One-step air bridge fabrication technique using 3D e-beam lithography

A new technique is demonstrated for the realisation of air bridges using one lithographic step. Gray scale lithography is used for the formation of 3D profiles on polymethyl methacrylate (PMMA) where a variable dose exposure is applied to create a trapezoid profile for the air bridge. In previous 3D electron beam lithography (EBL) methods the span area was exposed to a low dose or a low acceleration voltage [1,2,3]. Thus, the required discontinuity with the surrounding area for the lift-off process was created. In this technique, no exposure of the span area is needed. Another exposure of a gradient dose is applied to the sides of the highest part of the air bridge. The created profile, after developing the resist, is depicted in Figure 1. The surfaces with red and blue colour represent the metal to form the air bridge and the metal to lift-off, respectively. Using this configuration, the deposited metal at the sides of the top part of the air bridge is connected to the surrounding metal to lift-off and disconnect from the air bridge. The electron dose used in this area has to be smaller than the minimum dose that penetrates the total resist layer, so that the deposited metal does not reach the substrate. This method takes maximum advantage of the resist thickness for the fabrication of high structures, as no part of the resist is sacrifice

Seeking the Truth Beyond the Data. An Unsupervised Machine Learning Approach

Clustering is an unsupervised machine learning methodology where unlabeled elements/objects are grouped together aiming to the construction of well-established clusters that their elements are classified according to their similarity. The goal of this process is to provide a useful aid to the researcher that will help her/him to identify patterns among the data. Dealing with large databases, such patterns may not be easily detectable without the contribution of a clustering algorithm. This article provides a deep description of the most widely used clustering methodologies accompanied by useful presentations concerning suitable parameter selection and initializations. Simultaneously, this article not only represents a review highlighting the major elements of examined clustering techniques but emphasizes the comparison of these algorithms' clustering efficiency based on 3 datasets, revealing their existing weaknesses and capabilities through accuracy and complexity, during the confrontation of discrete and continuous observations. The produced results help us extract valuable conclusions about the appropriateness of the examined clustering techniques in accordance with the dataset's size.Comment: This paper has been accepted for publication in the proceedings of the 3rd International Scientific Forum on Computer and Energy Sciences (WFCES 2022

A Hybrid SEIHCRDV-UKF Model for COVID-19 Prediction. Application on real-time data

The prevalence of COVID-19 has been the most serious health challenge of the 21th century to date, concerning national health systems on a daily basis, since December 2019 when it appeared in Wuhan City. Prediction of pandemic spread plays an important role in effectively reducing this highly contagious disease. Nevertheless, most of the proposed mathematical methodologies, which aim to describe the dynamics of the pandemic, rely on deterministic models that are not able to reflect the true nature of the spread of COVID. In this paper, we propose a SEIHCRDV model - an extension of the classic SIR compartmental model - which also takes into consideration the populations of exposed, hospitalized, admitted in intensive care units (ICU), deceased and vaccinated cases, in combination with an unscented Kalman filter (UKF), providing a dynamic estimation of the time dependent parameters of the system. Apparently, this new consideration could be useful for examining also other pandemics. We examine the reliability of our model over a long period of 265 days, where we observe two major waves of infection, starting in January 2021 which signified the start of vaccinations in Europe, providing quite encouraging predictive performance. Finally, special emphasis is given to proving the non-negativity of SEIHCRDV model, to achieve a representative basic reproductive number R0 and to investigating the existence and stability of disease equilibriums in accordance with the formula produced to estimate R0.Comment: 31 pages, 13 figure

Co-fabrication of planar Gunn diode and HEMT on InP substrate

We present the co-fabrication of planar Gunn diodes and high electron mobility transistors (HEMTs) on an Indium Phosphide (InP) substrate for the first time. Electron beam lithography (EBL) has been used extensively for the complete fabrication procedure and a 70 nm T-gate technology was incorporated for the enhancement of the small-signal characteristics of the HEMT. Diodes with anode-to-cathode separation (Lac) down to 1 μm and 120 μm width where shown to oscillate up to 204 GHz. The transistor presents a cut-off frequency (fT) of 220 GHz, with power gain up to 330 GHz (f<sub>max</sub>). The integration of the two devices creates the potential for the realisation of high-power, high-frequency MMIC Gunn oscillators, circuits and systems

