This paper describes device simulation studies of surface and buffer trapping effects on static I-V, output-admittance (Y22), and transient characteristics of AlGaN/GaN HEMTs. The TCAD simulation model considering surface donors at EC − 0.5 eV and buffer traps at EC − 0.47 eV have been used to quantitatively reproduce the measured DC, Y22 frequency dispersion, gate-lag (GL) and drain-lag (DL) transients of AlGaN/GaN HEMT with 0.25 µm gate length. Moreover, simulated GL and DL transient responses of AlGaN/GaN HEMT with a longer gate length (0.5 µm) are validated with the reported experimental results. The impact of barrier trap at EC − 0.45 eV on the HEMT properties is also explored. It is shown that by matching simulation results with experimental data, it is possible to identify the trap (surface or buffer) responsible for a particular trapping induced degradation as well as its concentration and capture cross-section.

Trapping effects on AlGaN/GaN HEMT characteristics

Temperature-dependent dynamic degradation was investigated for C-doped GaN high electron mobility transistor (HEMT) from 300 to 20 K. Pulsed ${I}$ – ${V}$ measurements with various OFF-state quiescent bias voltages revealed that current collapse (C.C.) induced by charge-trapping effect at room temperature was greatly suppressed and monotonously declined as decreasing temperatures. This was attributed to reduced number of electrons which were injected into the C-doped layer and capable of overcoming capture potential barrier. Drain current transient measurements were employed to investigate temperature-dependent and time-resolved carrier capture/emission process. Based on the extracted time constants, an activation energy of 0.36 eV was identified for the electron capture process. For carrier emission process, both current-based and capacitance-based transient analysis indicated an activation energy around 0.20 eV. Furthermore, the traps were confirmed to be located in the C-doped layer by varying the pulse stimulus in deep-level transient spectroscope (DLTS). The measurement results showed that C-doped HEMTs grown on Si substrates exhibited high-saturation current, stabilized threshold voltage, and minor C.C. at cryogenic temperatures, particularly in comparison with those at room temperature.

Temperature-Dependent Dynamic Degradation of Carbon-Doped GaN HEMTs

Surface passivation in detectors is designed to improve the performance and stabilize the characteristics over time and ambient. In Cd 1−x Zn x Te radiation detectors, oxidation by plasma is a well-known passivation method, but its physics is not fully understood. This study focuses on the macroscopic and microscopic effects of plasma treatment. It is shown that plasma oxidation before contact deposition causes a considerable decrease in the device’s dark current and that this reduction is in a strong correlation with the lowering of the surface potential. X-ray photoelectron spectroscopy (XPS) measurements revealed a significant increase in TeO 2 fraction on the plasma-treated surface. The main conclusion of this study is that the plasma-related effects are not confined to the surface layer. Furthermore, the deep penetration of plasma treatment byproduct takes place over long time periods, which may lead to variations in device characteristics. The effects were corroborated by several characterization techniques. DC current–voltage measurements combined with layer-by-layer stripping indicate plasma signature deep into the bulk. Transient current technique (TCT) results demonstrate that the plasma treatment affects the electric field distribution millimeters into the bulk. Photo-induced current transient spectroscopy (PICTS) and thermoelectric emission spectroscopy (TEES) measurements reveal plasma-related traps in-depth of the CdZnTe crystals.

Effects of oxygen plasma treatment on Cd1−xZnxTe material and devices

Further development of harsh environment electronics capable of uncooled operation under Venus surface atmospheric conditions (~460°C, ~92 bar, corrosive) would enable future missions to the surface of Venus to operate for up to a year. Wide band-gap gallium nitride (GaN) heterostructure devices are attractive candidates for Venus lander missions due to their ability to withstand high-temperature exposure. Here, we present the first assessment of the electrical integrity of GaN-based devices subject to Venus surface atmospheric conditions. Three unique device architectures were fabricated at the Stanford Nanofabrication Facility and exposed in a Venus simulation chamber for 244 hours at the University of Arkansas Center for Space and Planetary Sciences. The three device architectures tested were InAlN/GaN high electron mobility transistors (HEMTs), InAlN/GaN Hall-effect sensors, and AlGaN/GaN UV photo detectors, which all have potential applications in the collection and readout of sensor data from Venusian landers. After exposure, HEMT threshold voltage had shifted only ~1% and gate leakage current remained on the same order of magnitude, demonstrating stability of the IrOx gate under supercritical CO2 ambient. Fluctuations in drain current after exposure are attributed to thermal detrapping and electrically-activated trapping processes. Measurements of the InAlN/GaN 2DEG properties in virgin and exposed Hall-effect sensors were comparable. Furthermore, the Hall-effect sensors exhibited a maximum change of only +11.4% in current-scaled sensitivity and −6.6% in voltage-scaled sensitivity post-exposure. The UV photodetectors with 362 nm peak wavelength exhibited an average decrease in responsivity of 38% after exposure, which is thought to be due to strain relaxation or ohmic contact degradation. Similar performance of the InAlN/GaN HEMTs and Hall-effect sensors before and after exposure highlights the viability of this material platform for development of Venus surface electronics, while the decrease in AlGaN/GaN UV photocurrent requires further analysis to assess whether the AlGaN/Ga heterostructure is suitable for robust, Venus-capable electronics.

