Multivalued distributions of hot electrons between equivalent valleys
The MED among equivalent valleys treated in the present chapter was at the origin of a triple-valued dependence of the Sasaki field, theoretically predicted in 1962 by Reik and Risken [5.231 in n-Ge when the current density was oriented near a 110 direction for the limiting case of negligible intervalley scattering. However, these authors paid no attention to this effect, since it vanished for an appropriate choice of the scattering rate which provided better agreement between experimental and numerical results.
The problem of the possible existence of multivalued transverse fields was theoretically considered in 1970 by Gribnikov et al. [5.3], and the first experimental indication with respect to this subject was reported in 1972 by Astrov and Kastalskii [5.29]. In the mean time, most of the interest was devoted to the similar problem of the transverse negative resistance [5.24-28]. These authors, recognizing the necessity of a strong intervalley repopulation rate, required in [5.3], performed measurements in Ge at low temperatures. By choosing the heating field along a 〈100〉 direction, they investigated the electric response in the <I 〈10〉 direction for both the closed and open circuit conditions. In a certain region of applied fields, they observed the appearance of transverse potential differences, which they correlated with MED on account of fluctuations. The experimental data, however, do not allow one to draw such conclusions unambiguously [5.15].
Early attempts to measure transverse instabilities in n-Si were made in 1972 by Gram et al. [5.30]. However, their efforts were unsuccessful because the lattice temperature they chose was not low enough to satisfy the required steep increase of the intervalley repopulation rate with heating field strength.
As reported in this chapter, the existence of MED has been definitely confirmed in recent years. Therefore, in perspective its consequences with respect to other phenomena should open fruitful areas of investigation. These pertain to the optical properties with the interaction of light with free carriers as well as microwave transduction phenomena probably accompanied by resonances.
KeywordsNontrivial Solution Transverse Field Transverse Electric Field Negative Differential Conductivity Carrier Heating
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