The temperature effect on current gain is presented for GalnP/GaAs heterojunction and heterostructure-emitter bipolar transistors (HBT's and HEBT's). Experimental results showed that the current gain of the HEBT increases with the increase of temperature in the temperature range of 25-125 °C and decreases slightly at temperatures above 150 °C. The smaller the collector current, the larger is the positive differential temperature coefficient. At high current levels, the current gain dependence on temperature is significantly reduced. On the other hand, a large negative coefficient is observed in the HBT in all current range. This finding indicates that the HEBT is a better candidate than the HBT for power devices. © 1999 IEEE Publisher Item Identifier S 0018-9383(99)00257-9.published_or_final_versio

Hsu, CC

Lo, HB

Ou, HJ

Yang, ES

Yang, YF

IEEE Transactions on Electron Devices

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Title Temperature dependence of current gain of GalnP/GaAs heterojunction and heterostructure-emitter bipolar transistorsAuthor(s) Yang, ES; Yang, YF; Hsu, CC; Ou, HJ; Lo, HBCitation Ieee Transactions On Electron Devices, 1999, v. 46 n. 2, p. 320-323Issued Date 1999URL http://hdl.handle.net/10722/42794Rights©1999 IEEE. Personal use of this material is permitted. However,permission to reprint/republish this material for advertising orpromotional purposes or for creating new collective works forresale or redistribution to servers or lists, or to reuse anycopyrighted component of this work in other works must beobtained from the IEEE.320 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 46, NO. 2, FEBRUARY 1999Temperature Dependence of Current Gainof GaInP/GaAs Heterojunction andHeterostructure-Emitter Bipolar TransistorsEdward S. Yang, Fellow, IEEE, Yue-Fei Yang, Member, IEEE, Chung-Chi Hsu, Hai-Jiang Ou, and H. B. LoAbstract—The temperature effect on current gain is presentedfor GaInP/GaAs heterojunction and heterostructure-emitterbipolar transistors (HBT’s and HEBT’s). Experimental resultsshowed that the current gain of the HEBT increases withthe increase of temperature in the temperature range of25–125 C and decreases slightly at temperatures above 150 C.The smaller the collector current, the larger is the positivedifferential temperature coefficient. At high current levels, thecurrent gain dependence on temperature is significantly reduced.On the other hand, a large negative coefficient is observed in theHBT in all current range. This finding indicates that the HEBTis a better candidate than the HBT for power devices.I. INTRODUCTIONIN A BIPOLAR transistor, the current gain has a definitetemperature coefficient. When current crowding exists in adevice, the temperature dependence can provide an electrical-thermal positive feedback loop giving rise to thermal runawayfor a positive coefficient and current collapse for a negativecoefficient. To avoid the catastrophic effect one could attemptto eliminate current crowding or making the current insensitiveto temperature variation. Current crowding may come fromthe nonuniformity of material, device structure or the lateralfield established by the base current. For the latter, a heavybase doping such as in a typical heterojunction bipolar tran-sistor (HBT) produces a small and negligible lateral potentialdrop in the active base. Although the difference betweenfingers is unavoidable, the temperature coefficient, however,may be controllable by manipulation of the heterojunction.This paper reports some experimental results using a GaInPheterostructure-emitter to make the gain less sensitive totemperature variation.In a power GaAs HBT the current gain decreases withthe increase of junction temperature which results in a neg-ative differential resistance and current collapse in thecurves [1], [2]. Although GaInP/GaAs power HBT’s haveless temperature dependence on current gain than AlGaAsManuscript received May 13, 1998; revised September 7, 1998. This workwas supported by CRCG Grant A/C 337/062/0047, large equipment Grant011/062, and RGC Grant HKU 7057/98E from The University of Hong Kong.The review of this paper was arranged by Editor M. F. Chang.E. S. Yang, H.-J. Ou, and H. B. Lo are with the Department of Electricaland Electronic Engineering, The University of Hong Kong, Hong Kong.Y.-F. Yang is with the Global Communication Semiconductors, Inc.,Torrance, CA 90505 USA.C.