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.