A transistor and a method for forming the transistor are provided. The transistor can be formed over a substrate including a first region and second regions on opposite sides of the first region. On the substrate, a first SiGe layer can be formed, followed by forming a first silicon layer on the first SiGe layer and forming a second SiGe layer on the first silicon layer. The second SiGe layer and the first silicon layer within the second regions are removed. The first silicon layer within the first region is removed to form a cavity such that the second SiGe layer is floated. An isolating layer is formed in the cavity. Second silicon layers are formed in the second regions. A gate structure is formed on the second SiGe layer within the first region and the second silicon layers are doped to form a source and a drain.