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

A method of making a high electron mobility transistor (HEMT) device comprising:

• providing a substrate;
• disposing epitaxial layers onto the substrate;
• masking the epitaxial layers with an etch mask;
• etching a source contact recess and a drain contact recess to expose an interface surface on the epitaxial layers, wherein the source contact recess and the drain contact recess are spaced apart;
• disposing an n-type GaN layer within the source contact recess and the drain contact recess;
• and
• co-doping the n-type GaN layer with both silicon (Si) and germanium (Ge) to form a source contact within the source contact recess and form a drain contact within the drain contact recess such that the source contact and the drain contact each have 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.