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Ultrahigh-voltage thyristors with a blocking voltage of 4-10 kV were developed by double diffusion technique. A new surface contouring technique realized in this development gives excellent blocking characteristics and is effective in minimizing the reduction of current-carrying area. To study the feasibility of ultrahigh-voltage thyristors, their electrical characteristics were estimated. According to the experimental results, the blocking voltage is restricted by their dynamic properties, especially turn-off time. The limit for the rated voltage may be about 5 kV at the present stage.