  We show that oblique propagation of electrons in crystals of Ge and Si, where
the electron velocity does not follow the electric field even on average, can
be explained using standard anisotropic theory for indirect gap semiconductors.
These effects are pronounced at temperatures below ~1K and for electric fields
below ~5V/cm because inter-valley transitions are energetically suppressed
forcing electrons to remain in the same band valley throughout their motion and
the valleys to separate in position space. To model, we start with an isotropic
approximation which incorporates the average properties of the crystals with
one phonon mode, and include the ellipsoidal electron valleys by transforming
into a momentum space where constant energy surfaces are spheres. We include
comparisons of simulated versus measured drift velocities for holes and
electrons, and explain the large discrepancy between electrons and holes for
shared events in adjacent electrodes.
