Investigation of the lateral turn-on process of thyristors with an epitaxial p-base

In this paper, the effect of the p-base doping concentration NAon the spreading velocity vsin power thyristors is examined. Chemical vapor deposition (CVD) techniques are used to produce the p-base layer, in order to change the p-base doping concentration and thickness independently. The results show a large reduction of vswith growing p-base doping concentration. At a doping concentration higher than 5 × 10¹⁶/cm³the spreading velocity follows a power law with an exponent of -0,9. The introduction of a sandwiched low-doped player between the p⁺-base and the n-emitter slows down the plasma propagation. The decrease of vs, in both cases, is attributed to the reduction of the current gain β2of the n-p-n transistor with doping concentration. In order to explain this behavior, a simple expression for the spreading velocity is derived, which relates the spreading velocity to the time constant of current rise trand consequently to the feedback loop gain (β1β2) of the two transistor components. In this derivation, only the lateral drift current in the p-base is taken into account. It was found that vsis given byv_{s} sim 1/t_{r} sim ln beta_{1}beta_{2}, in good agreement with the experiment.