A method of forming a semiconductor device is provided. The method includes preparing a substrate having a transistor region and an alignment region, forming a first trench and a second trench in the substrate of the transistor region and in the substrate of the alignment region, respectively, forming a drift region in the substrate of the transistor region, forming two third trenches respectively adjacent to two ends of the drift region, and forming an isolation pattern in the first trench, a buried dielectric pattern in the second trench, and dielectric patterns in the two third trenches, respectively. A depth of the first trench is less than a depth of the third trenches, and the depth of the first trench is equal or substantially equal to a depth of the second trench.