Semiconductor devices and methods for manufacturing the same are provided. An example method may include: forming a sacrificial gate stack on a substrate; forming a gate spacer on sidewalls of the sacrificial gate stack; forming an interlayer dielectric layer on the substrate and planarizing it to expose the sacrificial gate stack; partially etching back the sacrificial gate stack to form an opening; expanding the resultant opening so that the opening is in a shape whose size gradually increases from a side adjacent to the substrate towards an opposite side away from the substrate; and removing a remaining portion of the sacrificial gate stack and forming a gate stack in a space defined by the gate spacer.