A. Íñiguez-de-la-Torre

Searching for THz Gunn oscillations in GaN planar nanodiodes

A detailed study of GaN-based planar asymmetric nanodiodes, promising devices for the fabrication of room temperature THz Gunn oscillators, is reported. By using Monte Carlo simulations, an analysis of the static I-V curves and the time-domain evolution of the current obtained when varying some simulation parameters in the diodes has been made. Oscillation frequencies of hundreds of GHz are predicted by the simulations in diodes with micrometric channel lengths. Following simulation guidelines, a first batch of diodes was fabricated. It was found that surface charge depletion effects are stronger than expected and inhibit the onset of the oscillations. Indeed, a simple standard constant surface charge model is not able to reproduce experimental measurements and a self-consistent model must be included in the simulations. Using a self-consistent model, it was found that to achieve oscillations, wider channels and improved geometries are necessary.

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.

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.

Correlation between low-frequency current-noise enhancement and high-frequency oscillations in GaN-based planar nanodiodes: A Monte Carlo study

We present a spectral analysis of time sequences of current, calculated by means of Monte Carlo simulations, in GaN-based asymmetric nanodiodes, devices that are potential candidates to exhibit Gunn oscillations. It is found that the low-frequency noise increases significantly for biases close to the threshold voltage of Gunn oscillations, taking place at much higher frequencies of hundreds of gigahertz. Due to the inherent difficulty in detecting so fast fluctuations, the measurement of the low-frequency noise can be a quite useful tool for predicting current oscillations at sub-terahertz frequencies in these devices.

Terahertz current oscillations assisted by optical phonon emission in GaN n+nn+ diodes: Monte Carlo simulations

Under certain conditions, plasma instabilities associated with streaming motion of carriers taking place in n+nn+ diodes can lead to current oscillations. The origin of the phenomenon, known as optical phonon transit time resonance, is characterized by a frequency related to the transit time between consecutive optical phonon emissions by electrons along the active region of the diode. By means of Monte Carlo simulations, the possibility to obtaining current oscillations in GaN n+nn+ diodes is analyzed. The optimum conditions for the onset of such mechanism are investigated: applied bias, temperature, doping, and length of the active n region. Simulations show that current oscillations at frequencies in the terahertz range can be obtained at very low temperatures. Moreover, by choosing the appropriate applied voltage and length of the n region, some degree of tunability can be achieved for frequencies close to the plasma frequency of the n region of the n+nn+ diode.

Toward THz Gunn oscillations in planar GaN nanodiodes

By means of Monte Carlo simulations we show the feasibility of asymmetric nonlinear planar GaN nanodiodes for the development of high-frequency Gunn oscillations. For channel lengths of 1 μm, oscillation frequencies around 300 GHz are predicted, reaching more than 650 GHz for 0.4 μm. The intrinsic DC to AC conversion efficiency is found to be higher than 1% for the fundamental and second harmonic frequencies.

Room temperature THz detection and emission with semiconductor nanodevices

In this paper we present the advances on the fabrication of THz emitters and detectors obtained within the framework of the European ROOTHz project. Two types of devices are explored, self-switching diodes and slot-diodes, using both narrow bandgap and wide bandgap semiconductors. This broad approach allows us to improve the frequency and power generated by Gunn diodes and the responsivity and noise of detectors at THz frequencies.

Human-Powered Vehicles - Aerodynamics of Cycling

Human-powered transport can be defined as that type of transport that only uses the human muscle power. When considering non-vehicular human-powered transport, it should to be indicated that it exists from the beginning of the human history (walking, running, swimming or climbing to trees). On the contrary, if we refer to vehicular transport we must necessary mention the wheel invention. It is superfluous to remark that the wheel is the most essential element of any form of vehicular land transportation and perhaps the greatest mechanical invention of the human civilization. Although archaeologists say that the wheel was discovered around 8000 BC in Asia, today is generally accepted that the oldest wheel known was discovered around the year 3500 BC in Sumeria, part of modern day Iraq. It was manufactured of slats of wood linked together. The wheel allowed the people to travel with greater speed and efficiency than walking: In a first step with the help of animal-powered rolled chariots and from the 19th century in human-powered vehicles also.

OPTTR induced current oscillations in GaN diodes Monte Carlo simulations

At very low temperature, when optical phonon emission is the dominant scattering mechanism, the phenomenon known as Optical Phonon Transit Time Resonance (OPTTR) may originate current oscillations in n+ nn+ diodes at frequencies in the terahertz range. In this work, by means of Monte Carlo simulations, we study the optimum conditions for the onset of such mechanism in GaN diodes. For this purpose, we analyze the influence of the following quantities: applied bias, temperature, length and doping of the n region. Under certain conditions, simulations show clear current oscillations which are enhanced when providing a characteristic frequency close to the plasma frequency of the active region. Even so, it is possible to achieve some degree of tunability of that oscillation frequency by varying the applied voltage and the length of the n region.

Current oscillations excited by optical phonon emission in GaN n + nn + diodes: Monte Carlo simulations

Under certain conditions, usually at low temperature, when optical phonon emission is the dominant scattering mechanism, plasma instabilities, driven by the streaming motion of carriers, can appear in n+nn+ diodes and may originate current oscillations. In this work, by means of Monte Carlo simulations, we investigate the optimum conditions for the onset of such mechanism in GaN n+nn+ diodes: applied bias, temperature, doping and length of the active n region. Simulations show that current oscillations at frequencies as high as 400–600 GHz can be achieved for temperatures below 40 K, with some degree of tunability by means of the applied voltage and length of the n region. Oscillations are enhanced for bias conditions providing a characteristic frequency close to the plasma frequency of the n region.