Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, Italymore straightforward approaches (i.e., those relying on thesimple measurement of DC characteristics) are preferredwhen the mere determination of the (steady-state) thermalresistance R TH is required. Thermal resistance is defined asthe proportionality constant between temperature increaseover ambient and power dissipationR THTj−T0= , (1)PDFig. 1. Schematic layout of the silicon area surrounded by trenches. Thecollector contact is directly under the emitter as made possible by the twosidedcontacting in the SOG process.DevicedevAdevBdevCdevDEmitter area(A E) = W×L[μm×μm]TABLE IDEVICES UNDER TESTAspect ratio(AR) = L/W[μm/μm]1×2.5, 2×5, 3×7.5 2.51×5, 2×10, 3×15 51×10, 2×20, 3×30 101×20, 2×40, 3×60 201×30, 2×60, 3×90 303×6.6 2.24×5 1.251×2.5, 2×5, 3×7.5 2.51×5, 2×10, 3×15 51×10, 2×20, 3×30 101×20, 2×40, 3×60 201×30, 2×60, 3×90 303×6.6 2.24×5 1.251×2.5, 2×5, 3×7.5 2.51×5, 2×10, 3×15 51×10, 2×20, 3×30 101×20, 2×40, 3×60 201×30, 2×60, 3×90 303×6.6 2.24×5 1.251×2.5, 2×5, 3×7.5 2.51×5, 2×10, 3×15 51×10, 2×20, 3×30 101×20, 2×40, 3×60 201×30, 2×60, 3×90 303×6.6 2.24×5 1.25B. Thermal resistance extraction techniqueEmitter-totrenchdistanceS 1 S 2[µm] [µm]2.5 5.52.5 104 146 14Most of the experimental methods used for the extractionof the thermal impedance are based on the application ofpower pulses (see e.g., [10], [11]), and allow the evaluationof thermal transients in semiconductor devices. However,where T j is the junction temperature, T 0 is the ambienttemperature, and P D is the power dissipated by the deviceunder test.Most of the available DC methods (e.g., [12]-[14]) havebeen proposed for heterojunction bipolar transistors (HBTs)where the Early effect is negligible. In principle, however,these methods could be employed also for Si BJTs, but theyare not suitable to properly evaluate the thermal resistancewhen the devices under analysis are significantly affected bythe Early effect. The approach developed in [6] allows thedetermination of R TH from the experimental detection of thebiasing conditions that lead to thermal instability. Such aprocedure – which will be referred to as the “flyback”technique in the following – has the advantage of onlyrequiring a single measurement. Nevertheless, in whatconcerns all the analysis presented in the next Section, a“differential” procedure [15], based on the techniqueemployed by Dawson et al. in [14], is exploited, whichallows the elimination of the Early effect influence, therebyguaranteeing a high degree of accuracy. A comparisonbetween the DC techniques used for the extraction ofthermal resistances is presented in Fig. 2. In the samepicture, results obtained from thermal-only 3-D FEMsimulations are also reported, which have been performed asexplained in [16]. The values of the thermal resistancesobtained by using the different methods, as well as bysimulations, are shown for two different aspect ratios (AR)Thermal resistance [K/W]2400022000200001800016000140001200010000DifferentialFlybackDawson et al.SimulationsAR=10AR=3080001 10 100 1000Emitter area [µm 2 ]Fig. 2. Thermal resistance as a function of emitter area for two values ofthe aspect ratio of the emitter stripe, namely, 10 and 30, as evaluated byresorting to the experimental approaches described in [6], [14], [15], andnumerical simulations [9]. All data refer to geometry devB.©EDA Publishing/THERMINIC 2008 102ISBN: 978-2-35500-008-9