A semiconductor device and method for fabricating a semiconductor device is disclosed. An exemplary semiconductor device includes a substrate including a first region and a second region. The semiconductor device further includes a first buffer layer formed over the substrate and between first and second isolation regions in the first region and a second buffer layer formed over the substrate and between first and second isolation regions in the second region. The semiconductor device further includes a first fin structure formed over the first buffer layer and between the first and second isolation regions in the first region and a second fin structure formed over the second buffer layer and between the first and second isolation regions in the second region. The first buffer layer includes a top surface different from a top surface of the second buffer layer.