L. Varani

Voltage tuneable terahertz emission from a ballistic nanometer InGaAs∕InAlAs transistor

Terahertz emission from InGaAs∕InAlAs lattice-matched high electron mobility transistors was observed. The emission appears in a threshold-like manner when the applied drain-to-source voltage UDS is larger than a threshold value UTH. The spectrum of the emitted signal consists of two maxima. The spectral position of the lower-frequency maximum (around 1 THz) is sensitive to UDS and UGS, while that of the higher frequency one (around 5 THz) is not. The lower-frequency maximum is interpreted as resulting from the Dyakonov–Shur instability of the gated two-dimensional electron fluid, while the higher frequency is supposed to result from current-driven plasma instability in the ungated part of the channel. The experimental results are confirmed by and discussed within Monte Carlo calculations of the high-frequency current noise spectra.

Microscopic simulation of electronic noise in semiconductor materials and devices

We present a microscopic interpretation of electronic noise in semiconductor materials and two-terminal devices. The theory is based on Monte Carlo simulations of the carrier motion self-consistently coupled with a Poisson solver. Current and voltage noise operations are applied and their respective representations discussed. As application we consider the cases of homogeneous materials, resistors, n/sup +/nn/sup +/ structures, and Schottky-barrier diodes. Phenomena associated with coupling between fluctuations in carrier velocity and self-consistent electric field are quantitatively investigated for the first time. At increasing applied fields hot-carrier effects are found to be of relevant importance in all the cases considered here. As a general result, noise spectroscopy is found to be a source of valuable information to investigate and characterize transport properties of semiconductor materials and devices. >

Terahertz spectroscopy of plasma waves in high electron mobility transistors

We report on systematic measurements of resonant plasma waves oscillations in several gate-length InGaAs high electron mobility transistors (HEMTs) and compare them with numerical results from a specially developed model. A great concern of experiments has been to ensure that HEMTs were not subject to any spurious electronic oscillation that may interfere with the desired plasma-wave spectroscopy excited via a terahertz optical beating. The influence of geometrical HEMTs parameters as well as biasing conditions is then explored extensively owing to many different devices. Plasma resonances up to the terahertz are observed. A numerical approach, based on hydrodynamic equations coupled to a pseudo-two-dimensional Poisson solver, has been developed and is shown to render accurately from experiments. Using a combination of experimental results and numerical simulations all at once, a comprehensive spectroscopy of plasma waves in HEMTs is provided with a deep insight into the physical processes that are involved.

Hydrodynamic modeling of optically excited terahertz plasma oscillations in nanometric field effect transistors

We present a hydrodynamic model to simulate the excitation by optical beating of plasma waves in nanometric field effect transistors. The biasing conditions are whatever possible from Ohmic to saturation conditions. The model provides a direct calculation of the time-dependent voltage response of the transistors, which can be separated into an average and a harmonic component. These quantities are interpreted by generalizing the concepts of plasma transit time and wave increment to the case of nonuniform channels. The possibilities to tune and to optimize the plasma resonance at room temperature by varying the drain voltage are demonstrated.

Monte Carlo simulation of the generation of terahertz radiation in GaN

The conditions for microwave power generation at low temperatures under optical phonon emission are analyzed by Monte Carlo simulations of both small- and large-signal responses in bulk zinc blende and wurtzite GaN. As a result of the high optical phonon energy and the strong interaction of electrons with optical phonons in GaN a general improvement on the transit-time resonance and a considerable increase in the maximum generation frequency and power can be achieved in comparison to the widely studied III–V materials such as GaAs and InP. A dynamic negative differential mobility caused by transit-time resonance occurs in a wide frequency range of about 0.05–3 THz and persists in the THz frequency range up to the liquid nitrogen temperature with doping levels up to about 5×1016 cm−3. The efficiency of the amplification and generation is found to depend nonmonotonously on static and microwave electric field amplitudes, generation frequency, and doping level so that for each generation frequency there exist...

Modelling of small-signal response and electronic noise in semiconductor high-field transport

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 Ohms law and Nyquists 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.

Monte Carlo analysis of the behavior and spatial origin of electronic noise in GaAs MESFET's

We present a Monte Carlo (MC) analysis of electronic noise associated with velocity and field fluctuations in GaAs MESFETs. To this end, an accurate estimator of the instantaneous currents at the terminals is used, which improves the precision of the method. Both the current and voltage fluctuations at the different terminals of the device are investigated, thus allowing for the spatial localization of the noise sources. Three different MESFET geometries are analyzed. The results so found compare well with experimental results and confirm the general trend provided by existing phenomenological noise modeling. As a general result, the noise in the drain current is found to increase with the level of the current and remain constant with frequency at least up to 100 GHz. In the case of the gate current, the noise is null at low frequency and then increases quadratically. Under saturation conditions, the source of the drain-voltage fluctuations is localized at the drain end of the n channel, and even penetrates the drain n/sup +/ region due to the presence of hot carriers in the upper valleys. >

Plasma-Wave Detectors for Terahertz Wireless Communication

We report on terahertz wireless communication experiments at 0.3 THz using 250-nm gate-length GaAs/AlGaAs field-effect transistor (FET) as a detector and unitraveling-carrier photodiode as a source. The physical mechanism of the detection process is terahertz wave rectification on nonlinearities related to overdamped plasma oscillations in the transistor channel. We present an experimental study of rectification bandwidth and show for the first time that room-temperature direct detection with modulation bandwidth of up to 8 GHz can be achieved, thus showing that nanometer-sized FETs can be used as valuable detectors in all-solid-state terahertz wireless communication systems.

Linear and nonlinear analysis of microwave power generation in submicrometer n+nn+ InP diodes

A closed hydrodynamic model and the associated numerical procedures are developed for simulating hot‐carrier transport in submicron semiconductor devices. To check the validity of the model, the steady‐state characteristics of near‐micron n+nn+ InP diodes are compared with a standard Monte Carlo approach. The excellent agreement found fully validates the physical reliability of our model which has been further developed to investigate linear and nonlinear time‐dependent characteristics. The contribution of each part of the device, when operating as microwave power‐generation, is analyzed through the spatial profiles of the impedance‐field spectrum. The usual subdivision of the n‐region into a passive (dead‐zone) and active zone is carried out. The dead zone is found to manifest itself as a purely real resistance which is practically independent of the frequency. One or more spatial zones which are responsible for the generation are shown to be formed in the active region of the diode. By reducing the leng...

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.