Lino Reggiani

Diffusion coefficient of electrons in silicon

This paper reports an experimental and theoretical analysis of the diffusivity of electrons in Si as function of temperature, field strength, and field direction. Results for the longitudinal diffusion coefficient have been obtained experimentally for fields applied along 〈111〉 and 〈100〉 directions with time‐of‐flight and noise measurements. Calculations have been performed with the Monte Carlo procedure. The theoretical analysis, which includes an extensive discussion of the intervalley diffusion process, has yielded a revised version of the silicon model which correctly interprets both the new diffusion data and other well‐established electron transport properties. The revision of the model is mainly concerned with the relative weights of f and g intervalley scattering mechanisms. In fact the interpretation of the anisotropy of the diffusion allows separate estimates of the two types of scattering through their different effects on the intervalley diffusion which comes about when electrons have differen...

Bulk hot-electron properties of cubic semiconductors

Abstract This paper contains a review of charge transport properties at high electric fields in bulk cubic semiconductors. The microscopic theoretical interpretation follows a semi-classical approach and is based on the knowledge of the band structure and scattering mechanism of the material under investigation. For the solution of the Boltzmann equation, the Monte-Carlo simulation technique is considered, which provides an ‘exact’ numerical solution limited only by the simplifying assumptions inherent in the physical model assumed. Experimental techniques for the measurements of the most important transport quantities are briefly surveyed. Comparison between theory and experiment is reported for both electron and hole transport properties in Si, Ge and GaAs. These substances, besides being the best known materials, can in fact be considered as models for any other cubic semiconductor.

Monte Carlo algorithm for hot phonons in polar semiconductors

We present a novel ensemble Monte Carlo procedure for the study of electron and phonon dynamics during the relaxation of photoexcited hot carriers. For the first time hot‐electron and hot‐phonon effects are included together in the same Monte Carlo simulation. The algorithm is applied to a simplified model of GaAs, consisting of one‐type carriers (electrons) in a two‐valley system (L and Γ valleys). The buildup of the phonon population on a picosecond scale is monitored, in parallel with the cooling of the electron distribution. As expected, the presence of nonequilibrium phonons is found to slow down the electron relaxation.

Drift and diffusion of charge carriers in silicon and their empirical relation to the electric field

Abstract Simple empirical formulae, with three adjustable parameters, are employed in describing the drift velocity and longitudinal diffusion coefficient of charge carriers in purified silicon, over a wide range of electric field (10–10 5 V/cm). The effects of the lattice temperature for the range 77–430 K are also considered and a comparison with the experimental data is made. The proposed formulae can be useful for device simulation using silicon.

Trapping-detrapping fluctuations in organic space-charge layers

A trapping-detrapping model is proposed for explaining the current fluctuation behavior in organic semiconductors (polyacenes) operating under current-injection conditions. The fraction of ionized traps obtained from the current-voltage characteristics, is related to the relative current noise spectral density at the trap-filling transition. The agreement between theory and experiments validates the model and provides an estimate of the concentration and energy level of deep traps.

A network model to correlate conformational change and the impedance spectrum of single proteins

One of the main trend in to date research and development is the miniaturization of electronic devices. In this perspective, integrated nanodevices based on proteins or biomolecules are attracting a major interest. In fact, it has been shown that proteins like bacteriorhodopsin and azurin, manifest electrical properties which are promising for the development of active components in the field of molecular electronics. Here we focus on two relevant kinds of proteins: The bovine rhodopsin, prototype of GPCR protein, and the enzyme acetylcholinesterase (AChE), whose inhibition is one of the most qualified treatments of Alzheimer disease. Both these proteins exert their functioning starting with a conformational change of their native structure. Our guess is that such a change should be accompanied with a detectable variation of their electrical properties. To investigate this conjecture, we present an impedance network model of proteins, able to estimate the different electrical response associated with the different configurations. The model resolution of the electrical response is found able to monitor the structure and the conformational change of the given protein. In this respect, rhodopsin exhibits a better differential response than AChE. This result gives room to different interpretations of the degree of conformational change and in particular supports a recent hypothesis on the existence of a mixed state already in the native configuration of the protein.

Field-assisted capture of electrons in semi-insulating GaAs

We present a drift-diffusion modeling of the electric-field profile in semi-insulating n-GaAs detectors accounting for hot-carrier dynamics and the associated kinetics of electrical active traps. From the fitting of the detector active thickness we infer unambiguous evidence of a field-enhanced capture cross section from the two deepest electron traps we attribute to EL2 and EL3 centers.

Current and number fluctuations in submicron n+nn+ structures

Abstract We present a detailed analysis of current and number fluctuations in submicron n + nn + Si structures at different bias voltages and lengths of the active region. The calculation is carried out by coupling self-consistently a one-dimensional Poisson solver with a three-dimensional Ensemble Monte Carlo simulator. The coupling between fluctuations in carrier velocity and in the self-consistent field is found to be responsible for a negative part (a minimum) in the autocorrelation function of current fluctuations at equilibrium. The fluctuations of the carrier number in different slices and in the whole structure are influenced by space-charge effects and by the inhomogeneity of the structure. They are found to be a sensitive probe for characterizing different contact models. With increasing applied voltage the coupling between velocity and electric-field fluctuations weakens due to a less effective screening.

Monte Carlo algorithms for collisional broadening and intracollisional field effect in semiconductor high‐field transport

We present a computational scheme which can be readily included in a standard Monte Carlo code to give quantitative estimates of the intracollisional field effect (ICFE) or of collisional broadening (CB). The central quantity in our theory is the joint spectral density K(e i , e f  ) which connects the initial and final kinetic energy of a carrier in a scattering event. Crucial approximations, connected with the algorithms which model the joint spectral densities obtained from the appropriate Dyson equation, are clearly stated and discussed. Numerical results for a simple modelsemiconductor are provided for illustrative purposes. These show that both ICFE and CB lead to a significant increase of carriers in the high‐energy tail of the distribution function.

Problems of noise modeling in the presence of total current branching in high electron mobility transistor and field-effect transistor channels

In the framework of analytical and hydrodynamic models for the description of carrier transport and noise in high electron mobility transistor/field-effect transistor channels the main features of the intrinsic noise of transistors are investigated under continuous branching of the current between channel and gate. It is shown that the current-noise and voltage-noise spectra at the transistor terminals contain an excess noise related to thermal excitation of plasma wave modes in the dielectric layer between the channel and gate. It is found that the set of modes of excited plasma waves can be governed by the external embedding circuits, thus violating a universal description of noise in terms of Norton and Thevenin noise generators.