Structures for field-effect transistors and methods of forming a structure for field-effect transistors. A semiconductor layer includes first and second channel regions, a first field-effect transistor has a first gate dielectric layer over the first channel region, and a second field-effect transistor has a second gate dielectric layer over the second channel region. The first and second channel regions are each composed of an undoped section of an intrinsic semiconductor material, the first gate dielectric layer contains a first atomic concentration of a work function metal, and the second gate dielectric layer contains a second atomic concentration of the work function metal that is greater than the first atomic concentration of the work function metal in the first gate dielectric layer.