We investigated the effect of AlN nucleation layers (NLs) on the structural and electrical properties of N-face GaN grown on C-face 6H-SiC substrates by plasma-assisted molecular beam epitaxy. The GaN films were characterized by secondary ion mass spectroscopy, x-ray diffraction, and transistor electrical measurements. It was found that an AlN NL grown in the N-rich regime was essential for realizing highly resistive GaN buffers. The mosaic structure of the GaN epilayers was systematically correlated with the AlN nucleation conditions. N-face high electron mobility transistors fabricated on these low-leakage buffers demonstrated the highest output power density at 4 GHz to date of 8.1 W/mm with an associated power-added efficiency of 54%.