DC Degradation Research Articles

Investigating DC and AC degradation behaviors to P-type low temperature polycrystalline silicon thin film transistor with fin-like structure

Currently, thin film transistor (TFT) based on planar structure is widely used in the applications of display panels. However, when the device is scaling-down, new device structures or novel materials should be introduced. In this work, the low temperature polycrystalline silicon (LTPS) TFT with fin-like structure is investigated. On the basis of electrical measurements, the device features good switching characteristics with low operating voltages. However, the I d–V g sweeping result exhibits off-state leakage, which is known as the gate-induced drain leakage (GIDL) current. The GIDL current is increased with the drain bias and the temperature increasing. To observe the GIDL current degradations, different gate biases with negative bias stress and positive bias stress are applied. The results show different degradation behaviors. In addition, bias stresses with DC and AC methods are also applied to verify the device reliability. Both threshold voltage (V t) shift and sub-threshold swing (S.S.) are also extracted to verify the degradations of the device. Finally, the physical models are also proposed to illustrate the degradation behaviors of the LTPS device with a fin-like structure, which can be beneficial to future related development of LTPS-based devices.

Importance of uniformity of grain size to reduce dc degradation and improve reliability of ultra-thin BaTiO3-based MLCCs

Excellent direct current (dc)-bias and reliability have become increasingly important for ultra-thin BaTiO3-based multilayer ceramic capacitors (MLCCs). Herein, X5R-MLCCs with a thickness of ∼1 μm are fabricated using BaTiO3 with varied grain size. It is shown that the uniformity of the grain size plays an important impact on the direct current (dc)-bias and the reliability of ultra-thin MLCCs. Uniform grain size, which is indicative of good distribution of doping element contributes to improved temperature stability. By contrast, abnormal large grains induce reinforced space charge polarization. Chips with non-uniform grains exhibit dramatic dielectric constant change under dc-bias due to the high tetragonality and irreversible domain-wall motion. Weibull distribution and highly accelerated life test (HALT) reveal that non-uniform grains contain more oxygen vacancies supported by impedance spectra analysis at 300–450 °C. This study provides a feasible strategy to improve the dc-bias and the reliability of the ultra-thin MLCCs.

Degradation of doxycycline antibiotics using lanthanum copper oxide microspheres under simulated sunlight.

In this study, lanthanum copper oxide was synthesized under hydrothermal techniques and characterized for doxycycline degradation. The catalyst exhibited enhanced photocatalytic doxycycline degradation under visible light owing to its compatible bandgap energy (1.7eV). The XRD data revealed high crystallinity of the material with no noticeable impurities. Three-dimensional microspheres of varying sizes (average diameter of 2.52μm) were observed from SEM. EDX confirms the successful synthesis of La2CuO4. The effect of DC concentration, catalyst dosage, and initial pH on the degradation rate of DC was studied methodically. Interestingly, about 85% of doxycycline (10mg/L) was degraded within 120min of light-emitting diode irradiation at pH 10. Oxygen vacancies and surface defects were determined through photoluminescence spectra. The recyclability experiments suggested that the catalyst is capable of degrading DC for three consecutive runs. Radical trapping trials suggested that holes (h+), superoxide radicals (●O2-), and hydroxyl radicals (●OH) are involved in the photodegradation of DC. Herein, the novel approach of La2CuO4 synthesis and the efficient visible-light harvesting capability of as-prepared catalyst reveal the potentiality for DC degradation thereby opening a new horizon of research employing La2CuO4 used for various environmental applications.

Thermal Annealing Improved Stability of Amorphous InGaZnO Thin-Film Transistors Under AC Bias Stresses

Amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) severely degrade under AC gate bias stress and AC drain bias stress, whose transfer curves respectively undergo positive shifts of 6.3 and 14.2 V after a stress time of 3000 s with a stress amplitude of 20 V. In this study, annealing at 400 °C in O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> atmosphere is performed to effectively reduce the acceptor-like trap states in the a-IGZO channel and the electric field in the etching-stop layer under the extended drain electrode, which participate in dynamic and DC degradation mechanisms, respectively. Thus, a-IGZO TFTs exhibiting excellent stability without any shift of the transfer curve under the same AC gate bias stress and AC drain bias stress are experimentally demonstrated simultaneously.

