Understanding the physics of hard and soft failure in silicon-germanium heterojunction bipolar transistors
Perez Martinez, Rafael
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The objective of this work is to investigate hard and soft failure mechanisms observed when silicon-germanium heterojunction bipolar transistors (SiGe HBTs) are operated outside of traditionally defined electrothermal safe operating areas (SOAs). In Chapter 1, the need for studying the reliability physics of SiGe HBTs is motivated. Important physical mechanisms in SiGe HBTs such as avalanche multiplication, breakdown mechanisms, and base current reversal are briefly discussed to provide some insight on the physics of failure in later chapters. Chapter 2 provides an overview of the reliability physics of SiGe HBTs. The damage spectrum of the SiGe HBT is introduced in order to showcase the different soft failure mechanisms when operating a SiGe HBT in typical RF and mixed-signal circuits. All of the relevant damage mechanisms in scaled SiGe HBTs are described in greater detail to understand the impact on overall device performance. Chapter 3 introduces the concept of hard failures in SiGe HBTs. A new failure metric in SiGe HBTs, known as the “hard failure” point, is proposed in order to provide additional understanding to the physics of hard failure. This failure metric represents the bias condition at which the device transistor action fails, and the device becomes purely resistive as a result of catastrophic junction failure. An advanced SiGe HBT technology (GlobalFoundries 9HP) was chosen and measured in a variety of ways as a function of geometry, layout configuration, and temperature. Through TCAD simulations and experimental data, two modes of failures in SiGe HBTs were shown. The takeaway is that knowing specific details, such as the exact location of hard failures, the physics that drives them, and what they depend upon, can greatly aid circuit designers to exploit the design space between PDK specified SOAs and hard failures. The analysis presented in Chapter 3 has been published and presented at the 2017 IEEE Bipolar/BiCMOS Circuits and Technology Meeting. Chapter 4 presents the use of high-breakdown SiGe HBTs in RF integrated circuits. A medium-breakdown variant in a fourth-generation SiGe BiCMOS technology is described from a processing perspective to understand any subtle differences between its high-performance counterpart. Reliability stress measurement data is shown to provide insight into the differences between high-performance and medium-breakdown SiGe HBTs when biased under high-current and mixed-mode stress conditions. Trade-offs in reliability and performance are described to showcase advantages and disadvantages when using either device variant. Part of the analysis presented in Chapter 4 will be submitted for publication to the 2019 IEEE BiCMOS and Compound Semiconductor Integrated Circuits and Technology Symposium. Lastly, Chapter 5 summarizes the contributions presented in this thesis. A future work section is provided to showcase any additional research work that will result from the analysis done in Chapter 3 and 4.