G. Sabatini

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.

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.

Plasma waves subterahertz optical beating detection and enhancement in long-channel high-electron-mobility transistors: experiments and modeling

A photomixed laser beam of two 1.55 mum continuous-wave lasers is used for interband photoexcitation in submicron gate length InAlAs/InGaAs transistors. Results show the clear excitation of plasma oscillation modes in the transistor channel. A strong amplification of the optical beating detection in the 0-600 GHz range is observed as a function of drain-source voltage. Numerical results, using hydrodynamic model coupled to a pseudo-2D Poisson equation, are in good agreement with experiments concerning the plasma frequency dependence with gate voltage. Moreover, this model confirms both optical beating detection at subterahertz frequencies and the enhancement observed when drain-source voltage increases.

Terahertz three-dimensional plasma resonances in InGaAs diodes: A hydrodynamic study

We investigate 3D plasma resonances in InGaAs n+ − n − n+ diodes undergoing an optical beating excitation at room temperature. For this sake, we calculate the electric field response in the middle of the diode regions by using a hydrodynamic approach coupled to a one-dimensional Poisson solver. The results show clearly the presence of three-dimensional plasma resonances in the terahertz frequency domain for the two region types. We also emphasize a strong coupling between the plasma modes. Finally, the influence of both geometry and doping profile on the amplitude and the frequency of the resonances is evaluated.

Conception and fabrication of InAs-based hot electron transistor

An innovative hot electron transistor based on InAs/AlSb heterostructures is proposed and studied. First fabricated devices demonstrated static current gain of 5 at room temperature. The potential of this device for high frequency operation is also discussed.

Anomalous behavior of electronic noise related to the onset of current instabilities inn+nn+ diodes

We investigate the noise behavior in the presence of current instabilities induced by optical phonon emission. Calculations are carried out by means of Monte Carlo particle simulations of InP n+nn+ diodes at lattice temperatures below about 80 K. A giant enhancement of the low-frequency noise is observed in a narrow voltage region, which precedes the onset of current self-oscillations caused by the instability. A possible origin of this enhancement is discussed. The impact of the non-linear effects on the anomalous low-frequency noise behavior is stressed.

Monte Carlo simulation of ballistic transport in high-mobility channels

By means of Monte Carlo simulations coupled with a two-dimensional Poisson solver, we evaluate directly the possibility to use high mobility materials in ultra fast devices exploiting ballistic transport. To this purpose, we have calculated specific physical quantities such as the transit time, the transit velocity, the free flight time and the mean free path as functions of applied voltage in InAs channels with different lengths, from 2000 nm down to 50 nm. In this way the transition from diffusive to ballistic transport is carefully described. We remark a high value of the mean transit velocity with a maximum of 14×105 m/s for a 50 nm-long channel and a transit time shorter than 0.1 ps, corresponding to a cutoff frequency in the terahertz domain. The percentage of ballistic electrons and the number of scatterings as functions of distance are also reported, showing the strong influence of quasi-ballistic transport in the shorter channels.

Plasma waves induced by the optical beating in HEMT channels as an expected source of TeraHertz radiation generation

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.

Plasmonic noise in silicon nanolayers

We report a microscopic investigation of the spectrum of voltage fluctuations in nanometric n-Si layers. Theory makes use of a Monte Carlo simulator self-consistently coupled with a two-dimensional Poisson solver. We consider layers of variable thickness W in the range of 2–100 nm and variable length L in the range of 10–1000 nm embedded in an external dielectric medium. Calculations are performed at T=300 K for different doping levels and in the presence of an applied voltage of increasing strength. The spectra are found to exhibit peaks centered on the terahertz region. For W≥100 nm and carrier densities of 5×1017 and 5×1018 cm−3, the frequency peaks agree with the value of the three dimensional plasma frequency. For W≤100 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 of 0.2–10 THz and is in agreement with the values of the two-dimensional plasma frequency predicted by existin...

Electronic, optical and thermal excitation of plasma waves in HEMTs: A theoretical study

We investigate the influence of collective plasma modes in a field-effect transistor channel under different excitations and biasing conditions. First, we study the case of a device externally-excited by a harmonic optical beating or an electronic excitation and current-driven at the drain. The harmonic and continuous responses of the transistor are calculated using a pseudo-two-dimensional hydrodynamic approach. They show sharp resonances related to the first odd plasma modes, whose frequencies and amplitudes can be modified by playing on the drain bias. Then, through the generalized impedance-field method we calculate also the spectral density of drain voltage fluctuations in the absence of external excitations. Also these noise spectra exhibit peaks corresponding to the odd plasma modes.