A method for forming a semiconductor structure includes forming a first FET device and a second FET device over a substrate. Forming a first gate trench in the first FET device and a second gate trench in the second FET device. Forming a first high-k gate dielectric layer in the first gate trench, and a second high-k gate dielectric layer in the second gate trench. Forming a first barrier layer over the first high-k gate dielectric layer, and a second barrier layer over the second high-k gate dielectric layer. Increasing N nitridations in the first and second barrier layers. Removing the second barrier layer to expose the second high-k gate dielectric layer. Forming a first work function metal layer over the first barrier layer and a second work function metal layer over the second high-k gate dielectric layer.