Ozone treatment of AlN on AlN/GaN heterostructures produces effective surface passivation and chemical resistance to the AZ positive photoresist developer used for subsequent device fabrication. The ozone-passivated AlN/GaN high electron mobility transistors (HEMTs) exhibited low gate leakage currents, high gate modulation voltage, and minimal drain current degradation during gate pulse measurements. With an additional oxygen plasma treatment on the gate area prior to the gate metal deposition, enhancement-mode AlN/GaN high electron mobility transistors were realized. The gate characteristics of the HEMTs treated with the ozone and oxygen plasma behaved in a manner similar to a metal oxide semiconductor diodelike gate current-voltage characteristic instead of a Schottky diode. Drain breakdown voltages of 23 and 43 V for d- and e-mode HEMTs were obtained, respectively. For d-mode HEMTs, there was no reduction in the drain current during the gate pulse measurements at frequencies of 1, 10, and 100 kHz. For the e-mode HEMT, the drain current was reduced 5% at 100 kHz.