-C. Hsu is with the Department of Electronic Engineering, The ChineseUniversity of Hong Kong, Hong Kong.Publisher Item Identifier S 0018-9383(99)00257-9.HBT’s [3], there is still observable gain degradation andnegative differential resistance, especially for the semi-orderedGaInP/GaAs HBT’s [4]. The main reason of gain degradationin HBT’s is believed to be the low valence band offsetat the heterojunction [3]. It is based on the assumption thatdiffusion dominates the hole current injected from the base tothe emitter. If, however, the mechanism of the hole currentis controlled by thermionic emission rather than diffusion,the temperature dependence of current gain will be modifiedsignificantly.The heterostructure-emitter bipolar transistor (HEBT) withthe separation of electron injection and hole confinement [5]has been proposed by taking advantage of both the hetero-junction and homojunction. It can effectively eliminate theemitter potential spike and offers a low offset voltage [6].The heterostructure-emitter can also accommodate the basedopant outdiffusion which increases the device life time [7].We have also found that the HEBT provides better current gainuniformity [8]. Moreover, the hole current injected from baseto emitter is likely to be dominated by thermionic emissioncurrent at high temperature similar to the poly-Si emitter[9]. Since the thermionic emission current has a temperaturedependence of instead of in a diffusion current(excluding the exponential term contained in both) [10], thecurrent gain should be less dependent on the junction temper-ature. In this paper, we present the experimental results of thetemperature dependence on the current gain of GaInP/GaAsHBT’s and HEBT’s.II. EXPERIMENTGaInP/GaAs HEBT’s were grown by MOCVD on a SIGaAs substrate. The HEBT structure consists of a 5000 A˚GaAs subcollector layer cm a 5000 A˚GaAs collector layer cm a 1000 A˚ GaAsbase layer cm a 150 A˚ GaAs emitter layercm a 500 A˚ GaInP confinement layercm a 1500 A˚ GaAs emitter cap layercm and a 600 A˚ graded In Ga Asfrom 0 to 0.5) contact layer cmA GaInP/GaAs HBT without emitter setback layer was alsofabricated for comparison. Si and C were used as n- and p-typedopants, respectively.HEBT’s were fabricated using the conventional mesa struc-ture described in [8]. A thin depleted GaInP ledge was0018–9383/99$10.00  1999 IEEEYANG et al.: TEMPERATURE DEPENDENCE OF CURRENT GAIN 321Fig. 1. Current gain versus collector current for the GaInP/GaAs HEBT withemitter size of 3  16 m2 at various substrate temperatures.employed as a passivation layer for the extrinsic base surfaceto reduce the surface recombination current. A large emit-ter size of 40 100 m was used to further cut downthe periphery effect on the current gain. After the HEBT’swere fabricated, the devices were packaged in a chip carrier.Gummel plots at a base-collector bias of 0 V were measuredat the temperature range from 25 to 175 C by putting thesample in a temperature-controlled chamber. Because the base-collector was biased at 0 V, the device junction temperaturewas assumed to be the same as the substrate temperature,III. RESULTS AND DISCUSSIONThe fabricated HEBT has a common emitter current gainof 35 at room temperature. RF devices with a small emitterhave a cutoff frequency of 45 GHz and maximum oscillationfrequency of 80 GHz.Fig. 1 shows the current gain versus the collector currentfor the GaInP/GaAs HEBT at various substrate temperatures.The emitter size is 3 16 m . As shown, the current gainchanges only slightly with temperature for the wide range ofcollector current. The increase of current gain with temperatureis also observed. For clarity, Fig. 2 shows the current gain,normalized by its value at room temperature, as a function ofthe substrate temperature for the HEBT at collector current of1 10 , 1 10 , and 1 10 A. It is noted that thecurrent gain increases with the increase of temperature atbelow 100 C and starts to decrease at 100–150 Cdepending on the collector current. The smaller the collectorcurrent, the more the current gain increases with temperature.In comparison, Fig. 3 shows the current gain versus col-lector current of a GaInP/GaAs HBT for the temperaturerange of 25–200 C. The HBT has a similar structure andwas grown under the same conditions as that of the HEBTexcept without an emitter setback layer. Some of the featuresin Fig. 3 are replotted in Fig. 4 for of 10 , 10 , and10 A, respectively. It is noted that the current gain of theFig. 2. Current gain versus substrate temperature for the GaInP/GaAs HEBTat collector current of 1  10 5, 1  10 3, and 1  10 2 A.Fig. 3. Current gain versus collector current for the GaInP/GaAs HBT withemitter size of 3  16 m2 at various substrate temperatures.HBT’s has a negative temperature coefficient as defined bythe slope of the curves. This coefficient is current dependentbut is negative in all cases. On the other hand, the currentgain of the HEBT shown in Fig. 2 shows a positive slope atlow temperatures and it levels off before turning to a negativeslope. As a comparison, the gain variation is within 10% atA for the HEBT in Fig. 2 and a factor of two (i.e.,100%) for the HBT in Fig. 3. The contrast is even greater fora lower collector current.For the HBT, the temperature coefficient is not far frombeing a constant and a thermal-electrical feedback factor canbe defined [11]. However, the same coefficient in an HEBTis