The large polarization difference between AlN and GaN provides extremely high electron densities at the heterointerface covered by only 3-4 nm AlN barrier, which makes AlN/GaN heterojunction the ultimate nitride structure for high-frequency applications. This work includes the systematic study of the MBE growth of AlN/GaN HEMTs, theoretical study of 2DEG scattering mechanisms, and device issues of in-situ buffer leakage removal with polarization engineering and decreasing contact resistance with band diagram engineering. This book shows the approach to achieve the record high 2DEG density (5e13/cm2) and the record-low sheet resistance (128 ohm/sq) in high-quality AlN/GaN HEMTs. As a theoretical study, electron scattering mechanisms are reviewed in this book. A novel scattering mechanism, remote surface roughness scattering, is proposed. Large buffer leakage and ohmic contact resistance are two factors that heavily degrade high-speed device performance. Polarization engineering was applied in the buffer leakage study, which increased the ON/OFF ratio by >4 orders. Regrown Si-doped GaN and graded InGaN/InN contacts have been demonstrated with a comprehensive X-ray diffraction study.