The temperature dependence of current gain was investigated for AlGaAs/GaAs heterostructure-emitter bipolar transistors (HEBT). The current gain of the HEBT was found much less sensitive to temperature variation than that of a heterojunction bipolar transistor. In particular, the HEBT current gain was more or less constant with increasing temperature at the high current regime, indicating great potentials for power applications.published_or_final_versio

Lo, HB

Yang, ES

Yang, YF

Applied Physics Letters

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Title Thermal effect on current gains of an AlGaAs/GaAsheterostructure-emitter bipolar transistorAuthor(s) Lo, HB; Yang, ES; Yang, YFCitation Applied Physics Letters, 1999, v. 74 n. 12, p. 1725-1727Issued Date 1999URL http://hdl.handle.net/10722/42106Rights Creative Commons: Attribution 3.0 Hong Kong LicenseAPPLIED PHYSICS LETTERS VOLUME 74, NUMBER 12 22 MARCH 1999Thermal effect on current gains of an AlGaAs/GaAsheterostructure-emitter bipolar transistorH. B. Lo and E. S. YangDepartment of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, ChinaY. F. Yanga)Global Communication Semiconductors, Incorporated, Torrance, California 90505~Received 23 November 1998; accepted for publication 29 January 1999!The temperature effect on current gains is presented for an AlGaAs/GaAs heterostructure-emitterbipolar transistor ~HEBT!. Experimental results show that the HEBT has much less temperaturesensitivity in current gain than a heterojunction bipolar transistor. The current gains for the HEBTare almost constant with the substrate temperature at a high current regime. This indicates that theHEBT could be a good candidate for power applications. © 1999 American Institute of Physics.@S0003-6951~99!04312-0#Recent development indicates that GaAs heterojunctionbipolar transistors ~HBTs! will play an increasing role inpresent and future microwave and millimeter-wave powerapplications.1 Although AlGaAs/GaAs power HBTs havebeen widely used,2 the issue of the power AlGaAs/GaAsHBT is that the current gain decreases with the increase oftemperature which results in the negative differential resis-tance. The main reason of gain degradation for AlGaAs/GaAs HBTs is the low valence band offset (DEv) at theheterojunction,3 assuming that the hole current injected fromthe base to the emitter is diffusion limited. If, however, themechanism of the hole current is dominated by thermionicemission rather than diffusion, the temperature dependenceof current gain will be modified. The heterostructure-emitterbipolar transistor ~HEBT!, with separate electron injectionand hole confinement,4 has been proposed by taking advan-tage of both special features of heterojunction and homo-junction. It can effectively eliminate the emitter potentialspike and offer low offset voltage.5 The heterostructure emit-ter can also slow down the degradation caused by base dop-ant outdiffusion which improves the device’s reliability.6Moreover, HEBT structure is like a poly-Si BJT in which thehole current injected from the base to the emitter is likely tobe dominated by thermionic emission at high temperature.7Since the thermionic emission current has a temperature de-pendence of ;T2 plus an exponential factor, the current gaincan become less dependent on the junction temperature. Itwas recently demonstrated that the current gain of an InGaP/GaAs HEBT increase along with the increase of temperaturewere observed due to thermionic emission currentdominant.8 In this letter, we present experimental results ofthe temperature dependence of the current gain for AlGaAs/GaAs HEBTs. It is shown that the current gain of the HEBTis much less sensitive to temperature variation than that of anormal AlGaAs/GaAs HBT. The current gain is found to bealmost constant with increasing temperature for the HEBT atthe high current regime.a!Electronic mail: yfyang@gscincorp.com1720003-6951/99/74(12)/1725/3/$15.00Downloaded 13 Nov 2006 to 147.8.21.97. Redistribution subject to AlGaAs/GaAs HEBTs were grown by molecular beamepitaxy on a Sl GaAs substrate. The HEBT structure consistsof a 5000 Å GaAs subcollector layer (n5331018 cm23), a5000 Å GaAs collector layer (n5131016 cm23), a 800 ÅGaAs base layer (p5131019 cm23), a 300 Å GaAs emitterlayer (n5131017 cm23), a 500 Å AlxGa12xAs confine-ment layer (n5331017 cm23, x50.3), a 500 ÅAlxGa12xAs graded layer (n5331017 cm23, x50.3– 0), a1000 Å GaAs emitter cap layer (n5431018 cm23), and a600 Å graded InxGa12xAs ~x from 0 to 0.5! contact layer(n5531018 cm23).Si and Be were used as n- and p-typedopants, respectively.HEBTs were fabricated using the conventional mesastructure. A thin depleted AlGaAs ledge was employed as apassivation layer on the extrinsic base surface to reduce thesurface recombination current. Figure 1 shows the cross sec-tion of a typical HEBT. A large emitter size of 403100 mm2was used to further reduce the periphery effect on the currentgain. After the HEBTs were fabricated, the devices werepackaged in a chip