The thermal stability of current gain in InGaP/GaAsSb/GaAs double-heterojunction bipolar transistors (DHBTs) is investigated. The experimental results show that the current gain in the InGaP/GaAsSb/GaAs DHBTs is nearly independent of the substrate temperature at collector current densities > 10 A/cm2, indicating that the InGaP/GaAsSb/GaAs DHBTs have excellent thermal stability. This finding suggests that the InGaP/GaAsSb/GaAs DHBTs have larger emitter-base junction valence-band discontinuity than traditional GaAs-based HBTs. © 2004 American Institute of Physics.published_or_final_versio

Hsu, CC

Wang, XQ

Yan, BP

Yang, ES

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Title Thermal stability of current gain in InGaP/GaAsSb/GaAs double-heterojunction bipolar transistorsAuthor(s) Yan, BP; Hsu, CC; Wang, XQ; Yang, ESCitation Applied Physics Letters, 2004, v. 85 n. 19, p. 4505-4507Issued Date 2004URL http://hdl.handle.net/10722/42101Rights Creative Commons: Attribution 3.0 Hong Kong LicenseAPPLIED PHYSICS LETTERS VOLUME 85, NUMBER 19 8 NOVEMBER 2004Thermal stability of current gain in InGaP/GaAsSb/GaAsdouble-heterojunction bipolar transistorsB. P. Yan,a) C. C. Hsu, X. Q. Wang, and E. S. YangDepartment of Electrical and Electronic Engineering, The University of Hong Kong, Pok Fu Lam Road,Hong Kong SAR, People’s Republic of China(Received 6 July 2004; accepted 28 September 2004)The thermal stability of current gain in InGaP/GaAsSb/GaAs double-heterojunction bipolartransistors (DHBTs) is investigated. The experimental results show that the current gain in theInGaP/GaAsSb/GaAs DHBTs is nearly independent of the substrate temperature at collectorcurrent densities .10 A/cm2, indicating that the InGaP/GaAsSb/GaAs DHBTs have excellentthermal stability. This finding suggests that the InGaP/GaAsSb/GaAs DHBTs have largeremitter-base junction valence-band discontinuity than traditional GaAs-based HBTs. © 2004American Institute of Physics. [DOI: 10.1063/1.1819505]GaAs-based heterojunction bipolar transistors (HBTs)have been widely used for power amplifiers in wireless andhigh-speed digital applications. Compared with rival siliconbipolar and GaAs field-effect transistor devices, HBTs haveseveral advantages, including high linearity, high transcon-ductance, high power added efficiency, and low 1/ f noise.These advantages have made GaAs HBT the preferred tech-nology for many military and commercial applications.However, GaAs HBTs have a relatively large base-emitterturn-on voltage due to the large band gap of GaAs used asthe base layer, and this limits the minimum operating voltageand increases the power consumption in circuit applications.In addition, AlGaAs/GaAs HBTs suffer from current gaindegradation at elevated temperature, which leads to a nega-tive differential resistance and current collapse in I–Vcurves.1,2 Although InGaP/GaAs HBTs have less tempera-ture dependence on current gain than AlGaAs/GaAs HBTs,3there is still observable gain degradation and negative differ-ential resistance, especially for the semi-orderedInGaP/GaAs HBTs.4 The main reason for current gain deg-radation in GaAs-based HBTs is believed to be the lowvalence-band offset sDEVd at the emitter-base (EB)heterojunction.3 In order to realize low-voltage, low-powerand high reliable operation, it is essential to develop newGaAs-based HBTs with low turn-on voltage and better ther-mal stability. In terms of this requirement, future GaAs HBTsshould use low energy band gap materials as the base layerto reduce the turn-on voltage. At the same time, the emitter-base junction should keep a large valence-band offset DEV,which would prevent the degradation of the current gain atelevated temperature.Two different narrow band gap materials have been de-veloped. The first is InGaAsN, which has been used as thebase layer by several groups.5–8 However, InGaAsN materialtypically displays degraded minority carrier properties com-pared with GaAs,5,6 leading to the reduction of dc currentgain and high frequency performance. Although these unfa-vorable characteristics can be suppressed by the insertion ofgraded layers between the base and collector junction,8 thiscomplicates the transistor design and fabrication.a)Electronic mail: bpyan@eee.hku.hk0003-6951/2004/85(19)/4505/3/$22.00 4505Downloaded 08 Nov 2006 to 147.8.21.97. Redistribution subject to AAnother attractive narrow band gap base material isGaAsSb, which has been used in InP/GaAsSb/ InP HBTs byBolognesi et al.9,10 The lattice-matched InP/GaAsSb/ InPHBT has demonstrated excellent high frequency and high-speed performance. However, GaAsSb as the base layer inthe GaAs technology has had limited success because