Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyNew approach for thermal investigation ofa III – V power transistorM. Fontaine, E. Joubert, O. Latry, P. Dherbecourt, M. KetataLEMI – Université de Rouenrue Thomas Becket76821 Mont Saint Aignan, FranceAbstract- in this paper is presented a new method forcharacterisation of temperature of AlGaN – GaN transistor. Anellipsometer is also explained for measure of refractive indexand so propagation time constant.I. INTRODUCTIONCurrent applications of electronic (radars) require transistorsalways more efficient in terms of power, frequency of use,noise factor, reliability and miniaturization. Because of allthese constraints, the behaviour of those components, whichare subject to very high thermal stresses, is becomingincreasingly difficult to model.There is a need today to understand the impact of thermalparameters on their performance and lifetime. For this, thedevelopment of measurement tools to tackle these phenomenaheat becomes a necessity.Many technologies are identified in literature to reach thisaim. As a first step we propose to make a tour of some of them.Then, in a second step, an original measurement technique,currently under development in LEMI (Laboratory ofElectronics, Instrumentation and Microtechnology), ispresented and finally current results for this approach arediscussed.II.EXISTING TECHNOLOGIESTwo families of techniques for the measurement oftemperature exist. The first uses changes in the electricalcharacteristics of component while the second, morewidespread, uses optics.The first electrical method uses the characteristic I - V(Current - Voltage) component [1]. When RF power amplifier,for example, is active, a part of electric power it receives istransformed into heat energy : its temperature is changing. As aresult of this evolution in temperature, its electricalcharacteristics are altered. It is therefore possible, in light ofchanging parameters I – V, to quantify the temperature of thecomponent.The second method is to integrate within the package one ormore electronic sensors (spies), which electrical characteristicsevolve as a function of temperature [2],[3]. The characteristicsI - V of these components, placed in a strategic way, canmeasure the temperature inside the package at variouslocations, the number of measuring points corresponding to thenumber of spies implanted.As for the electrical methods, six optical techniques arepresented below. Those methods use different thermo - opticalor electro - optical principles. It is important to establish somepoints that characterize an ideal measuring instrument. Amongthese items :• Type of measure (absolute or differential)• Measurement time• Measurement accuracy• Measuring range• Temperature measured (on the surface or inside thecomponent)• Spatial resolution• Localisation of measure (punctual, 2D or 3D)• Degradation due to the measure (destructive measurementor not)Six optical method for thermal investigation are existing :• IR (Infrared) thermography : the thermal radiationsemitted by the device are absorbed by using a sensor or asensor array (eg CCD camera) [4]. The amplitude of theelectrical signal converted by the photo-detector isproportional to the surface temperature of the component.Its main advantage resides in its maturity.• Liquid crystal thermography : the polarization of anelectromagnetic wave is amended by Liquid Crystals [5].This change in polarisation is function of energy absorbedby LC as electrical energy (LCD) or what is interesting,thermal energy. Its main advantage lies in the accuracy ofmeasurement.©EDA Publishing/THERMINIC 2008 26ISBN: 978-2-35500-008-9