C. Palermo

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.

Room temperature coherent and voltage tunable terahertz emission from nanometer-sized field effect transistors

We report on reflective electro-optic sampling measurements of terahertz emission from nanometer-gate-length InGaAs-based high electron mobility transistors. The room temperature coherent gate-voltage tunable emission is demonstrated. We establish that the physical mechanism of the coherent terahertz emission is related to the plasma waves driven by simultaneous current and optical excitation. A significant shift of the plasma frequency and the narrowing of the emission with increasing channel’s current are observed and explained as due to the increase in the carriers’ density and drift velocity.

Room-temperature terahertz mixer based on the simultaneous electronic and optical excitations of plasma waves in a field effect transistor

A method for the heterodyne detection of terahertz (THz) signals is proposed. A high electron mobility transistor is used as a nonlinear element, while the optical beating of two laser beams exciting plasma waves in the transistor channel plays the role of the THz local oscillator. High efficiency and room-temperature operation of such a mixer are demonstrated by numerical simulations.

Terahertz generation in nitrides due to transit-time resonance assisted by optical phonon emission

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.

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.

Monte Carlo investigation of terahertz plasma oscillations in ultrathin layers of n-type In0.53Ga0.47As

By numerical simulations we investigate the dispersion of the plasma frequency in a n-type In0.53Ga0.47As layer of thickness W and submicron length at T=300K. For W=100nm and carrier concentrations of 1016–1018cm−3 the results are in good agreement with the standard three-dimensional (3D) expression of the plasma frequency. For W⩽10nm the results exhibit a plasma frequency that depends on L, thus implying that the oscillation mode is dispersive. The corresponding frequency values are in good agreement with the two-dimensional (2D) expression of the plasma frequency obtained for a ballistic regime within the in-plane approximation for the electric field. A region of cross over between the 2D and 3D behaviors of the plasma frequency is evidenced for W>10nm.

Enhanced THz Detection Through Phase-Controlled Current Response in Field-Effect Transistors

A field effect transistor can be used as a nonlinear element for the resonant detection of incident terahertz (THz) radiation at room temperature. The excitation of the plasma modes in the channel significantly increases the detection efficiency in the THz range. By means of a numerical hydrodynamic model, we study the drain-current response of a high electron mobility transistor to a THz signal applied on its gate and/or on its drain contacts to obtain the optimal configuration in terms of detection. We demonstrate that the amplitudes of the harmonic and average drain-current responses associated with the presence of plasma modes in the channel strongly depend on which transistor terminal collects the incident THz radiation and that a maximum dcresponse can be obtained by appropriately dephasing the two electrode signals.

Terahertz emission induced by optical beating in nanometer-length field-effect transistors

We report on photo-induced terahertz radiation with a high spectral purity generated by a submicron sized InGaAs-based high-electron-mobility transistor. The emission peak is due to the electron-hole pairs photocreated in the transistor channel at the frequency of the beating of two cw-laser sources. The radiation frequency corresponds to the lowest fundamental plasma mode in the gated region of the transistor channel. The observed high emission quality factor at 200 K is interpreted as a result of stream-plasma instability in the two-dimensional electron gas whose appearance is emphasized by the reduction of the velocity relaxation rate with the temperature.

Plasma resonances in a gated semiconductor slab of arbitrary thickness

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.

Monte Carlo calculations of static and dynamic electron transport in nitrides

Monte Carlo simulation of high-field transport in semiconductor nitrides GaN and InN is used to calculate the velocity field and the high-frequency behavior of differential mobility, spectral density of velocity fluctuations, and noise temperature. The spectra of hot-carrier differential mobility and velocity noise are found to exhibit a plateau in the low-frequency region, a peak at intermediate frequencies, and a 1∕f2 decay at the highest frequencies. The comparison with standard A3B5 compounds shows that the characteristic frequencies associated with extreme and cutoff decay of the negative differential mobility, etc., are shifted to a higher-frequency range for the case of nitrides. This property is favorable for applications of nitrides in the terahertz frequency range.