A semiconductor device includes a semiconductor substrate including a memory cell region and a peripheral circuit region, a first trench formed in the memory cell region and having a first depth and a first opening width, and a second trench formed in the peripheral circuit region and including a pair of bottom edge portions and a bottom middle portion located between the bottom edge portions. The second trench has a second opening width that is larger than the first opening width. Each bottom edge portion has a second depth that is larger than the first depth. The bottom middle portion has a third depth that is same as the first depth.