Time Constant Spectra Research Articles

Investigation of trap characteristics under the inverse piezoelectric effect in AlGaN/GaN HEMT devices at room temperature and low temperature

Changes in the electrical properties and the trap characteristics of AlGaN/GaN high electron mobility transistors under the application of reverse bias stress at both room temperature and low temperature were investigated. When the critical stress voltage was reached, the gate current, which complied with the Poole–Frenkel conduction conditions, showed an abrupt increase. Furthermore, the magnitude of the critical stress voltage for occurrence of the inverse piezoelectric effect can be increased at 83 K. The transient current method was used to establish that the detrapping peak amplitudes of the traps increased, but the trap activation energy remained unchanged. The changes observed in both the time constant spectra and the pulsed current–voltage curves confirmed that the trap densities in the AlGaN barrier layer increased as a result of the inverse piezoelectric effect. However, the different degrees to which the numbers of traps increased at room temperature and at 83 K contributed to the occurrence of different degradations in the device.

Trap characteristics of hafnium oxide-based ferroelectric field-effect transistors measured by using a current transient method

The trap characteristics and polarization effect on the trapping behavior in Hf0.5Zr0.5O2 ferroelectric field-effect transistors were analyzed. The current transient that corresponds to the trapping/detrapping of charge carriers was measured and the exact time constant spectra were extracted. In accordance with the different time constants and activation energies as well as the dependence of the trapping behavior on the filling conditions, traps that originated from the oxygen vacancies in the Hf0.5Zr0.5O2 layer and from the Si/SiO2 interface trap states were identified. The detrapping peaks in time constant spectra showed a consistent changing trend with the variation of remanent polarization, confirming that the positive polarization enhanced the trapping of charge carriers injected from channel side to the ferroelectric layer.

Effects of Static and Repetitive Uniaxial Bending Strains on the Electrical Properties and Trap Characteristics of Flexible Low-Temperature Polysilicon Thin-Film Transistors

Changes in the electrical properties and trap characteristics of flexible low-temperature polysilicon thin-film transistors (TFTs) under the application of uniaxial bending strains and during repetitive bending cycles were investigated. When the bending strain increased, the transfer curve showed a negative shift, the output current decreased, and the subthreshold slope increased. After the bending strains were removed, the electrical performance showed recovery behavior, but the device performance could not return fully to its original state. The devices were also subjected to repetitive bending cycles. The transfer curves showed a positive shift after <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{{4}}$ </tex-math></inline-formula> bending cycles and the output current increased. Using the transient current method, it was established that the detrapping time constant of the traps decreased and the peak amplitudes increased as a result of both the bending strain and the bending cycles. However, while the trap activation energy was not changed by static bending, it did decrease after the repetitive bending cycles. Both the changes in the electrical properties and the time constant spectra confirm that the trap state density increased as a result of the bending processes. However, the different trapping behaviors contributed to different degradations in the device’s electrical properties.

Identification of Traps in p-GaN Gate HEMTs During OFF-State Stress by Current Transient Method

In this work, the current transient method was conducted for trap analysis in the p-GaN gate high-electron-mobility transistors (HEMTs) in the OFF-state. Based on the traditional detrapping transient measurements, the pure recovery transients can be isolated by subtracting the undetected part caused by the measurement conditions. A comparison of the measured and actual recovery transients under different drain filling voltages was presented. It suggested that this method can be effective to analyze the unregular transient curves and distinguish the charge trapping type preliminarily. In addition, three traps were identified based on the time constant spectra and the hidden absolute amplitudes of traps can be corrected using the differential amplitude spectra. The information of trap levels in the buffer layer and AlGaN barrier layer was revealed, consisting of three electron traps with energy levels of 0.313, 0.265, and 0.467 eV. The identification of the traps may provide a physical foundation for better understanding of the drain-induced trapping effect during the OFF-state in p-GaN HEMTs.

Compact Thermal Modeling of Modules Containing Multiple Power LEDs

Temperature is an essential factor affecting the operation of light-emitting diodes (LEDs), which are often used in circuits containing multiple devices influencing each other. Therefore, the thermal models of such circuits should take into account not only the self-heating effects, but also the mutual thermal influences among devices. This problem is illustrated here based on the example of a module containing six LEDs forming on the substrate a hexagon. This module is supposed to operate without any heat sink in the natural convection cooling conditions, hence it has been proposed to increase the thermal pad area in order to lower the device-operating temperature. In the experimental part of the paper, the recorded diode-heating curves are processed using the network identification by deconvolution method. This allows for the computation of the thermal time constant spectra and the generation of device-compact thermal models. Moreover, the influence of the thermal pad surface area on the device temperature and the thermal coupling between LEDs is investigated.

