Direct Parameter Extraction Of Base and Emitter Resistances For SiGe HBTs Using DC Data Only

Abstract

A new method to directly extract base and emitter resistance parameters for SiGe HBTs using the Mextram bipolar compact transistor model has been presented. The method only requires a standard forward Gummel measurement and accounts for several second order effects like high-injection base conductivity modulation, Early effect and emitter current crowding. Base and emitter resistance parameters have been extracted over a wide temperature range (300K -93K) for a first generation 0.5μm SiGe HBT with a peak cut-off frequency of 50 GHz at 300K. The HBT features a heavily doped poly-silicon emitter, an extrinsic base region doped well above the Mott's transition (͵ ൈ ͳͲ ଵ଼ cm -3 for boron in silicon) and an intrinsic base region doped very close to the Mott's transition. The extracted parameters have been verified by comparison with several different methods. IntroductionParasitic base and emitter series resistances are very important parameters for bipolar transistors, and their measurement is difficult because of several second order effects taking place in the base like conductivity modulation and high injection. Accurate base and resistance parameters are needed by various bipolar compact models like Mextram (1) and VBIC (2) to predict the different transistor characteristics accurately. In most of the compact models, the series resistances are distributed into an emitter resistance ‫,)ܧܴ(‬ an extrinsic base resistance ‫)ܥܤܴ(‬ and a bias-dependent intrinsic base resistance ‫,)ܸܤܴ(‬ as shown in figure 1.Various methods have been proposed to extract the base and emitter resistances of bipolar transistors, and they can be classified based on measurement techniques: noise, AC and DC. Methods based on noise (3) and AC (4)-(5) data can yield fairly accurate values of the resistances, but they require expensive characterization equipment, are sensitive to the accuracy of the measured data, and are often very time consuming. DC methods are generally preferred, because they are straightforward and easier to execute.Several DC methods have been proposed based on the dual-base test structures (6)-(8) which provide a means of accurate extraction of base and emitter resistance values A popular method to extract the emitter resistance is the open-collector method (9). In this method, the collector-emitter voltage versus emitter current is measured, while driving the transistor in saturation. The slope of the ܸ ா െ ‫ܫ‬ ா curve gives the emitter resistance ‫.ܧܴ‬ However, the method does not account for high level injection effects, and the slope of the ܸ ா െ ‫ܫ‬ ா characteristics is bias-dependent as reported in (10). As a result, the extracted emitter resistance is often over-estimated. The classical Ning-Tang method (10) is based on relating the deviation of base current from its ideal exponential value at higher base-emitter biases in the forward Gummel setup. However, it does not account for emitter current crowding and a prior knowledge of pinched base sheet resistance is needed in order to separate the extracted emitter and...