1. J. B. Gunn, 1964, I.B.M.Journ.Res.Dev., 8, 141–9.
2. P. N. Butcher, 1967, Rep.Prog.Phys. 30, Part I, 97–148.
3. I. B. Bott and W. Fawcett, 1968, Advances in Microwaves 3, 223–300.
4. C. Hilsum, 1968, Brit.J.Appl.Phys. (J.Phys.D) 1, 265–81.
5. E. G. S. Paige, 1964, Prog.in Semiconductors 8, 1–244 (London: Heywood, Eds. A. F. Gibson and R. E. Burgess).