-tron concentration, an increase in ETi and increase in N willtend to increase USRH, but such a change will also tend todecrease tB and therefore decrease the electron concentrationand USRH in the QNB. This compensating mechanism leadsto a less significant effect of ETi and N on IBASE , as ob-served in the region of VBE.0.8 V in Fig. 2. To furtherdemonstrate this, we show in Fig. 3 IBASE vs VBE calculatedwith and without trap-assisted tunneling. It can be seen thatthe current component resulted from trap-assisted tunnelingis negligible if VBE is greater than 0.8 V. For this bias region,recombination current in the QNB is the dominant current,and IBASE is less insensitive to NTi and N , as observed in Fig.2. Also note that the abnormality of n'3 is more evident inIBASE with trap-assisted tunneling.The dependence of USRH(x) on VBE is illustrated in Fig.4. A logarithmic scale has been used for the x axis to illus-trate the details of USRH(x) in the base side of SCR ~farleft-hand side of the figure! due to trap-assisted tunneling.For relatively small VBE ~i.e., VBE50.4 and 0.8 V!, the re-combination rate in the SCR decreases with increasing VBEbecause of a smaller electric field and thus a smaller trap-assisted tunneling factor in the region. The trend is reversedif VBE is further increased ~i.e., VBE51.2 V!, however, due tothe fact that the SCR is vanishing, and USRH becomes theQNB recombination rate.Figure 5 shows the effect of NTi on the recombinationcurrent in the base. Here, we have arbitrarily chosen a singletrap with ETi50.7 eV in calculations. Clearly, the value ofNTi affects IBASE significantly, and NTi will be the mainparameter in fitting the model calculations with experimentaldata.Figure 6 shows the total base currents of pre- and post-burn-in HBT-1 ~device makeup and its leakage current pa-rameters are given in Table I! calculated from the model andobtained from measurements. The plateaulike current forVBE,0.8 V in the pre-burn-in HBT is the base leakage cur-rent. For the post-stress HBT the current behavior forVBE.0.2 V is changed to that of n'3. This is due to the factthat, in addition to the base leakage current, there is a largeIBASE in the post-burn-in HBT. NTi58.7531018 cm23 hasbeen used to fit the model to measured data, suggesting thestress-induced defect density in such a HBT is 8.7531018cm23. A single trap with ETi50.7 eV has also been used.Figure 7 shows the total base currents of pre- and post-burn-in HBT-2 ~see Table I! calculated from the model andobtained from measurements.3 For this device, we found thatthe burn-in test resulted in NTi5231018 cm23 in the base.This is smaller than NTi in HBT-1, due perhaps to the factthat HBT-2 is subjected to a less severe burn-in test ~200 °Cand 73103 A/cm2! than HBT-1 ~240 °C and 104 A/cm2!.IV. CONCLUSIONSA model has been developed to investigate the physicalmechanisms underlying the abnormal base current ~i.e., withan ideality factor of about 3! observed in the post-burn-inAlGaAs/GaAs heterojunction bipolar transistor ~HBT!. Ourstudy confirms the finding of recent experimental work thatsuch a current resulted from the significant electron–holerecombination via stress-induced defect centers in the base ofthe HBT. Furthermore, it has been shown that the trap-assisted tunneling is an important mechanism for recombina-tion in the space-charge region when the bias voltage is rela-tively low. The model calculations compare favorably withdata measured from two different HBTs.FIG. 6. Pre- and post-burn-in base currents of HBT-1 calculated from themodel and obtained from measurements.FIG. 7. Pre- and post-burn-in base currents of HBT-2 calculated from themodel and obtained from measurements ~Ref. 3!.TABLE I. HBT structures and leakage current parameters.Parameters HBT-1 HBT-2Emitter doping ~cm23! 531017 531017Emitter thickness ~mm! 0.17 0.18Emitter area ~mm2! 100 30Base doping ~cm23! 131019 131019Base thickness ~mm! 0.1 0.14JBL8 ~A/cm! 131025 1.3331026FL 0.005 0.0057351J. Appl. Phys., Vol. 79, No. 9, 1 May 1996 Sheu et al.ACKNOWLEDGMENTThis work was supported in part by a research grantfunded by the Air Force Office of Scientific Research.1M. E. Hafizi, L. M. Pawlowicz, L. T. Tran, D. K. Umemoto, D. C. Streit,A. K. Oki, M. E. Kim, and K. H. Yen, in Digest of the IEEE GaAs ICSymposium, 1990, p. 329.2T. Henderson, D. Hill, W. Liu, D. Costa, H. F. Chau, T. S. Kim, and A.Khatibzadeh, in Digest IEEE IEDM, 1994.3H. Sugahara, J. Nagano, T. Nittono, and K. Ogawa, in Digest IEEE GaAsIC Symposium 1993, p. 115.4 J. J. Liou and C. I. Huang, Solid-State Electron. 37, 1349 ~1994!.5 J. L. Benton, M. Levinson, A. T. Macrander, H. Temkin, and L. C. Ki-merling, Appl. Phys. Lett. 45, 566 ~1984!.6 J. J. Liou, C. I. Huang, B. Bayraktaroglu, D. C. Willamson, and K. B.Parab, J. Appl. Phys. 76, 1349 ~1994!.7For example, see J. J. Liou, Advanced Semiconductor Device Physics andModeling ~Artech, Boston, 1994!.8G. A. M. Hurkx, D. B. M. Klaassen, and M. P. G., IEEE Trans. ElectronDevices ED-39, 331 ~1992!.9C.-T. Sah, Fundamentals of Solid-State Electronics ~World Scientific, Sin-gapore, 1991!, Chap. 3.10M. S. Shur, GaAs Devices and Circuits ~Plenum, New York, 1987!.7352 J. Appl. Phys., Vol. 79, No. 9, 1 May 1996 Sheu et al.

Modeling The Post-Burn-In Abnormal Base Current In Algaas/Gaas Heterojunction Bipolar Transistors

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Modeling The Post-Burn-In Abnormal Base Current In Algaas/Gaas Heterojunction Bipolar Transistors

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