  The direction and impact parameter dependence of electronic stopping power,
along with its velocity threshold behavior, is investigated in a prototypical
small band gap semiconductor. We calculate the electronic stopping power of H
in Ge, a semiconductor with relatively low packing density, using time-evolving
time-dependent density-functional theory. The calculations are carried out in
channeling conditions with different impact parameters and in different crystal
directions, for projectile velocities ranging from 0.05 to 0.6 atomic units.
The satisfactory comparison with available experiments supports the results and
conclusions beyond experimental reach. The calculated electronic stopping power
is found to be different in different crystal directions; however, strong
impact parameter dependence is observed only in one of these directions. The
distinct velocity threshold observed in experiments is well reproduced, and its
non-trivial relation with the band gap follows a perturbation theory argument
surprisingly well. This simple model is also successful in explaining why
different density functionals give the same threshold even with substantially
different band gaps.
