We systematically investigated characteristics of Pt-gated capacitors with four kinds of gate insulators such as Hfo.57Sio.43Ox, Hf0.64Si0.36Ox, HfO2, and Al2O3. The Hf0.64Si0.36Ox films fabricated by post-deposition annealing (PDA) at 800°C in O2 (PDO), N2 (PDN), and 3%H2 (PDH) ambient exhibited quite different characteristics. After PDO, the Hf0.64Si0.36Ox film was partially crystallized and had a thick interfacial layer at the n-GaN/Hf0.64Si0.36Ox interface, while the Hf0.64Si0.36Ox films after PDN and PDH maintained an amorphous structure. The PDH capacitor exhibited an order of magnitude larger interface state density than the PDN capacitor, indicating that the PDN process can produce superior Hf0.64Si0.36Ox film. The Hf0.57Si0.43Ox, Hf0.64Si0.36Ox, HfO2, and Al2O3 films after PDA at 800°C in N2 were 15.4, 15.9, 17.6, and 9, respectively. The HfO2 and Al2O3 films consist of polycrystalline structure while the Hf0.57Si0.43Ox and Hf0.64Si0.36Ox films maintain an amorphous structure. The different structure strongly affected to electrical properties. The Hf0.57Si0.43Ox and Hf0.64Si0.36Ox capacitors showed superior electrical properties such as a minimal flatband voltage (Vfo) hysteresis (≤ +70 mV) and a small Vfo, shift (≤ -0.46 V), as well as a low interface state density (∼ 3 × 1011 cm-2eV-1 at -0.45 eV from conduction band), and a high breakdown electric field (≥ 8.6 MV/cm) compared to those of the HfO2 and Al2O3 capacitors.