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International audience; We propose in the present work a numerical solution employed to treat the coupled Monte Carlo method and Poisson equations. This technique is capable of capturing some important features of semiconductor devices.Numerical results are presented for one-dimensional Hg0.8Cd0.2Te n nn structure, the presence of velocity overshoot has been observed and it is recognized that the fluctuation of velocity and energy term plays an important role in the simulation of semiconductor devices.

Results of hydrodynamic simulation of HEMT performance under photo‐mixing excitation of free carriers in the conducting channel are presented. A resonant enhancement of current amplitudes at the beating frequency due to excitation of 2D plasma waves is demonstrated.

The spatio‐temporal evolution of Gunn and Ryzhii current instabilities and accompanied noise behavior in n+nn+ structures is investigated by the Monte Carlo particle technique. It is found that the transition from a passive‐state to a generation‐state is accompanied by an anomalous giant enhancement of the noise at frequencies quite lower than those corresponding to generation. By comparing various transport and noise features it is demonstrated that such a giant enhancement of low‐frequency noise can be considered as an indicator for the onset of a high‐frequency instability.

In this paper we propose an analytical model for the calculation of the spectral density of current fluctuations in Heterostructure-barrier varactors . The structures of the calculated spectra are analyzed in terms of physical processes useful to optimize the device parameters for the extraction of the high-order harmonics.

We simulate electron transport in ultra small mercury-cadmium-telluride n+-n-n+ diodes using a hydrodynamic approach. A numerical staggered solution is employed to treat the coupled hydrodynamic and Poisson equations, where the spatial profiles of the main transport parameters within the diodes are analyzed including the Auger generation-recombination processes. Our numerical results show that, even for low applied voltages, impact ionization processes are activated and affect dramatically the current-voltage characteristics of the Hg0.8Cd0.2Te diode.

By Monte Carlo simulations we investigate the spectrum of voltage fluctuations of Si layers of variable thickness W in the range 2 ÷ 100 nm and variable length L in the range 10÷1000 nm embedded in an external dielectric medium. Calculations are performed at T = 300 K for di!erent doping levels and in the presence of an external bias of increasing strength. For W ! 100 nm and carrier densities of 5 " 10 17 and 5 " 10 18 cm ! 3 the peaks agree with the value of the three dimensional (3D) plasma frequency. For W # 10 nm the results exhibit a plasma frequency that depends on L, thus implying that the oscillation mode is dispersive. The corresponding frequency covers a wide range of values 0.2 ÷ 10 THz and is in agreement with the values predicted by existing analytical models. At su"ciently high bias the 2D plasma peak is washed out and we observe the onset of a peak in the subTHz region which, by analogy with the results obtained in n-InGaAs layers, is associated with transit time instabilities induced by current saturation conditions.

We survey recent results on electronic noise in nanostructured devices. Three kinds of excess noise referring to hot-carriers in submicron n+nn+ diodes, shot noise in mesoscopic structures, resistance fluctuations in thin film conductors, are considered. The influence of down-sizing on fluctuations is illustrated in each case.

The conditions for THz radiation generation caused by electron transit-time resonance in momentum and real spaces under optical phonon emission are analyzed for nitride-based materials and their structures. It is shown that such a mechanism provides a unique possibility to realize sub-THz and THz radiation generation at the border between the electro-optical and electronic techniques by using two alternative approaches: (i) amplification of transverse electromagnetic waves in 3D bulk materials and 2D quantum wells, and (ii) longitudinal current-field instabilities in sub-micron and micron n(+)nn(+) diodes. Estimations of frequency regions, output power and efficiency of the generation demonstrate that nitrides are promising materials for THz radiation generation.

We present an analytical model suitable for the study of the plasma modes in gated semiconductor slabs of arbitrary thickness. A pseudo-two-dimensional Poisson equation allows us to consider both transverse and longitudinal electric field variations. We calculate the dispersion relation demonstrating the dispersive nature of the slab. We express the frequencies of the plasma modes appearing in a cavity. A transition from a two-dimensional to a three-dimensional behavior is revealed when the transverse dimension of the device or the order of modes grow. These analytical results show a good agreement with Monte Carlo calculations of the voltage noise spectrum.

