AbstractWe report on the growth of Al0.25Ga0.75N/GaN heterostructures on low dislocation density semi‐insulating c‐axis GaN substrates by metalorganic vapor phase epitaxy (MOVPE). A room temperature (RT) Hall mobility (µRT) up to 2065 cm2 V−1 s−1 at sheet density (ns) of 8.25 × 1012 cm−2 has been measured. This work compliments prior studies in which we observed a buffer‐induced modulation of the RT two‐dimensional electron gas (2DEG) ns and µRT by varying the GaN buffer layer thickness. Here, we focus on the optimization of the heterostructure 2DEG properties by elimination of silicon doping in the Al0.25Ga0.75N barrier and unintentional Al in the not‐intentionally doped (n.i.d.) GaN buffer layer. The 15% improvement in µRT and ns relative to previous results is consistent with those predicted by Poisson solver calculations. Use of thick GaN buffers has minimized the theoretical mobility reduction based on intersubband scattering and has enabled us to determine the 2DEG sheet density associated with the polarization field ($n_{{\rm s}}^{{\rm polar}} $) to be ∼5 × 1012 cm−2.