Impact ionization fronts in semiconductors: superfast propagation due to "nonlocalized" preionization
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 $λ$, the front velocity is determined by $v_f \approx 2 β_{\rm m}/λ$, where $β_{\rm m} \equiv β(E_{\rm m})$ 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.