highly nonlinear as it changes from positive to negative.This nonlinearity makes it difficult to define a feedback factor.Nevertheless, there is no question as to the less sensitivetemperature dependence in a HEBT. The smaller coefficientgives rise to a smaller thermal-electrical feedback and a less322 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 46, NO. 2, FEBRUARY 1999Fig. 4. Current gain versus substrate temperature for the GaInP/GaAs HBTat collector current of 1  10 5, 1  10 4, and 1  10 3 A.likelihood of current collapse in the device. The current gainof the HBT decreases with increasing temperature in the entirecurrent range. The lower the collector current, the more is thecurrent gain reduction. As can be seen in Fig. 3, the currentgain decreases in magnitude by a factor of 20 at of 10A but only 2 at 10 A, respectively, for the temperaturerange of 25–200 C.From the data shown in Figs. 2 and 4, the contrast ofthe thermal effect on the current gain between the HEBTand HBT is very striking. In general, current crowding willproduce a thermal gradient within the transistor. However, ifthe current gain were independent of temperature, there wouldnot be a feedback mechanism to produce a catastrophic effect.In reality, when the temperature coefficient of the gain ispositive, thermal runaway will take place as in power siliconBJT. On the other hand, a negative temperature coefficientgives rise to gain depression thus the collapse of the collectorcurrent in the plot of the HBT. By assuming thatthe heterostructure-emitter is similar to the poly-Si emitter,the hole current from the base to emitter is dominated bythermionic emission at high temperature [9]. The temperaturedependence of the diffusion current is described by[10, Ch. 2, Eq. (46)]. The term describes the temperatureeffect of the ratio of the diffusion coefficient and diffusionlength that is negligible in most cases. Ignoring the exponentialterm which cancels out, the thermionic emission currenthas a temperature dependence of and the diffusion currenthas a temperature dependence of Hence, the currentgain is less dependent on the junction temperature when thehole current is dominated by thermionic emission. A detailedanalysis is too long to include here and will be published ina separate paper in the future.One advantage of the current gain increase with temperatureis for power applications. The negative differential resistanceand current collapse in HBT’s are commonly observed as dueto the self-heating effect. In our devices, we have obtaineda positive differential resistance in the curve. Thispositive differential resistance results directly from the positivetemperature coefficient of the current gain. However, thecurrent gain decrease at high temperature brings back thenegative differential resistance at high current and high bias.This effect prevents the device from thermal runaway.In summary, we have reported a significant difference in thethermal coefficient of the current gain for GaInP/GaAs HEBTand a HBT. It is showed that the current gain for the HEBT iscontrollable by design and can be much less dependent on thejunction temperature. A positive differential resistance incurve is observed corresponding to the current gain increasewith temperature. The results suggest that the HEBT structurecan overcome the problem of current collapse and is a bettercandidate than the HBT for power devices.REFERENCES[1] B. Bayraktaroglu, “GaAs HBT’s for microwave integrated circuits,”Proc. IEEE, vol. 81, pp. 1762–1785, 1993.[2] G. B. Gao, H. Morkoc, and M. F. Chang, “Heterojunction bipolartransistor design for power applications,” IEEE Trans. Electron Devices,vol. 39, pp. 1987–1997, 1992.[3] W. Liu, S. K. Fan, T. Henderson, and D. Davito, “Temperature de-pendence of current gains in GaInP/GaAs and AlGaAs/GaAs hetero-junction bipolar transistors,” IEEE Trans. Electron Devices, vol. 40, pp.1351–1353, 1993.[4] W. Liu, E. Beam, III, T. Kim, and A. Khatibzadeh, “Recent devel-opments in GaInP/GaAs heterojunction bipolar transistors,” in CurrentTrends in Heterojunction Bipolar Transistors, M. F. Chang, Ed. Sin-gapore: World Scientific, 1995.[5] L. F. Luo, H. L. Evans, and E. S. Yang, “A heterojunction bipolartransistor with separate carrier injection and confinement,” IEEE Trans.Electron Devices, vol. 36, pp. 1844–1846, 1989.[6] W. S. Lour, W. C. Liu, D. F. Guo, and R. C. Liu, “Modeling the DCperformance of heterostructure-emitter bipolar transistor,” Jpn. J. Appl.Phys., vol. 31, pp. 2388–2393, 1992.[7] Y. F. Yang, C. C. Hsu, and E. S Yang, “Prevention of base dopantoutdiffusion using heterostructure-emitter in GaInP/GaAs heterojunctionbipolar transistors,” Semicond. Sci. Technol., vol. 10, pp. 339–343, 1995.[8] Y. F. Yang, C. C. Hsu, E. S Yang, and Y. K. Chen, “Comparison ofGaInP/GaAs heterostructure-emitter bipolar transistors and heterojunc-tion bipolar transistors,” IEEE Trans. Electron Devices, vol. 42, pp.1210–1215, 1995.