Low Al composition (<40 %) AlGaN high-electron mobility transistors (HEMTs) have been an excellent technology platform for both high power and high frequency applications. Deep sub-micrometer HEMTs with decreasing barrier thickness are required to further boost the device performance. For AlN barrier, low contact resistance comparable to AlGaN barrier is hampered by the wide band gap of AlN (6.2 eV). A high Al composition AlGaN barrier layer can provide a high 2-D electron-gas (2DEG) density, and allow AlGaN thickness scaling. However, few groups reported on the growth of high Al composition AlGaN/GaN HEMT structures and the room temperature (RT) electron mobility has been limited to 500 cm<SUP>2</SUP>/Vs with Al composition higher than 70%. Molecular beam epitaxy (MBE) grown high Al composition AlGaN has been observed to suffer spontaneous phase modulation. We report the study of 2DEG structures with high Al composition (>70%) AlGaN grown by plasma-assisted MBE. In metal-rich regime, with the Ga flux of (1.1 - 1.3) × 10<SUP>-7</SUP> torr, Al flux of 4.3 × 10<SUP>-8</SUP> torr, RF power of 275 W and thermocouple temperature ranging from 580 to 680°C, the epitaxial growth of relatively thick (>3 nm) high Al composition AlGaN directly on top of GaN always results in hexagonal stripes. The origin of the stripes is still under investigation, and we suspect it might be related to lateral phase separation. However, by inserting an ultrathin (<1 nm) AlN spacer, the surface morphology was dramatically modified and consisted of atomic steps. The growth rate and Al composition of the AlGaN layer were calibrated by high-resolution X-ray diffraction (XRD) measurement of a nine-period 2.7 nm/0.6 nm/26.7 nm Al<SUB>0.72</SUB>Ga<SUB>0.28</SUB>N /AlN/GaN multiple quantum well calibration structure. The AlN spacer thickness was kept around 0.6 nm. The polarization-induced 2DEG density increases with total barrier thickness for samples with AlN spacer and 72% AlGaN. For HEMTs heterostructures with 3.9 nm/0.6 nm Al<SUB>0.72</SUB>Ga<SUB>0.28</SUB>N /AlN barrier, at room temperature the 2DEG mobility reaches 1140 cm<SUP>2</SUP>/Vs at a sheet density of 2.3 × 10<SUP>13</SUP> cm<SUP>-2</SUP>, which yields low sheet resistance of 238 Ω/□. For the samples without AlN spacer the electron mobility is typically about 120 cm<SUP>2</SUP>/Vs at room temperature and there was no clear increase at 77 K. In conclusion, we have succeeded in demonstrating high Al composition alloy AlGaN/GaN HEMT structures with high carrier mobility. An ultrathin AlN interlayer was crucial to suppress phase separation and yield high mobilities, paving the way for deep submicron HEMTs.