Field-enhanced ionization of deep-level centers as a triggering mechanism for superfast impact ionization fronts in Si structures

We investigate the origin of free carriers that initiate impact ionization in depleted high-voltage p-n junctions under dynamic breakdown conditions and deterministically trigger superfast ionization fronts that propagate several times faster than the saturated drift velocity. We argue that in Si structures triggering occurs due to the field-enhanced ionization of process-induced deep-level centers identified as sulfur impurities. This impurity is a double-level electron trap with low recombination activity. It is present in high-voltage Si structures due to the side effect of widely used fabrication technology. We calculate the field and temperature dependences of the ionization probability for the upper midgap level (0.28 eV) and midgap level (0.54 eV) in electric fields up to 5x10(5) V/cm as well as the occupation of these levels at different temperatures. The emission of free electrons is sufficient to trigger the ionization front from zero temperature to similar to 400 K, in agreement with experiments. At room temperature the front is triggered due to the phonon-assisted tunneling from the midgap level with an ionization energy of 0.54 eV. For temperatures below 200 K all double-level centers are in the ground state and the front is triggered due to the direct tunneling from the upper midgap level with an ionization energy of 0.28 eV. (c) 2005 American Institute of Physics.