A semiconductor device based on split multi-gate field-effect transistor radio frequency devices is provided. The semiconductor device includes a substrate and a gate structure above the substrate and orthogonal to a channel axis. The semiconductor device also includes a semiconductor fin structure above the substrate along the channel axis. The semiconductor also includes a gate oxide region beneath the gate structure and in contact with the gate structure and the semiconductor fin structure. The gate oxide region has a first region with a first thickness and a first length. The gate oxide region also has a second region with a second thickness and a second length. The first thickness is greater than the second thickness. The first region and the second region are formed side-by-side along the channel axis.