Impact ionization fronts in semiconductors: Numerical evidence of superfast propagation due to nonlocalized preionization

We present numerical evidence of superfast propagation of ionizing fronts that occurs due to nonlocalized preionization of the depleted high-field region. In nonlinear dynamics terms, this traveling front mode of avalanche breakdown in a semiconductor corresponds to a pulled front propagating into an unstable state in the regime of nonlocalized initial conditions. Our simulations reveal excitation and propagation of such fronts in a Si p+-n-n+ structure. The front is triggered by applying a sharp voltage ramp to a reversely biased structure. Before the front starts to travel, field-ehanced emission of electrons from deep-level impurities preionizes initially depleted n base creating spatially nonuniform profile of free carriers. Impact ionization takes place in the whole high-field region the front propagates to. We find two ionizing fronts that propagate in opposite directions with velocities up to ten times higher than the saturated drift velocity.