Modeling of fast ionization waves in silicon p-n junctions under breakdown

This paper presents the results of modeling of the delayed breakdown and the generation of a fast ionization wave in silicon p(+)-n-n(+) structures with concentrations of 10(18) cm(-3) in the p layer and 1.25 x 10(14) cm(-3) in the n layer. The thickness of the n layer varied from 100 to 350 mu m, the area of the structure varied from 0.005 to 0.022 cm(2), and the voltage-growth rates were 2 and 4 kV/ns. The modeling shows that, when the wave reaches the neutral region, the current-growth rate decreases, and the current is limited for some time t(d) by the low conductivity of the neutral region and remains almost constant. Rapid ionization then occurs uniformly over the entire neutral region, and the current-growth rate again increases sharply. The experiments never show such two-step dependence. The indicated discrepancy is associated with spatial inhomogeneity of the process (current filamentation), and the following model of the inhomogeneity is proposed: the whole area of the structure breaks up into two parts - one ballast part, through which no conductivity current flows, while uniform ionization occurs in the second part. As expected, the constant-current ''shelf'' becomes shorter for a certain ratio of the switched-on area to the total area, equal to K = 0.1-0.25, and it almost disappears when K < 0.1. (C) 1996 American Institute of Physics.