A field effect transistor includes a substrate and spacers over the substrate. The field effect transistor includes a channel recess cavity between the spacers, wherein a bottom-most surface of the channel recess cavity is parallel to the substrate top surface. The field effect transistor includes a gate stack, wherein the gate stack includes a bottom portion in the channel recess cavity and a top portion outside the channel recess cavity, the gate stack further includes a gate dielectric layer extending from the channel recess cavity along sidewalls of each of the pair of spacers, and the gate dielectric layer directly contacts the substrate below substrate top surface. The field effect transistor includes a strained source/drain (S/D) below the substrate top surface, wherein the strained S/D extends below the gate stack. The field effect transistor further includes a source/drain (S/D) extension substantially conformably surrounding the strained S/D.