We have developed InGaP/GaAsSb/GaAs double-heterojunction bipolar transistors (DHBTs) with low turn-on voltage and high current gain by using a narrow energy bandgap GaAsSb layer as the base and an InGaP layer as the emitter. The current transport mechanism is examined by measuring both of the terminal currents in forward and reverse mode. The results show that the dominant current transport mechanism in the InGaP/GaAsSb/GaAs DHBTs is the transport of carriers across the base layer. This finding suggests that the bandgap offset produced by incorporating Sb composition into GaAs mainly appears on the valence band and the conduction-band offset in InGaP/GaAsSb heterojunction is very small. © 2004 American Institute of Physics.published_or_final_versio

Hsu, CC

Wang, XQ

Yan, BP

Yang, ES

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Title Current transport mechanism in InGaP/GaAsSb/GaAs double-heterojunction bipolar transistorsAuthor(s) Yan, BP; Hsu, CC; Wang, XQ; Yang, ESCitation Applied Physics Letters, 2004, v. 85 n. 17, p. 3884-3886Issued Date 2004URL http://hdl.handle.net/10722/42102Rights Applied Physics Letters. Copyright © American Institute ofPhysics.APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 17 25 OCTOBER 2004Current transport mechanism in InGaP/GaAsSb/GaAs double-heterojunctionbipolar transistorsB. P. Yan,a) C. C. Hsu, X. Q. Wang, and E. S. YangDepartment of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road,Hong Kong SAR, People’s Republic of China(Received 15 June 2004; accepted 24 August 2004)We have developed InGaP/GaAsSb/GaAs double-heterojunction bipolar transistors (DHBTs) withlow turn-on voltage and high current gain by using a narrow energy bandgap GaAsSb layer as thebase and an InGaP layer as the emitter. The current transport mechanism is examined by measuringboth of the terminal currents in forward and reverse mode. The results show that the dominantcurrent transport mechanism in the InGaP/GaAsSb/GaAs DHBTs is the transport of carriers acrossthe base layer. This finding suggests that the bandgap offset produced by incorporating Sbcomposition into GaAs mainly appears on the valence band and the conduction-band offset inInGaP/GaAsSb heterojunction is very small. © 2004 American Institute of Physics.[DOI: 10.1063/1.1808891]GaAs-based heterojunction bipolar transistors (HBTs)have been widely used for power amplifiers in wireless andhigh-speed digital applications. However, GaAs-based HBTshave a relatively large base-emitter turn-on voltage due tothe large bandgap of GaAs used as the base layer, and thislimits the minimum operating voltage and increases thepower consumption in circuit applications. In addition, bothAlGaAs/GaAs and semi-ordered InGaP/GaAs HBTs sufferfrom current gain degradation at elevated temperature be-cause of a low valence-band offset DEv,1,2reducing powercapacity. Therefore, it is important to develop material com-bination HBTs with low turn-on voltage and thermal stabil-ity. The material combination HBTs should meet followingrequirements: (1) the base layer should use lower energybandgap materials to reduce the turn-on voltage; (2) theemitter-base (EB) junction should have a large valence-bandoffset DEv and a small conduction-band offset DEc; (3) theconduction-band discontinuity DEc in the base-collector(BC) junction should be very small such that no collectorcurrent blocking effect occurs. Recently, several groups havedemonstrated the potential of using narrow energy bandgapInGaAsN material in the base layer to reduce the turn-onvoltage.3–6 However, InGaAsN material typically displaysdegraded minority carrier properties compared with GaAs,leading to the reduction of dc current gain and high fre-quency performance. Although these unfavorable character-istics can be suppressed by the insertion of graded layersbetween the base and collector junction,4 this complicates thetransistor design and fabrication.Another