[9] C. C. Ng and E. S. Yang, “A thermionic-diffusion model of polysiliconemitter,” in IEDM Tech. Dig., 1986, pp. 32–35.[10] S. M. Sze, Physics of Semiconductor Devices. New York: Wiley, 1981,ch. 2.[11] W. Liu, Handbook of III-V Heterojunction Bipolar Transistors. NewYork: McGraw-Hill, 1998.Edward S. Yang (S’60–M’61–SM’74–F’92) re-ceived the B.S. degree from Cheng-Kung Univer-sity, Tainan, Taiwan, R.O.C., in 1957, the M.S.degree from Oklahoma State University, Stillwater,in 1961, and the Ph.D. degree from Yale University,New Haven, CT, in 1966, all in electrical engineer-ing.He was an Engineer at IBM Corporation, Pough-keepsie, NY, from 1961 to 1963. From 1965 to1998, he was with the Department of ElectricalEngineering, Columbia University, New York, NY.Currently, he is Professor of Microelectronics at the University of Hong Kong.His research activities are in high-speed and high-power HBT’s, HTS for MRIapplications, and LED’s. He has published more than 160 technical papersand written two books: Fundamentals of Semiconductor Devices (New York:McGraw-Hill, 1978) and Microelectronic Devices (New York: McGraw-Hill,1988).YANG et al.: TEMPERATURE DEPENDENCE OF CURRENT GAIN 323Yue-Fei Yang (M’95) received the B.S. degreein physics from East China Normal University,Shanghai, in 1982, the M.S. degree in electricalengineering from Shanghai Institute of Metallurgy,Chinese Academy of Sciences, Shanghai, in 1985,and the Ph.D. degree in electrical engineering fromColumbia University, New York, NY, in 1995.From 1985 to 1990, he was a Research Associateat the Shanghai Institute of Metallurgy, where heconducted research on III–V compounds and short-channel GaAs MESFET. From 1991 to 1992, heworked on hot electron transistors in the Department of Electronic andElectrical Engineering, Sheffield University, Shiffield, U.K. From 1995 to1997, he was a Postdoctoral Fellow and later Associate Research Scientistin the Department of Electrical Engineering, Columbia University, where heworked on HBT’s, HEMT’s and RTD’s. Since 1998, he has been a Memberof Technical Staff, Global Communication Semiconductors, Inc., Torrance,CA. His main interests include developing high-speed heterostructure devicessuch as HBT’s, HEMT’s and integrated circuits for digital and microwaveapplications.Chung-Chi Hsu graduated from National TaiwanUniversity, Taipei, Taiwan, R.O.C., in 1976,majoring in electrical engineering. He receivedthe M.S.E.E. degree from Purdue University, WestLafayette, IN, in 1979, and the Ph.D. degree fromthe University of Utah, Salt Lake City, in 1985.He worked for Xerox Palo Alto Research Center,Palo Alto, CA, as a Research Intern from 1979 to1980. In 1985, he joined Lasertron, Inc., workingon organometallic vapor phase epitaxy for longwavelength laser development. In 1987, he joinedITRI Material Research Laboratories, Hsinchu, Taiwan. He has been anAssociate Professor at the Chinese University of Hong Kong since 1989.His current research interests are high-speed optoelectronics, high-speedheterostructure devices, crystal growth, and organometallic vapor phaseepitaxy.Hai-Jiang Ou received the B.S. degree in physicsfrom Hangzhou University, Zhejiang, China, in1984 and the M.S. degree in electrical engineeringfrom the Shanghai Institute of Metallurgy, ChineseAcademy of Sciences, Shanghai, China, in 1987.He joined the Shanghai Institute of TechnicalPhysics, Chinese Academy of Sciences, in 1987,where he was an Assistant Professor and lateran Associate Professor, working on InSb andAlGaAs/GaAs MQW infrared focal plane arrays.He has been a Visiting Scientist in the Departmentof Electrical Engineering at Columbia University, New York, NY. He is nowwith the Department of Electrical and Electronic Engineering, University ofHong Kong. His current research interests are in compound semiconductorheterostructure devices.H. B. Lo received the B.Eng. and M.Eng degreesin engineering physics from McMaster University,Hamilton, Ont., Canada, in 1970 and 1972, respec-tively.From 1970 to 1972, he was a Research Engi-neer in the Atomic Power Division and the SolidState Devices Department, Westinghouse Canada,Hamilton. From 1974 to 1980, he was with MicroElectronics, Ltd., Hong Kong, where he was en-gaged in both development and production of waferprocessing of bipolar devices. In 1980, he joined theDepartment of Electrical and Electronic Engineering, The University of HongKong, where he is currently a Lecturer. His current research interests are onthe design and processing of semiconductor devices.

Temperature dependence of current gain of GalnP/GaAs heteroj unction and heterostructure-emitter bipolar transistors

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Temperature dependence of current gain of GalnP/GaAs heteroj unction and heterostructure-emitter bipolar transistors

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