  Mott transitions induced by strong electric fields are receiving a growing
interest. Recent theoretical proposals have focused on the Zener dielectric
breakdown in Mott insulators, however experimental studies are still too scarce
to conclude about the mechanism. Here we report a study of the dielectric
breakdown in the narrow gap Mott insulators GaTa$_4$Se$_{8-x}$Te$_x$. We find
that the I-V characteristics and the magnitude of the threshold electric field
(E$_{th}$) do not correspond to a Zener breakdown, but rather to an avalanche
breakdown. E$_{th}$ increases as a power law of the Mott Hubbard gap (E$_g$),
in surprising agreement with the universal law E$_{th}$ $\propto$E$_g$$^{2.5}$
reported for avalanche breakdown in semiconductors. However, the delay time for
the avalanche that we observe in Mott insulators is over three orders of
magnitude longer than in conventional semiconductors. Our results suggest that
the electric field induces local insulator-to-metal Mott transitions that
create conductive domains which grow to form filamentary paths across the
sample.