attractive narrow bandgap base material isGaAsSb, which has been successfully used in InP-basedHBTs by Bolognesi et al.7,8 In comparison with a lattice-matched GaAs base, the smaller band gap of GaAsSb canreduce the turn-on voltage, thus the power dissipation in cir-cuits. Moreover, the band lineup at the GaAsSb/GaAs inter-face is inferred to be a staggered (“type II”) lineup,9 thatwould eliminate the current blocking. Although GaAs-basedHBTs with GaAsSb base layers have been reported,10–12 onlya)Author to whom correspondence should be addressed; electronic mail:bpyan@eee.hku.hk0003-6951/2004/85(17)/3884/3/$22.00 3884Downloaded 08 Nov 2006 to 147.8.21.97. Redistribution subject to Alimited information was given. In the previous work, thegrown emitter-base junction was either anAlGaAs/GaAsSb10,11 or a GaAs/GaAsSb heterojunction,12and the devices showed either poor current gains10,11 or largebase recombination current with a base current ideality factorof 2.12 Recently, we have demonstrated a material combina-tion, i.e., InGaP/GaAsSb/GaAs double-heterojunction bipo-lar transistors (DHBTs) with low turn-on voltage.13,14 For theInGaP/GaAsSb/GaAs DHBTs, an interesting issue is whetheror not the InGaP/GaAsSb EB junction and the GaAsSb/GaAsSb/GaAs BC junction confine carrier transport. In thisletter, we study the current transport mechanism in theInGaP/GaAsSb/GaAs DHBTs.The InGaP/GaAsSb/GaAs DHBT structure was grownon a semi-insulating (100) GaAs substrate by metalorganicchemical vapor disposition. Trimethylgallium, trimethylin-dium, trimethylantimony, tertiarybutyphosphine, and tertia-rybutylarsine were used as the organometallic sources. Car-bon and silicon were used as p- and n-type dopants,respectively. The device structure consists of a500 nm n.331018 cm−3 GaAs subcollector, a 500 nm n=531016 cm−3 GaAs collector, a 50 nm p=831018 cm−3GaAsSb base (Sb composition: 10.4%), a 50 nm n=331017 cm−3 InGaP emitter, a 150 nm n=431018 cm−3GaAs layer, a 50 nm n.131019 cm−3 compositionallygraded InxGaAs1−xcap layer sx=0–0.5d, and a 50 nm n.131019 cm−3, In0.5GaAs0.5 cap ohmic contact layer. The Sbcomposition was confirmed by high-resolution x-ray diffrac-tion measurement. The surface morphology was observed byatomic force microscope and no crosshatched patterns asso-ciated with misfit dislocations were observed. This suggeststhat the GaAsSb base layer is fully strained. The structurewas fabricated into devices using optical lithography andchemical wet selective etching for mesa definition.Figure 1 shows the dependence of current gain on thecollector current for a large area device of 1003100 mm2.The inset in Fig. 1 shows the common-emitter I–V character-istics of the device. The device demonstrates excellent dcperformance. The offset voltage is 0.21 V and the breakdownvoltage BVCEO is about 6 V. It should be noticed that there isstill a current gain more than unity even at a collector current© 2004 American Institute of PhysicsIP license or copyright, see http://apl.aip.org/apl/copyright.jspAppl. Phys. Lett., Vol. 85, No. 17, 25 October 2004 Yan et al. 3885of 10−10 A, which is probably due to the large valence-banddiscontinuity and the lower interface recombination at theInGaP/GaAsSb interface. A maximum dc current gain b of207 has been obtained at a collector current of 95 mA. TheInGaP/GaAsSb/GaAs DHBTs display a turn-on voltage of0.914 V at JC=1 A/cm2, which is 0.18 V lower than that ofconventional InGaP/GaAs HBTs, which usually have aturn-on voltage of 1.1 V at the same collector current density.In order to determine the current-transport mechanism inthe InGaP/GaAsSb/GaAs DHBTs, we have measured the ter-minal currents of the DHBT both in the forward active modeand in the reverse active mode. It is well known that thereare two kinds of current transport mechanisms in bipolartransistors. The first one is the diffusion limitation mecha-nism, i.e., the terminal current is limited by