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A metal-oxide-semiconductor field-effect transistor (MOSFET) structure called FITMOS (floating island and thick bottom oxide trench gate MOSFET) has been successfully developed that exhibits outstanding low loss. This unique feature of the FITMOS is realised by fabricating deep trenches in the n-type drift layer, by forming p-type floating islands below the individual trenches using self-aligned ion implantation, and by fabricating trench gates with a thick oxide layer on the bottom. Then, the trade-off between the drain-to-source breakdown voltage and on-resistance has been further optimised by changing the shape of the floating islands from spherical to ellipsoidal. By improving the device structure, a breakdown voltage of 91-V and specific on-resistance of 41.5 mΩ mm2 have been obtained in a 4.5 × 4.5 mm square device with ellipsoidal floating islands. The relationship between the device structure and the reverse recovery characteristics is also investigated.