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Si-MOSFETs with the breakdown voltage of over 1000 V were demonstrated, for the first time, realizing low on-resistance below the theoretical Si limit. The fabricated MOSFETs have a semi-superjunction (SemiSJ) structure, which is the combination of a superjunction (SJ) structure and an n-bottom assisted layer. The SemiSJ MOSFETs realize both the high breakdown voltage of 1110 and 1400 V and the low on-resistance of 54 and 163 mΩcm2, respectively. The fabrication process for the high-voltage SemiSJ-MOSFET was completely equivalent to a 600-V class SJ-MOSFET process, which implies that a single optimized process for forming SJ structure for 600 V-class MOSFET can be used for a wide voltage range extending up to 1200 V MOSFET. Additionally the fabricated MOSFETs realized low RonQgd of 4.6 ΩnC for a 1110-V device and 13.1 ΩnC for a 1400-V device, and recovery characteristics of the body diode were softer than those for the SJ MOSFET. These results show the possibility of new Si power-MOSFET with a higher application voltage range.