A field effect transistor includes a semiconductor substrate, source and drain regions, lower source and drain contacts, a metal gate, a first interlayer dielectric layer, a capping layer, and an etch stop layer. The source and drain regions are disposed on the semiconductor substrate. The lower source and drain contacts are disposed on the source and drain regions. The metal gate is disposed in between the lower source and drain contacts. The first interlayer dielectric layer encircles the metal gate and the lower source and drain contacts. The capping layer is disposed on the metal gate. The etch stop layer extends on the first interlayer dielectric layer. An etching selectivity for the etch stop layer over the capping layer is greater than 10.