Deep-level traps in AlGaN/GaN- and AlInN/GaN-based HEMTs with different buffer doping technologies are identified by drain current transient spectroscopy (DCTS) and low-frequency (LF) output admittance ( Y_22 ) dispersion techniques. TCAD simulations are also carried out to determine the spatial location and type of traps. The DCTS and LF Y_22 measurements on Al0.25Ga0.75N/GaN HEMT (Fe-doped buffer) reveal a single electron trap at E_C - 0.47 eV. On the other hand, an electron trap at E_C - (0.53-0.59) eV and a deep hole trap at E_V + 0.82 eV are detected in Al0.845In0.155N/AlN/GaN HEMT with unintentionally doped (UID) buffer, while a slow detrapping behavior is noticed at E_C - 0.6 eV in Al0.83In0.17N/AlN/GaN HEMT with C-doped buffer. The DCTS and LF Y_22 measurements yield nearly the same trap signatures, indicating the reliability of the trap characterization techniques used in this article. The simulated LF Y_22 characteristics show that all these traps are acceptor-like states located in the buffer layer. The identified trap parameters in various buffers may be helpful to improve the crystalline quality of the epitaxial buffer layers. © 1963-2012 IEEE.