Step-stress Test Research Articles

Reliability comparison of AlGaN/GaN HEMTs with different carbon doping concentration

The reliability of AlGaN/GaN HEMTs adopting Fe and C co-doping, with high and low carbon doping concentration was investigated by means of different stress tests. Firstly, DC and pulsed I-V characterization at room temperature are discussed, then drain step stress tests at different gate voltages are compared, afterwards, the constant stress at different bias points are discussed. Results show that the high C HEMTs showed reduced DIBL, smaller leakage current, as well as decreased electric field, leading to an improved robustness during on-state stress testing, with respect to the reference ones. Failure modes during constant voltage stress consist in a decrease of drain current and transconductance, accelerated by temperature and electric field.

Investigation into trapping modes and threshold instabilities of state-of-art commercial GaN HEMTs

This work is aimed at investigating the performance and reliability limits of a commercially available state-of-the-art RF GaN HEMT technology. Measurement strategies to recognize and assess trap-induced degradations, prevalent in mature technologies, are discussed. Double pulsed measurements are used to capture and quantify threshold instabilities, and their evolution with temperature, for short quiescent near-threshold and off-state stress conditions. A versatile transient measurement technique evaluates threshold voltage fluctuations in the 10 μs–100 s temporal range, during stress or recovery phases, for different trap-filling configurations. Corresponding recovery transients between 27 °C to 130 °C are analysed to extract a trap activation energy of 0.53–0.56 eV for this technology. Finally, drain and gate step-stress tests are performed, providing an overview into the robustness of the gate contact under high electric fields.

Planning step-stress test plans under Type-I hybrid censoring for the log-location-scale distribution

The optimal design of a k-level step-stress accelerated life-testing (ALT) experiment with unequal duration steps under Type-I hybrid censoring scheme for a general log-location-scale lifetime distribution is discussed here. Censoring is allowed only at the change-stress point in the final stage. Based on the cumulative exposure model, the determination of the optimal choice for Weibull, lognormal and log-logistic lifetime distributions are considered by minimization of the asymptotic variance of the maximum likelihood estimate of the pth percentile of the lifetime at the normal operating condition. Numerical results show that for these lifetime distributions, the optimal k-step-stress ALT design with unequal duration steps under Type-I hybrid censoring scheme reduces just to a 2-step-stress ALT design.

Method of selecting step stress test parameters for XLPE insulation DC voltage endurance coefficient

Cross-linked polyethylene (XLPE) insulation DC voltage endurance coefficient (VEC) is an important basis in the selection of DC cable insulation design and factory test parameters. Studies on the degradation of XLPE insulation performance under DC electric field can also provide technical support for the reliability evaluation of cable operation. The step stress method as an efficient electrical life test method is widely used to obtain VEC. The parameters of step stress method present considerable influences on VEC. An analytical method based on the trajectory of the damage curve and equivalence between step stress test and constant stress test within zone division is proposed in this paper. According to the proposed method, several sets of step stress tests with different test parameters have been carried out. The accuracy of the test results has been confirmed via analyzing test data. The experimental results show that the VEC (n=12.0) obtained by the step stress test whose parameters are selected by proposed method has good equivalence with the result (n=12.2) under constant-stress method.

Zirconia-reinforced lithium silicate crowns: Effect of thickness on survival and failure mode

ObjectiveTo evaluate the reliability and failure mode of zirconia-reinforced lithium silicate (ZLS) molar crowns of different thicknesses. MethodsMonolithic ZLS molar crowns (0.5mm, 1.0mm, and 1.5 mm thickness) were modeled and milled using a CAD/CAM system (n = 21/group). Crowns were cemented on dentin-like epoxy resin replicas with a resin cement. The specimens were subjected to single load-to-failure test for step-stress profiles designing. Mouth-motion step-stress accelerated-life test was performed under water by sliding an indenter 0.7 mm lingually down on the distobuccal cusp until specimen fracture or suspension. Use level probability Weibull curves and reliability were calculated and plotted. Polarized-light optical microscope and scanning electron microscope (SEM) were used to characterize fracture patterns. ResultsIrrespective of crown thickness, beta (β) values were higher than 1 and fatigue accelerated failures. While 0.5 mm ZLS crowns exhibited a significant reduction in the probability of survival at 200N, 300N and 400 N mission loads (69%, 41% and 19%, respectively), no significant difference was observed between 1.0 mm and 1.5 mm crowns. Both thicknesses have maintained the survivability at approximately 90%. Failure primarily comprised bulk fracture where radial cracks originated from the cementation surface beneath the indenter loading trail and propagated towards the cervical margin. Significance1.5 mm- and 1.0 mm-thickness monolithic ZLS crowns presented higher probability of survival compared to 0.5 mm crowns. Bulk fracture was the chief failure mode, regardless of thickness.

