The present disclosure provides a method for fabricating a semiconductor device that includes forming a gate stack over a silicon substrate, forming dummy spacers on sidewalls of the gate stack, isotropically etching the silicon substrate to form recess regions on either side of the gate stack, forming a semiconductor material in the recess regions, the semiconductor material being different from the silicon substrate, removing the dummy spacers, forming spacer layers having an oxide-nitride-oxide configuration over the gate stack and the semiconductor material, and etching the spacer layers to form gate spacers on the sidewalls of the gate stack.