To satisfy the increasing demand for high-power, high-performance power electronics systems, new semiconductor devices have to be developed. In this thesis the concept of MOS-Turn-Off Thyristors (MTOs) will be studied in detail. This device structure enables the active turn-off of the power switch with a greatly reduced complexity of the gate driver unit. The thesis comprises two different parts. In the first part the MTO device structure is studied theoretically. The second part of the thesis deals with the manufacturing processes which are needed to build a disc-type MTO. Finally, test structures are manufactured and the electrical characteristics are presented and discussed.