Embodiments of the present disclosure describe a semiconductor multi-gate transistor having a semi-conductor fin extending from a substrate and including a sub-fin region and an active region. The sub-fin region may include a dielectric material region under the gate to provide improved isolation. The dielectric material region may be formed during a replacement gate process by replacing a portion of a sub-fin region under the gate with the dielectric material region, followed by fabrication of a replacement gate structure. The sub-fin region may be comprised of group III-V semiconductor materials in various combinations and concentrations. The active region may be comprised of a different group III-V semiconductor material. The dielectric material region may be comprised of amorphous silicon. Other embodiments may be described and/or claimed.