A method of manufacturing a semiconductor device including forming a first conductive-type buried layer in a substrate; forming a first conductive-type drift area on the first conductive-type buried layer; forming a gate insulating layer and gate electrodes by selectively removing the first conductive-type drift area; forming a first oxide layer on the substrate and gate electrodes; implanting second conductive-type impurity ions into the substrate; forming a nitride layer on the first oxide layer; forming a second conductive-type well by diffusing the second conductive-type impurity ions while forming a second oxide layer; removing the nitride layer, the second oxide layer, and portions of the first oxide layer; forming first conductive-type source areas at sides of the gate electrode(s); forming a dielectric layer on the oxide layer; forming a trench in the dielectric layer and the oxide layer; forming a source contact in the trench; and forming a drain.