The present disclosure provides a method for fabricating semiconductor device. The method includes forming a first dielectric layer over a substrate, forming a gate structure over a first portion of the first dielectric layer, forming a sidewall spacer over a second portion of the first dielectric layer and on the gate structure, converting the second portion of the first dielectric layer and an exposed third portion of the first dielectric layer to a first portion of a second dielectric layer and a second portion of the second dielectric layer, respectively, removing the second portion of the second dielectric layer and a portion of the substrate to form a recess in the substrate adjacent the sidewall spacer, forming a source/drain (S/D) feature in the recess and removing the gate electrode and the first portion of the first dielectric layer to form a gate trench.