Fermi level pinning in Co‐doped BaTiO3: Part I. DC and AC electrical conductivities and degradation behavior

AbstractWe explore the synergistic effects of co‐doping BaTiO3 with a judicious combination of acceptors and donors to control the point defect chemistry and electrical properties, with the goal of simultaneously limiting the electronic and ionic conductivities over broad temperature and oxygen partial pressure (pO2) ranges. Specifically, we compare the temperature‐ and pO2‐dependent electrical properties of BaTiO3 ceramics acceptor‐doped with either Mn or Mg and co‐doped with a Y donor. This study, which is the first of a two‐part series, presents the electrical properties as a function of pO2, temperature, and time, focusing on the grain‐interior electrical response. The DC and AC electrical conductivity measurements reveal that co‐doping with Mn and Y can result in (1) increased electrical resistivity over a broad temperature range, (2) pO2‐independent electrical conductivity in oxidizing conditions, and (3) improved time‐dependent dielectric degradation resistance. These behaviors are attributed to a Fermi level pinning effect, as is explained in the companion paper, which presents complementary density functional theory (DFT)‐based grand‐canonical defect chemistry models. The collective experimental and computational studies demonstrate that the pO2‐independent electrical conductivity in the Mn and Y co‐doped BaTiO3 is attributed to a Fermi level pinning mechanism arising from the multivalent Mn dopant, and the background reservoir of positive charge provided by the predominant substitution of Y on the Ba sites. The enhanced degradation resistance is attributed to a reduced oxygen vacancy concentration relative to the other doping chemistries.

An overview of oxygen vacancy dynamics in (1 − x)(Bi1/2Na1/2)TiO3–xBaTiO3 solid solution

Across the phase diagram of (Bi1/2Na1/2)TiO3-xBaTiO3 solid solution, the conduction mechanism changes from ionic to electronic through the mixed case at the morphotropic phase boundary, giving rise to distinct DC degradation behaviors.

Proton Irradiation of High Aluminum Content AlGaN Polarization Doped Field Effect Transistors

The effects of proton irradiation dose on the DC and switching properties of high aluminum content, polarization-doped field effect transistors (POLFETs) were studied. The POLFETs were irradiated at proton energy of 10 MeV at fluences of 1 × 1014 cm−2 and 3 × 1014 cm−2. The DC saturation current exhibited a 21 and 36% reduction at fluences of 1 × 1014 cm−2 and 3 × 1014 cm−2, respectively. The carrier removal rates for this energy was 677 cm−1. However, switching current at 100 kHz demonstrated no change, with near ideal performance, as opposed to significant degradation in their GaN HEMT counterparts. This near ideal performance is attributed to the volume of the 3D electron gas in the POLFETs reducing the likelihood of negatively impacting scattering events, as opposed to the narrow 2D electron gas of the HEMT. The DC degradation and carrier removal rates are on par with reported traditional GaN HEMTs, but the switching performance is exceptionally improved.

TCAD Analysis of the Four-Terminal Poly-Si TFTs on Suppression Mechanisms of the DC and AC Hot-Carrier Degradation

Four-terminal poly-Si thin-film transistors (TFTs), with a counter-doped body terminal connected to the floating channel, can suppress both dc and dynamic hot-carrier (HC) degradation of TFTs. With 3-D TCAD simulation, we clarify the underlying mechanisms of the suppression effect and analyze its dependence on the position and width of the body terminal. Under dc HC condition, a wider body terminal or that closer to the drain collects more holes generated from impact ionization in the drain depletion region and suppresses the parasitic bipolar junction transistor effect, subsequently reduces the kink current more effectively. Under dynamic HC condition, the body terminal injects holes at the end of the falling time of the gate pulse, partially removes the non-equilibrium state, significantly relieves the transient maximum electric field in the drain depletion region, thus suppresses the dynamic HC degradation. A wider body terminal can inject more holes and the holes injected by that closer to the drain diffuse to drain depletion region first, thus suppress the dynamic HC degradation better.

