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Gallium nitride/aluminium gallium nitride (GaN/AlGaN) etching in BCl3/Cl2-based inductively coupled plasma (ICP) is investigated for high electron mobility transistor (HEMT) mesa etching using rarely preferred mask-photoresist. The critical issues related to photresist burning/deforming, resist removal, selectivity, mesa edge roughening, and nonuniform etching of GaN and AlGaN layers are discussed in detail using plasma of BCl3/ Cl2 gases. The effect of ICP process parameters like ICP power, RF power, pressure, and BCl3/Cl2 flow rate ratio on etch rate of GaN/AlGaN layers and mask is studied systematically for the optimization of a HEMT mesa etching process that results in smooth etched surface with sharp and highly anisotropic mesa edges. The photoresist mask selectivity is found to depend strongly on pressure and RF power, whereas the etched surface morphology changes significantly with the gas flow rate ratio and chamber pressure. The AlGaN etch rate and selectivity with respect to GaN is also characterized for different Al concentrations varying up to 33%. The etch process is finally applied to GaN/AlGaN HEMT mesa etching, where the mesa features with depth of ~ 1500 A° are etched successfully. The resultant process etch uniformity is found to be better than 5% over 2-in wafer.