1. Introduction

AVAILABILITY OF ENERGY is an important cornerstone for wealth and social stability. The continuous economical growth in the last two centuries went hand in hand with a steady growth of energy demand in the industrial, public, and private sector. While very early facilities had to deal with locally available energy sources, the beginning of the oil era with the vision of apparently unlimited resources has brought an immense impact on industry, culture, lifestyle, but also environment. These days, the end of fossil fuels as the major energy source for the civilization in its current way is foreseeable, and environmental concerns have become topics of international political discussions and daily news.

In the last decades, uprising efforts in science and engineering to increase fuel efficiency was by far not able to compensate economical growth in order to at least keep the consumption of limited fossil reserves on a constant level. The current scenario incorporates increasing energy demand facing short running resources, which implies the need for tremendous efforts on an exit strategy. While the awareness about the worth of our energy supply has to generally increase, a new consciousness of consumption has to gain impact on the international markets. Politics has to honestly deal with the problem in a holistic point of view and provide the background for the right measures.

The role of science and engineering to develop technologies for providing electric energy from renewable sources on a broad basis incorporates an immense responsibility for environment, living space, wealth, and security for the next generations. Therefore, ecological, economical, and social considerations have to enter engineering processes beside technical excellence. On the one hand, decentralized power facilities based on renewable resources will play a significant role in the future. On the other hand, the potentials of improvement in efficiency has to be exhausted in a wide range of applications. Therefore, both existing technologies have to be further developed as well as new innovative and unconventional ideas have to enter well established solutions. Besides the power consumption optimization of single devices and components, interdisciplinary research has to combine single optimization potentials by system wide considerations to intelligent global solutions.

Thermoelectric energy conversion is one of the technologies with a potential to play a role in future energy technology. It incorporates the direct energy conversion from temperature gradients to electric energy, whereby the fundamental existence of its underlying physical effect has been well known for almost two centuries. However, in spite of ongoing efforts, their low conversion efficiency currently limits the application of thermoelectric devices to a few highly specialized niches. In the last decades, tremendous efforts on material research has provided both novel materials for thermoelectric energy conversion as well as a principle understanding of the demands for higher efficiencies. Thus, a directed search for according materials has been made possible. Furthermore, recent developments in nanotechnology and low dimensional systems include promising potentials for increased efficiencies.

The contribution of this thesis is a fruitful extension and application of semiconductor device simulation to thermoelectric devices. Technology Computer Aided Design (TCAD) has been established as an important tool to shorten development cycles in mainstream microelectronics. Based on a physically rigorous framework, it incorporates the possibility of device investigations by elaborate simulation studies as well as the optimization of several device parameters for given constraints.

Chapter 2 starts with an overview of important milestones in the history of thermoelectrics. Furthermore, the three thermoelectric effects as well as their relationships are discussed on a phenomenological basis. A glimpse on current as well as potential future applications accomplishes this chapter.

The focus of Chapter 3 is directed to the proper description of electrical transport in semiconductors. After a delimitation within the hierarchy of physical simulation approaches, the fundamentals of macroscopic transport models are sketched. The main part incorporates the systematic derivation of macroscopic transport models from Boltzmann's equation by the method of moments. Thereby, different approximations of the scattering operator are investigated and the resulting equations are summarized in a final comparison. Based on the equations of up to the third moment, a transport model is formulated assuming local thermal equilibrium. The compatibility of this model with the principles of phenomenological irreversible thermodynamics is demonstrated. Special attention is payed to the Seebeck coefficient by a comparison of measurement data with its theoretical formulation inherently contained in the transport model.

Chapter 4 is devoted to materials for thermoelectric energy conversion. Based on the thermoelectric figure of merit, the influence of single material properties on device characteristics are investigated and possibilities for performance optimization are discussed. In addition, relevant parameters of the three most important material classes for thermoelectric applications are presented. Besides silicon-germanium alloys, the material systems of lead telluride and bismuth telluride are briefly introduced.

In Chapter 5, the material properties of lead telluride are collected. Thereby, special attention is drawn to the temperature dependence of several physical quantities. One of its ternary alloys, lead tin telluride is used to highlight interesting aspects of the physical behavior. Models of all relevant parameters for device simulation are formulated based on comprehensive measurement data available in literature as well as theoretical considerations.

Finally, Chapter 6 contains case studies of both classical thermoelectric devices and a novel structure incorporating a large scale pn-junction. The influences of several design parameters on device behavior are assessed in elaborate simulation studies. Results for both silicon and lead telluride devices are compared to measurement data, whereby excellent agreement is achieved. Furthermore, the influence of non-ideal thermal environments on device performance are discussed. Chapter 7 gives some closing remarks.

M. Wagner: Simulation of Thermoelectric Devices