A semiconductor device includes a source region disposed in a substrate and having a first conductivity type, a drain region disposed in the substrate and having the first conductivity type, a first drift region having the first conductivity type and extending in a channel length direction between the source and drain regions, a second drift region having a second conductivity type and extending parallel to the first drift region, a field plate region disposed in an upper portion of the second drift region, an auxiliary electrode disposed in an upper portion of the field plate region, and a gate electrode disposed on the substrate and electrically connected with the auxiliary electrode. Such devices can reduce the specific on-resistance while also reducing electric field concentrations at the edge portions of the gate electrode, and the breakdown voltage of the device can therefore be significantly improved.