A semiconductor device includes a substrate, a core device, and an input/output (I/O) device. The core device is disposed on the substrate. The core device includes a first gate electrode having a bottom surface and at least one sidewall. The bottom surface of the first gate electrode and the sidewall of the first gate electrode intersect to form a first interior angle. The I/O device is disposed on the substrate. The I/O device includes a second gate electrode having a bottom surface and at least one sidewall. The bottom surface of the second gate electrode and the sidewall of the second gate electrode intersect to form a second interior angle greater than the first interior angle of the first gate electrode.