The effects of emitter-shorts on plasma propagation in thyristors

Abstract

In this paper, the effects of the emitter-shorts on the propagation of the turned-on area in rectangular bar-shaped thyristors are examined in detail. It was found that the plasma propagation can be described completely by the three physical quantities: 1. the critical anode current density in the turned-on area at the front of the short, 2. the plasma propagation stoptime at the short, and 3. the spreading velocity in the homogeneous thyristor. Different biasing mechanisms are proposed to determine the critical current density as a function of the emitter short length in the direction of propagation and the short-short distance. The biasing mechanism of the cathode junction is only effective for relatively small emitter-short length while the pnp-transistor biasing is dominant for relatively large emitter-short length. A comparison between the theoretical and experimental results reveals that the pnp-transistor biasing is completely sufficient for the determination of the critical current density for all practical short geometrics. One of the main results is that the critical current density rises exponentially with the emitter-short length. It was found also that the short-short distance must be made greater than twice the high injection diffusion length otherwise an excessively large critical current density will appear. This can be considered as a design rule for the emitter shorts in thyristors. Physically, the plasma stoptime is the time needed to build up the critical charge in the four-layer structure at the firing point behind the emitter short by the current supplied from the turned-on area preceding the emitter short. It can be calculated as a function of the current density in the turned-on area using the two transistor model of the thyristor at the firing point behind the emitter short. There is good agreement between the theoretical and the measured values of the plasma stoptime. A general expression is derived for the plasma spreading velocity in thyristors without emitter shorts. This allows the determination of the average spreading velocity for the plasma propagation in shorted-emitter thyristors. Again, good agreement between the measured average spreading velocity and the calculated value at the same current density could be achieved.