1. University of Oslo, Physics Department, Physical Electronics, P.O. Box 1048 Blindern, 0316 Oslo, Norway
2. SINTEF, Electronics and Cybernetics, P.O. Box 124 Blindern, 0314 Oslo, Norway
3. Royal Institute of Technology, Solid State Electronics, P.O. Box E229, 164 40 Kista‐Stockholm, Sweden