AlN/GaN high electron mobility transistor (HEMT) structures with a 3.5 nm AlN barrier were grown with a rf plasma-assisted molecular beam epitaxy (MBE) system on sapphire and SiC substrates, resulting in high electron mobility of >1800 cm²/V s and 2DEG carrier density of 2.8×10¹³ cm^-2. A near record-low sheet resistance of ~167 ohm/sq was achieved using Ti/Al based Ohmic metallization annealed at 850 °C for 30s. The sheet resistance of AlN/GaN structure is about 57% lower than a AlGaN/GaN structure. UV ozone treatment was used to oxidize the AlN surface layer and this oxide layer served as the gate oxide layer and a protective layer during the device fabrication. AlN/GaN HEMTs exhibited a drain current density of 1.3 A/mm at gate voltage of +4 V and excellent pinch-off characteristics at gate voltage of -4 V. The current gain cut-off frequency, fT, and maximum frequency of oscillation, fmax, were 19.6 GHz and 30.9 GHz, respectively for 0.4× 200 μm² gate HEMTs.