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 (Y₂₂) dispersion techniques. TCAD simulations are also carried out to determine the spatial location and type of traps. The DCTS and LF Y₂₂ measurements on Al0.25Ga0.75N/GaN HEMT (Fe-doped buffer) reveal a single electron trap at EC - 0.47 eV. On the other hand, an electron trap at EC - (0.53-0.59) eV and a deep hole trap at EV + 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 EC - 0.6 eV in Al0.83In0.17N/AlN/GaN HEMT with C-doped buffer. The DCTS and LF Y₂₂ measurements yield nearly the same trap signatures, indicating the reliability of the trap characterization techniques used in this article. The simulated LF Y₂₂ 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.

The buffer and surface trapping effects on low-frequency (LF) Y-parameters of Fe-doped AlGaN/GaN high-electron mobility transistors (HEMTs) are analyzed through experimental and simulation studies. The drain current transient (DCT) characterization is also carried out to complement the trapping investigation. The Y₂₂ and DCT measurements reveal the presence of an electron trap at 0.45–0.5 eV in the HEMT structure. On the other hand, two electron trap states at 0.2 eV and 0.45 eV are identified from the LF Y₂₁ dispersion properties of the same device. The Y-parameter simulations are performed in Sentaurus TCAD in order to detect the spatial location of the traps. As an effective approach, physics-based TCAD models are calibrated by matching the simulated I-V with the measured DC data. The effect of surface donor energy level and trap density on the two-dimensional electron gas (2DEG) density is examined. The validated Y₂₁ simulation results indicate the existence of both acceptor-like traps at E_C –0.45 eV in the GaN buffer and surface donor states at E_C –0.2 eV in the GaN/nitride interface. Thus, it is shown that LF Y₂₁ characteristics could help in differentiating the defects present in the buffer and surface region, while the DCT and Y₂₂ are mostly sensitive to the buffer traps.

We derived a simplified analytical expression to estimate the dynamic reduction in the 2DEG density due to the buffer trapping ( ${n}_{T}$ ) in GaN-based high-electron-mobility transistors (HEMTs). To compute ${n}_{T}$ , gate-lag drain current transient (DCT) measurements are carried out at a fixed temperature for different ${V}_{GS}$ points. The dynamic decrease in the 2DEG due to the Fe-related trap at ${E}_{C}$ -0.5 eV is found to be ~ $3.4\times10$ ¹¹ cm ⁻² in AlGaN/GaN HEMT; thus, ${n}_{T}$ is an essential quantity to evaluate the current collapse effects in HEMT. The validity of the expression is further demonstrated in the C-doped HEMTs. The calculated ${n}_{T}$ for the trap at ${E}_{C}$ -0.14 eV is about $9\times10$ ¹² cm ⁻⁰ in InAlN/GaN HEMT with C-doped buffer, and the calculated ${n}_{T}$ for the trap at ${E}_{C}$ -0.33 eV is ~ $1.2\times10$ ¹³ cm ⁻² in AlN/GaN HEMT with C-doped buffer. These values reveal that current collapse effects are more pronounced in the C-doped HEMTs than the Fe-doped HEMTs. Thus, this work is an important first step before further improvements in the model for computing buffer trap concentration.