1. F. I. Vergunas and K. Sh. Enikeeva, FTT,6, 2100, 1964.
2. L. Eisenmann, Ann. Phys.,10, 129, 1952.
3. R. Bube, Photoconductivity of Solids [Russian translation] IIL, 1962.
4. E. M. Trykhan, FTT,4, 3496, 1962.
5. B. A. Sillars. J. Elec. Engr. (London),80, 378, 1937.