1. W. Shockley, U.S. Patent 2 569, 347 (1951).
2. H. Kroemer, RCA Rev. 18, 332 (1957).
3. For recent reviews on band-gap engineering see F. Capasso, in Gallium Arsenide Technology, D. K. Ferry, Ed. (Sams, Indianapolis, 1985), chap 8; F. Capasso, in Picosecond Electronics and Optoelectronics, G. Mourou, D. M. Bloom, C. H. Lee, Eds. ( Springer-Verlag, Berlin, 1985 ), p. 112.
4. H. Kroemer, Proc. IEEE 51, 1782 (1963).
5. Zh. I. Alferov etal., Sov. Phys. Semicond. 4, 1573 (1971) [translated from Fiz. Tekh. Pouprovodn. 4, 1826 (1970)]; I. Hayashi, M. B. Panish, P. W. Foy, S. Sumski, Appl. Phys. Lett. 17, 109 (1970).