AMONG the numerous people without who this work would have been impossible I would like to thank especially:

Prof. Dr. Siegfried Selberherr for giving me the opportunity to join the Institute for Microelectronics at Technische Universität Wien. As my advisor he was always available for discussion, and was also a constant source of encouragement and motivation (even when the deadlines seemed impossible to meet). He showed belief and patience throughout the whole course of work.

Dr. Vassil Palankovski for accepting me to the Advanced Material and Device Analysis Group and the financial support from his START project. I owe him my very special thanks for introducing me to device modeling and simulation software and guiding and advising my work through all the years. He was available seven days a week with great ideas and solutions and was the one from whose vast experience I learned the most.

Dr. Rüdiger Quay, Fraunhofer IAF, for the sustained collaboration, for providing experimental data, and for serving on my examination comitee. I greatly benefitted from his extensive knowledge in the area of GaN-based HEMTs.

Dipl.-Ing. Stephan Maroldt, Fraunhofer IAF, for fruitful discussions and RF measurements.

Prof. Dr. Dionyz Pogany, Dr. Jan Kuzmik, and Dr. Gianmauro Pozzovivo, FKE, TU Wien, for the collaboration on InAlN/GaN structures.

Dr. Johann Cervenka for the programming support, for thorough proofreading, and for the head-to-head competition for the ``most messy desk'' award.

Prof. Dr. Tibor Grasser for his help on the simulation software. His laconic advices were always spot-on.

Prof. Dr. Erasmus Langer for providing me the opportunity to attend several workshops and courses.

Dr. Martin Vasicek for many discussions including but not limited to semiconductor physics.

Above all, I would like to thank my father. I am greatly indebted to him for the continuous support through all the years of my studies. Even when things were looking bad his belief kept me going. For the endless patience this work is dedicated to him.

S. Vitanov: Simulation of High Electron Mobility Transistors