A universal description of the OPTTR-assisted generation band of THz radiation is developed in the framework of a 3D transport in bulk materials and 2D transport in QWs and HLs. On the one hand, in the framework of an ideal streaming motion we have carried out an analytical investigation based on phonon scattering rates: namely, elastic deformation acoustic phonons in the passive energy region below the optical-phonon energy 0 and emission of polar optical phonons in the active energy region, 0. On the another hand, quantitative estimations of the qualitative constraints given by Eqs. 1 and 2 for the streaming conditions are obtained on the basis of numerical calculations of the OPTTR DNDM by the Monte Carlo method. It is found that i for Eq. 1 the average momentum relaxation time in the passive region must satisfy the condition −1–2E and ii for Eq. 2 carrier penetration into the active region must occurr at a level less than 14%–15%. In the framework of such a model we have provided simple analytical expressions that estimate the low- and high-frequency limits of the generation band determined by, respectively, the average relaxation time − see Eq. 25 and the carrier penetration into the active region see Eq. 33. Having in mind that for THz radiation generation the high-frequency limit is of the most interest, for the highfrequency cutoff of the DNDM we have found that i for 3D bulk materials the relevant physical quantities are the carrier effective mass, the optical-phonon energy, and the polaroptical coupling strength; ii in passing from 3D to 2D vertical transport, for the same material the influence of 2D transport on the OPTTR is characterized entirely by the dimensionless parameter k0d related to both the radius of the optical-phonon sphere in wave-vector space k0 and the effective width of the electron localization d associated with the lowest miniband of the QW/HL structure; iii in going from 3D to 2D transport, the change of the energy dependence of the density of states is responsible for an extension of the maximum generation frequency for up to a factor of 5 times. In essence, such a model gives the “upper” estimation of the generation band limits determined primarily by the parameters of a bulk material and a 2D structure. Any other scatterings not incorporated directly into our model— namely, impurity, electron-electron, interface roughness, etc.—act mainly on the low-frequency limit since one can speak about the upper limit only in the case when an electron runaway from the regime of low-energy scatterings in the passive region takes place. Of course, the presence of additional low-energy scatterings will increase the low-frequency limit and eventually destroy the generation. Nevertheless, additional scattering mechanisms can be simply incorporated into the model by taking into account their contribution to the average momentum relaxation time in the passive region − as 1/−=1/DA+1/imp +1/ee. For example, such a procedure was used to estimate − in the experimental observation of the OPTTR generation in bulk InP.20 These estimations give relaxation times, which is reasonably close to the values estimated above.

International audience; The conditions for THz radiation generation caused by electron transite-time resonance in momentum and real spaces under low-temperature optical-phonon emission are analysed. Main attention is paid to transverse electromagnetic waves generation in: (i) 3D bulk materials and (ii) 2D quantum heterosctructures as well as (iii) longitudinal current instabilities in submicron and overmicron n+nn+ diodes. It is found that in all the cases considered the nitride-based materials and structures are the most preferable for the THz radiation generation as compared with III-V compounds. Furthermore, a considerable increase up to 5 times of the high-frequency cutoff for THz radiation generation is predicted in going from 3D to 2D transport.

A possibility to develop the so called TASER (Terahertz-Amplification-by-the-Stimulated-Emission-of- Radiation) by using two-dimensional (2D) electron transport in quantum well (QW) structures is investigated by Monte Carlo simulation of the optical-phonon-emission assisted transit-time resonance (OPTTR) of 2D electrons in momentum space under the low lattice temperature. A considerable extension of the frequency region for THz radiation generation (upto 5 times) when going from 3D- to 2D-case is predicted.

International audience; We report Monte Carlo simulations of electronic noise in heavily doped nanometric GaAs Schottky-barrier diodes (SBDs) recently proposed as promising devices for THz applications. We consider a SBD operating in series with a parallel output resonant circuit when a high-frequency large-signal voltage is applied to the whole system. Significant modifications of the noise spectrum with respect to the diode subjected to a constant applied voltage are found to occur in the THz-region. To interpret such behaviour, we have developed a simple analytical approach based on the static I-V and C-V relations as well as on the series resistance of the SBD

