I found the IXGX400N30A3 at Digikey. The datasheet says the device is rated for 400A @ 25C, 1200A @ 25C for 1ms, with a voltage rating of 300V and PD of 1000W.
Really? This TO-264 package can control 400A of current all day long? I can short out my TIG welder with it in DC mode? How do those leads even carry 400A of current?