In the modern world, electronic components based on semiconductor devices have become a very important part of an uncountable number of applications. The performance gain and the miniaturization of semiconductor devices continuously open up new possibilities. Of comparable importance for the entire development is the decreasing costs per device. The main factor for cost reduction is the dramatic increase of the device density on a silicon waver [1,2]. Therefore, a considerable amount of industrial and academical research is being performed to enable the continued shrinking of the devices.

Another important method for the cost reduction of electronic components is to combine different functional groups in a single integrated semiconductor die. This technique is applied, for example, in smart power devices which incorporate power and high-voltage devices with additional functions, such as power control, sensing and protection, and interfacing. To accomplish this, different technologies have to be combined. A majority of the smart power applications integrates low-voltage CMOS logic with high-voltage and/or power device technologies [3,4,5,6].

When talking about high-voltage devices, one has to clarify more specifically
the voltage range, because the terminus ``high-voltage'' strongly depends on
the application field. In power engineering and in the area of transmission and
distribution, for example, voltages above 1,000V are considered as
high-voltages [7]. In long-distance electric power transmission
lines this lower limit is as high as 100kV. At the other end of the voltage
range, especially in smart power devices in which CMOS logic is integrated, one
considers voltages starting from 5V as high voltages [5]. In
the automotive industry, for example, the high-voltage part of smart power
devices is dominated by the 14V vehicle power supply. Contributing to the
disturbances on the supply and signal lines encountered in automotive
environments, these devices are often rated in the range of 50-100V
[8,9,10]. In this thesis the term high-voltage
refers to the range from 5 to 60V. In **Chapter 2** various device
architectures, which are used in this voltage range, are presented. Due to the
wide usage in industry, the focus is put on the double-diffused
metal-oxide-semiconductor field-effect transistor (DMOSFET) and its variants in
lateral and vertical orientation. Also several device design approaches such as
field shaping and isolation techniques are shortly described.

Modern semiconductor devices have to fulfill many requirements in terms of
performance, reliability, and costs. Certain reliability goals must be met,
which depend on the field of application and other considerations, for example,
safety, security, and liability issues.
An overview on reliability in general followed by some specific reliability
concerns found in semiconductor devices is given in
**Chapter 3**. However, the down-scaling and the increasing complexity
of devices and integrated circuits make it very challenging to reach all
specified design goals. Therefore, more and more often device simulation tools
are employed in development, research, and optimization. These tools, commonly
referred to as technology computer aided design (TCAD) tools, aim to reproduce
the physical mechanisms and hope to predict the device behavior
[11,12,13,14]. The most important
formulas and physical models which are needed for device simulation within the
drift-diffusion model are described in **Chapter 4**. This chapter also discusses
the possibilities and limitations of this model to describe hot-carrier
phenomena, which are of crucial importance for high-voltage device reliability.
Strictly speaking, a physics-based modeling approach of these effects requires
an exact solution of the Boltzmann transport equation. In this context the
Monte Carlo method [15,16] proved to be one of the most
popular approaches. In fact, this method gives accurate results and allows to
easily incorporate various physical models. Unfortunately, the computational
cost required for this method are very high, which make them not too appealing
for industrial use. On the other hand, the drift-diffusion model is numerically stable
and can be solved efficiently [11,14]. The modeling
approaches presented in this thesis aim to deliver good results in reasonable
simulation times and, therefore, are based on the drift-diffusion model.

The two hot-carrier related reliability effects discussed in this thesis are
the impact-ionization generation and hot-carrier degradation. **Chapter 5** is
devoted to the physical phenomenon of impact-ionization and presents different modeling
strategies and aspects. The importance of impact-ionization for the reliability of smart
power devices is demonstrated in a case study. In **Chapter 6**
hot-carrier degradation in MOS devices is discussed. After a review of
currently used modeling techniques, a distribution function based model is
presented, which is currently under development
[17,18]. This model is based on results
obtained within the Monte Carlo simulations and is therefore computationally very
demanding. To overcome this disadvantage, possible approximations using the
drift-diffusion scheme are presented and discussed.

The simulation of semiconductor devices in TCAD tools requires the solution of
a system of non-linear differential equations. To solve this system, a spatial
discretization scheme has to be used to transform the equations into a system
of difference equations. In the context of device simulation, the box
integration method proved to be very reliable. Iterative solution techniques
are required to obtain a solution for this numerical problem.
Simulations in high-voltage devices turned out to be numerically challenging,
especially in combination with impact-ionization.
Hence, investigations on various vector discretization schemes were done, which
are presented in **Chapter 7**.

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