A study of design influence on anode-shorted GTO thyristor turn-on and turn-off

Anode-shorted GTO thyristor samples were investigated by means of the free-carrier absorption (FCA) technique. Both the turn-on and turn-off processes were investigated as regards the two-dimensional carrier distribution for different stages of the transient cycles. The results are presented as carrier-map sequences, i.e., 3-D pictures of measured 2-D carrier distributions. Samples were formed as unit segments cut out from large-area devices, and associated with different degrees of anode shorting, silicon thickness, and lifetime treatment. During investigation, the samples were inductively anode loaded, and as regards the turn-off process they were operated near the safe-operation limit. The measurements clearly illustrate the way carriers are transported in the sample when firing the device, and the turn-on process is visualized in steps by means of carrier-map sequences. These measurements are supported by computer simulations. The turn-off process is also visualized in carrier-map sequences measured from two perpendicular directions, and the maps show the critical electric-field expansion which always precedes a turn-off failure due to dynamic breakdown mechanisms. Further on, the effect of design-parameter variations, e.g., anode shorting pattern and carrier lifetime reductions, on destructive GTO turn-off phenomena are discussed