  Current-voltage characteristics of resonant-tunneling structures often
exhibit intrinsic bistabilities. In the bistable region of the I-V curve one of
the two current states is metastable. The system switches from the metastable
state to the stable one at a random moment in time. The mean switching time
\tau depends exponentially on the bias measured from the boundary of the
bistable region V_{th}. We find full expressions for \tau (including
prefactors) as functions of bias, sample geometry, and in-plane conductivity.
Our results take universal form upon appropriate renormalization of the
threshold voltage V_{th}. We also show that in large samples the switching
initiates inside, at the edge, or at a corner of the sample depending on the
parameters of the system.
