Anisotropy of conductivity due to warm holes inp-type germanium andp-type silicon

The dependence of the electric conductivity on a d.c. electric field of medium intensity is measured for 3.5 ohm-cmp-type silicon and 0.9 ohm-cmp-type germanium. From these data and the symmetry properties of the cubic crystal lattice the dependence of the conductivity on crystal orientation is determined. Also, an analytical treatment is made for silicon by assuming a model of constant energy surfaces near momentum?k=0 which consists of spheroids of rotational symmetry penetrating each other. The usually accepted model of warped surfaces does not seem to be tractable. The ratio of effective masses parallel and perpendicular to the axis of rotation enters as a parameter which is determined by the experimental value of the conductivity anisotropy. This mass ratio for silicon at 275 °K is 3.25, at 193 °K 4.64 and at 77 °K 37. With germanium the observed anisotropy is stronger than the anisotropy calculated with any finite mass ratio which means that the model is not adequate.