First of all I would like to express my thanks to Prof. Siegfried Selberherr, who provided me with a perfect working environment. His guidance and advise were crucial for this work. I would also like to point out his financial support to attend several important international conferences all over the world.
I owe gratitude to number of my colleagues at the Institute for Microelectronics for the interesting discussions and their support and cooperation.
Finally I wish to thank my family for their continuous support during all the years of my study.