Shear transformation zone analysis of anelastic relaxation of a metallic glass reveals distinct properties of α and β relaxations.

Metallic glasses with pronounced high-frequency β relaxation in their dynamic-mechanical response have been observed to exhibit large plasticity. Due to their disordered atomic structure, it is challenging to identify the microscopic mechanisms of their relaxation behavior. Quasistatic anelastic relaxation measurements have been performed over 10 orders of magnitude of time on La_{55}Ni_{20}Al_{25} metallic glass, which exhibits a strong β relaxation. The corresponding time-constant spectra were computed from the data-they contain a series of peaks corresponding to an atomically quantized hierarchy of shear transformation zones (STZs), where both the α and β relaxations are consistent with the STZ model. Two different regimes of activation-volume increment between the peaks are observed, suggesting the involvement of different elements in STZs corresponding to α vs β relaxations. Room-temperature structural relaxation significantly affects the former but not the latter.

A global approach for a consistent identification of static and dynamic phenomena in a PEM Fuel Cell

In this paper, we propose a parameterization process for static and dynamics models dedicated to the analysis of two typical characterizations of a Proton Exchange Membrane Fuel Cell (PEMFC): the polarization curve (V–Icurve) and a set of Electrochemical Impedance Spectroscopies (EIS) carried out for several current densities of this curve. The specificity of the proposed approach is to consider conjointly both characterizations during all the proposed analysis process. This global strategy ensures the separation of the different fuel cell phenomena (activation, diffusion and ohmic) in the static and dynamic domains by imposing, with different ways, an expected consistency between both characterizations.Starting from the measured polarization curve, a first parametric identification is carried out leading to a static model of the Fuel Cell (FC). The EIS data are, at this stage, used to obtain the ohmic resistance of the static model. This is the first consistency forced between both characterizations. By fixing the transfer coefficient to 0.5 (platinum catalysts), it is possible to separate unequivocally the different fuel cell phenomena (activation, diffusion and ohmic) and to identify the different parameters of the laws which describe them in steady state for a given Membrane Electrode Assembly (MEA).In the objective of describing the dynamic of these three phenomena, an identification process for generic models involving RC cells in series and without a priori (number of RC cells not presupposed) is secondly applied to the set of measured EIS. Thanks to time-constant spectra (not classically used in fuel cell world) handled in this approach, the time-constants related to the three phenomena are extracted. The first specificity is to force the consistency between V–I curve and EIS by calculating the activation and diffusion resistances thanks to the derivation of the physical laws used for the static model and parameterized in the first step of our approach. The second specificity consists in forcing the equality between the V–I slope and the “zero-frequency resistor” of the EIS for a given characterized density current. This results in the apparition of a residual part for the impedance involving different suffered phenomena (potentially platinum oxidation, channel pressure oscillations…). The characteristics of this residue are analyzed.To calibrate the proposed method and to demonstrate its sensitivity to changes that may occur in the FC components, experiments concerning a single cell with different sets of components (different membrane thicknesses and different platinum loadings in the Active Layer (AL)) were achieved and analyzed by applying this method. Cyclic voltammetries were carried out in addition of V–I curves and EIS to check the relevancy of the parameters identified through our approach.

Microscopic characterization of structural relaxation and cryogenic rejuvenation in metallic glasses

Plasticity improvement in metallic glasses resulting from cycling treatment between room and liquid nitrogen temperature has been reported and attributed to rejuvenation due to a non-uniform thermal expansion coefficient. However, the detailed microscopic effect is still unclear. The present study focuses on the microscopic effect of room-temperature ageing and cryogenic cycling. La70Cu15Al15 and La70Ni15Al15 metallic glasses were subjected to varying ageing times, after which some were also cryogenically cycled. Quasi-static anelastic relaxation measurements were then employed to characterize the time-constant spectra over seven orders of magnitude. The overall anelastic strain decreases with increasing ageing time, but is not noticeably affected by cryogenic cycling. On the other hand, while ageing also causes an increase in characteristic time constants, cycling reverses this effect. The new details shed light on the effect of structural relaxation and rejuvenation on the properties of shear transformation zones.

Identifying the spatial position and properties of traps in GaN HEMTs using current transient spectroscopy

Time constant spectra are extracted from current transients based on the Bayesian deconvolution and used to characterize traps in GaN high-electron mobility transistors. Two kinds of traps with different time constants in an actual device were identified in the AlGaN barrier layer and the GaN layer, respectively. In particular, the trapping process in the AlGaN barrier layer was identified at the region near the drain side under gate contact. Trapping mechanisms of both two traps are discussed. Additionally, we observe that the trap in the AlGaN barrier layer requires sufficient electric field to activate the trapping process and a high drain voltage (Vds) accelerates the trapping processes both in the AlGaN barrier layer and the GaN layer. In addition, detrapping experiments with different filling conditions were performed to confirm their spatial positions. The influence of self-heating is excluded during the experiment by keeping the power density at a very low level, and the trapping effect is the sole factor accounting for the current transients.