the transport ofcarriers across the base layer. In this case, the terminal cur-rent only depends on the doping profile and thickness of thebase layer, independent of the structure of the emitter-base orthe base-collector junction. Therefore, both the terminal cur-rents in the forward active mode and in the reverse activemode should overlap each other and have an ideality factorof unity,15 because the base layer is the same in either opera-tion mode. Another transport mechanism is the conductionband barrier limitation of the emitter-base or the base-collector heterojunction, which usually takes place in abruptheterojunctions.15,16 In the case of conduction-band barrierlimitation, the terminal current critically depends on themagnitude of the conduction-band barrier. Therefore, the twoterminal currents in forward and reverse mode would beapart from each other and have an ideality factor more thanunity.17Figure 2(a) shows measured terminal currents of a largearea device s1003100 mm2d in both the forward activemode, and the reverse active mode. When the device is in theforward active mode, the conduction carriers are emittedfrom the InGaP/GaAsSb EB heterojunction. In contrast,when the device is in the reverse active mode, the conductioncarriers are emitted from the GaAsSb/GaAs BC heterojunc-tion. Despite the operational differences in the forward activeand reverse active modes of the GaAsSb DHBT, Fig. 2(a)demonstrates that the two terminal currents overlap eachother in a large proportion of current range. This findingFIG. 1. Direct Current current gain b and incremental current gain hfe as afunction of collector current IC.shows that carriers emitted from either the InGaP/GaAsSbDownloaded 08 Nov 2006 to 147.8.21.97. Redistribution subject to AEB junction or GaAsSb/GaAs BC junction in the GaAsSbDHBTs are limited by the transport across the base layer.This finding is further corroborated from the ideality factorsof the terminal currents that are very close to unity. To fur-ther verify our conclusion, we also measured the terminalcurrents in a small area device of 2330 mm2, as shown inFig. 2(b). Figure 2(b) demonstrates again that the two termi-nal currents are nearly identical in almost the entire currentrange. It should be noticed that, the overlapping of the ter-minal currents in Fig. 2(a) does not take place in entire cur-rent range. This is attributed to large base and collector re-sistances, which take a large proportion of VBC, causing theavailable junction voltage, VBCj, to decrease. Since the car-rier transport is not limited by the conduction-band barrier, itis impossible to determine the exact magnitude of the bandoffset in the emitter-base and the base-collector heterojunc-tions from this investigation. However, it can still be inferredthat the conduction-band offsets at both InGaP/GaAsSb EBjunction and GaAsS/aAs BC heterojunction are small enoughsuch that the terminal currents have nearly identical charac-teristics.To find whether there is the collector current blocking inthe InGaP/GaAsSb/GaAs DHBTs, we measured theircommon-base I–V characteristics. Figure 3 shows thecommon-base I–V characteristics of a large area transistor2FIG. 2. Measured terminal currents of the InGaP/GaAsSb DHBTs in bothforward active mode and the reverse active mode for (a) a large area devices1003100 mm2d and (b) a small area device s2330 mm2d.with an emitter area of 1003100 mm . At small currentIP license or copyright, see http://apl.aip.org/apl/copyright.jsp3886 Appl. Phys. Lett., Vol. 85, No. 17, 25 October 2004 Yan et al.level, as shown in Fig. 3(a), the transistor is free of collectorcurrent blocking because the turn on near VCB=−1 V isabrupt and the collector current is very nearly independent ofVCB in the turn-on region. At large current level, however,the situation is different. As shown in Fig. 3(b), the turn onnear VCB=−1 V is no longer abrupt and the collector currentis dependent