1. W. Schockley:Electrons and holes in semiconductors (New York, 1950), pp. 29 ff.
2. W. Schockley andG. L. Pearson:Phys. Rev.,74, 232 (1948).
3. E. Low:Proc. Phys. Soc., B68, 10 (1954).
4. G. Bonfiglioli, E. Coen andR. Malvano: in press in thePhys. Rev.
5. G. Configlioli, E. Coen andR. Malvano: in press in theJ.A.P.