Digital etching is an effective method to lower dry etch damages in A1GaN/GaN HEMTs. This work systematically investigated O₂-plasma-based digital etching of AlGaN and p-GaN. AlN layers were used as the etch stop layers in the AlGaN etch. Important process aspects such as the use of the AlN layers, the RF power, the oxygen flow rate, the oxidation time and the resulting roughness were studied. These are technically relevant to obtain controllable, uniform etch surfaces with low surface damages for better HEMT performance.