Measurements of the velocity of plasma spread in switched thyristors

The method employed in the measurement of plasma spreading velocity ν_s, in power thyristors and the results obtained are reported in detail. To induce ν_s to be primarily one-dimensional the thyristors wore rectangular in configuration with gate contacts to measure the arrival of the plasma, being placed at regular intervals around the rectangular (alloyed) cathode. Gold-doped devices were compared with geometrically similar thyristors nominally gold free. Values of ν_s for gold-doped thyristors were persistently, though not always, lower. The current gain of the constituent transistors has also been measured and attempts are made to correlate these to the measured ν_s values. The currant, gains, particularly of the n-p-n section, prevailing in the ‘ off ’ region of a thyristor during a switch-on transient are contrasted with those obtaining during current gain measurements and are thought to be responsible, in conjunction with the lowered life-time in the wide n-basr regoin of the p-n-p section, for the lowered ν_s values of gold-doped thyriators.