High electron mobility transistor (HEMT) device and method of making the same

A high electron mobility transistor (HEMT) device with epitaxial layers that include a gallium nitride (GaN) layer co-doped with silicon (Si) and germanium Ge and a method of making the same is disclosed. The HEMT device includes a substrate with epitaxial layers over the substrate. An n-type gallium nitride (GaN) layer is disposed on an interface surface of the epitaxial layers, wherein the n-type GaN layer is co-doped with silicon (Si) and germanium (Ge) that provide a carrier concentration of at least 1×1020 cm−3 and a root mean square (RMS) surface roughness that is no greater than 2 nm for a contact surface of the n-type GaN layer that is interfaced with the interface surface of the epitaxial layers.