Exact Likelihood-Ratio Tests for a Simple Step-Stress Cumulative Exposure Model with Censored Exponential Data

An accelerated life test (ALT) is commonly used in reliability studies of high quality products. A step-stress experiment is one such ALT that is often used in practice. In this paper, we develop exact inference based on likelihood-ratio test for a simple step-stress model with exponential lifetimes under Type-II censoring. We consider both unrestricted and order-restricted maximum likelihood estimators (MLEs) for this purpose. We also derive the exact power function of this test, which we then use for designing an optimal step-stress testing experiment. By using a similar approach, we extend these results to other forms of censoring. Monte Carlo simulations are then carried out to evaluate the performance of the inferential methods developed here. Finally, an illustrative example is presented for demonstrating all the inferential results.

Optimal design of an electrical connector intermittent fault–accelerated reproduction test based on a genetic algorithm

The intermittent fault of an electrical connector is a latent threat to the reliability of an electromechanical system. For electrical connector intermittent fault diagnosis, an intermittent fault must be reproduced. Reproducing an intermittent fault by a traditional test has a low efficiency and adds some damage to the product, which is not conducive to intermittent fault diagnosis. To further improve the reproduction efficiency of an intermittent fault and reduce the damage, optimal design of a step-stress-accelerated intermittent fault reproduction test is carried out. First, the number of intermittent faults and the degree of damage in the reproduction test are estimated, and reproduction and damage models of an intermittent fault during the step-stress reproduction test are constructed. Then, based on the intermittent fault and damage models, an optimized method based on a genetic algorithm is established. Finally, the validity and applicability of the theoretical model and the optimized method of the step-stress-accelerated test based on a genetic algorithm are verified by comparing data from a contrast test.

A new rheological model for thixoelastic materials in subaqueous gravity driven flows

A new viscoelastic constitutive model for subaqueous clay-rich gravity flows is presented. It is explained that for the materials which exhibit a minimum in their strain controlled flow curves the structure parameter must be a symmetric function of the strain rate and the stress. Therefore, the destruction of structure within the material is modeled using the dissipation energy. An expression for the elastic strain of the flowing structure is derived. The final set of equations can reproduce the viscosity bifurcation that clay suspensions may exhibit under a given load. This is explained to be important for the prediction of the run-out distance of clay-rich gravity flows. The ability of the model to reproduce the general response of pasty materials to step stress and step shear rate tests is examined. The model requires four empirical parameters. A methodology is presented for obtaining these parameters and power law functions are given for their calculations for a limited rest time of 3000 s. The ability of the model to reproduce the rheological behavior that clay-rich suspensions exhibit under both stress and strain controlled conditions is examined using rheometry tests.

Acids generated and influence on electrical lifetime of natural ester impregnated paper insulation

Acids have attracted an increasing interest from researchers for the standard evaluation of transformer oil, especially in natural ester. This study aims to investigate the types of generated acids in natural ester under hydrolysis, oxidation, and accelerated thermal aging tests with varying paper moistures. The oxidation and hydrolysis degrees were evaluated by acid number and kinematic viscosity, and acids were distinguished from aging condition hydrolysis of oil paper insulation by combining pH values. Meanwhile, the electrical lifetime of oil-paper insulation with seven types of acid were analyzed by step-stress test. The two-parameter Weibull distribution was used to analyze the mean lifetime of specimens statistically. Experimental results reveal that the majority of acids generated from hydrolysis and the main acids are long-chain fatty acids of natural ester impregnated paper insulation. Furthermore, the electrical lifetime models of oil-paper insulation with different types acid were established, and the voltage endurance coefficient with seven types of acids were also obtained. The electrical lifetime test indicated that acids can reduce the electrical lifetime of natural ester impregnated paper insulation and that the effect of low-molecular-weight acids on insulation lifetime is approximately 2–3 times higher than high-molecular-weight acids.

Analysis of the reliability of LED lamps during accelerated thermal aging test by online method

In order to shorten the accelerated aging time, it is necessary to select a reasonable thermal stress without changing the failure mechanism. In this paper, a short-cycle step-stress test is introduced to determine the stress limitation of LED, which is about 90 °C for the samples used in this work. The online method which means the lumen degradation of samples is obtained at aging condition is applied, and the error of measured luminous flux is simulated by the software of Tracepro. In data procession, the exponential fitting and Bayesian method are firstly carried out to get the lifetime and failure probability of each lamp under accelerated stress. Then the characteristic lifetime and shape parameter of samples is calculated according to Weibull distribution. Finally, the Weibull distribution curve at room temperature can be obtained by using the Arrhenius model. For the samples in this work, the correspondent lifetimes are respectively obtained to be 32,251 h with the failure probability of 63.2%. And compared with the method of Energy Star standards, it is confirmed that the method is effective.