thematerials are not lattice matched. Early work showed a poorcurrent gain due to the use of a thick GaAsSb layer wellabove the critical layer thickness.11,12 Although the currentgain could be improved by using a pseudomorphic GaAsSblayer as the base and a GaAs layer as the emitter,13 the de-vices showed a large recombination current at the GaAsemitter depletion region. Recently, we have developed aInGaP/GaAsSb/GaAs double-heterojunction bipolar transis-tor (DHBT) with low turn-on voltage and high currentgain.14,15 By using a narrow-energy band gap GaAsSb layeras the base, the turn-on voltage of the device is reduced. Thelow turn-on voltage makes possible better power-added effi-ciency and lower operating voltage, which in turn allows theuse of smaller and lighter batteries. This is of critical impor-tance for any battery-operated device such as a cell phone.On the other hand, by using an InGaP layer as the emitter,the desirable band lineup at the emitter-base heterojunction isobtained, for the band gap offset at the InGaP/GaAsSb in-terface mainly appears on the valence band.14 This suggeststhat the blocking effect of DEV on the base-to-emitter back-injection hole current would be more significant and, there-fore, the InGaP/GaAsSb/GaAs DHBT is expected to havehigher current gain and better thermal stability than tradi-tional GaAs HBTs. In this letter, we study the temperaturedependence of current gain of the InGaP/GaAsSb/GaAsDHBTs.The InGaP/GaAsSb/GaAs DHBT structure was grownon a semi-insulating (100) GaAs substrate by metalorganicchemical vapor deposition. Trimethylgallium, trimethylin-dium, trimethylantimony, tertiarybutylphosphine, and tertia-rybutylarsine were used as the organometallic sources. Car-bon and silicon were used as p- and n-type dopants,respectively. The device nominal structure consists of a500 nm GaAs subcollector sn.331018 cm−3d, a 500 nmGaAs collector sn=531016 cm−3d, a 50 nm GaAsSb base(Sb composition =10.4%, p=831018 cm−3), a 50 nm InGaP17 −3emitter sn=3310 cm d, a 150 nm GaAs layer sn=4© 2004 American Institute of PhysicsIP license or copyright, see http://apl.aip.org/apl/copyright.jsp4506 Appl. Phys. Lett., Vol. 85, No. 19, 8 November 2004 Yan et al.31018 cm−3d, a 50 nm compositionally gradedInxGaAs1−xcap layer sx=0–0.5,n.131019 cm−3d and a50 nm In0.5GaAs0.5 cap ohmic contact layer sn.131019 cm−3d. The Sb composition was confirmed by high-resolution x-ray diffraction measurement. The surface mor-phology was observed by atomic force microscope and nocross-hatched patterns associated with misfit dislocationswere observed. This suggests that the GaAsSb base layer isfully strained. The structure was fabricated into devices us-ing optical lithography and chemical wet selective etchingfor mesa definition.Figure 1 shows the Gummel plots of a large area deviceof 1003100 mm2. The device demonstrates a current gaingreater than unity at ultra-small collector current density of1310−6 A/cm2. The device also displays a low turn-on volt-age, 0.914 V, at JC=1 A/cm2, which is 0.18 V lower thanthat of conventional InGaP/GaAs HBTs.15 Measured Gum-mel plots are near-ideal. The ideality factor of the collectorcurrent is 1.03. The base current has different ideality factorsdepending on the current level. The ideality factor of basecurrent is near 2.0 at the low current level. At high currentlevel, the ideality factor is 1.30. In comparison withGaAs/GaAsSb/GaAs DHBT,13 the recombination current ofInGaP/GaAsSb/GaAs DHBT is greatly reduced due to theuse of InGaP as emitter layer instead of GaAs.Figure 2 illustrates the measured current gain as a func-FIG. 1. (Color online) Representative Gummel plots forInGaP/GaAsSb/GaAs DHBT with an emitter area of 1003100 mm2.FIG. 2. (Color online) Measured current gain vs collector current ofInGaP/GaAsSb/GaAs DHBT at different substrate temperatures.Downloaded 08 Nov 2006 to 147.8.21.97. Redistribution subject to Ation of collector current for a InGaP/GaAsSb/GaAs DHBTwith emitter size of 1003100 mm2, as the substrate tem-perature was varied from 20 to 180 °C. It can be seen that,at current densities JC.10 A/cm2, the measured currentgains nearly overlap for different substrate temperature, in-dicating that the current gain of the DHBTs is basically in-dependent of temperature. For clarity, Fig. 3 shows the cur-rent gain, normalized by its value at room temperature, as afunction of the substrate temperature at collector current of 1and 10 mA, respectively. The current gain at 180 °C variesonly 3% for JC of 10 A/cm2, and 6% for a JC of 100 A/cm2.This result is significantly different from that of the conven-tional AlGaAs/GaAs3 and semi-ordered InGaP/GaAsHBTs4 in which the