Giovanni Basso

Ultralow-noise programmable voltage source

To avoid introducing additional noise sources while making low-frequency noise measurements, batteries are normally used instead of electronic power supplies. This paper presents an alternative solution by describing the design, construction, and testing of an ultralow-noise voltage source. Such a power supply can be computer controlled and has a typical noise level two orders of magnitude below that of similar commercial instruments. Some typical values of the spectral density of the voltage fluctuations at its output are: (10/sup -12/, 10/sup -15/, 10/sup -16/) V/sup 2//Hz at (0.01, 0.1, and 1) Hz, respectively. These noise performances are almost independent of the supplied current, with a degradation of less than 3 dB up to 400 mA. A special algorithm for digital-to-analog conversion, using passive devices with 1% tolerance, ensures a resolution of 2.5 mV and an accuracy better than /spl plusmn/1.5 mV over the entire output range from 0 to 8 V.

Effect of potential fluctuations on shot noise suppression in mesoscopic cavities

We perform a numerical investigation of the effect of the disorder associated with randomly located impurities on shot noise in mesoscopic cavities. We show that such a disorder becomes dominant in determining the noise behavior when the amplitude of the potential fluctuations is comparable to the value of the Fermi energy and for a large enough density of impurities. In contrast to existing conjectures, random potential fluctuations are shown not to contribute to achieving the chaotic regime whose signature is a Fano factor of 1/4, but, rather, to the diffusive behavior typical of disordered conductors. In particular, the 1/4 suppression factor expected for a symmetric cavity can be achieved only in high-quality material, with a very low density of impurities. As the disorder strength is increased, a relatively rapid transition of the suppression factor from 1/4 to values typical of diffusive or quasi-diffusive transport is observed. Finally, on the basis of a comparison between a hard-wall and a realist...

Very long current transients in reverse-biased almost ideal n/sup +/-p junctions

The current transients due to a change in reverse-bias voltage in almost ideal n/sup +/-p junctions obtained by proper gettering processes are discussed. Very long relaxation phenomena are observed. The transient current is fitted by a t/sup -v/ law with v approximately=0.7 and by an exponential with a relaxation time of 45 min. The transient current amplitude is virtually proportional to the volume of the depletion region swept by changing the reverse bias.<<ETX>>

A detailed analysis of the pre-breakdown current fluctuations in thin oxide MOS capacitors

The on-off fluctuations of the tunnel current preceding the breakdown of thin oxide MOS capacitors are analyzed using a low noise measurement system controlled by a personal computer. An automated procedure has been implemented which is capable of interrupting the stress a few seconds before breakdown, in order to perform the electrical characterization of the structure in pre-breakdown conditions. The relationships with other phenomena preceding breakdown (stress induced leakage current, SILC, and soft breakdown, SBD) are discussed.

A novel characterization tool for the study of dielectric breakdown of ultra-thin oxide MOS structures

Dielectric breakdown of ultra-thin oxide MOS structures of integrated circuits is preceded by a precursory stage characterized by random on-off fluctuations of the current tunneling through the oxide. In this paper a new version of a low noise measurement system capable of monitoring these phenomena in a band of 1 kHz is presented. The instrument, controlled by a Personal Computer which stores and elaborates the acquired data, is capable of recognizing the current fluctuations announcing the proximity of the breakdown, so allowing the interruption of the test just a few seconds before the destruction of the sample. Some preliminary observations, made possible by the use of this new analysis tool, are presented in the paper.

Flicker noise in graphene-based Hall sensors

We present an investigation of shot noise in graphene-based Hall sensors, with the purpose of evaluating their detection limit and of developing strategies to improve their characteristics. We perform measurements on two devices, one with a defective contact that could however be used as a gate to control the carrier density in the device and the other with all contacts working. We discuss the results in terms of the dependence of the noise power spectral density on the carrier density and on the DC bias current. Finally, we provide an estimate of the detection limit, pointing out that the approach commonly used in the literature for its evaluation is flawed.

Performance analysis of correlation techniques for noise measurements

The cross-correlation technique makes it possible to perform noise measurements with a sensitivity that would otherwise be unreachable, well below the noise floor of the amplifiers. Not all noise contributions from the amplifiers can however be eliminated or even just attenuated by cross-correlation: therefore it is important to take into consideration the detailed characteristics of the DUT (Device Under Test) and of the amplifiers when setting up the measurement system. Here we discuss the relative advantages of the different (“series” and “parallel”) configurations coupled with our technique for the accurate evaluation of the transimpedance between the noise source to be measured and the amplifier output. In particular, we show (i) the importance of the comparison between the real and the imaginary part of the cross-spectrum due to the asymmetry of the correlation amplifiers and (ii) how to estimate the maximum number of averages in the cross-spectrum evaluation that leads to an actual advantage from the point of view of the measurement accuracy. Finally we discuss the issue of shielding from external spurious signals, whose relevance is often underestimated.

Shot noise suppression in p‐n junctions due to carrier recombination

We have investigated shot noise suppression as a function of bias current in gallium arsenide and silicon p‐n junctions, focusing on the effect of generation‐recombination phenomena. The availability of the cross‐correlation technique and of ultra‐low‐noise amplifiers has allowed us to significantly extend the range of bias values for which results were available in the literature. We have then developed a numerical model, based on the Monte Carlo method, which provides a qualitative explanation of the observed noise behavior, and, with the adjustment of a fitting parameter, exhibits satisfactory agreement with the experimental results.

MESFET cryogenic front-end for cross-correlation noise measurements,

We present a dual‐channel transimpedance amplifier for cross‐correlation noise measurements with a MESFET cryogenic front‐end and cooled feedback resistors. In order to test the noise performance of the system, devices with known noise power spectral density (i.e. resistors) have been measured. In particular, such a system has been able to reliably measure a noise current spectral density lower than 1 fA2/Hz in a bandwidth larger than 3 kHz.

Transition between Pauli exclusion and Coulomb interaction in the noise behavior of resonant tunneling devices

We present results from noise measurements on double barrier resonant tunnel diodes, down to extremely low current bias values. Such measurements were performed for current levels between 0.8 pA and 10 nA, in order to investigate the theoretically predicted deviations from full shot noise due to the interplay of Coulomb repulsion and Pauli exclusion. While Coulomb effects are visible also in the proximity of the current peak, Pauli exclusion is expected to contribute only at very low current levels, which has motivated our effort towards an increase in the sensitivity of the measuring equipment.