Fluctuation and Noise Letters

The distinction between "physical" and "equivalent" noise sources in bipolar junction transistors and other semiconductor devices has become blurred in the current engineering textbooks. An unfortunate consequence of this is the emergence in the literature of fictitious noise sources such as the "the collector-current shot noise" and the "base-current shot noise". These are often assigned a physical reality and incorrectly treated as real physical noise sources, independent of circuit topology. Text-books have encouraged successive generations of students in this belief. Non-physical noise sources such as these are convenient and legitimate, even essential, for the effective modeling and simulation of circuit noise. However their naïve use in teaching and research is likely to continue to give rise to fallacious concepts and misleading conclusions. The physical. The physical modeling of the light-emitting diode, the photon transport transistor and the bipolar junction transistor are briefly discussed to illustrate this view.

1/4 scaling of various biological variables in relation to body mass is universal in biology. Recent models provided evidence that such a scaling is a result of optimization of the processes in the system. We present evidence for 1/4 scaling in dynamical processes from two different types of experiments: fluctuation of the erythrocyte diameter and a task of random generation of series of numbers by human subjects. The time series obtained for different cells and human subjects respectively were analyzed by detrended fluctuation analysis. The exponents from different individuals presented a gaussian distribution centered at a scaling exponent equivalent to 1/4 for both types of experiments. While this proves the existence of 1/4 scaling in dynamics, it also shows that processes do not proceed equally efficient in various individuals. Consequently the deviation from the 1/4 scaling individuals can be used as a universal tool of investigation for the efficiency, or the optimization of various biological, psychological or other types of variables.

As was pointed out in some recent papers, the fluctuations in the secondary particle distribution of extensive air showers at observation level exhibit a complex structure which can be interpreted as a white noise plus a 1/f noise. The resulting coloured noise fits very well into the framework of the universal multitractal theory. In this paper we compare the e⁺ and e^- density fluctuations generated by different primary cosmic rays, namely, high energy γ rays, protons, and helium, oxygen and iron nuclei, and the universal multifractal parameters for all these primaries are calculated for several samples. The performed analysis reveals that the Levy index and the mean codimension depend monotonically on the mass of the primary cosmic ray. Some future applications of the multifractal properties of extensive air showers are suggested.

In this paper, we show how noise measurements can be used for the characterization of a dielectric material. This nonperturbative technique allows the determination of the real and imaginary parts of the complex permittivity from current and voltage noise measurements. This technique is illustrated hereafter for the case of an antiferroelectric liquid crystal.

We investigated, analytically and by analog simulation, a simple model system, an overdamped Kramers oscillator with two additive noise sources, internal white and external colored. We found that the magnetude of noise background in the spectrum of system outpit can decrease when the amplitude of external noise or periodic signal increases.

In this paper, we propose an original statistical explanation to interpret 1/f noise consistently. It is shown that a sequence of noise signals is necessarily non-stationary if it is generated by stationary random walks at random time points. The variance of the signal distribution is random and the probability of the appearance of random increments in the k'th time interval (k > 1) is based on the intensities of a family of Poisson processes in the current time interval and on conditional probabilities generated in all previous time intervals. This leads to increasing expectations of the random variances in consecutive time intervals. As a consequence, the correlation of signals is random with time dependent expectations. The noise process is non-stationary and not first-order Markovian. Common estimation yields biased frequency spectra with increasing error variances appearing as 1/f error spectra. Families of Poisson processes with jump probabilities characterized by a priori Beta distributions are investigated and the second moments of the resulting noise processes are conisdered. Simulated process yield spectral estimates with linearly increasing error variances.

Resistance noise data from a single gas sensor can be utilized to identify gas mixtures. We calculated the power spectral density. higher order probability densities and the bispectrum function of the recorded noise samples; these functions are sensitive to different natural vapors and can be employed to select a proper detection criterion for gas composites and odors.

The temperature dependence of discrete spin fluctuation modes is investigated in the spin glass phase of CuMn, doped with Au to produce strong local anisotropy. As with the locally anisotropic spin glass AuFe, and unlike the nearly isotropic CuMn, individual few-state fluctuators are found and can be tracked vs. temperature. The kinetics follow Arrhenius laws, with normal attempt rates.

A multimode model is proposed to simulate the mode-competition low-frequency noise in semiconductor lasers taking account of the possible mechanisms of nonlinear gain saturation. A new technique is reported to generate the Langevin noise sources that induce spontaneous-emission fluctuations in dynamics of the lasing modes. The model is applied to simulate the intensity noise in AlGaAs lasers. Agreement of the simulated noise results with the experimental measurements is found. Correlation of the noise characteristics with dynamics of the mode competition is newly introduced. The low-frequency noise is enhanced when the dynamics exhibit mode hopping or a jittering single mode, and is suppressed when the stable single-mode operation is achieved.

Transpassive electrodissolution of copper in phosphoric acid was studied froma dynamical point of view. A new electrochemical oscillation named corrosion oscillation as the form of electrochemical noise appears at the corrosion region. The pre-passivated copper electrode in phosphoric acid media shows a fluctuated current within the active potential region. The current oscillation in this system, and generally in electrochemical systems, is sensitive to the initial condition of the electrode surface from heterogeneity point of view. We show that the noise in the current-potential curve is caused by a tiny instability of the passive film, a transition from the passive to extrapassive states and vice versa. Here we demonstrate that the current oscillations can also be observed in the low scale of electrochemical noise. Moreover, the possible mechanism for the electrochemical noise and generally the other noises are also described. The study is of interest from an interdisciplinary point of view as shown by complicated processes such as noises, can be recognized in a dynamical system by applying a closer view in a lower scale. Thus, it is a typical (but not general) study for investigation and monitoring tiny processes in scale, whereas they are so complicated.

We demonstrate that signal-to-noise ratio (SNR) can be significantly improved by stochastic resonance in a double well potential. The overdamped dynamical system was studied using mixed signal simulation techniques. The system was driven by wideband Gaussian white noise and a periodic pulse train with variable amplitude and duty cycle. Operating the system in the non-linear response range, we obtained SNR gains much greater than unity. In addition to the classical SNR definition, the ratio of the total power of the signal to the power of the noise part was also measured and it showed better signal improvement.

The conversion mechanisms of microscopic low frequency noise sources (e.g. generation-recombination noise sources) located in the channel of a FET (Field Effect Transistor), in the presence of a large RF signal, are investigated. It is shown that the base-band (low frequency) input gate noise voltage spectral density is strongly dependent on the magnitude of the input RF power applied to the FET. Moreover, the microscopic generation-recombination noise sources distributed along the channel are also responsible of up-converted input gate noise voltage spectral density around the RF frequency.

This paper presents a survey on the topical aspects of the noise resistance in field-effect transistors (FET) at microwave frequencies. Such noise parameter represents the sensitivity of the device noise figure to the departure from the minimum noise condition and is therefore important in all low-noise applications. The performance of the noise resistance in FETs has been reviewed since the first noise modeling analysis of short-gate devices were presented in the early '70s. The authors also comment and compare their own results on this subject as obtained by extensive experimental activity in the field of noisy device characterization vs. frequency, bias and temperature conditions.