A semiconductor device includes a substrate, a plurality of memory cells, a logic gate electrode and a high-voltage gate electrode. The substrate at least includes a memory area, a high-voltage area and a logic area. The memory cells are disposed in the memory area. The logic gate electrode is disposed on the logic area. The high-voltage gate electrode has a first portion and a second portion in contact with each other and stacked on the high-voltage area. The high-voltage gate electrode has a thickness substantially greater than that of the logic gate electrode.