Extended Exposure of Gallium Nitride Heterostructure Devices to a Simulated Venus Environment

https://hal.science/hal-03826217/document

Comprehensive characterization of vertical GaN-on-GaN Schottky barrier diodes

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.

Deep-Level Traps in AlGaN/GaN- and AlInN/GaN-Based HEMTs With Different Buffer Doping Technologies

Effect of GaN cap layer on the performance of AlInN/GaN-based HEMTs

Kink effect is observed in Al 0.83In 0.17N/GaN high electron mobility transistor by measuring I D-V DS characteristics at a low sweep rate. It is inferred that the kink is induced due to the trapping/detrapping of charge carriers at deep levels present in the GaN buffer in the gate–drain access region. The detrapping of charge carriers from the deep levels is by the hot-electron-assisted mechanism. Two types of traps with activation energies, 0.29 eV (donor-like) and 0.57 eV (acceptor-like) were extracted by temperature-dependent transient drain current analysis. It is concluded that the deep-acceptor-like trap with a large emission time constant is responsible for the kink effect.

Origin of the Kink Effect in AlInN/GaN High Electron-Mobility Transistor

This work presents a comparative study of the subthreshold swing (SS) in InAlN/GaN-based high electron mobility transistor (HEMT), with and without carbon doping in the buffer layer. It is observed that devices with carbon-doped (C-doped) buffer exhibit near-ideal SS of close to 60-mV/decade over a wide range of drain voltage for both up and downsweep. The C-doped buffer layer reduces the OFF-state drain leakage current and increases the drain current ON/OFF ratio. TCAD simulation of the vertical electric field under the gate shows that the sample with a C-doped buffer layer has a much lower value of the peak electric field at the drain side edge of the gate, resulting in lower reverse-biased gate leakage current and smaller SS compared to that of the undoped buffer. Moreover, the junction capacitance at the interface of the unintentionally doped GaN (UID:GAN) and the C-doped buffer layer, in series with the depletion capacitance, reduces the overall capacitance, which further improves the SS in the C-doped wafer.

Near-Ideal Subthreshold Swing in InAlN/GaN Schottky Gate High Electron Mobility Transistor Using Carbon-Doped GaN Buffer

In this paper, we report the effect of post-gate metallization annealing on the performance of GaN-based High Electron Mobility Transistors (HEMTs). The performances of HEMTs annealed at 200 °C (HEMT1) and at 400 °C (HEMT2) for 5 minutes in N2 ambient are compared. While there is a kink in the output characteristics of HEMT1, there is no such kink in the output characteristics of HEMT2. The kink is attributed to impact ionization in the GaN channel. Surface and interface traps of HEMT1 increase the peak electric field at the drain side gate edge and cause impact ionization. The post-gate metallization annealing at a higher temperature reduces the surface and interface traps, which reduces the peak electric field in HEMT2 and suppresses impact ionization. This is substantiated by TCAD simulations. Threshold voltage instability on the application of negative gate bias stress was also examined for these devices. A positive shift in threshold voltage was observed in HEMT1 on the application of negative gate bias stress, whereas the corresponding shift was negative in HEMT2, indicating the presence of two different types of traps in HEMT1 and HEMT2.

https://ieeexplore.ieee.org/ielx7/6245494/6423298/09963604.pdf

Sujan Sarkar

Papers

Deep-Level Traps in AlGaN/GaN- and AlInN/GaN-Based HEMTs With Different Buffer Doping Technologies

Effect of GaN cap layer on the performance of AlInN/GaN-based HEMTs

Origin of the Kink Effect in AlInN/GaN High Electron-Mobility Transistor

Near-Ideal Subthreshold Swing in InAlN/GaN Schottky Gate High Electron Mobility Transistor Using Carbon-Doped GaN Buffer

Suppression of Kink in the Output Characteristics of AlInN/GaN High Electron Mobility Transistors by Post-Gate Metallization Annealing