We propose the development of a very-high-voltage, large area MOS Turn-Off Thyristor (MTO) to replace the Gate Turn-Off Thyristor (GTO) in high power converter systems ranging from 1 to 20 MVA. Because a GTO rated to turn off 2000 A typically requires a turn-off current of 800 A, the most unreliable feature in GTO applications is the gate circuit that provides this high current turn-off pulse. The MTO incorporates the main features of proven GTO technology to achieve the highest available voltage, current, and thermal stability. However, the MTO incorporates a high impedance turn-off gate structure based on an MOS transistor which allows for a much smaller, lower cost, and potentially much more reliable gate circuit. Furthermore, the turn-on gate of the GTO can be preserved in the MTO or, optionally, it can be replaced by a buried gate structure for faster operation. SPCO has already demonstrated a 6kV / 1200 A hybrid version of the MTO. The integrated MTO, which will provide the highest system reliability, requires that an MOS transistor be fabricated directly on high-voltage device wafers. This necessitates further detailed simulation and design efforts for both the device physical properties and for its fabrications processes.
