Turn-off failure mechanisms in large (2.2 kV, 20 A) MCT devices

In this paper we present the phenomenological mechanisms leading to turn-off failures for large size MCT devices under inductive loading. Depending on the device current and the clamping voltage two different mechanisms lead to turn-off failures. At lower clamping voltages or in soft switching conditions the SOA is limited by the MOSFET and cathode design rules. At higher voltages and in hard switched conditions the dynamic formation of filaments limits the SOA. The failure phenomena were found to be independent, of cathode layout details, anode structuring, device size, and load circuit, thus representing the fundamental limits of the MCT. However, for optimal selection of these parameters, the failure level can be pushed to high power permitting industrial application of the device. The experimental investigation was carried out with MCT samples of different sizes and layout features and by analysis of destructed devices. The device simulator SIMUL_/sub ISE/ was used to identify the failure mechanisms.