We present a model for electron injection from thermal reservoirs which is applied to particle simulations of one-dimensional mesoscopic conductors. The statistics of injected carriers is correctly described from nondegenerate to completely degenerate conditions. The model is validated by comparing Monte Carlo simulations with existing analytical results for the case of ballistic conductors. An excellent agreement is found for average and noise characteristics, in particular, the fundamental unities of electrical and thermal conductances are exactly reproduced., Comment: 4 pages, revtex, 4 PS figures, accepted Semicond. Sci. Technol

The intra- and inter-collisional time scales are investigated in terms of relations between the spectral densities of velocity and acceleration fluctuations, S v (ω) and S a (ω) , caused by the carrier interaction with the thermal bath. From the quantum Nyquist form of S v (ω) it is shown that the high-frequency cut-off of S a (ω)=ω 2 S v (ω) indicates the non-Markovian nature of the carrier interaction with the thermal bath on the time scale t k B T 0 . In the extreme quantum limit when the interaction process becomes continuous in time the variances of acceleration and velocity fluctuations along a given direction are found to satisfy the universal relation a 2 / v 2 =(2 π 2 /5) Ω 2 with Ω = k B T 0 /ℏ the thermal quantum frequency.

We show that within the transfer impedance method, the spectral strength of the local noise source originally defined by Van Vliet et al. [J. Appl. Phys. 46, 1804 (1975)] should be more generally replaced by the two-point spectral density of current fluctuations. The theoretical arguments are further supported by numerical results for the voltage noise spectrum associated with conduction–current fluctuations in submicron n+nn+ structures. The above two-point spectral strength is, in general, a complex quantity related to fluctuations induced by velocity fluctuations only.

We propose a novel numerical procedure to calculate the hot-carrier thermal conductivity in bulk semiconductors. The method is based on combining Monte Carlo and hydrodynamic simulations of carrier transport in submicron inhomogeneous structures. Application to a Si structure indicates a decrease over one order of magnitude of the thermal conductivity of electrons at electric fields over , in close agreement with recent results obtained within the correlation-function formalism.

A microscopic analysis of current and voltage fluctuations in GaAs Schottky barrier diodes under forward-bias conditions in the absence of 1/f contributions is presented. Calculations are performed by coupling self-consistently an ensemble Monte Carlo simulator with a one-dimensional Poisson solver. By using current- and voltage-operation modes the microscopic origin and the spatial location of the noise sources, respectively, is provided. At different voltages the device exhibits different types of noise (shot, thermal, and excess), which are explained as a result of the coupling between fluctuations in carrier velocity and self-consistent field. The essential role of the field fluctuations to correctly determine the noise properties in these diodes is demonstrated. The results obtained for the equivalent noise temperature are found to reproduce the typical behavior of experimental measurements. An equivalent circuit is proposed to predict and explain the noise spectra of the device under thermionic emis...

We present a survey on the theoretical modelling of the small-signal response and noise associated with velocity fluctuations in semiconductor high-field transport. Because of the high values of the applied electric field, current - voltage characteristics and electrical noise are found to deviate strongly from Ohm's law and Nyquist's relation respectively. Accordingly, in the case of homogeneous (bulk) structures the field and frequency dependence of the differential mobility, diffusivity and electronic noise temperature are investigated within a rigorous microscopic approach which solves exactly the appropriate kinetic equations through analytical and Monte Carlo techniques. Spectral functions in the frequency domain are obtained from their correspondent response and correlation functions in the time domain. The subject is also analysed within a balance-equation approach which enables us to obtain simple analytical expressions which can provide a direct microscopic interpretation and can be applied to device modelling. For validation purposes calculations are applied to the relevant case of holes in Si and electrons in GaAs. In the latter material the presence of negative differential conductivity (Gunn effect) leads to interesting behaviour of the small-signal response and noise spectra which are also investigated for the simplest prototype of non-homogeneous structures, that is the diode. The comparison between the different approaches so developed and between calculations and experiments is found to be quite good, thus providing a quantitative microscopic interpretation of the main features associated with small-signal response and fluctuations in semiconductors under high-field conditions.

A universal thermal conductance of charge carriersK=2π2 k B 2 T/(3h) is rigorously derived within a correlation-function formalism. Similar to the case of the universal electrical conductanceG=2e 2 /h this result pertains to one-dimensional, ballistic, and degenerate conditions for non-interacting particles.

