Systematic Characterization and Modeling of Small and Large Signal Performance Of 50 - 200 Ghz SiGe HBTs

Abstract

Over the last ten years, SiGe BiCMOS technology has become the leading technology in analog circuit design for both wired and wireless telecommunication applications. However, the endless pursuit for high performance is fraught with difficulties in characterizing and modeling the SiGe HBTs. In order to obtain a high cut off frequency fT, devices are scaled to extremes and collector doping is increased to allow more current flow before the onset of the Kirk effect, allowing the high speed benefit of a smaller base transit time to be realized. As a result, self-heating plays an important role in the already complicated HBT characteristics. A good example of this is the characterization of avalanche multiplication. As an inevitable result of pursuing high fT , the breakdown voltage is decreased, which makes the characterization of avalanche multiplication more important. However, with severe self-heating, conventional methods fail at a practical bias (below peak fT current). Chapter 2 gives a review of the measurement methods currently available for characterizing avalanche multiplication in SiGe HBTs. With the scaling of devices, conventional methods can no longer be applied. New methods are proposed to accurately measure avalanche multiplication factor (M-1) even in the severe self-heating region. The current dependence of M-1 is demonstrated. The results show that the CB breakdown voltage at the JE of peak fT is higher than that at either low JE or in the off state by a significant 1 V in a 120 GHz peak fT device. Also in Chapter 2, the current dependence of M-1 is found to be considerably smaller by taking into account the extrinsic collector resistance. Later, in Chapter 5, a simplified model for the current dependent M-1 is proposed. In Chapter 3, RF characterization methods are discussed, including S-parameters characterization, large signal power characterization and third order intermodulation characterization. The large signal performance characterization of SiGe HBTs is of great importance to both circuit design and process design. Chapter 4 experimentally investigates SiGe profile and collector profile optimization from a large signal performance standpoint, as well as the impact of technology scaling. The results show that device and circuit designs that only consider optimum small signal performance could inadvertently degrade large signal performance. The tradeoffs in SiGe profile design between small signal and large signal performance, as well as the impact of speed-breakdown tradeoff on large signal performance, are experimentally examined. The SiGe HBTs from a 200 GHz technology show impressive small and large signal performance at 20 GHz, demonstrating the benefits of technology scaling, despite decreased breakdown voltage. Intermodulation linearity is another important figure-of-merit for SiGe HBTs, as it relates to the selectivity of an RF receiver and the spectral purity of an RF transmitter. Chapter 5 presents a systematic characterization of the intermodulation linearity for SiGe HBTs in order to gain insight into the device physics underlying linearity behavior, and to construct guidelines to optimal sizing, biasing, and device selection (e.g. high breakdown versus low breakdown versions). The input 3rd order intercept point, IIP3, is measured on IC † VCE plane for devices of various size, breakdown voltage, Ge profile, and technology generation. Later in this chapter, problems of VBIC model for simulating IIP3 are presented. Improvements for base collector capacitance and avalanche modelling in the VBIC model are suggested and implemented in Verilog-A to give a much better fit to the measurement results.