966 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 58, NO. 4, APRIL 2011 Analysis of Electrothermal Effects in Bipolar Differential Pairs Vincenzo d’Alessandro, Luigi La Spina, Member, IEEE, Lis K. Nanver, Member, IEEE, and Niccolò Rinaldi, Member, IEEE Abstract—An extensive experimental and theoretical analysis of bipolar differential pairs subject to radical electrothermal feed- back is presented. Measurements demonstrate that considerable thermally-induced degradation of circuit characteristics may oc- cur, eventually turning into the full disappearance of a linear region, which is replaced by a hysteresis behavior under voltage- controlled conditions. An analytical model is derived for a simple yet reliable prediction of the distortion of I –V curves. A more elaborated circuit approach is employed to accurately quantify the concurrent destabilizing action of electrothermal and impact ion- ization effects, as well as to evaluate the impact of layout asymme- tries and examine the beneficial influence of emitter degeneration resistors. Simulation results are found to compare favorably with experiments performed on silicon-on-glass test structures with various layouts and isolation schemes, from which the benefits of thermally coupling the two devices become evident. Index Terms—Analog circuits, bipolar junction transistor (BJT), breakdown voltage, differential pair, electrothermal sim- ulation, emitter-coupled logic, heterojunction bipolar transistor (HBT), impact ionization, self-heating, silicon germanium (SiGe), silicon-on-glass (SOG), thermal coupling, thermal instability, thermal resistance. I. I NTRODUCTION I T HAS LONG been reported that electrothermal effects may generate heavy distortion in the characteristics of bipolar analog circuits, thus severely degrading functional operation and adversely affecting long-term reliability [1]–[4]. Although such issues were traditionally associated with either discrete or integrated circuits (ICs) subject to large power dissipation, nowadays they also plague low-power electronic systems due to the technology trends devised to achieve high-speed perfor- mance, such as the adoption of aggressive isolation schemes in- volving low-k dielectrics [5] and shallow/deep trenches [6]–[8] often combined with buried oxide layers [9], [10], as well as the use of III–V substrates [11], [12]. The thermal resistances of individual bipolar transistors have indeed grown to thousands of K/W, thereby entailing considerable electrothermal feedback. Manuscript received September 26, 2010; revised December 30, 2010; accepted January 3, 2011. Date of publication March 7, 2011; date of current version March 23, 2011. The review of this paper was arranged by Editor J. D. Cressler. V. d’Alessandro and N. Rinaldi are with the University of Naples Federico II, 80138 Naples, Italy (e-mail: vindales@unina.it). L. La Spina was with the Laboratory of Electronic Components, Technology, and Materials (ECTM), Delft Institute of Microsystems and Nanoelectronics (DIMES), Delft University of Technology, 2628 CT Delft, The Netherlands. He is now with Iszgro Diodes b.v., 2719 CA Zoetermeer, The Netherlands. L. K. Nanver is with the Laboratory of Electrical Components, Technology and Materials, Faculty of Electrical Engineering, Mathematics and Computer Science, Delft University of Technology, 2628 CT Delft, The Netherlands. Digital Object Identifier 10.1109/TED.2011.2106132 These issues are pushed to the extreme when adopting the back- wafer-contacted silicon-on-glass (SOG) technology, in which resistive and capacitive parasitics are drastically reduced by surrounding the whole active device area with electrically insu- lating materials and applying a substrate transfer from silicon to glass [13], [14]. As a consequence, a clear understanding of the detrimental impact of electrothermal effects has become crucial, and several recent works have been devoted to the analysis of the behavior of single-finger [15]–[23], two-finger [19], [24]–[27], and multifinger [11], [28], [29] bipolar transis- tors. However, only few papers have been focused on thermal issues in basic state-of-the-art bipolar circuits. In a recent exam- ple, an experimental and theoretical study performed on GaAs and SOG current mirrors has evidenced that a combination of electrothermal and avalanche effects may lead to a marked degradation of the mirroring action, eventually turning into an instability behavior under severe operating conditions [30]. This paper is devoted to a systematic analysis of elec- trothermal effects in bipolar differential pairs, a schematic representation of which is given in Fig. 1. These circuits, often referred to as emitter-coupled pairs or basic differential gain stages, are commonly employed in typical analog/radio- frequency blocks such as bipolar low-noise and variable gain amplifiers, as well as Gilbert mixers and multipliers [31]–[34]. Their massive adoption in comparison to single-ended ampli- fiers is mainly due to the following attractive features. First and foremost, they provide a high degree of rejection to equal (common mode) voltages applied to both input terminals if transistors share closely matched electrical characteristics; as a result, differential pairs exhibit low sensitivity to interference of coupling effects. Second, since the common-mode input range of differential pairs is usually wider than the allowable input swing of the single-ended counterparts, they can be more easily cascaded without interstage coupling capacitors, which are costly in terms of occupied area in ICs [35]–[37]. Bipolar differential pairs are also employed in digital systems due to the property that small input voltages can switch the current from one side to the other in a circuit. In particular, they are the core of an utmost performance very-high-speed logic circuit family referred to as emitter-coupled logic, which is character- ized by the lowest propagation delay among logic gates [37], [38]. Furthermore, differential pairs have been adopted as basic components of bistable elements for sensor systems [39]. Here the aim is to combine and extend the theoretical analy- sis briefly sketched in [40] and the experimental/numerical in- vestigation presented in [41] so as to offer a complete treatment of the electrothermal behavior of bipolar differential pairs. 0018-9383/$26.00 © 2011 IEEE