Abstract: High quality GaN film and GaN/AlN/GaN heterostructures have been grown on (0001) sapphire by ammonia-molecular beam epitaxy. The epitaxial layers are all N-surface materials. The two-dimensional electron gases (2DEG) are formed in the GaN/AlN/GaN heterostructures and considered as " inverted ". Hall electron mobility in a GaN epitaxial layer with thickness of 1.2 μm is 300 cm²/Vs at room temperature. Hall electron mobilities in the GaN/AlN/GaN 2DEG heterostructure are 680 cm²/Vs at room temperature and 1750 cm²/Vs at 77K. Due to the lattice-mismatch between the GaN and AlN layers and the AlN layers being too thick, cracks are formed in the GaN/AlN/GaN hetrostructures.