Impact ionization fronts in semiconductors: superfast propagation due to "nonlocalized" preionization

TL;DR

This paper introduces a new mode of superfast ionization front propagation in semiconductors driven by nonlocalized preionization, surpassing traditional velocities and differing from TRAPATT fronts.

Contribution

It presents a novel propagation mechanism where preionization profiles control ionization front speed, enabling velocities much higher than the saturated drift velocity.

Findings

Front velocity can be approximated by v_f ≈ 2β_m/λ.

Proper preionization profiles can produce front velocities exceeding v_s by orders of magnitude.

The mechanism differs fundamentally from TRAPATT fronts.

Abstract

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.