A gate structure includes a gate dielectric over a substrate, and a gate electrode over the gate dielectric, wherein the gate dielectric contacts sidewalls of the gate electrode. The gate structure further includes a nitrogen-containing dielectric layer surrounding the gate electrode, and a contact etch stop layer (CESL) surrounding the nitrogen-containing dielectric layer. The gate structure further includes an interlayer dielectric layer surrounding the CESL and a lightly doped region in the substrate, the lightly doped region extends beyond an interface of the sidewalls of the gate electrode and the gate dielectric.