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Abstract


Semiconductor devices are used in many different application areas and play an important role in the modern world. Advances in technology, customer demands, and cost pressure lead to higher integration densities and to smart power structures, which incorporate high- and low-voltage devices on the same chip. Because of the down-scaling and the rising complexity of devices, it becomes an increasingly challenging task to obtain the required reliability demands. Therefore, technology computer-aided design (TCAD) tools are used to simulate semiconductor devices.


While the term high-voltage is often used for a wide range of devices, this thesis is focused on field-effect transistors with operating voltages ranging from 5 to 60V. The most important devices among this class type as well as relevant design techniques are presented. Since reliability in these high-voltage field-effect transistors is a major concern for the semiconductor industry, the physical processes behind the degradation occurring in semiconductor bulk, oxide, and their interfaces are discussed in this work. However, probably the most important degradation processes in high-voltage devices are those related to the hot-carrier phenomena impact-ionization and hot-carrier degradation. These two topics are addressed in detail from a modeling and simulation perspective. In particular, simulations based on the drift-diffusion (DD) framework are used and the possibilities and limitations of modeling hot-carrier induced phenomena herein are discussed.


Impact-ionization generation is the first hot-carrier process presented in this thesis, starting with a summary on the different modeling approaches. The importance of impact-ionization generation for the reliability of high-voltage smart power devices is demonstrated in a case-study. In this study the snap-back behavior of a parasitic bipolar structure is investigated and structure optimizations are discussed. The second process driven by high energetic carriers is hot-carrier degradation. A physics-based modeling approach relying on the carrier energy distribution function which is derived from Boltzmann's transport equation is presented. The long simulation times required to calculate the distribution function make this approach not very flexible for industrial use. Therefore, variations of this model based on the DD framework have been investigated and show to deliver good results for relevant devices.


Simulations of high-voltage devices often lead to numerical difficulties, especially if impact-ionization generation has to be considered. In the DD framework the modeling of impact-ionization requires an accurate discretization of vector quantities such as the current densities and the driving force, which is numerically very challenging. Different vector discretization schemes are presented and their influence on the convergence behavior and accuracy is analyzed.


next up previous contents
Next: Kurzfassung Up: Dissertation Oliver Triebl Previous: Dissertation Oliver Triebl

O. Triebl: Reliability Issues in High-Voltage Semiconductor Devices