Self-aligned normally-off n(++)GaN/InAlN/AlN/GaN MOS HEMTs with a recessed gate show scalable threshold voltage between 1.3 to 3.7V, which increases with the gate oxide thickness. Al2O3 or HfO2 gate insulators were grown by ALD at 100 degrees C so that one photoresist mask could be used for the gate recessing, ALD and metal lift-off. A low density of the barrier surface donors approximate to 1x10(13)cm(-2) stems from the low thermal budged during the HEMT processing and explains the threshold voltage behaviour. Maximal I-DS reaches approximate to 0.4 A/mm despite 2-m gate length and 8-m source-to-gate distance invariant to the threshold voltage. It is shown that for the present device Al2O3 provides better gate insulation than HfO2, however, the latter may be more appropriate for highly scaled short gate-length HEMTs. Schematic picture of the self-aligned InAlN/GaN MOS HEMT.