Atomic layer deposition (ALD) is supplied to grow ZrO2 high-k gate dielectrics and fabricate InAlN/GaN metal oxide semiconductor (MOS) high electron mobility transistors (HEMTs). Ex situ chemical surface cleaning and optimised in situ InAlN surface pre-treatment by ALD lead to a substantial suppression of drain-source current collapse owing to a high-quality InAlN/oxide interface. In addition, the gate leakage current was suppressed by about three orders of magnitude.