current gain degrades with the increaseof temperature. The excellent thermal stability of currentgain in the InGaP/GaAsSb/GaAs DHBT is attributed to theband alignment particular to the InGaP/GaAsSb heterojunc-tion. In general, the decrease of the current gain withtemperature depends on the valence band discontinuitysDEVd of the base-emitter heterojunction. For traditionalAlGaAs/GaAs and InGaP/GaAs HBTs, the valence banddiscontinuity at the EB heterojunction was estimated to be0.1916 and 0.43 eV,17 respectively. When Sb composition isincorporated into GaAs base, the energy band gap of the basematerial is reduced.13,14 The band gap reduction depends onincorporated Sb composition. For 10.4% Sb composition,calculated band gap reduction is about 0.186 eV. Thus thetotal valence discontinuity at the InGaP/GaAsSb heterojunc-tion should be the sum of 0.43 and 0.186 eV, i.e., 0.616 eV.That means that the valence discontinuity in theInGaP/GaAsSb heterojunction is much larger than that intraditional AlGaAs/GaAs and InGaP/GaAs heterojunction.The larger the valence band discontinuity, the moreremarkable the blocking effect on the base-to-emitter back-injection holes. Therefore, the current gain of theInGaP/GaAsSb/GaAs DHBT exhibits better thermal stabil-ity than traditional GaAs-based HBTs. On the other hand, thebase sheet resistance also influences the thermal stability ofthe current gain. The base sheet resistance was measured as8300 V / sq using transmission-line patterns. Although it islarge relative to that of traditional GaAs base, the influenceof it on thermal stability of the current gain is not significantcompared with the contribution from band alignment, be-cause the current gains measured at different substrate tem-FIG. 3. (Color online) Normalized current gain vs substrate temperature forthe InGaP/GaAsSb/GaAs DHBTs at collector current of 1 and 10 mA.peratures keep constant even at the small current of 0.1 mA.IP license or copyright, see http://apl.aip.org/apl/copyright.jspAppl. Phys. Lett., Vol. 85, No. 19, 8 November 2004 Yan et al. 4507The independence of the current gain on the temperatureis favorable to power devices. It is well known that there aretwo thermal phenomena in power HBTs, i.e., thermal run-away and the collector current collapse, which are caused bycurrent crowding effect. When the current crowding effectexists in a device, the temperature dependence can providean electrical-thermal positive feedback loop that gives rise tothermal runaway for a positive current gain coefficient andcurrent collapse for a negative current gain coefficient. If,however, the current gain is independent of temperature,there will not be a feedback mechanism to produce a cata-strophic effect. Therefore, from the point of view of thermalmanagement, the InGaP/GaAsSb/GaAs DHBT should havegreater potential for power application.In summary, we studied the thermal stability of currentgain in the InGaP/GaAsSb/GaAs DHBTs. The exper-imental results show that the current gain in theInGaP/GaAsSb/GaAs DHBTs is nearly independent of thesubstrate temperature at collector current densities.10 A/cm2, which indicates that the InGaP/GaAsSb/GaAsDHBTs have excellent thermal stability. This finding sug-gests that the InGaP/GaAsSb/GaAs DHBTs have largeremitter-base junction valence-band discontinuity than tradi-tional GaAs-based HBTs.The authors would like to thank the support of ResearchGrants Council of Hong Kong Special Administrative Re-gion, China (Project No. HKU7069/02E).Downloaded 08 Nov 2006 to 147.8.21.97. Redistribution subject to A1G. B. Gao, H. Morkoc, and M. F. Chang, IEEE Trans. Electron Devices39, 1987 (1992).2W. Liu, S. Nelson, D. Hill, and A. Khatibzadeh, IEEE Trans. ElectronDevices 40, 1917 (1993).3W. Liu, S. K. Fan, T. Henderson, and D. Davito, IEEE Trans. ElectronDevices 40, 1351 (1993).4W. Liu, E. Beam, III, T. Kim, and A. Khatibzadeh, in Current Trends inHeterojunction Bipolar Transistors, edited by M. F. Chang (World Scien-tific, Singapore, 1996), pp. 241–301.5R. J. Welty, H. P. Xin, K. Mochizuki, C. W. Tu, and P. M. Asbeck, Solid-State Electron. 46, 1 (2002).6P. C. Chang, C. Monier, A. G. Baca, N. Y. Li, F. Newman, E. Armour, andH. Q. 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Kobayashi, K. Taira, F. Nakamura, and H. Kawai, J. Appl. Phys. 65,4898 (1989).IP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Thermal stability of current gain in InGaP/GaAsSb/GaAs double-heterojunction bipolar transistors

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Thermal stability of current gain in InGaP/GaAsSb/GaAs double-heterojunction bipolar transistors

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