A systematic study of the selective etching of p-GaN over AlGaN was carried out using a BCl₃/SF₆ inductively coupled plasma (ICP) process. Compared to similar chemistry, a record high etch selectivity of 41:1 with a p-GaN etch rate of 3.4 nm/min was realized by optimizing the SF₆ concentration, chamber pressure, ICP and bias power. The surface morphology after p-GaN etching was characterized by AFM for both selective and nonselective processes, showing the exposed AlGaN surface RMS values of 0.43 nm and 0.99 nm, respectively. MIS-capacitor devices fabricated on the AlGaN surface with ALD-Al₂O₃ as the gate dielectric after p-GaN etch showed the significant benefit of BCl₃/SF₆ selective etch process.