Invented in the late 1950s, the power thyristor is a semiconductor device which has shown a remarkable ability to accept the challenge of technological advancement. Since it conception as a switching device with the then high power ratings of a few hundred watts, modern thyristors have been developed capable of megawatts of switching power. Furthermore, novel design concepts and improvements in semiconductor silicon processing techniques have led to special thyristor designs such as the gate turn-OFF thyristors, light triggered thyristors and more recently the field controlled thyristors or static induction thyristor. The design of both the basic thyristor and these several special types of thyristor is the subject of this book, which covers design from the selection of the starting silicon through to the design of the device encapsulation.