Josef Sikula

New automatic localization technique of acoustic emission signals in thin metal plates.

In acoustic emission (AE) measurement, the information of the arrival time is very important for event location, event identification and source mechanism analysis. Manual picks are time-consuming and sometimes subjective, especially in the case of large volumes of digital data. Various techniques have been presented in the literature and are routinely used in practice such as amplitude threshold, analysis of the long-term average/short-term average (LTA/STA), high-order statistics or artificial neural networks. A new automatic determination technique of the first arrival times of AE signals is presented for thin metal plates. Based on Akaikes information criterion, proposed algorithm of the first arrival detection uses a specific characteristic function, which is sensitive to change of frequency in contrast to others such as envelope of the signal. The approach is applied to data sets of three different tests. Reliable results show the potential of our approach.

Characterization of acoustic and electromagnetic emission sources

The creation of cracks is accompanied by electric charge redistribution due to loosened chemical bonds. Electric charges on crack walls create dipole moments. Vibrations of crack walls produce time-dependent dipole moments and, consequently, electric and magnetic fields are generated. An electric signal is induced on metal electrodes. Information about the vibration of crack walls was obtained from this signal analysis. For crack lengths below 1 mm the electrical signal has a frequency of over 2 MHz. In this paper the frequency analysis was performed in a frequency band of up to 5 MHz.

Symmetrical current-voltage characteristic of a metal-semiconductor-metal structure of Schottky contacts and parameter retrieval of a CdTe structure

Symmetrical, non-linear and current–voltage (I–V) characteristics of a metal–semiconductor–metal (M-S-M) structure of two metallic Schottky contacts fabricated to a p-type semiconductor were modeled by treating the semiconductor as a resistor sandwiched between two identical head-to-head Schottky barriers. The voltage distributions along the M-S-M structure were numerically determined and found that the voltage drop across the reverse-biased Schottky barrier is dominating at the low bias voltage, and the dominant range depends on the value of the resistor of the semiconductor bulk. The field dependence of barrier height due to the image force was proposed to be the mechanism for the current through the M-S-M structure when the voltage drop across the reverse-biased barrier is dominating. The proposed model was applied to the I–V curves measured at different temperatures on low-resistivity p-type CdTe with Au contacts and the density of the effective acceptors calculated, and the zero-field Schottky barrier height and the Richardson constant were extracted using the activation energy method. The extracted parameters fitted well with that published for the same material structure.

Low-frequency noise of thick-film resistors as quality and reliability indicator

Abstract The non-linearity and the noise of thick-film resistors are parameters that can be used to make a prediction of resistor reliability. The noise spectroscopy measurements of thick-film resistors are proposed as a diagnostic tool for the prediction of possible types of failure. The correlation between noise spectral density data and the results of accelerated aging of thick-film resistors at high temperature were made for HS80 and 2000 resistor pastes.

Empirical model for the low-frequency noise of hot-carrier degraded submicron LDD MOSFETs

This paper discusses the empirical low-frequency (LP) noise behavior of hot-carrier degraded Lowly-Doped Drain (LDD) n-MOSFETs, which have been fabricated in a 0.7-/spl mu/m CMOS technology. It is shown that the increase of the noise spectral density follows a t/sup 0.3/ power law dependence with stress time. Additionally, an empirical relationship will be shown between the input-referred noise spectral density S/sub VG/ and the transconductance g/sub m/ of the stressed devices. The practical consequences of this exponential dependence will be briefly discussed.

Acoustic and electromagnetic emission as a tool for crack localization

The creation of cracks is accompanied by electric charge redistribution due to loosened chemical bounds. Electric charge on a crack wall creates dipole moments. Vibrations of crack walls produce time-dependent dipole moments and, consequently, electric and magnetic fields are generated. An electric signal is induced on metal electrodes. Simultaneously with the electromagnetic emission (EME) signal, an acoustic emission (AE) signal is generated, but due to the different velocities of propagation of both waves, the detection of the AE signal is delayed. This time delay presents the time of the wave propagation from the individual acoustic emission sensor to the crack. The defect can be located by means of these time intervals. This paper describes the localization using acoustic and electromagnetic emission signals for the two-dimensional case.

Noise and transport characterisation of tantalum capacitors

Abstract A low frequency noise and charge carrier transport mechanisms were investigated on tantalum capacitors made by various producers. The model of Ta–Ta2O5–MnO2 MIS structure was used to give physical interpretation of I–V characteristics in normal and reverse modes. The noise in time and frequency domain was examined and noise sources were identified. We evaluated correlation between leakage current and noise spectral density and discussed corresponding quality and reliability indicators.

REVERSE BIASED P-N JUNCTION NOISE IN GaAsP DIODES WITH AVALANCHE BREAKDOWN INDUCED MICROPLASMAS

Random two-level or multiple-level current impulses may occur in electronic devices containing reverse biased p-n junctions in a certain operating mode. These impulses are usually rectangular, featuring constant amplitude, random pulse width and pulse origin time points. This phenomenon is generally ascribed to local avalanche breakdowns originating in p-n junction defect regions called microplasma regions. Based on experiment results, a two-state model of stochastic generation-recombination process has been elaborated for the two-level impulse noise allowing to derive some statistical characteristics of this process. It can be shown that the distribution of the probability density w(τ0) of the impulse separation τ0 and the probability density w(τ1) of the impulse width τ1 have exponential courses. The power spectral density of the noise current is of a G-R process type and depends on the particular microplasma properties. From the viewpoint of noise diagnostics, the most important features are the spectral density Su and noise current IN versus reverse current IR plots, because each local extreme of these plots corresponds to an active microplasma region. Thus obtained results may be used for p-n junction non-destructive diagnostics and quality assessment.

1/f noise in metallic thin films

Studies of 1/fa noise in vast ensembles of thin film resistors are described. The distribution of the exponent a in the spectral density power law is found to be normal, the mean value being near unity and depending on the sample preparation technology. The scattering range of the values of a is typical of the resistor manufacturers. The noise voltage spectral density distribution is shown to depend on the frequency, being close to normal. The number of fluctuators is shown to be much less than the number of free electrons in the specimen.

Failure modes of tantalum capacitors made by different technologies

Abstract Tantalum capacitor failure modes have been discussed both for the standard manganese dioxide cathode and the new conductive polymer (CP) type. For standard tantalum in the normal operation mode, an electrical breakdown can be stimulated by an increase of the electrical conductance in channel by an electrical pulse or voltage level. This leads to capacitor destruction followed by thermal breakdown. In the reverse mode, we have reported that thermal breakdown is initiated by an increase of the electrical conductance by Joule heating at a relatively low voltage level. Consequently, a feedback cycle consisting of temperature–conductivity–current–Joule heat–temperature, ending with electrical breakdown was created. Both of these breakdown modes possess a stochastic behavior and can be hardly localized in advance. CP capacitors have shown a slightly different current conductivity mechanism compared to standard tantalum capacitors. The breakdown of CP dielectrics is similar to avalanche and field emission breaks. It is an electromechanical collapse due to the attractive forces between electrodes, electrochemical deterioration, dendrite formation, and so on. However, some self-healing of the cathode film has been reported. This can be attributed to film evaporation, carbonizing or reoxidation. Not all of the breakdowns of CP capacitors can lead to self-healing or an open circuit state. Short circuits can also occur.