Emission of terahertz (THz) radiations from interdigitated GaN quantum-wells structures under DC-bias has
been measured at room temperature. This measurements has been performed by a 4K Si-Bolometer associated
with a Fourier Transform Spectrometer. Using an analytical model, we have shown that the observed peak
at approximately 3 THz due to 2D ungated plasma-waves oscillations in the quantum well, is emitted by the
metallic contacts of our device acting as antennas.
We report on measurements of radiation transmission in the 0.220-0.325 THz and 0.75-1.1 THz
frequency ranges through GaN quantum wells grown on sapphire substrates at nitrogen and room
temperatures. Significant enhancement of the transmitted beam intensity with applied voltage is
found at nitrogen temperature. This effect is explained by changes in the mobility of two-dimensional
electrons under electric bias. We have clarified which physical mechanism modifies the electron mobility
and we suggest that the effect of voltage-controlled sub-terahertz transmission can be used for
the development of electro-optic modulators operating in the sub-THz frequency range.
Two-dimensional (2D) plasma waves in field effect transistors are well known since the pioneer work of Dyakonov
and Shur. The application to terahertz (THz) detection was proven recently both at cryogenic and room temperatures.
Aside from these experiments, we used the interband photoexcitation brought by the difference-frequency
component of a photomixed laser beam to excite very efficiently plasma waves in HEMT channel at room temperature.
Owing to a specific experimental setup avoiding unwanted high-frequency electrical oscillations of the
HEMTs, we obtained the spectral profiles of THz 2D plasma waves resonances of InGaAs HEMTs for many
experimental conditions. The effect of geometrical HEMTs parameters (lengths of the gate and surrounding
regions) as well as biasing conditions (drain and gate voltages) was evaluated on both plasma oscillations frequencies
and amplitudes. Simultaneously, a numerical approach, based on hydrodynamic equations coupled to a
pseudo-2D Poisson solver, was developed that compares well with experiments. Using this unique combination
of experiments and numerical simulations, a comprehensive spectroscopy of plasma waves in HEMTs is thus
obtained. It provides a deeper insight into the physical processes involved in plasma wave excitation and allows
predicting for mixer operation at THz frequency only using the plasma wave nonlinearity. Mixing experiments
are under progress.
To analyze the main features of THz radiation generation caused by optical-phonon transit-time resonance a
simplified analytical model is developed in terms of some generalized parameters of bulk materials and 2D
structures. In the framework of such a model and direct Monte Carlo simulations an increase up to 5 times of
the cutoff frequency for THz radiation generation is predicted going from 3D and 2D transport.
This paper overviews and implements the transfer-field method
applied to the calculation of electronic noise in small
semiconductor structures. Two basic schemes are used and developed
in detail. The former considers velocity fluctuations and the
latter acceleration fluctuations as microcopic noise sources. We
show that the latter scheme has several advantages with respect to
the former one. Indeed, starting from Markovian noise sources, the
latter scheme separates the time and spatial evolution of the
local noise sources. In this way, the dual representation of the
noise spectral density in terms of impedance and admittance fields
is recovered. A remarkable achievement is that from the knowledge
of the bulk Langevin sources at a hydrodynamic level it is
possible to calculate the noise spectra of non-homogeneous
structures even for nanometric devices. The method is validated by
comparing the results of the present scheme with those obtained
from self-consistent Monte Carlo approaches for different
structures of interest.
We report Monte Carlo particle (MCP) simulations of the current response and noise spectrum in heavily doped nanometric GaAs Schottky-barrier diodes (SBDs) operating under static, cyclostationary and resonant-circuit conditions in the forward bias region. Main attention is paid to the SBDs application in the THz frequency region. General features of the regular response and noise as well as their modifications under various operation modes are obtained from MCP simulations and analyzed in the framework of a simple analytical model based on the static I-V and C-V relations obtained from simulations.
The investigation of noise in electronic devices operating under large-signal conditions is attracting increasing attention in recent years. Theoretical analyses on this subject are typically performed in the framework of the impedance field method, implemented under the drift-diffusion approximation. As an alternative, a more general microscopic approach including a more detailed physical description of the systems is mandatory. This work reviews recent results of Monte Carlo simulations of electronic noise in bulk materials and submicron semiconductor structures subject to high-frequency large-amplitude periodic electric fields or applied voltages/currents.
The peculiarity of the noise analysis under large-signal operation is that velocity or current/voltage fluctuations appear simultaneously with the regular response of the nonlinear medium or device, so that the regular response and noise spectra are overlapped in the whole frequency range of interest. Here, various correlation functions of fluctuations, their instantaneous and integrated spectral densities, etc. are calculated under large-signal operation for compound semiconductors, such as GaAs, and InN, as well as for GaAs Schottky-barrrier diodes and n+nn+ structures. A comparison with the results obtained under stationary conditions is performed. Under these large-signal cyclostationary working conditions, when the system response becomes nonlinear, several modifications and anomalies appear in the noise spectra with respect to static stationary conditions. In particular, an increase of the low-frequency noise and a resonant-like enhancement of the spectra near the fundamental frequency (and eventually high-order harmonics) of the applied signal is observed under some specific conditions. These anomalies are interpreted as a manifestation of dynamical effects under sufficiently high frequency and amplitude of the applied signal. Similarities and differences of the noise resonant-like enhancement around the fundamental frequency with noise upconversion processes are discussed.
Amplification and generation of microwave radiation by optical phonon transit-time resonance GaN THz maser is investigated theoretically by Monte Carlo simulations. Results confirm that GaN is a promising material for THz power generation. The amplification and generation occurs in the wide frequency range of 0.3 to 3 THz and persists in the THz frequency range up to liquid nitrogen temperatures and doping levels of about 5 X 1016 cm-3.