Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyDual Approach for HBT Thermal ImpedanceA. Xiong, R. Sommet, A.A.L. de Souza and R. QuéréXLIM-UMR CNRS n°6172, University of Limoges7, rue Jules Vallès19100 Brive La Gaillarde FRANCEAbstract- This paper presents a dual approach for a coherentdetermination and validation of HBT thermal impedance. Thisstudy relies both on an experimental characterization methodand a 3D finite element simulation approach. The first sectionreminds briefly the experimental approach. The second sectiondescribes the 3D device modeling used for the physic-basedthermal simulations. Thereafter, details on the reductionmethod used for the numerical computation of the thermalimpedance are explained. The last section compares the finalresults and validates this dual approach for power HBTs.I. INTRODUCTIONThermal phenomena, such as self-heating, thermalcoupling are known to impact and affect the semiconductordevices efficiency especially in the high power applicationsdomain. In that case, the thermal modeling becomes crucial.Several models have been investigated [1] [2] and manycharacterization methods have been developed [3] [4] [5] [6][7]. In this paper, simulations based on 3D Finite Elements(FE) have been performed. An automatic model orderreduction [11] technique has been applied enabling an easydetermination of the thermal impedance of the transistordevices. The results have been compared to the experimentalmeasurements [7] extracted for GaAs Heterojunction BipolarTransistors (HBT) brazed onto two different carriers.II. THERMAL IMPEDANCE MEASUREMENT METHODThe study led in this paper concerns a 10x2x110_10 GaAsHBT featuring 20 emitter fingers (10 bi-cells) of 2x110 µm²and with a pitch of 10 µm between each finger of a bi-cell.This transistor has been brazed onto Copper carriers andKovar carriers.The experimental characterization of the thermalimpedance is based on the low frequency measurement ofthe input impedance of microwave bipolar transistor incommon emitter configuration [7].base-emitter voltage V BE as a nonlinear function of the basecurrent I B and the temperature T, the frequency-dependentexpression of the thermal impedance Z th (ω) can be extractedfrom equation (1).~Z = Z + Z ( ϖ ). ϕ.h .( V − R . I )in inISO th fe CE0L C0(1)The establishment of this equation is described in moredetails in [7].The extraction of Z th requires the determination of allterms of (1). Z inISO represents the ratio between ∂V BE and ∂I Bunder isothermal condition. The physical quantity φ, whichrepresents the variation of the base-emitter voltage with thetemperature (value typically around -1.5 mV/°C for GaAsHBTs [8]) can be measured. The small signal current gain h febetween the collector current I C and the base current I B canalso be measured. The last measurements for Z th extractionconsists in two steps and is performed in the following way:− The first step aims to measure Z inISO by annulling thesecond term of (1) with R L = V CE0 / I C0 .− A second measurement of Z in is performed to proceed toZ th extraction by choosing R L = 0Ω while applying V D0 =V CE0 .The measurement can be performed from 1 Hz up to 100KHz. The bias point used for the measurements is V CE0 =10Vand I C0 =50mA which corresponds to a total DC power ofaround 0.5 W. Fig. 2 shows the input impedance curvesmeasured versus the load charge R L for a transistor brazed ona Copper carrier. Fig. 3 shows the thermal impedanceextracted from the input impedance measurements.403020100-101 1E1 1E2 1E3 1E4 1E5Fig. 1 Input impedance measurement protocol for bipolar transistor incommon emitter configurationFrom the measurement protocol (Fig. 1) and defining theFig. 2Measured input impedances Z inISO and Z in of a GaAs HBT brazed ona Copper carrier for chosen values of respectively R L = V CE0 / I C0=200Ω and R L = 0Ω©EDA Publishing/THERMINIC 2008 190ISBN: 978-2-35500-008-9