Papers by TOMAS LOPEZ GONZALEZ
Nanotechnology, 2004
We present a Monte Carlo study of an InGaAs based four-terminal ballistic rectifier operating at ... more We present a Monte Carlo study of an InGaAs based four-terminal ballistic rectifier operating at different temperatures. The rectifying effect is due to the vertical asymmetry of the electron concentration originated, in the presence of ballistic transport, by the action of an obstacle located in the centre of a ballistic cross junction. An increase of temperature degrades the efficiency of the device, since transport becomes more diffusive. However, it shows an intrinsic capability for rectification up to a frequency of 1.0 THz almost independently of the temperature.
2019 IEEE Asia-Pacific Microwave Conference (APMC), 2019
We report on the growth optimization of GaNbased self switching diode (SSD) structure on SiC, des... more We report on the growth optimization of GaNbased self switching diode (SSD) structure on SiC, designed using Monte Carlo simulations, for the fabrication of nano-scale SSDs to reach THz emission as a result of Gunn oscillations. Crack-free epistructure with good epi-characteristics and uniformity on 2inch SiC substrate was achieved. High carrier density of 2 ×10 18 cm-3 resulted in a low contact resistance of 0.35 Ω.mm.
Electronics, 2021
Despite high costs and lengthy deployments, satellite communications have traditionally been used... more Despite high costs and lengthy deployments, satellite communications have traditionally been used to provide coverage in remote areas. However, given the fact that there is no radio infrastructure available in these areas, Near Vertical Incidence Skywave (NVIS) technology has positioned itself as an attractive alternative to communicate with low-power nodes in remote areas. This type of communication works in the HF frequency range complying with STANAG and MIL-STD standards, which define a physical layer for scenarios that differ from NVIS and low-power communication. The purpose of this paper was to present the definition of a new communication physical layer based on single-carrier frequency-domain equalization (SC-FDE) based on these standards but adapted to the ionospheric communication channel. This physical layer was compared to an OFDM-based layer from a previous study. The experiments performed show that this new approach achieves better results than OFDM in terms of a high...
Sensors, 2021
Sensor networks have become more popular in recent years, now featuring plenty of options and cap... more Sensor networks have become more popular in recent years, now featuring plenty of options and capabilities. Notwithstanding this, remote locations present many difficulties for their study and monitoring. High-frequency (HF) communications are presented as an alternative to satellite communications, being a low-cost and easy-to-deploy solution. Near vertical incidence skywave (NVIS) technology provides a coverage of approximately 250 km (depending on the frequency being used and the ionospheric conditions) without a line of sight using the ionosphere as a communication channel. This paper centers on the study of the ionosphere and its characteristic waves as two independent channels in order to improve any NVIS link, increasing its robustness or decreasing the size of the node antennas through the appliance of specific techniques. We studied the channel sounding of both the ordinary and extraordinary waves and their respective channels, analyzing parameters such as the delay spread ...
2021 13th Spanish Conference on Electron Devices (CDE), 2021
In this work, with the help of a semi-classical twodimensional Monte Carlo (MC) simulator, we stu... more In this work, with the help of a semi-classical twodimensional Monte Carlo (MC) simulator, we study the DC current-voltage curves of Self-Switching Diodes (SSDs) fabricated on an AlGaN/GaN heterostructure from 100 K up to room temperature. Due to the very narrow channel of the SSDs, the presence of surface effects plays a key role not only on their DC behavior but also on their RF detection performance. The evolution with temperature of the negative surface charge density σ at the etched sidewalls of the SSD is the key quantity to explain the measurements. At 300 K, MC simulations with a constant value of σ are able to replicate very satisfactorily the experiments. However, to reproduce the shape of the I-V curve at low temperatures, a more realistic approach, where σ depends not only on T, but also on the applied bias V, is necessary.
