Method of manufacturing semiconductor device and semiconductor device

Gate profile tuning techniques are disclosed herein. An exemplary gate profile tuning method includes forming a gate structure over a channel layer. The gate structure includes a dummy gate and gate spacers disposed along sidewalls of the dummy gate. The method further includes partially removing the dummy gate to form a gate opening that defines a gate profile. The gate profile is then modified by treating portions of the gate spacers (for example, by oxygen plasma treatment) and removing the treated portions of the gate spacers (for example, by oxide removal). After removing a remainder of the dummy gate to expose the channel layer, a gate stack of the gate structure is formed in the gate opening. The gate stack has a funnel-shaped profile. In some embodiments, a width of the gate stack above the channel layer is greater than a width of the gate stack below the channel layer.