An integrated circuit structure includes a substrate and a first and a second plurality of III-V semiconductor layers. The first plurality of III-V semiconductor layers includes a first bottom barrier over the substrate; a first channel layer over the first bottom barrier; and a first top barrier over the first channel layer. A first field-effect transistor (FET) includes a first channel region, which includes a portion of the first channel layer. The second plurality of III-V semiconductor layers is over the first plurality of III-V semiconductor layers and includes a second bottom barrier; a second channel layer over the second bottom barrier; and a second top barrier over the second channel layer. A second FET includes a second channel region, which includes a portion of the second channel layer.