1. N. C. Ford and E. W. Pugh, J. Appl. Phys.,30, no. 4, 270, 1959.
2. G. P. Zhigal'skii and V. V. Potemkin, Vestnik MGU, ser. fiz. [Moscow University Physics Bulletin], no.4, 105, 1966.
3. K. A. Goronina and A. A. Grachev, Izv. VUZ, Radiofizika,2, 581, 1959.
4. F. V. Bunkin, Radiotekhnika i elektronika,4, no. 11, 1913, 1959.
5. G. Marie, Acta Electronica,8, no. 1, 1964.