CALCULATION OF TURN-ON CHARACTERISTICS OF p-n-p-n STRUCTURES WITH AN ALLOWANCE FOR BASE SPREADING RESISTANCE.

A study is made of the initial part (the collector junction is under reverse bias) of the transient characteristic governing the turn-on of a p-n-p-n structure. A simple two-dimensional model is used and it is assumed that the injection level is low and the injection efficiencies of the emitter p-n junctions are independent of the current. An approximate method of solution of the system of partial differential equations for the minority-carrier density in the bases is proposed which reduces this system to a boundary-value problem for a system of ordinary differential equations with time-dependent coefficients. The solution of this system of ordinary differential equations was obtained on a computer. Explicit formulas for the current density expressed in terms of the minority-carrier density are derived. An analysis of numerical results indicates that, during the turn-on process, the inhomogeneity in the current density in the bases of the device is governed by the base resistance and by the magnitude of the gate current i//g. For large i//g, the rate of increase of the current density depends on the longitudinal coordinate in the bases and, when the current i//g is increased, this rate at boundary and decreases near the center of the device. This leads to a nonuniformity in the turn-on of p-n-p-n n structures.