A simple optical method of in-process checking of GTO uniformity

For reliable functioning of high-power GTO thyristors it is very important that the parallel-connected segments of the GTO structure have the same characteristics, both static and dynamic. The authors study the homogeneity of large-area GTOs by measuring the current gain distribution over the GTO structure. The local current gains in individual segments of a large area GTO may be evaluated from the reverse current induced by light generated by a GaAs LED over the reverse biased thyristor structure. Measurements on all segments of a GTO structure under the condition of constant voltage between the gate and the anode enables one to evaluate the large area GTO homogeneity with respect to low injection carrier lifetime. The equipment developed enables the current gain distribution to be obtained in a very short time and no cathode contact of any segment could be damaged during checking the GTO homogeneity. Hot spots found within the tested GTO structures were in good accord with the prediction of the described method.