  In this paper, we propose a new electrical breakdown mechanism for exciton
insulators in the BCS limit, which differs fundamentally from the Zener
breakdown mechanism observed in traditional band insulators. Our new mechanism
results from the instability of the many-body ground state for exciton
condensation, caused by the strong competition between the polarization and
condensation energies in the presence of an electric field. We refer to this
mechanism as ``many-body breakdown''. To investigate this new mechanism, we
propose a BCS-type trial wave function under finite electric fields and use it
to study the many-body breakdown numerically. Our results reveal two different
types of electric breakdown behavior. If the system size is larger than a
critical value, the Zener tunneling process is first turned on when an
electrical field is applied, but the excitonic gap remains until the field
strength reaches the critical value of the many-body breakdown, after which the
excitonic gap disappears and the system becomes a highly conductive metallic
state. However, if the system size is much smaller than the critical value, the
intermediate tunneling phase disappears since the many-body breakdown happens
before the onset of Zener tunneling. The sudden disappearance of the local gap
leads to an ``off-on'' feature in the current-voltage ($I-V$) curve, providing
a straightforward way to distinguish excitonic insulators from normal
insulators.
