A method includes forming an active channel region, forming a dummy channel region, forming a first gate dielectric layer over the active channel region, forming a second gate dielectric layer over the dummy channel region, removing the second gate dielectric layer from the dummy channel region, forming a gate isolation region over and contacting the dummy channel region, and forming a first gate stack and a second gate stack. The first gate stack is on the active channel region. The gate isolation region separates the first gate stack from the second gate stack.