Base Resistance Scaling for Transistors of Various Geometries Yingying Yang and P.J. Zampardi* Skyworks Solutions, Inc., 2427 Hillcrest Drive, Newbury Park, CA 91320 yingying.yang@skyworksinc.com , (805) 480-4289 *formerly of Skyworks Abstract — Base resistance is an important parameter for bipolar transistor performance and modeling. It can be calculated from simple inputs: base sheet resistance, geometries, and contact metal characteristic impedance. Because a variety of device geometries are used, it is useful to develop scaling equations for different transistor geometries. In this work, we develop generalized equations for the base resistance of multi-finger rectangular devices, ring devices, and horseshoe devices. Index Terms — bipolar, base resistances, geometry scaling, power amplifiers. I. INTRODUCTION The base resistance of a bipolar device impacts RF performance metrics such as RF power gain and noise figure, making its determination important for geometry optimization as well as modeling. In Silicon technologies, the base resistance, R b , is often calculated from geometry [1] and well established geometrical corrections factors [2]. However, previous works focused solely on single- emitter (single finger, ring, or dot) transistors. In amplifier applications (power amplifiers or low-noise amplifiers using GaAs HBT) multi-finger unit cells are used to achieve compact layouts. These multi-finger devices may have various configurations, as shown in Fig.1. Other geometries such as ring or horseshoe devices may also be used. To allow design flexibility (such as geometry optimization), a simple set of equations that describes these configurations is useful. In this work, we develop and present such equations. II. STRAIGHT FIGURE DEVICES A. Compact Model Convention and Assumption In compact models, R btotal is split into two parts, the Rbi (intrinsic) and Rbx (extrinsic). Rbi represented the part under emitter (a.k.a. “spreading”, “turn”, or “pinch” resistance); and Rbx is the sum of “link” (from the base contact to the emitter) and contact resistance. For multi-finger devices, each finger has these resistance elements (Fig. 2), RTURN_y (y represents any of one finger in multi-finger structure, could be A, B, C…), RLAT_y and RCONT_y. (1) should be the R btotal associated with each individual finger. ࡾ࢈ ࢀࡻࢀ࡭ࡸ_࢟ ൌ ࡾ ࡯ࡻࡺࢀ_࢟ ൅ࡾ ࡸ࡭ࢀ_࢟ ൅ࡾ ࢀࢁࡾࡺ_࢟ (1) Assuming equal potentials (VA1=VB1=VC1…, VA2=VB2=VC2…) we can group (through scaling equations) all the “turn” resistances (RTURN_A, RTURN_B, RTURN_C,…) from each finger as a single model parameter “Rbi”, and all “link” resistances (RLAT_A, RLAT_B, RLAT_C…) and contact resistances (RCONT_A, RCONT_B , RCONT_C…) etc. from each finger as a single model parameter “Rbx” (Fig. 2). This assumption breaks down when NE>NB>1. Figure 2 Explanation of Assumption and Resistances Nb > Ne with tab Nb = Ne with tab Nb < Ne with tab Nb > Ne without tab Nb = Ne without tab Nb < Ne without tab Base Contact Metal Emitter Fingers Base Contact Metal Emitter Ring Ring Base Contact Metal Emitter Horse Shoe finger B’s resistive path finger A’s resistive path RCONT_A finger C’s resistive path A B C D Base Contact Tab Emitter Fingers Base Fingers RLAT_A RTURN_A RCONT_B RCONT_C RLAT_B RLAT_C RTURN_B RTURN_C WE LE Ldg A1 B1 C1 A2 B2 C2 A3 B3 C3 Figure 1 Definition of geometries studied. Top row is “tab” connected HBTs and ring emitter. Bottom row is no tab and horse shoe device. 978-1-4799-0583-6/13/$31.00 ©2013 IEEE