Effect of proton irradiation energy on SiNx/AlGaN/GaN metal-insulator semiconductor high electron mobility transistors

The effects of proton irradiation energy on the electrical properties of SiNx/AlGaN/GaN metal-insulator semiconductor high electron mobility transistors (MISHEMTs) using in situ grown silicon nitride as the gate dielectric were studied. The SiNx/AlGaN/GaN MISHEMT devices were irradiated with protons at energies of 5, 10, or 15 MeV at a fixed fluence of 2.5 × 1014 cm−2. The largest amount of device degradation was shown in the samples irradiated with the lowest irradiation energy of 5 MeV. The DC saturation current was reduced by 10.4%, 3.2%, and 0.5% for MISHEMTs irradiated with proton energies of 5, 10, and 15 MeV, respectively. Device performance degradation was more pronounced in the irradiated samples under high-frequency operation. At a frequency of 100 kHz, the percent saturation drain current reduction at a gate voltage of 3 V was 40%, 19%, and 17% after proton irradiation at 5, 10, and 15 MeV, respectively. The carrier removal rates for the MISHEMT devices were in the range of 21–144 cm−1 for the proton irradiation energies studied. The measured DC degradation and carrier removal rates are lower than the values reported for AlGaN/GaN metal-gate high electron mobility transistor devices irradiated at similar conditions, which can be attributed to the SiNx insulating layer reducing the total damage on the AlGaN surface.

Biomimetic synthesis of Bi2O3/Bi2WO6/MgAl-CLDH hybrids from lotus pollen and their enhanced adsorption and photocatalysis performance

Hierarchically porous Bi2O3/Bi2WO6/MgAl-CLDH hybrids were synthesized using a biotemplate route assisted with hydrothermal treatment. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), electronic differential system (EDS), transmission electron microscope (TEM), Brunner-Emmet-Teller (BET), fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and UV–vis spectra (UV–vis). It was demonstrated that the as-synthesized Bi2O3/Bi2WO6/MgAl-CLDH exhibited 3D hierarchically porous microsphere structure, which was composed of MgAl-LDH nanosheets, Bi2WO6 nanosheets and Bi2O3 and nanoparticles. The maximum adsorption capacities of Bi2O3/Bi2WO6/MgAl-CLDH toward CR and DC were 205.3 mg/g and 204.3 mg/g, respectively. These two adsorption processes all followed pseudo-second-order kinetic model. The as-prepared samples also exhibited superior catalytic activity toward CR or DC degradation under simulated sunlight irradiation due to the high charge separation ratio of Bi2O3/Bi2WO6 heterojunction, hierarchically porous structure and synergic effect of adsorption and photocatalysis.

Simulating RF Performance of Proton Irradiated AlGaN/GaN High Electron Mobility Transistors (HEMT)s

AlGaN/GaN High Electron Mobility Transistors (HEMTS) subjected to proton irradiation of fluence up to 1014 cm-2 were simulated to observe the impact on small signal and RF characteristics. While a lot of work has covered dc degradation of HEMTs, very few studies exist which focus exclusively on RF degradation despite their high frequency applications. This work establishes a simulation framework capable of frequency domain device simulation up to 10 GHz. The role of defects generated by proton irradiation will be studied by carrying out low frequency transconductance simulation and current gain frequency sweeps to examine degradation of cut-off frequency. The primary objective will be to understand the role of partially ionized donor traps near the AlGaN/GaN interface and how their bias dependence and capture time constants may significantly impact cut-off frequency and current gain estimations at high frequencies.

Simulating RF Performance of Proton Irradiated AlGaN/GaN High Electron Mobility Transistors (HEMT)s

Due to their relative radiation hardness, High Electron Mobility Transistors (HEMTs) with an AlGaN/GaN heterostructure are of high interest for space applications. Many studies have been performed to quantify the level of degradation due to proton radiation, with the majority being focused on the effects on the DC or steady-state performance. Fewer studies have evaluated degradation of the radio frequency (RF) performance due to proton radiation. Chen et al. caution that degradation due to proton irradiation has a more measurable impact on the RF performance in comparison to the DC performance [1]. In particular, gate lag and increases in both channel resistance and device capacitance due to fast bulk and surface traps contribute more notably to RF degradation than to DC degradation [1]. Understanding the dynamics of these traps is required for modeling and simulation of GaN HEMTs for RF applications. In this paper, we aim to identify the necessary input parameters for future RF simulation of radiation effects on AlGaN/GaN HEMTs. In previous work, we have found that donor traps have a larger role than previously thought on the magnitude of DC performance degradation. We use experimental techniques to identify the donor trap lifetimes and energies, both of which are incorporated into a transient simulation of the HEMT and compared to results obtained experimentally. The experiments are long-term transient measurements of drain current after a step in gate voltage for various temperatures, resulting in the establishment of capture and release time constants associated with donor traps. (1) J. Chen, E. X. Zhang, C. X. Zhang, M. W. McCurdy, D. M. Fleetwood, R. D. Schrimpf, S. W. Kaun, E. C. H. Kyle, and J. S. Speck, IEEE Trans. Nucl. Sci, 61 (6), 2959, (2014).