Thermal time-constant spectrum extraction method in AlGaN/GaN HEMTs

The transient temperature rise in the active region in AlGaN/GaN high electron mobility transistors (HEMTs) is measured using an electrical method. The original data are smoothed and denoised by a nonparametric fitting algorithm, called locally weighted scatterplot smoothing (LOWESS). The thermal time-constant spectrum is extracted to analyze the physical structure of the heat-conduction path in AlGaN/GaN HEMTs. The thermal time-constant spectra extracted using the LOWESS algorithm are richer and the RC network obtained is greater compared with those with the traditional denoising method (multi-exponential fitting). Thus, the analysis of the heat-flow path is more precise. The results show that the LOWESS nonparametric fitting algorithm can remove noise from measured data better than other methods and can retain the subtle variation tendency of the original discrete data. The thermal time-constant spectra extracted using this method can describe the subtle temperature variations in the AlGaN/GaN HEMT active region. This will help researchers to precisely analyze the layer composition of the heat-flow path.

Interpreting Impedance Spectra in Time-Constant Domain: Application to the Passive Film on Titanium

Electrochemical impedance spectroscopy (EIS) is a useful method for characterizing the interfacial characteristics of materials such as a passive layer underneath the surface of a titanium-based metal-oxide anode. Conventionally EIS data were interpreted by fitting them with electrically equivalent circuit. However, this method requires the initial knowledge of the structure of the tested material. It is also impossible to model a system with continuous variation of layers. Normally, an electrode with finite dielectric layers can be represented by a series of RC circuits plus an interfacial impedance. To cope with the difficulty of systems with continuous variation, a distribution function may be assumed to describe the distribution of time constant along the thickness of the layers [1]. Still, this approach needs to assume a profile to which the system is restricted. A reverse approach was proposed [2]. Within a layered material, a distribution function g(τ) of the resistance over the time-constant (τ=RC) may result from the discrete or continuous variation of the composition (Figure1). By simply fitting the EIS data into Equation 1, one may obtain the function g(τ), which is directly related to the layered structures of the electrode. This method was previously applied to study a transient passive layer in the Ti-supported metal oxide anode, and showed a continuum over the time constant spectrum [2]. However, direct correlation between the bands and the physically observed layers was not well established. Thus the interpretation method by time-constant domain needs to be validated. In the present work, the growth of a titanium oxide film on top of Ti was studied. This reaction is well studied. The growth kinetics and characterization of the titanium oxide film is well established [3]. Therefore it is considered to be a suitable system to test and validate the use of time-constant spectra for the interpretation of EIS. Commercially pure titanium was polarized in a pH=7 buffer containing phosphate. Preliminary data confirmed that the system showed the growth of a resistance band with a characteristic time constant (Figure 2). This band is different from the other peak representing the surface charge-transfer process. Its position was almost stationary at all voltages. Judging by its increase, it was considered to be related to the dielectric film developed on top of the Ti. Further work will focus on correlating the data with physical data, including thickness and composition information from the literature.

Effect of electrolyte composition on TiO2 nanotubular structure formation and its electrochemical evaluation

Nanotubular structures of TiO2 were synthesized on Ti6Al4V alloy in various electrolytes with and without fluoride ions and were then evaluated by potentiodynamic polarization tests and electrochemical impedance spectroscopy. The resultant surface features were revealed through scanning electron microscopy. It was observed that barrier type oxide film is formed in halide free acidic electrolyte. From potentiodynamic polarization scans it was deduced that high current density in 0.15M NH4F containing electrolyte corresponded to the formation of TiO2 nanotubes by slow field driven Ti4+ ions diffusion in the electrolyte than field assisted TiO2 growth. Two time constant impedance spectra were simulated to the field assisted charge transfer reactions at high frequency (dissolution of TiO2 by fluoride ions) and at low frequency (adsorption of oxidase ions) regime. X-ray diffraction pattern of as anodized surface revealed oxygen deficient Ti6O phase, which was converted to rutile/anatase phases by heat treatment in air.