Stability and robustness of InAlGaN/GaN HEMT in short-term DC tests for different passivation schemes

On-wafer short term step-stress tests were carried out to evaluate InAlGaN/GaN HEMT devices. Three types of transistor were studied, each one having a specific two dielectric layer passivation. The results of these tests demonstrate that the upper layer of the passivation has a strong impact on the ageing of the devices. When the upper layer is an Al2O3 Atomic Layer Deposited one, the transistors show a better stability of their electrical parameters than those passivated with the other stacks.

Catastrophic Optical Bulk Damage – A New Failure Mode in High-Power InGaAs-AlGaAs Strained Quantum Well Lasers

ABSTRACTHigh-power single-mode (SM) and multi-mode (MM) InGaAs-AlGaAs strained quantum well (QW) lasers with emission wavelengths of 910 − 980 nm are extensively used in various fiber lasers and amplifiers for both telecom and defense applications. In particular, underseas and satellite communication systems require stringent reliability from these lasers. Since these lasers predominantly fail by catastrophic and sudden degradation due to COD, it is crucial especially for space satellite applications to investigate reliability, failure modes, and degradation mechanisms of these lasers. Catastrophic optical mirror damage (COMD) was known to be the only failure mode until our group reported a new failure mode in MM and SM InGaAs-AlGaAs strained QW lasers in 2009 and 2016, respectively. Our group reported that bulk failure due to catastrophic optical bulk damage (COBD) has become the dominant failure mode of both SM and MM lasers. Since there have been limited reports on COBD compared to COMD, the intent of this paper is to introduce our studies on COBD that have spanned the last decade. We investigated reliability, failure modes, and degradation processes in SM and MM InGaAs-AlGaAs strained QW lasers by performing short-term step-stress tests and long-term accelerated life-tests as well as failure mode analyses using various nondestructive and destructive micro-analytical techniques including electron beam induced current (EBIC), time-resolved electroluminescence (EL), deep level transient spectroscopy (DLTS), focused ion beam (FIB), and high-resolution TEM. EBIC and EL techniques were employed to study dark line defects generated in degraded lasers stressed under different test conditions. Time-resolved EL techniques were employed to study initiation and progressions of dark spots and dark lines in real time as lasers were aged. DLTS techniques were employed to study electron traps in both pristine and degraded lasers. Lastly, FIB and high-resolution TEM were employed to prepare cross sectional and plan view TEM specimens to study DLD areas in post-aged lasers. We also report our current understanding on degradation mechanisms responsible for COBD in both SM and MM lasers.

A Note on the Prediction of Censored Exponential Lifetimes in a Simple Step-stress Model with Type-II Censoring

In this article, we consider the problem of predicting survival times of units from the exponential distribution which are censored under a simple step-stress testing experiment. In this article, Type-II censoring are considered for the form of censoring. Two kinds of predictors—the maximum likelihood predictors (MLP) and the conditional median predictors (CMPs)—are derived. Some numerical examples are presented to illustrate the prediction methods developed here. Using simulation studies, prediction intervals are generated for these examples. We then compare the MLP and the CMP with respect to mean squared prediction error (MSPE) and the prediction interval. Finally, we used a real data to apply the prediction methods developed in the article.

Joint modeling of linear degradation and failure time data with masked causes of failure under simple step-stress test

In this paper, we propose a method to model the relationship between degradation and failure time for a simple step-stress test where the underlying degradation path is linear and different causes of failure are possible. It is assumed that the intensity function depends only on the degradation value. No assumptions are made about the distribution of the failure times. A simple step-stress test is used to induce failure experimentally and a tampered failure rate model is proposed to describe the effect of the changing stress on the intensities. We assume that some of the products that fail during the test have a cause of failure that is only known to belong to a certain subset of all possible failures. This case is known as masking. In the presence of masking, the maximum likelihood estimates of the model parameters are obtained through the expectation–maximization algorithm by treating the causes of failure as missing values. The effect of incomplete information on the estimation of parameters is studied through a Monte-Carlo simulation. Finally, a real-world example is analysed to illustrate the application of the proposed methods.

Analysis and Optimum Plan for 3-Step Step-Stress Accelerated Life Tests with Lomax Model Under Progressive Type-I Censoring

In this paper, the optimum test plan and parameter estimation for 3-step step-stress accelerated life tests in the presence of modified progressive Type-I censoring are discussed. It is assumed that the lifetime of test units follows a Lomax distribution with log of characteristic life being quadratic function of stress level. The maximum likelihood and Bayesian method are used to obtain the point and interval estimators of the model parameters. The Bayes estimates are obtained using Markov chain Monte Carlo simulation based on Gibbs sampling. The optimum plan for 3-step step-stress test under modified progressive Type-I censoring is developed which minimizes the asymptotic variance of the maximum likelihood estimators of log of scale parameter at design stress. Finally, the numerical study with sensitivity analysis is presented to illustrate the proposed study.