Analysis of high power microwave induced degradation and damage effects in AlGaAs/InGaAs pHEMTs

The high power microwave (HPM) induced effects in AlGaAs/InGaAs pseudomorphic high electron mobility transistors (pHEMTs) are investigated by simulation and experiments. Simulated results suggest that the HPM damage mechanism is device burnout, which is caused by emerging current path and strong electric field intensity beneath the gate metal. Besides, analysis points out that the gate metal diffusion may be thermally activated, resulting in DC and RF performance degradation. Specifically, a positive shift will occur to the pinch-off voltage while accordingly the small-signal gain will decrease. The HPM injection experiments on the dual-stage pHEMT low noise amplifiers (LNAs) are carried out. Experimental results substantiate that deterioration happens both in the noise figure and in the small-signal gain, which is in agreement with the simulated results. Failure analysis indicates that the HPM induced failure of LNA is attributed to the failure of the first stage pHEMT. Finally, samples dissection analysis using the scanning electron microscopy (SEM) verifies the simulation analysis of the damage mechanism and the location susceptible to burnout. Meanwhile, the assumption of gate metal diffusion is validated by the observation of pits after removing the gate metal. The performance parameters deterioration can be utilized as the degradation (or damage) criteria, and the mechanisms analysis facilitates making reinforcing design.

Impact of proton fluence on DC and trapping characteristics in InAlN/GaN HEMTs

Robustness of InAlN/GaN devices under proton radiation is investigated. Several proton fluences ranging from 1×1014 to 4×1014 have been considered on two typologies of devices. Displacement damage is found to be the major responsible of device DC degradation leading to threshold voltage positive shift, ON resistance increase and drain current decrease, in all cases well correlated with proton fluence. Negligible difference is noticed in displacement damage effects measured on different device typologies. Furthermore, device geometry does not influence the impact of proton radiation on main DC parameters, either if gate width or length are considered. Radiation significantly affects trapping properties. A good correlation between the so-called current collapse increase and proton fluence is demonstrated when a high gate drain voltage value is imposed as trapping condition. Moreover radiation enhances the contribution of dynamic ON resistance and transconductance peak variation on current collapse increase.

Electrical characterization and analysis of the degradation of electrode Schottky barriers in BaTiO3 dielectric materials due to hydrogen exposure

Hydrogen gas creates a highly damaging environment that degrades electrical properties in oxide based dielectrics and piezoelectrics. In this study, the degradation resistivity due to hydrogen gas in a barium titanate X7R dielectric is designed and processed for base metal electrode capacitors. The present paper is devoted to I-V measurements and the loss of resistivity in the electrode Schottky barriers. The DC degradation and asymmetries noted in I-V forward and reverse biasing conditions were assumed to be hydrogen ion interstitials, locally creating donor substitutions. Thermionic and field emission conductivity mechanisms are applied to model the I-V data; the conductivity is controlled by the Schottky barrier heights and hydrogen ions localizing at the interfaces. Finally, a mechanism was proposed for resistivity degradation due to exposure to hydrogen gas. The proposed mechanism predicts the degradation should be reversible, and its validity was examined by recovery tests.

Percolation effects in dc degradation ofZnOvaristors

For quantitative estimation of the degree of electrical disorder (electrical inhomogeneity) in ZnO varistor ceramics caused by a variation in the barrier height at different grain boundaries in a sample, the comparison of threshold electric fields (onsets of highly nonlinear current–voltage characteristics) in ceramics and single grain boundary (GB) is suggested and approved. At dc degradation similar behavior of the current–voltage characteristics of ZnO varistor ceramics and single GB is observed. The percolation model of Shklovskii–De Gennes is applicable for the description of a disorder in ZnO varistor ceramics. The degree of the disorder in ZnO varistor ceramics is not dependent on the duration of dc degradation at least at degradation time below 60 h. At voltages close to the onset of a highly nonlinear region of current–voltage characteristic the correlation radius of infinite cluster is ∼ 5 times greater than the average grain size.