carrier. Gummel plots at a base-collectorbias of 0 V were measured at the temperature range from 25to 175 °C. Since the base collector was biased at 0 V, thedevice junction temperature was assumed to be the same asthe substrate temperature, Tsub .A typical Gummel plot of the HEBT at room tempera-FIG. 1. Cross section of the AlGaAs/GaAs HEBT.5 © 1999 American Institute of PhysicsAIP license or copyright, see http://apl.aip.org/apl/copyright.jsp1726 Appl. Phys. Lett., Vol. 74, No. 12, 22 March 1999 Lo, Yang, and Yangture is shown in Fig. 2. A current gain ~b! of 52 at roomtemperature was obtained. The ideality factors of collectorand base currents are 1.16 and 1.80, respectively. Figure 4shows the current gain versus collector current at varioussubstrate temperatures for the AlGaAs/GaAs HEBT. Thecurrent gain is almost constant at Tsub,100 °C and becomesslightly lower at Tsub.100 °C. It is noted that b remains thesame at the high current regime. Figure 3 shows the currentgain, normalized by its value at room temperature, as a func-tion of the substrate temperatures at the collector currentdensities (Jc) of 250 A/cm2. The experimental data fromRef. 9 are also shown in Fig. 4 for comparison. The currentgain of the HEBT at Tsub of 175 °C is reduced by only 10%for Jc of 250 A/cm2. In comparison, b for the HBT decreasesby 65% at a higher Jc and its drop is even more pronouncedat low Jc .3,9 It is noted that the current gain of the HEBTshown in Fig. 4 is at a Jc about a hundred times lower thanthat of the HBT.9 If the gain was obtained at a higher Jc , thedrop in gain for the HEBT would have been even less sig-nificant. It is evident that less temperature sensitivity in theFIG. 3. Current gain vs collector current for the HEBT at various substratetemperatures.FIG. 2. Gummel plot of the HEBT at room temperature.Downloaded 13 Nov 2006 to 147.8.21.97. Redistribution subject to current gain was obtained for the HEBT than that of theHBT.The basic reason for the current gain degradation withtemperature in an AlGaAs/GaAs HBT is the small value ofDEv , based on the assumption of the diffusion dominatedhole current.9 Since the heterostructure emitter is similar tothe poly-Si emitter, the hole injection from the base to theemitter is controlled by thermionic emission at hightemperature.7 We note that the thermionic emission currenthas a temperature dependence of ;T2 plus an exponentialfactor, while the diffusion current has a temperature depen-dence of ;T3 plus an exponential factor. Hence, the currentgain will be less dependent on the junction temperature whenthe hole current is dominated by thermionic emission. Thedetailed analysis will be given in a future publication. Theabove results showed that even though the value of DEv forAlGaAs/GaAs material system is lower than that of theGaInP/GaAs system, the degree of temperature dependenceof b for AlGaAs/GaAs HEBT is comparable to that ofGaInP/GaAs HBT.In summary, we have reported a significant difference inthe temperature dependence of the current gain betweenHEBTs and HBTs. It is shown that the current gain for theHEBT is much less temperature sensitive and almost con-stant with increasing junction temperature at a high currentlevel. This suggests that the performance of an HEBT is notobviously degraded at high temperature. If HEBTs are usedfor power amplifier, the power performance could be im-proved because the power device often operates at high tem-perature.This work was supported by CRCG grant to two of theauthors ~H.B.L. and E.S.Y.! and RGC Grant No. HKU7057/98E.1 B. Bayraktaroglu, Proc. IEEE 81, 1762 ~1993!.2 G. B. Gao, H. Morkoc, and M. F. Chang, IEEE Trans. Electron Devices39, 1987 ~1992!.3 W. Liu, S. K. Fan, T. Henderson, and D. Davito, IEEE Trans. ElectronDevices 40, 1351 ~1993!.4 L. F. Luo, H. L. Evans, and E. S. Yang, IEEE Trans. Electron Devices 36,1844 ~1989!.FIG. 4. Normalized current gain vs substrate temperature for the HEBT ofthis work at Jc5250 A/cm2 and HBT Jc52.53104 A/cm2 from Ref. 9.The current gain is normalized by its value at room temperature.AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp1727Appl. Phys. Lett., Vol. 74, No. 12, 22 March 1999 Lo, Yang, and Yang5 W. S. Lour, W. C. Liu, D. F. Guo, and R. C. Liu, Jpn. J. Appl. Phys., Part1 31, 2388 ~1992!.6 Y. F. Yang, C. C. Hsu, and E. S. Yang, Semicond. Sci. Technol. 10, 339~1995!.7 C. C. Ng and E. S. Yang, Tech. Dig. Int. Electron Devices Meet. 32~1986!.Downloaded 13 Nov 2006 to 147.8.21.97. Redistribution subject to 8 E. S. Yang, Y. F. Yang, C. C. Hsu, H. J. Ou, and H. B. Lo, IEEE Trans.Electron Devices 46, 320 ~1999!.9 W. 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.AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Thermal effect on current gains of an AlGaAs/GaAs heterostructure-emitter bipolar transistor

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Thermal effect on current gains of an AlGaAs/GaAs heterostructure-emitter bipolar transistor

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