Sensors, 2020
The number of Internet of Things (IoT) devices has experienced a large growth during the last dec... more The number of Internet of Things (IoT) devices has experienced a large growth during the last decade, as well as the data volume gathered from remote sensors. Satellites are still a suitable communication method and may be preferable for a remote ubiquitous sensor network (USN), which sometimes are located in places without much communications infrastructure where coverage is the principal drawback. Alternatively, the proposed solution for this article aims at a near-vertical incidence skywave (NVIS) channel for high frequencies (HF) with a low-cost platform, allowing a low-power transmissions coverage area up to 250 km for USN. The HF standards are focused on generic communication channels not being robust for NVIS communications. In this article we study and test an alternative based on orthogonal frequency-division multiplexing (OFDM) modulations to make them more robust and less dependent on the channel NVIS communications. For that purpose, we test the HF standard modulations a...
Journal of Computational Electronics, 2018
Ion shot noise, the noise associated to the random passage of ions across the cell membrane, is s... more Ion shot noise, the noise associated to the random passage of ions across the cell membrane, is studied by means of a stochastic model based on the Hodgkin-Huxley equations, which includes gating channels for sodium and potassium cations and leakage channels through the biological membrane. Apart from shot noise, other sources such as extrinsic and channel noise are taken into account. Ion shot noise, of increasing influence for smaller membrane patch sizes S, can lead to the emergence of action potentials in the membrane voltage in the presence of sinusoidal excitation currents below the threshold for the onset of spikes. The spiking activity in the presence of noise has been analyzed in terms of the coefficient of variation CV, the inter-spike interval histogram, the spectrum of membrane voltage fluctuations and the signal-to-noise ratio SNR. CV shows improved coherence in the sequence of randomly generated spikes due to the presence of shot noise. The voltage noise spectra show a common signature of the presence of spikes under different operating conditions, even in the absence of excitation. The SNR exhibits intrinsic stochastic resonance when varying S. For a sinusoidal excitation current with amplitude 1.5 µA/cm 2 and frequency 50 Hz, the SNR presents optimal values around 0.2 µm 2. When considering the presence of ambient noise in the excitation current, extrinsic stochastic resonance is found for S > 0.6 µm 2 .
IEEE Transactions on Electron Devices, 2016
Revista De Ingenieria, Oct 1, 2011
Cuadernos Unimetanos, 2010
Birkbeck Working Papers in Economics and Finance, 2006
This paper presents a model of conflict that combines popular support and asymmetric fighting tec... more This paper presents a model of conflict that combines popular support and asymmetric fighting technologies in a civil war setting. Starting with different endowments, two parties must decide on the amount of resources to divert from production to fighting. The conditions for conflict to arise are derived and civil war is shown to be subject to efficiency and distributive costs. Two other equilibria can occur, the first involving only one side choosing to arm, and the other a peace equilibrium where both groups choose zero fighting effort. The model is consistent with various historical accounts of the different roads to war and with recent empirical evidence on the determinants of conflict. Although the model focuses on civil wars, it can easily be extended to other situations that involve conflict such as rent seeking, political campaigning or litigation.
Tiempo Y Espacio, Jun 1, 2014
Th e present study aims to show the position of the group of invisible by calling national histor... more Th e present study aims to show the position of the group of invisible by calling national history, those that judging the facts of the April 19, 1810, were not taken into account in assessing the diffi culties suff ered all actors made.
IEEE Transactions on Electron Devices, 2016
An experimental demonstration of GaN-based asymmetric nanodiodes as direct and heterodyne detecto... more An experimental demonstration of GaN-based asymmetric nanodiodes as direct and heterodyne detectors up to 0.69 THz has been performed at room temperature. Responsivities of 2 and 0.3 V/W in a free-space configuration were obtained at 0.30 and 0.69 THz, respectively. An intermediate frequency (IF) signal has been measured up to 40 and 13 GHz in the same frequency ranges. The characterization of the nanodiodes as mixers did not show any deviation from linearity between the RF input power and the IF output. Monte Carlo simulations, used to estimate nanodevice intrinsic conversion losses of 27 dB at 0.69 THz, have confirmed these results.