Hot-electron reliability improvement using perhydropolysilazane spin-on-dielectric passivation buffer layers for AlGaN/GaN HEMTs

We investigate the effects of perhydropolysilazane spin-on-dielectric (SOD) buffer layer adopted prior to Si3N4 passivation on the dc drain current level and degradation after the electrical stress in the AlGaN-GaN high electron mobility transistors (HEMTs). The SOD-buffered HEMTs show ∼1.6 times greater drain current densities (∼257 mA/mm) than those of the devices with conventional-Si3N4 passivations (∼155 mA/mm). After the hot electron stresses (step-wise and constant) applied to the devices, it is also found that the SOD-buffered structure produces greatly improved device reliability in terms of the dc current collapse (15% for step-stress and constant stress) compared to the conventional structure (25% for each case). We propose that the enhancement of SOD-buffered structure in dc current collapse is due to the reduction in surface state density at the passivation interface and the suppressed electron trapping.

(Invited) Fundamental Modeling of Radiation Effects in AlGaN/GaN HEMTs

AlGaN/GaN HEMTs have high radiation tolerance compared to Si and GaAs transistors. They are also well suited for high power and high frequency applications and thus may be advantageous for satellite-based communication systems. Radiation studies have shown the following DC performance changes: threshold voltage shift (positive), transconductance reduction, and saturation drain current reduction.The basic phenomenon leading to degradation of DC performance is the creation of point defects such as Ga or N vacancies and subsequent ionization of the traps. We have successfully modeled DC degradation due to radiation effects with FLOODS, a TCAD finite-element simulator. The simulation of trapped negative charges in the GaN buffer layer explained experimental results. As Figure 1 shows, there is a positive shift in threshold voltage that increases with the concentration of negatively charged traps.Another phenomenon that occurs in irradiated AlGaN/GaN HEMTs is an increase in breakdown voltage and apparent enhanced reliability. The fundamental mechanism for this phenomenon is unclear. We have shown that the negatively-trapped charges in the GaN buffer are not the primary mechanism and that charging at the AlGaN surface is a plausible reason. This work explores the fundamental mechanisms for charging of the AlGaN surface. We look at the transient effects of electron and hole creation during radiation and subsequent interfacial trapping mechanisms.Figure 1. Positive shift in threshold voltage with respect to trap concentration in GaN buffer layer.

The Thermally Stimulated Current (TSC) Technique on Evaluating Degradation of ZnO Varistors

Thermally stimulated current (TSC) is a simple and effective test technique to study the thermal activated charge, electron trap and activation energy of dielectric and semiconducting materials. It is well known that the addition of glass frits can improve the degradation property of ZnO varistors. An activation energy of 0.45 eV has been found on the ZnO varistors with/without glass frits after the DC degradation by using TSC method. The tested quantity of thermal activated charges, QTSC, may represent the numbers of migrated zinc interstitials. ZnO visitors doped with glass frits have less QTSC. Experiment indicates that TSC test is a convenient method to evaluate the degradation behavior of ZnO varistors.

The Effect of DC degradation and heat-treatment on defects in ZnO varistor

In this research, DC degradation for ZnO varistors at 3.2 kV/cm and 50 mA/cm2 for 115 hours was performed, and its effect on electrical properties and defects of ZnO varistors was investigated. It was found that the breakdown field and nonlinear coefficient drops sharply from 2845 V/cm to 51.6 V/cm and 38.3 to 1.1, respectively, when the DC degradaion time reaches 115 hours. For the degraded sample, the dielectric loss was dominated by the increase of conductivity so that some defect relaxation peaks cannot be observed is the DC degraded ZnO varistors. However, in electrical modulus plot, one relaxation peak can be observed. The conductivity in low frequency range increases greatly and the conductance activation energy drops from 0.84 to 0.083 eV. Additionally, the heat-treatment process of ZnO varistors at 800 ℃ for 24 hours was also performed. It is interesting to note that the electrical properties and the relaxation processes of ZnO varistor is restorable completely again after heat-treatment. The breakdown field and the nonlinear coefficient increase to 3085 V/cm and 50.8, respectively, and the activation energy of conductance increases to 0.88 eV. It is also found that the defect relaxation peak, which is shown in dielectric spectra corresponding to oxygen vacancy defect, is suppressed evidently by heat-retreating. Therefore, it is proposed that oxygen is likely to diffuse into the ZnO grain boundaries at the heat-treatment process, which can play an important role in restorability of the DC degraded ZnO varistor.