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Two-dimensional model of a thyristor turn-on channel

The goal of this contribution is to give some insight into the carrier and the current distribution at turn-on of a high-voltage thyristor in both axial and lateral dimensions. By means of a quasi-static two-dimensional theoretical model, the influence of the mobile charge on the potential distribution at the verge of turn-on is calculated. The result shows a bulging out of the space-charge layer (SCL) into the neutral part of the n-base region near the anode, as well as a divergence of the current lines, in the shape of a trumpet. An additional sharp increase of the current-carrying area occurs immediately outside the SCL. While the bulging out of the SCL is more pronounced when the applied voltage is higher, the size of the current-carrying area near the anode emitter junction decreases. These theoretical results have been verified by means of infrared (IR) recombination radiation experiments using sections of reverse blocking as well as reverse nonblocking thyristors. In addition, it can be shown that the lateral plasma distribution, being induced electrostatically, does not undergo any change due to breakdown of the applied voltage.