Details of the plasma spreading process in thyristors

In this paper an exact two-dimensional numerical model is used to analyze the plasma spreading process in detail. The exact model offers the advantages that no assumptions have to be made about the nature of the spreading process as well as allowing all aspects of the device operation to be carefully examined. From careful study of the carrier densities, voltage distributions, and lateral current densities it is conclusively shown that the plasma spreading process occurs as a result of the spread in the area of the injecting (active) portion of the N emitter. It is also shown that the start of the plasma spread occurs when the lateral base resistance becomes conductivity modulated by the carrier buildup at the emitter periphery. The factor controlling this as well as the spreading velocity is the excess hole charge available in the P base. Using these concepts an approximate theory is developed which suggests that d/dt ln(X) ∝ J^l/n, where X is the size of the active region. This proposition is tested with good success on the devices under study and on data available from a literature source.