Semiconductor device with gate stack

A semiconductor device is provided. The semiconductor device includes a gate stack over a semiconductor substrate and a spacer element over a sidewall of the gate stack. The spacer element has a lower portion and an upper portion, the lower portion has a substantially uniform width. The upper portion becomes wider along a direction from a top of the spacer element towards the lower portion, and a bottom of the upper portion is higher than a top of the gate stack. The semiconductor device also includes a dielectric layer surrounding the gate stack and the spacer element. The semiconductor device further includes a conductive contact penetrating through the dielectric layer and electrically connected to a conductive feature over the semiconductor substrate.