Structures for a field-effect transistor and methods of forming a structure for a field-effect transistor. A first channel region containing a first semiconductor material and a second channel region containing a second semiconductor material are formed over a buried insulating layer of a silicon-on-insulator substrate. A first gate electrode of a first field-effect transistor is formed over the first channel region. A second gate electrode of a second field-effect transistor is formed over the second channel region. The first semiconductor material of the first channel region has a first germanium concentration. The second semiconductor material of the second channel region has a second germanium concentration that is greater than the first germanium concentration in the first semiconductor material of the first channel region.