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impedance behaviour and on the other hand, it has beenshown the great influence of the carrier and generally thedevice environment for the good operating temperatureprediction.The aim of the study led in this paper shows the difficultyto interpret in a coherent way the comparison between purethermal simulations and results of an electric methodbecause the electrothermal coupling can hardly be evaluatedwithin the transistor. For further works, a distributedelectrothermal model should be used and would be moresuitable for a better description of the phenomena within themulti-fingers transistors.AKNOWLEDGEMENTThe authors would like to thank UMS for providingHBTs.24-26 September 2008, Rome, ItalyREFERENCES[1] M. N. Sabry, “Compact thermal models for electronic systems”, inTHERMINIC, 2001, pp. 197-202.[2] D. T. Zweidinger, S. G. Lee, and R. M. Fox, “Compact modelling ofbjt self-heating in spice”, IEEE Trans. On CAD of Intergated Circuitsand Systems, vol. 12, pp. 1368-1375, 1993.[3] O. Mueller, “Internal thermal feedback in four-poles especially intransistors”, Proceedings of the IEEE, vol. 52, no. 8, pp. 924-930,Aug. 1964.[4] S. Marsh, “Direct extraction technique to derive the junctiontemperature of HBT's under high self-heating bias conditions”,Electron Devices, IEEE Transactions on, vol. 47, no. 2, pp. 288-291,Feb 2000.[5] N. Bovolon, P. Baureis, J.-E. Muller, P. Zwicknagl, R. Schultheis andZanoni, “A simple method for the thermal resistance measurement ofAlGaAs/GaAs heterojunction bipolar transistors“,Electron Devices,IEEE Transactions on, vol. 45, no. 8, pp. 1846-1848, Aug 1998.[6] J. Lonac, A. Sabtarelli, I. Melczarsky and F. Filicori, “A simpletechnique for measuring the thermal impedance and the thermalresistance of HBTs”, in Gallium Arsenide and Other SemiconductorApplication Symposium, 2005. EGAAS 2005. European, 3-4 Oct2005, pp. 197-200.[7] A.A.L. de Souza, J.-C.Nallatamby, M. Prigent, and R. Quéré,“Dynamic impact of self-heating on input impedance of bipolartransistors”, in Electronics Letters, vol. 42, no. 13, 22 June 2006, pp.777-778.[8] T. Peyretaillade, M. Perez, S. Mons, R. Sommet, P. Auxemery, J.Lalaurie, and R. Quere, “A pulsed-measurement based electrothermalmodel of hbt with thermal stability prediction capabilities,” inMicrowave Symposium Digest, 1997., IEEE MTT-S International,vol. 3, 1997, pp. 1515–1518 vol.3.[9] E. B. Rudnyi, J. Lienemann, A. Greiner, and J. G. Korvink,“mor4ansys: Generating compact models directly from ansysmodels,” Technical Proceedings of the 2004 NanotechnologyConference and Trade Show, Nanotech 2004, Boston, Massachusetts,USA, vol. 2, pp. 279–282, March 7-11, 2004.[10] A. Xiong, A. A. Lisboa de Souza, R. Sommet, R. Quéré and B.Barbalat, “Détermination d’impédance thermique de TBH SiGe parmesures électriques basses fréquences”, 15 ème Journées NationalesMicroondes, Toulouse 2007.[11] D. Lopez, R. Sommet, and R. Quere, “Spice thermal subcircuit ofmultifinger HBT derived from ritz vector technique of 3D thermalsimulation for electrothermal modelling”, in GAAS – Londres, 2001,pp. 207-210.[12] E. Wilson and M.W.Yuan, “Dynamic Analysis by directsuperposition of Ritz vectors”, in Earthquake Eng. StructuralDynamics, vol. 10, no. 6, pp. 813-821, 1982.[13] E. Koenig, S. Ulrich, J. Schneider, U. Erben and H. Schumacher,“Impact of thermal distribution and emitter length on theperformance of microwave heterojunction bipolar transistors”, inElectron Devices, IEEE Transactions on, vol. 38, no. 4, pp.775-779,Apr. 1995.©EDA Publishing/THERMINIC 2008 194ISBN: 978-2-35500-008-9