This dissertation is the outcome of three years work in the semiconductor Physics group at the Cavendish Laboratory, during which time I was financially _supported by the Science and Engineering Research Council. Many thanks are due to my supervisor, Mike Pepper, for his advice and enthusiastic encouragement. I would also like to warmly thank the past and present members of the group who have helped to make the these three years an enjoyable experience. I cannot mention everyone here, but particular thanks are due to Alan Marsh, Colin Dean, Richard Newbury for his efforts to revive the Rutherford fridge, and to Normand Paquin and Donald Pooke for their excellent proof reading of this dissertation. This work would not have been possible without the invaluable technical assistance provided by the members of the Low Temperature Physics workshop, and Dr. s. Read at .the Rutherford Appleton Laboratory. Useful discussions were held during the course of this work with Professors M.Ya. Azbel, K.F. Berggren, and M. Kaveh. This dissertation is the result of my own work and includes nothing which is the outcome of work done in collaboration. It is not the same as any other that I have submitted, or am submitting, for a degree, diploma or any other qualification to any other university.