Techniques are disclosed for gallium nitride (GaN) oxide isolation and formation of GaN transistor structures on a substrate. In some cases, the GaN transistor structures can be used for system-on-chip integration of high-voltage GaN front-end radio frequency (RF) switches on a bulk silicon substrate. The techniques can include, for example, forming multiple fins in a substrate, depositing the GaN layer on the fins, oxidizing at least a portion of each fin in a gap below the GaN layer, and forming one or more transistors on and/or from the GaN layer. In some cases, the GaN layer is a plurality of GaN islands, each island corresponding to a given fin. The techniques can be used to form various non-planar isolated GaN transistor architectures having a relatively small form factor, low on-state resistance, and low off-state leakage, in some cases.