1. Introduction

SOME refer to the age we are living in, as the Information Age. It is the age of global information exchange and instant access to knowledge, the age of computers, mobile telephones, email, and on-line social networking services. The technological revolution has provided us with enough computational power to satisfy one's every day needs at acceptable costs. Even handheld devices are able to handle computationally intensive tasks, such as complex encoding algorithms, for instance.

However, one should evaluate technological evolution not only by the ability to process information, but also by the ability to transfer it. The progress in the latter has been also nothing short of impressive: while the first transatlantic telephone cable was able to carry only 36 telephone connections some 55 years ago, a single mobile telephone can achieve the hundred-fold bandwidth nowadays. Both the cable-bound and wireless telecommunications have experienced a rapid development. Due to the nature of the transport medium there are different hurdles which apply to the two techniques. For cable-bound communications one of the major obstacles is the lack of broadband cable networks (optical cables still are not an established replacement for classic telephone lines in the last-mile), while the technology has been available for some years already.

Wireless communications on the other hand face still some technology challenges, despite the steady development in the last decade. One of the key technologies, that made this evolution possible was the Gallium Nitride (GaN) based High Electron Mobility Transistor (HEMT). It offers a high current density at radio frequencies that, combined with its high breakdown voltage, makes it an excellent choice for high-power amplifiers. While this was the application, which granted the HEMT technology entry into the mass market, other emerging areas include automotive power inverters for hybrid vehicles, for instance. An overview of the different present and future application areas is included in Chapter 2. It is followed by a review of the current state-of-the-art and an overview of device simulation tools able to deal with the specifics of HEMTs.

A profound knowledge of the material system is a key requirement for proper device modeling. Chapter 3 summarizes some of the properties of the III-V materials used in HEMT structures, with focus on the carrier transport. It features an extensive study of available experimental and simulation data compared against own Monte Carlo simulations.

Chapter 4 introduces the basics of device simulation. Relevant models and model parameters are discussed, with focus on the carrier transport models. Subsequently, different simulation studies are presented in Chapter 5. These include optimization for various applications, high-temperature performance analysis, and transconductance collapse investigation of normally-on devices. Further, several techniques for normally-off devices are studied and compared.

The last chapter provides a short summary and points out several possible directions for future work.