of VCB in the turn-on region. We cannot simplyregard this situation as the current blocking, because a highbase resistance also causes this phenomenon. Using thetransmission line method, the base sheet resistance and thebase contact resistance were measured as 8300 V / sq and3.4310−3 V cm2, much higher than that of traditional GaAsHBTs. At low current level, the voltage across the base re-FIG. 3. Common-base I–V characteristics of InGaP/GaAsSb/GaAs DHBTwith an emitter size of 1003100 mm2; (a) at small current and (b) at largecurrent.sistance can be negligible and the applied voltage totallyDownloaded 08 Nov 2006 to 147.8.21.97. Redistribution subject to Adrops across the BC junction. At large current level, how-ever, due to high base resistance, the large proportion of theapplied voltage would drop across the parasitic base resis-tance, causing the available junction voltage to decrease.Further studies by other measurement methods are needed toclarify this point. The work is under way.In summary, we studied the current transport mechanismof the InGaP/GaAsSb/GaAs DHBTs by measuring both ofthe terminal currents in forward and reverse mode. The re-sults show that the dominant current transport mechanism inthe InGaP/GaAsSb/GaAs DHBTs is the transport of carriersacross the base layer. This finding suggests that theconduction-band offset in the InGaP/GaAsSb heterojunctionis very small, and the energy band gap offset produced byincorporating Sb composition into GaAs mainly appears onthe valence band, i.e., there is a larger valence band offset inthe InGaP/GaAsSb heterojunction compared to traditionalInGaP/GaAs heterojunction, which effectively blocks holeback-injection current and improves the current gain.The authors would like to thank the support of the Re-search Grants Council of Hong Kong Special AdministrativeRegion, China (Project No. HKU7069/02E).1W. Liu, S. K. Fan, T. Henderson, and D. Davito, IEEE Trans. ElectronDevices 40, 1351 (1993).2W. Liu, E. Beam III, T. Kim, and A. Khatibzadeh, Current Trends inHeterojunction Bipolar Transistors, edited by M. F. Chang (World Scien-tific, Singapore, 1996), pp. 241–301.3R. J. Welty, H. P. Xin, K. Mochizuki, C. W. Tu, and P. M. Asbeck, J. Elast.46, 1 (2002).4P. M. DeLuca, C. R. Lutz, R. E. Welser, T. Y. Chi, E. K. Huang, R. J.Welty, and P. M. Asbeck, IEEE Electron Device Lett. 23, 582 (2002).5P. C. Chang, C. Monier, A. G. Baca, N. Y. Li, F. Newman, E. Armour, andH. Q. Hou, J. Elast. 46, 581 (2002).6R. E. Welser, P. M. DeLuca, and N. Pan, IEEE Electron Device Lett. 21,554 (2000).7C. R. Bolognesi, N. Matine, M. W. Dvorak, P. Yeo, X. G. Xu, and S. P.Watkins, IEEE Trans. Electron Devices 48, 2631 (2001).8M. W. Dvorak, C. R. Bolognesi, O. J. Pitts, and S. P. Watkins, IEEEElectron Device Lett. 22, 361 (2001).9R. Teissier, D. Sicault, J. C. Harmand, G. Ungaro, G. Le Roux, and L.Largeau, J. Appl. Phys. 89, 5473 (2001).10B. Khamsehpour and K. E. Singer, Electron. Lett. 26, 965 (1990).11K. Ikossi-Anastasiou, IEEE Trans. Electron Devices 40, 878 (1993).12T. Oka, T. Mishima, and M. Kudo, Appl. Phys. Lett. 78, 483 (2001).13B. P. Yan, C. C. Hsu, X. Q. Wang, and E. S. Yang, IEEE Electron DeviceLett. 23, 170 (2002).14B. P. Yan, C. C. Hsu, X. Q. Wang, and E. S. Yang, in the 14th IndiumPhosphide and Related Materials Conference, 2002, pp. 169–172.15P. Asbeck, in High-Speed Semiconductor Devices, edited by S. M. Sze(Wiley, New York, 1990), pp. 364–365.16H. Kroemer, Solid-State Electron. 28, 1101 (1985).17W. Liu, S. K. Fan, T. S. Kim, E. A. Beam III, and D. B. Davito, IEEETrans. Electron Devices 40, 1378 (1993).IP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Current transport mechanism in InGaP/GaAsSb/GaAs double-heterojunction bipolar transistors

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Current transport mechanism in InGaP/GaAsSb/GaAs double-heterojunction bipolar transistors

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