Impact ionization fronts in semiconductors: Superfast propagation due to nonlocalized preionization

We present a theoretical study of impact ionization fronts propagating into semiconductor with a constant electric field Em in presence of a small concentration of free nonequilibrium carriers—so-called preionization. We show that if this concentration decays in the direction of the front propagation with a small characteristic exponent λ, the front velocity is determined by vf≈2βm∕λ, where βm≡β(Em) is the corresponding ionization frequency. The propagation velocity vf can exceed the saturated drift velocity vs by several orders of magnitude even in moderate electric fields.