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The time‐and‐frequency behavior of hot‐carrier noise in submicrometer n + nn + GaAs diodes is investigated theoretically using the Monte Carlo method. We have continuously investigated the noise from current‐to‐voltage operation mode by calculating the noise‐power spectrum at the terminals of a noiseless load‐resistance R connected in series with the diode. By varying appropriately the value of R we have calculated the small‐signal impedance of the diode and then obtained the full spectrum of the noise temperature. Under voltage‐operation mode the current–noise spectrum exhibits two resonant peaks at the transit‐time and plasma frequencies, respectively. Under current operation mode, all current oscillations are effectively damped, and the voltage–noise spectrum exhibits a quasi‐Lorentzian shape, which vanishes at the transit‐time frequency. The behavior of hot‐carrier noise closely parallels the frequency dependence of the diode small‐signal impedance, which exhibits a dynamic negative differential resistance in the frequency range corresponding to the transit‐time resonance.

A double S‐type instability in an original semiconductor n+/n‐GaAs/i‐AlGaAs/i‐GaAs/p+ heterostructure is investigated theoretically by a Monte Carlo simulation. The mechanism responsible for the second instability is the hot‐electron current which, besides itself being a source of an S‐type instability, drives a hot‐hole current whose sharp switching‐on provokes a sudden increase of the heterostructure conductance. An oscillatory behavior of the voltage drop between the heterostructure contacts is predicted. The maximum frequency of the generation so obtained is found to increase from 100 to 500 GHz when the temperature is lowered from 300 to 4 K.

International audience

We analyse the effects of a non-equilibrium phonon population on noise and diffusion phenomena in polar semiconductors. We calculate the current and energy autocorrelation functions and the current-energy cross-correlation functions, for both the case with and without hot-phonon effects. Owing to the presence of hot phonons, we find an increase of the noise equivalent temperature and of the longitudinal diffusion coefficient at intermediate field strengths below the threshold for negative differential mobility.

We present a theoretical study of generation-recombination processes from shallow impurities which includes self-consistently the contribution of the impurity excited levels. Making use of an original Monte Carlo simulation which accounts for both the kinetic energy of the carrier and its potential energy near the impurity, we have solved the semiclassical Boltzmann equation to investigate the dynamics of generation-recombination processes on a kinetic level. Calculations performed for the case of p-type Si clearly evidence a nonexponential distribution of the microscopic times spent by the carriers in the impurity centers and in the conducting band

By using the Monte Carlo method, submicron Si n+nn+ structures are analysed at different bias voltages and lengths of the n region. The autocorrelation function of current fluctuations and the variance of carrier number in different parts of the device is used to investigate the correlations induced by the self-consistent solution of the Boltzmann and Poisson equations and the influence of the modelling of the contacts.

We present a general theory to investigate the electronic noise in the presence of scattering as well as of generation-recombination processes. An exact decomposition procedure of the current spectral density is given that, in addition to fluctuations in carrier velocity and number, shows the presence of a cross term coupling both fluctuations. Four correlation functions are thus found to be needed to evaluate all terms. To this purpose, the Monte Carlo method is shown to provide a unifying microscopic calculation of these functions. We consider the case of p-type Si at 77 K with a generation-recombination mechanism given by the capture at shallow impurities assisted by acoustic phonons. Then, the theoretical results are compared with existing mesoscopic theories as well as with available experimental results.

We present an original Monte Carlo study of correlation functions for the general case of a doped semiconductor under the influence of an electric field of arbitrary strength. Number, velocity, and energy of the carriers are taken as relevant variables. These lead to a set of 25 correlation functions that, for the case of a cubic semiconductor and an electric field in the 〈100〉 direction as investigated in this paper, reduce to 11 nonvanishing functions. By using an analytical model based on coupled Langevin equations together with the results of the Monte Carlo simulations, we have quantitatively analyzed the different coupling processes as functions of the electric field. The simulations are performed for lightly doped p-type Si at 77 K. We demonstrate that even at equilibrium the coupling between energy relaxation and generation-recombination processes can lead to a strongly nonexponential decay of the corresponding correlation functions. At higher fields, we find the interesting result of a transition from two real relaxation rates for energy and longitudinal velocity to a pair of complex-conjugate values, which indicates some kind of ordering in the system driven by the electric field. At the highest fields, this ordering disappears and the rates again become real. The theory so developed is shown to provide a rigorous scheme for a microscopic interpretation of noise-spectroscopy measurement.

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