The present invention provides a method for fabricating a semiconductor device, comprising at least the steps of: providing a substrate in which a memory region and a peripheral region are defined, the memory region includes a plurality of memory cells, each memory cell includes at least a first transistor and a capacitor, the peripheral region compress a second transistor, a first insulating layer is formed within the memory region and the peripheral region by an atomic layer deposition process, covering the capacitor of the memory cells in the memory region and the second transistor in the peripheral region, and a second insulating layer is formed, overlying the first insulating layer and the peripheral region. Finally, a contact structure is formed within the second insulating layer, and electrically connecting the second transistor.