Reliability Characteristics and Mechanisms of HRL’s T3 GaN Technology

HRL’s T3 GaN MMIC technology is evaluated using dc reliability experiments, including a voltage step-stress test, a temperature step-stress test, and a 3-temperature life test. The drain voltage step-stress test revealed three distinct regions of operation through gate leakage characteristic changes: burn-in stabilization up to 5 V, followed by stable operation up to 20 V, then voltage-dependent degradation up to catastrophic failure around 30 V. As a result, a de-rated recommended safe-operating limit of Vds = 12 V was established. The temperature step-stress test resulted in clear Arrhenius temperature-dependent degradation of the device on-resistance above 250 °C. Similarly, a recommended safe-operating limit of 150 °C was established for the technology. Finally, the 3-temperature life test resulted in clear temperature group separation and an activation energy of 2.52 eV, with a mean time-to-failure of 4 million hours at 150 °C with only a +0.2 ohm.mm increase in device on-resistance. Physical failure analysis of degraded parts showed the mechanism was a defect formation in the epitaxial barrier layer at the drain edge of the gate. Finite-element electrical simulations duplicated this degradation mechanism by modeling the defect as a void, which depleted the 2DEG and therefore increased device on-resistance.

Weibull Step-Stress Model with a Lagged Effect

SYNOPTIC ABSTRACTIn survival analysis and reliability, researchers are often interested in assessing the effects of different stress factors on the lifetime of experimental units. The model introduced in this article is motivated by a study of the effects of altitude and other risk factors on decompression sickness, a condition encountered when individuals are exposed to significant changes in environmental pressure. Unlike standard life-testing experiments, in this study, the levels of the stress factor, viz. altitude, are changed during the exposure duration. This is known as a step-stress test, a class of accelerated testing, widely used in material testing. Recently Kannan, Kundu, and Balakrishnan (2010) introduced the cumulative risk model as an alternative to the widely used cumulative exposure model. The new model allows for the inclusion of a lag period in the hazard function, a more realistic assumption in most applications. In this article, we consider the cumulative risk model assuming that the lifetime distributions of the experimental units follow Weibull distributions at the different levels of the risk factor. It is assumed that the level of the stress factor is changed only once during the exposure duration at a pre-fixed time τ1. The maximum likelihood and the least squares methods have been used to estimate the unknown parameters. Monte Carlo simulations are performed to compare the performances of the two different methods. We further propose the Bayes estimators of the unknown parameters of the model. To evaluate the performance of the model, one data set from the altitude decompression sickness experiment has been analyzed, and it is observed that the proposed model fits the data quite well.

Statistical inference for masked interval data with Weibull distribution under simple step-stress test and tampered failure rate model

In this article, we consider the planning of simple step-stress life test in the presence of competing risks with the possibility of masked causes of failure and interval censoring. It is assumed t...

Yellow laser emission at 578 nm by frequency doubling with diode lasers of high radiance at 1156 nm

For the first time, we demonstrate diode lasers consisting of a single quantum-well, which emit at 1156 nm. Ridge waveguide lasers and tapered lasers of this structure contain a monolithically integrated distributed Bragg reflector grating. Step-stress tests of both laser types demonstrate long lifetimes. The radiation of the laser sources is utilized for second-harmonic generation in a single-pass configuration with a 15.5 mm long MgO-doped periodically poled LiNbO\(_{3}\) ridge waveguide crystal. Optical output powers of up to 0.86 W are achieved at 578 nm.

Estimating the inverse power law aging exponent for the DC aging of XLPE and its nanocomposites at different temperatures

XLPE and its nanocomposites have been subjected to a progressive step voltage until failure occurred, and the results analyzed in terms of an inverse power law ageing expression. The step-stress tests have been conducted at three temperature 20°C,40°C and 60°C, and the exponent n of the inverse power law ageing expression has been estimated for the two materials by a new method, which fully considers the contribution to the failure of the whole sequence of voltage steps, including ageing processes at the lower voltages, the final step and the process of voltage increase. It was found that the value of exponent n increases slightly with increasing temperature for both XLPE and its nano-composites. More importantly the value of n for the nano-composites was found to be substantially lower than that for XLPE at all temperatures. A complementary experiment in which the space charge distribution was measured under the same step-stress protocol as the breakdown tests has also been carried out and the results of both experiments are discussed.