Interaction of Defects with Quantum Well States: Electrostatic-Dependant Response Time for Traps in AlGaN/GaN HEMTs

Recovery transients in high-voltage AlGaN/GaN HEMTs following both blocking voltage and ON-state stress show strong dependence on stress time. The defect time constant spectra exhibit a temperature-dependant component (TG1, Ea = 0.6 eV) and a temperature-independent component (TG2). With increased stress time and larger current collapse, the time constants for TG2 become progressively longer. The stress-time-dependent behavior of TG2 is shown to be consistent with the capture of trapped carriers in the AlGaN barrier directly by quantum well states, originating from the same defect giving rise to the 0.6 eV behavior. By modulating this alternate recombination pathway, the electric field normal to the AlGaN/GaN interface is shown to have a strong effect on reducing the response time of the trap to several orders of magnitude below its bulk response time. This demonstrates that barrier design may be utilized to tune the recovery characteristics of the HEMT.

Accuracy and boundary condition independence of Cauer RC ladder compact thermal models

This paper presents an experimental study aimed at the investigation of the influence of variable boundary conditions on the element values of compact thermal models. These models are generated from the time constant spectra of thermal responses in the form Cauer RC ladders in such a way that their individual components might have some physical interpretation and correspond to individual stages in the heat flow path. The accuracy of thermal simulation results obtained using the proposed models is validated with temperature measurements of discrete silicon carbide power devices.

Data processing method developments using TAU-transformation of Time-Domain IP data II. Interpretation results of field measured data

The paper presents field results of the data processing method developments based on approximate TAU-transform of the Time-Domain IP (Induced Polarization) curves. The theoretical basis of these developments was published in Part I by Turai and Dobróka (2011).Two ways for estimation of the type and the value of soil contaminations will be introduced. On first way the calculation of contamination type comes from the time constant value of the time constant spectra. The value estimation of the soil contamination is based on the Weighted Amplitude Value (WAV) of the time constant spectra. On second way we define the corrected apparent conductivity of the contaminated material (σcorr) as a product of time-constant spectrum and measured apparent conductivity. The σcorr parameter is useful for classification of contamination level.These methods were tested over several contaminated area (Offheim — 1992, Nyékládháza — 1997–2008, Ráckeve — 1997, Kecskemét — 1997, Győröcske — 1999, Pásztó — 2000, Tokaj — 2001, Balmazújváros — 2002, Szerencs — 2004, Tiszapalkonya —2004, Berekböszörmény —2007, Nagytárkány —2008, Darvastó — 2008, Miskolc-Hejőpart — 2008, Tiszavasvári — 2008, Nagytétény — 2008, Miskolc-Bedő hegy — 2009, Telkibánya — 2010, Miskolc-Salakbánya — 2010, Miskolc-Gózon L. út — 2010 and Miskolctapolca-Várhegy — 2010).Some results from interpreting field data collected over waste sites and other contaminated areas are presented. The contamination level and the main components of the contaminating material are characterized based on the time-constant spectra.

Generation of reduced dynamic thermal models of electronic systems from time constant spectra of transient temperature responses

This paper concerns the problem of generating reduced dynamic thermal models whose elements have physical interpretation. The compact models of an electronic system are generated here based on the analysis of the time constant spectra of system transient thermal responses. The proposed method is illustrated on a practical example of an integrated power amplifier with a heat sink. The experiments demonstrate the influence of contact resistance and cooling conditions on the resulting values of thermal model elements.

Impedance Spectroscopy and XPS Comparative Investigations of the State of Boron Atoms in an Amorphous Metallic Matrix

The purpose of this study is to compare the investigations of amorphous magnetic ribbons combining the usual structural methods with physical impedance spectroscopy (PIS) and X-ray photoelectron spectroscopy (XPS). The samples are commercially available rapidly quenched ribbons for torroidal transformers. Two types of ribbons were thermally treated below the temperature of amorphous–crystalline phase transition. Two binding energies of B 1selectrons are determined by XPS in one of the as-quenched ribbons, which show impedance time constant spectra without dispersion. The other ribbon, possessing only one value of binding energy of the B 1selectron (BIstate) in a rapidly as-quenched state, undergoes transition to a still amorphous state but acquires high electromagnetic losses and show another value of the binding energy of the B 1selectron (BIIstate) after the heat-treatment procedure. This latter type of ribbons show structural impedance spectra containing various time constant values.

Spectral IP – Past, present and future

These has been considerable research and development into Spectral IP over the last five years. Some of the earliest claims have been found to be erroneous. The dominant influence on IP spectra is rock texture and grain size and not the particular electrochemical reactions taking place at the metallic-ionic interface. In spite of the inability of Spectral IP to directly identify chemical reactions and thus, specific minerals, it has been found to be a useful tool in indirectly identifying minerals through their characteristic habit. The long time constant spectra associated with the typically large effective grain size of graphite is readily distinguished from the short time constant spectra of typical sugary volcanogenic massive sulfide Py-Cp-Sp-Pb mineral mineralization.