The effect of the AlN nucleation layer growth conditions on buffer leakage in unintentionally doped AlGaN/GaN high electron mobility transistors was investigated. The samples were grown by rf-plasma assisted molecular beam epitaxy on 4H–SiC (0001). Drain-source leakage currents were found to be markedly different for samples grown with different Al/N flux ratios during the AlN nucleation layer. Growth of N-rich nucleation layers (Al/N≪1) resulted in a significant reduction in buffer leakage. Secondary ion mass spectroscopy results showed that Si incorporation into Al-rich AlN layers (Al/N≫1) grown on SiC was as high as ∼1–2×1018atoms/cm3. In contrast, Si incorporation into N-rich AlN layers was two orders of magnitude lower, ∼2×1016atoms/cm3. Initial devices grown on low-leakage material realized via N-rich nucleation yielded output power densities at 4GHz of 4.8W/mm with a power added efficiency (PAE) of 62% at a drain bias of 30V, and 8.1W/mm with a PAE of 38% at a drain bias of 50V.