1. E. Kooi, Philips Res. Repts 20, 306–314, 1965 (chapter 5 of this book).
2. H. L. Hughes and R. G. Giroux, Electronics 37, 58–60, 1964.
3. H. Edagawa, Y. Morita, H. Ishikawa, S. Maekawa and Y. Inuishi, Jap. J. appl. Phys. 3, 644–660, 1964.
4. H. Edagawa, Y. Morita and Y. Inuishi, J. phys. Soc. Japan 18, 314–315, 1963.
5. M. V. Whelan, Philips Res. Repts 20, 562–577, 1965.