J.-F. Millithaler

Experimental demonstration of direct terahertz detection at room-temperature in AlGaN/GaN asymmetric nanochannels

The potentialities of AlGaN/GaN nanodevices as THz detectors are analyzed. Nanochannels with broken symmetry (so called self switching diodes) have been fabricated for the first time in this material system using both recess-etching and ion implantation technologies. The responsivities of both types of devices have been measured and explained using Monte Carlo simulations and non linear analysis. Sensitivities up to 100 V/W are obtained at 0.3 THz with a 280 pW/Hz1/2 noise equivalent power.

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.

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.

Optimized V-shape design of GaN nanodiodes for the generation of Gunn oscillations

In this work, recent advances in the design of GaN planar Gunn diodes with asymmetric shape, so-called self-switching diodes, are presented. A particular geometry for the nanodiode is proposed, referred as V-shape, where the width of the channel is intentionally increased as approaching the anode. This design, which reduces the effect of the surface-charges at the anode side, is the most favourable one for the onset of Gunn oscillations, which emerge at lower current levels and with lower threshold voltages as compared to the standard square geometry, thus enhancing the power efficiency of the self-switching diode as sub-millimeter wave emitters.

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 investigation of terahertz plasma oscillations in gated ultrathin channel of n-InGaAs

By numerical simulations we investigate the dispersion of the plasma frequency in a gated channel of n-type InGaAs layer of thickness W and submicron length L at T=300 K. In the presence of a source-drain voltage and for a carrier concentrations of 1018 cm−3 the spectra evidences a peaked shape with two main bumps, the former at high frequency corresponding to the three-dimensional plasma frequency and the latter at a low frequency. The frequency value of the latter peak exhibits a dispersion as the inverse of the channel length in agreement with the predictions of gradual channel approximation. At increasing drain voltages the instabilities associated with the presence of Gunn domains are responsible for a suppression of the plasma peak in favor of the onset of a peak in the subterahertz domain associated with transit time effects.

Study of surface charges in ballistic deflection transistors.

This paper presents a comprehensive study of the behavior of surface charges in ballistic deflection transistors, at room temperature, where the in-plane geometry associating two drains with two gates in push-pull modes allows the control of electron path. Monte Carlo simulations were performed and compared with experimental data by using different models for accounting for surface charge effects. The simple model which assumes a constant and uniform value of the surface charge provides good results at equilibrium, but it is not able to correctly reproduce the BDTs complex behavior when biased. We have confirmed that for a correct description of the device operation it is necessary to use a model allowing the surface charge to adapt itself locally to the carrier concentration in its surroundings.

Exploration of digital latch design using ballistic deflection transistors — Modeling and simulation

In this paper, we explore the feasibility of a digital latch design using ballistic deflection transistors (BDTs). An analytical model, based on Monte Carlo simulations for different sizes of BDTs, has been integrated into a behavioral Verilog A module to facilitate the investigation of digital latch design. The simulation results indicate interesting capability of latch operation by holding data in a stable operating state.

Optimization of Ballistic Deflection Transistors by Monte Carlo Simulations

This paper presents an optimization of the current-voltage characteristic of Ballistic Deflection Transistors. The implementation of an adequate surface charge model in a Monte Carlo tool shows a very good agreement with the available experimental data and allows us to predict the influence of different parameters, like temperature, channel and trench dimensions on the device output. These results are of importance for further use of this device in logical circuit applications.

Monte Carlo modeling of ultra-fast operating Ballistic Deflection Transistor

This paper presents Monte Carlo simulations of a unique non-linear device called Ballistic Deflection Transistor. A self-consistent model for surface charges has been used and has shown a remarkable efficiency to reproduce the non-linearity. A dynamic analysis is completing the transistor study. Results are showing very good response to an ultra-fast excitation. These are confirming Terahertz performances of our device, leading to the development of more efficient logical computing circuits.