I would like to express my gratitude to Prof. Siegfried Selberherr for the opportunity to study for a doctorate under his supervision, for his guidance, and all support throughout my work. I am grateful for his friendship and willingness to always make my life in Vienna easier.
I would like to thank Dr. Hajdin Ceric for his constant support, instructive suggestions, and interesting discussions during this work, and also, for always helping me with personal matters during this period, mainly when I first arrived in Austria.
I acknowledge all colleagues of the Institute for Microelectronics for the cooperation. In particular Thomas, Otmar, and Karli for the very nice discussions, help with the ``get used to Austrian life'', and German lessons.
I thank the staff of the Institute for Microelectronics for keeping a perfect environment of work. In particular Cerv, who has been always ready to fix all problems with my computer.
I am grateful to Prof. Jacobus W. Swart, who has been responsible for my first steps regarding scientific work.
I would like to thank my family for the continuous support during all the years of my study and for all efforts which allowed me to be where I am.
Finally, a special thanks to Anita, my beloved wife and best friend, for being always by my side. Without her support and encouragement everything would have been much harder.