D. Acknowledgement

This dissertation was carried out at the institute ISG1 of Forschungzentrum Jülich. The success of this work would not have been possible without an excellent guidance, support and cooperation. Among others, I would like to acknowledge and say thanks to the following people :

Prof. Dr. H. Lüth for the great opportunity to perform my PhD work in Germany. His great enthusiasm in scientific research combined with the friendly atmosphere made the last three years a wonderful experience.

Prof. Dr. R. Waser for kindly accepting to referee this work.

Prof. Dr. A. Förster for being my research advisor during the last three years. He has constantly amazed me with his ability to simplify complicated research problems. I would like to express my deepest gratitude to him for showing to an engineer how interesting solid state physics could be.

Dr. M. Lepsa for sharing with me his extraordinary experience in semiconductor processing and physics. Despite my repeated failures, his enthusiasm and strong support motivated me to finally succeed in this project. Also, many thanks for the fruitful discussions and for the critical review of this work.

Dr. M. Indlekofer for the helpful suggestions, the many explanations, the excellent simulation program Wingreen, and the review of this work.

Dr. J. Stock for the introduction to the clean room activities and for the helpful advices with plasma etching. The friendly atmosphere in the office during the first two years developed, later, in an excellent external collaboration.

Mr. J. Wensorra for many interesting not only scientific discussions.

Dr. R. Calarco, Dipl. Ing. M Bertelli, Dipl. Ing. C. Varini, Dipl. Phys. E. Dona and Prof. Dr. A. Rizzi for helping me not to forget the Italian language during the three long years in Germany. The topic did not matter: politics, sport, military service or cooking, there was always a reason for a funny remark.

Mr.C. Krause for the outstanding MBE growths and for the numerous metal depositions.

Dr. N. Kaluza, Dr. H. Hardtdegen and Dipl. Ing. R. Steins for the excellent epitaxial growth of GaN for nanocolumns and Gunn diodes.

Dr. R. Calarco, Dipl. Phys. R. Meijers, Mr. T. Richter and Dipl. Phys. N. Thillosen for sharing with me the nanocolumns topic. I am also grateful to R. Meijers for helping me with the AFM measurements.

Dr. D. Freundt and his group from Bosch GmbH for the intensive cooperation regarding the ACC project. The collaboration gave me the chance to familiarize with the working attitude of a big international group.

Mr. F. Mohammed for his diligent work about short pulse measurements and for his very nice personality.

Dipl. Ing. A. Fox, Dr. Ing. M. Marso and Prof. Dr. Ing. Kordoš for the interesting discussions in the field of high frequency devices and the disposal of the S-parameter characterization setup.

Mrs. I. Schumacher, Miss M. Gruber and Miss M. Tödt for the help and support in solving all kind of administrative problems.

Dipl. Ing. A. Steffen and J. Müller, as well as all co-workers of the clean-room for their help with processing of the samples. Dipl. Ing. F. Ringelmann for the introduction to plasma etching, the professional interest in my work and for the RIE maintenance. Miss. A. Pracht, Mr. J. Zillikens and Mr. H. Wingens for the numerous metal depositions. Miss. S. Bunte for precise cutting of the GaN samples.

Mrs. M. Nonn, an example of efficiency and accuracy, for the excellent lithography masks and Dr. A. van der Hart for the electron beam lithography with proximity-correction.

Dipl. Ing. H.P. Bochem and Mrs. E. Brauweiler-Reuters for the several and high quality SEM images.

Dr. S. Vitusevych, Dr. S. Danylyuk and Dr. A. Kluschin for the disposal of the high frequency mixers and spectrum analyzers.

Dipl. Ing. J. Bernát and Dr. Ing. P. Javorka for the helpful tips in the GaN device processing.

PD Dr. T. Schäpers, Dr. M. Mikulics, Mr. K. Wambach, Mr. K. Nicoll, Dr. J. Malindratos, Dr. V. Guzenko, Dr. M. Kocan, Dr. R. Adam, Dr. P. Vagner, Dr. M. Wolters, Dr. J. Moers, Dr I. Batov , Prof. Dr. P. Kordoš, Dr. L. Vescan, Dr. S. Trellenkamp, Dr. M. von der Ahe, Dr. A. Tönnesmann, Dr. T. Stoica, Dr. M. Goryll, Dipl. Ing. G. Heidelberg and Dr. G. Crecelius for attributing to this work in various ways.

Dipl. Geol. M. Kloppisch for her permanent moral support and the constant help in learning the German language. I am grateful for the review of my numerous German texts.

The administrative and technical coworker as well as all other not namely mentioned people for their versatile support and kindness.