A multi-gate high electron mobility transistor (HEMT) and its methods of formation are disclosed. The multi-gate HEMT includes a substrate and an adhesion layer on top of the substrate. A channel layer is disposed on top of the adhesion layer, and a first gate electrode is disposed on top of the channel layer. The first gate electrode has a first gate dielectric layer in between the first gate electrode and the channel layer. A second gate electrode is embedded within the substrate and beneath the channel layer. The second gate electrode has a second gate dielectric layer completely surrounding the second gate electrode. A pair of source and drain contacts are disposed on opposite sides of the first gate electrode.