  We discuss a new mode of ionization front passage in semiconductor
structures. The front of avalanche ionization propagates into an intrinsic
semiconductor with a constant electric field $E_{\rm m}$ in presence of a small
concentration of free nonequilibrium carriers - so called preionization. We
show that if the profile of these initial carriers decays in the direction of
the front propagation with a characteristic exponent $\lambda$, the front
velocity is determined by $v_f \approx 2 \beta_{\rm m}/\lambda$, where
$\beta_{\rm m} \equiv \beta(E_{\rm m})$ is the corresponding ionization
frequency. By a proper choice of the preionization profile one can achieve
front velocities $v_f$ that exceed the saturated drift velocity $v_s$ by
several orders of magnitude even in moderate electric fields. Our propagation
mechanism differs from the one for well-known TRAPATT fronts. Finally, we
discuss physical reasons for the appearance of preionization profiles with slow
spatial decay.
