A semiconductor device formed on a silicon-on-insulator substrate includes a gate electrode, a gate insulation film, a drain diffusion region, a drift region, a body region, a plurality of source diffusion regions, and a plurality of charge collection diffusion regions. The source diffusion regions and charge collection diffusion regions are of mutually opposite conductivity types, and alternate with one another in the direction paralleling the width of the gate electrode. The half-width of each source diffusion region is equal to or less than the length of the gate electrode plus the half-length of the drift region.