En diversos jaciments de l'edat del Ferro del nord-oest de la Península Ibèrica s'han rec... more En diversos jaciments de l'edat del Ferro del nord-oest de la Península Ibèrica s'han recuperat restes d'estructures de fang cuit i fons perforat que han estat objecte de diverses interpretacions. Atesa una nova interpretació d'elements d'aquest tipus del castro de Castrovite, es presenta un estudi d'una de les cambres de combustió més completes, apareguda en Castromao, i que ha suposat un primer pas per repensar aspectes de la seva manufactu-ra i funcionalitat. Les diferents perspectives establertes a través de la combinació de les anàlisis arqueomètriques, la reconstrucció experimental i la revisió de dades arqueològiques i etnogràfiques, han possibilitat establir preguntes i respostes en una reflexió en què ha participat un terrissaire de la ceràmica tradicional de Gundivós (Sober, Lugo) Edat del Ferro, nord-oest ibèric, càmeres de cocció, ceràmica. The remains of baked clay structures with perforated bases have been found on several Iron Age sites in NW I...
Revista de Ingeniería, 2011
Resumen En Colombia es evidente la necesidad de innovar en el sector público. En este artículo se... more Resumen En Colombia es evidente la necesidad de innovar en el sector público. En este artículo se presenta el caso específico de la innovación del gobierno en el sector minero. Para esto, se iniciará con la identificación del potencial minero en Colombia y las barreras para su explotación. En seguida se explorarán los planes de innovación del gobierno como una estrategia para sobrepasar aquellas barreras que impiden aprovechar este potencial.
Thin Solid Films, 2007
In this paper, we present a study on three-terminal ballistic junction and their applications to ... more In this paper, we present a study on three-terminal ballistic junction and their applications to rectifiers and MUX/DEMUX. Rectifying effect is observed up to 94 GHz at room temperature. Although THz frequency performance has been demonstrated by Monte Carlo simulation, the high impedance of the nano-device combined with the parasitic capacitances is a limiting factor.
Journal of Computational Electronics, 2015
IEEE Transactions on Terahertz Science and Technology, 2014
In this paper, we perform, by means of Monte Carlo simulations and experimental measurements, a g... more In this paper, we perform, by means of Monte Carlo simulations and experimental measurements, a geometry optimization of GaN-based nano-diodes for broadband Terahertz direct detection (in terms of responsivity) and mixing (in terms of output power). The capabilities of the so-called self-switching diode (SSD) are analyzed for different dimensions of the channel at room temperature. Signal detection up to the 690 GHz limit of the experimental setup has been achieved at zero bias. The reduction of the channel width increases the detection responsivity, while the reduction in length reduces the responsivity but increases the cutoff frequency. In the case of heterodyne detection an intrinsic bandwidth of at least 100 GHz has been found. The intermediate frequency (IF) power increases for short SSDs, while the optimization in terms of the channel width is a trade-off between a higher non-linearity (obtained for narrow SSDs) and a large current level (obtained for wide SSDs). Moreover, the RF performance can be improved by biasing, with optimum performances reached, as expected, when the DC non-linearity is maximum.
Solid-State Electronics, 1996
This paper investigates the problem of modelling ohmic contacts for Monte Carlo simulation of sem... more This paper investigates the problem of modelling ohmic contacts for Monte Carlo simulation of semiconductor devices. Several models are proposed with different velocity distributions for the injected carriers. The influence of each model on the device physics near the contact is discussed. As a prototype for this analysis we investigate the role of the ohmic contact on the electrical characteristics of a GaAs Schottky-barrier diode under forward-bias condition. To get accurate results from the simulations of this device, correct modelling of the ohmic contact is crucial. We have found that the best simulation of the carrier dynamics near the contact is achieved by using a velocity-weighted Maxwellian distribution for injecting the carriers, which provides flat profiles of the different magnitudes near the boundary and a zero voltage drop at the contact. In addition, an appropriate time and space algorithm for carrier injection must be applied.
PeerJ, 2014
Analyzing field data from pumping tests, we show that as with many other natural phenomena, groun... more Analyzing field data from pumping tests, we show that as with many other natural phenomena, groundwater flow exhibits a complex dynamics described by 1 ⁄ f power spectrum. This result is theoretically studied within an agent perspective. Using a traveling agent model, we prove that this statistical behavior emerges when the medium is complex. Some heuristic reasoning is provided to justify both spatial and dynamic complexity, as the result of the superposition of an infinite number of stochastic processes. Even more, we show that this implies that non-Kolmogorovian probability is needed for its study, and provide a set of new partial differential equations for groundwater flow.
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Papers by TOMAS LOPEZ GONZALEZ