We present a systematic study on the admittance characterization of surface trap states in unpassivated and SiN x -passivated Al 0.83In 0.17N/AlN/GaN heterostructures. C-V and G/ω-V measurements were carried out in the frequency range of 1 kHz to 1 MHz, and an equivalent circuit model was used to analyze the experimental data. A detailed analysis of the frequency-dependent capacitance and conductance data was performed, assuming models in which traps are located at the metal-AlInN surface. The density (D t) and time constant (τ t) of the surface trap states have been determined as a function of energy separation from the conduction-band edge (E c - E t). The D st and τ st values of the surface trap states for the unpassivated samples were found to be D st≅ (4 - 13)× 10 12 eV - 1 cm - 2 and τ st ≈ 3 μs to 7 μs, respectively. For the passivated sample, D st decreased to 1.5× 10 12eV - 1cm - 2 and τ st to 1.8 μs to 2 μs. The density of surface trap states in Al 0.83In 0.17N/AlN/GaN heterostructures decreased by approximately one order of magnitude with SiN x passivation, indicating that the SiN x insulator layer between the metal contact and the surface of the Al 0.83In 0.17N layer can passivate surface states.