  We report the observation of the steering of 855 MeV electrons by bent
silicon and germanium crystals at the MAinzer MIkrotron. 15 $\mu $m long
crystals, bent along (111) planes, were exploited to investigate orientational
coherent effects. By using a piezo-actuated mechanical holder, which allowed to
remotely change the crystal curvature, it was possible to study the steering
capability of planar channeling and volume reflection vs. the curvature radius
and the atomic number, Z. For silicon, the channeling efficiency exceeds 35 %,
a record for negatively charged particles. This was possible due to the
realization of a crystal with a thickness of the order of the dechanneling
length. On the other hand, for germanium the efficiency is slightly below 10 %
due to the stronger contribution of multiple scattering for a higher-Z
material. Nevertheless this is the first evidence of negative beam steering by
planar channeling in a Ge crystal. Having determined for the first time the
dechanneling length, one may design a Ge crystal based on such knowledge
providing nearly the same channeling efficiency of silicon. The presented
results are relevant for crystal-based beam manipulation as well as for the
generation of e.m. radiation in bent and periodically bent crystals.