Integration of planar Gunn diodes and HEMTs for high-power MMIC oscillators

This work has as main objective the integration of planar Gunn diodes and high electron mobility transistors (HEMTs) on the same chip for the realisation of high-power oscillators in the millimeter-wave regime. By integrating the two devices, we can reinforce the high frequency oscillations generated by the diode using a transistor-based amplifier. The integration of the planar Gunn diode and the pseudomorphic HEMT was initially attempted on a combined gallium arsenide (GaAs) wafer. In this approach, the active layers of the two devices were separated by a thick buffer layer. A second technique was examined afterwards where both devices were fabricated on the same wafer that included AlGaAs/InGaAs/GaAs heterostructures optimised for the fabrication of pHEMTs. The second approach demonstrated the successful implementation of both devices on the same substrate. Planar Gunn diodes with 1.3 μm anode-to-cathode separation (Lac) presented oscillations up to 87.6 GHz with a maximum power equal to -40 dBm. A new technique was developed for the fabrication of 70 nm long T-gates, improving the gain and the high frequency performance of the transistor. The pHEMT presented cut-off frequency (fT) equal to 90 GHz and 200 GHz maximum frequency of oscillation (fmax). The same side-by-side approach was applied afterwards for the implementation of both devices on an indium phosphide (InP) HEMT wafer for the first time. Planar Gunn diodes with Lac equal to 1 μm generated oscillations up to 204 GHz with -7.1 dBm maximum power. The developed 70 nm T-gate technology was applied for the fabrication of HEMTs with fT equal to 220 GHz and fmax equal to 330 GHz. In the end of this work, the two devices were combined in the same monolithic microwave integrated circuit (MMIC), where the diode was connected to the transistor based amplifier. The amplifier demonstrated a very promising performance with 10 dB of stable gain at 43 GHz. However, imperfections of the material caused large variations at the current density of the devices. As a consequence, no signals were detected at the output of the complete MMIC oscillators

Genetic and Geo-Epidemiological Analysis of the Zika Virus Pandemic; Learning Lessons from the Recent Ebola Outbreak

“Outbreak” is a term referring to a virus or a parasite that is transmitted very aggressively and therefore could potentially cause fatalities, as the recent Ebola and Zika epidemics did. Nevertheless, looking back through history, quite a few outbreaks have been reported, which turned out so deadly that essentially changed, molded and literally re-shaped the society as it is today. In the present chapter, differences and similarities between the two most recent outbreaks have been studied, in order to pinpoint and design a trace model that will allow us to draw some conclusions for the connection of those two epidemics. Due to the high dimensionality of the problem, modern and state of the art geo-epidemiological methods have been used in an effort to provide the means necessary to establish the abovementioned model. It is only through geo-epidemiological analysis that it is possible to analyze concurrently a multitude of variables, such as genetic, environmental, behavioral, socioeconomic and a series of related infection risk factors

Integration techniques of pHEMTs and planar Gunn diodes on GaAs substrates

This work presents two different approaches for the implementation of pseudomorphic high electron mobility transistors (pHEMTs) and planar Gunn diodes on the same gallium arsenide substrate. In the first approach, a combined wafer is used where a buffer layer separates the active layers of the two devices. A second approach was also examined using a single wafer where the AlGaAs/InGaAs/GaAs heterostructures were designed for the realisation of pHEMTs. The comparison between the two techniques showed that the devices fabricated on the single pHEMT wafer presented superior performance over the combined wafer technique. The DC and small-signal characteristics of the pHEMTs on the single wafer were enhanced after the use of T-gates with 70 nm length. The maximum transconductance of the transistors was equal to 780 mS/mm with 200 GHz maximum frequency of oscillation (fmax). Planar Gunn diodes fabricated in the pHEMT wafer, with 1.3 μm anode-to-cathode separation (LAC) presented oscillations at 87.6 GHz with maximum power of oscillation equal to -40 dBm

INTEGRATED TRAFFIC CONTROL FOR FREEWAYS USING VARIABLE SPEED LIMITS AND LANE CHANGE CONTROL ACTIONS

The wide deployment of vehicle automation and communication systems (VACS) in the next decade is expected to influence traffic performance on freeways. Apart from safety and comfort, one of the goals is the alleviation of traffic congestion which is a major and challenging problem for modern societies. The paper investigates the combined use of two feedback control strategies utilizing VACS at different penetration rates, aiming to maximize throughput at bottleneck locations. The first control strategy employs mainstream traffic flow control using appropriate variable speed limits as an actuator. The second control strategy delivers appropriate lane-changing actions to selected connected vehicles using a feedback-feedforward control law. Investigations of the proposed integrated scheme have been conducted using a microscopic simulation model for a hypothetical freeway featuring a lane-drop bottleneck. The results demonstrate significant improvements even for low penetration rates of connected vehicles. Document type: Articl

Molecular dynamics simulations through GPU video games technologies

Bioinformatics is the scientific field that focuses on the application of computer technology to the management of biological information. Over the years, bioinformatics applications have been used to store, process and integrate biological and genetic information, using a wide range of methodologies. One of the most de novo techniques used to understand the physical movements of atoms and molecules is molecular dynamics (MD). MD is an in silico method to simulate the physical motions of atoms and molecules under certain conditions. This has become a state strategic technique and now plays a key role in many areas of exact sciences, such as chemistry, biology, physics and medicine. Due to their complexity, MD calculations could require enormous amounts of computer memory and time and therefore their execution has been a big problem. Despite the huge computational cost, molecular dynamics have been implemented using traditional computers with a central memory unit (CPU). A graphics processing unit (GPU) computing technology was first designed with the goal to improve video games, by rapidly creating and displaying images in a frame buffer such as screens. The hybrid GPU-CPU implementation, combined with parallel computing is a novel technology to perform a wide range of calculations. GPUs have been proposed and used to accelerate many scientific computations including MD simulations. Herein, we describe the new methodologies developed initially as video games and how they are now applied in MD simulations