A high electron mobility transistor (HEMT) includes a silicon substrate, an unintentionally doped gallium nitride (UID GaN) layer over the silicon substrate. The HEMT further includes a donor-supply layer over the UID GaN layer, a gate structure, a drain, and a source over the donor-supply layer. The HEMT further includes a dielectric layer having one or more dielectric plug portions in the donor-supply layer and top portions between the gate structure and the drain over the donor-supply layer. A method for making the HEMT is also provided.