Group iii-n devices with buffer structures and methods of manufacturing the same

A semiconductor device comprising:

• a substrate;
• an aluminum gallium nitride (AlGaN) barrier layer over the substrate;
• a first gallium nitride (GaN) layer underneath the AlGaN barrier layer, the first GaN layer including 2-dimensional electron gas (2DEG);
• a second GaN layer underneath the first GaN layer, the second GaN layer having a carbon concentration in a first range of carbon concentrations, wherein the carbon concentration of the second GaN layer is greater than that of the first GaN layer by at least two orders of magnitude;
• and
• a first superlattice (SL) structure underneath the second GaN layer, the first SL structure having a plurality of pairs of alternating a GaN layer and an aluminum nitride (AlN) layer, wherein the first SL structure has a first aluminum average composition and a first carbon concentration in the first range of carbon concentrations.