High-frequency noise modeling of Si(Ge) bipolar transistors

Abstract

The design and the optimization of electronic systems often requires a detailed knowledge of the inherent noise generated within semiconductor active devices, constituting the core of such systems. Examples of applications in which noise is a key issue include receiver front-ends in radiofrequency (RF) and optoelectronic transmission systems, front-end stages in sensors and also oscillators and mixers. The rapid growth of the silicon- (Si-) based RF electronics has triggered increasing interest towards low-noise Si-based devices, such as silicon-germanium (SiGe) heterojunction bipolar transistors (HBT's). In this context, modeling of the high-frequency noise can be considered an essential tool for the optimization of noise performance at both technological level (device design) and circuit level (circuit design). This thesis addresses the problem of high-frequency noise modeling in Si(Ge) bipolar transistors. As remarked throughout the treatise, accurate and reliable noise predictions must rely on a careful analysis of the carrier transport in the intrinsic transistor as well as of the impact of the parasitics constituting the extrinsic part of the device. The relative impact of the extrinsic and intrinsic part on noise characteristics depends upon bias conditions. Chapter 2 begins with an experimental demonstration of the relevance of the extrinsic network to simulated noise characteristics, through a study of the impact of the base resistance distribution with respect to the base-collector capacitance. Furthermore, approximated analytical results for the real part of the feedback admittance y12 are presented, in terms of the small-signal equivalent circuit of the Mextram compact model. Such analytical results provide a method for the extraction of the base resistance distribution. From Chapter 3 on, the analysis steps into the intrinsic device level, focusing on the transport process of the minority carriers injected into the quasi-neutral base region. Chapter 3 provides a thorough review of selected most seminal contributions to the noise modeling theory of bipolar devices, namely Van der Ziel's collective approach, Polder-Baelde's equivalent network and Van Vliet's analytical approach based on the Green's function approach. Chapter 3 serves as theoretical basis for the subsequent chapters. In Chapter 4 a noise modeling approach is presented based on a lumped network. Such network is the result of a discretization of the partial differential equations governing the transport process of minority carriers injected into the quasi-neutral base region. The proposed approach is first verified based on analytical reasoning and then through the assessment of noise characteristics of a complete industrial SiGe HBT. Chapter 5 presents a comparative analysis of approximated high-frequency noise model formulations for the intrinsic transistor. The analysis includes the approximated transport noise model feasible for compact model implementations and a correlated noise model derived systematically from theoretical results discussed in Chapter 3, along with the non quasi-static theory of bipolar transistors. In the last part of the chapter a discussion over the impact of the high-frequency effects in the emitter region is profiled.