Formation of the space charge region in diffusion p-n junctions under high-density current interruption

Abstract

The recovery of diodes with diffusion p-n junctions in the case of high reverse current density j is analyzed. A condition for quasi-neutrality breaking in the diffusion layers with allowance for the dependence of charge carrier mobility μ on electric field strength E is obtained that is valid for a wide range of j. The problem of formation of the space charge region in a circuit with inductance L and resistance R is reduced to a system of two ordinary differential equations. Approximation of a numerical solution to this system makes it possible to derive crude analytical relationships between interrupted current density {ie88-1}, circuit parameters, diode parameters, and parameters of a forming voltage pulse (with amplitude V _m and pulse rise time t _p). The limiting parameters of a pulser with an inductive energy storage and current interrupter based on diffusion diodes are studied. The critical density of interrupted current {ie88-2} is determined at which the field in the space charge region near the anode reaches breakdown value E _b and intense impact ionization by holes begins. The impact ionization decreases the rates of current decay and voltage increase in the space charge region. As a result, at {ie88-3}, t _p starts increasing and the overvoltage factor of the pulser decreases. The value of V _m corresponding to {ie88-4} is roughly given by {ie88-5}, where m is the number of diodes in the interrupter, ɛ is the permittivity of the semiconductor, {ie88-6} is the saturated drift velocity of holes, and l _p is the depth of the p-n junction (diffusion depth). Theoretical predictions are confirmed by exact numerical simulation of the recovery process and qualitatively agree with the available experimental data.