The District Energy-Efficient Retrofitting of Torrelago (Laguna de Duero – Spain)

The urban growth is estimated to reach up the 66 % by 2050 and consequently the need of resources within the cities will increase significantly. This, combined with the 40 % of energy consumption and 36 % of CO2 emissions of the building sector, makes necessary to accelerate the transition towards more sustainable cities. The CITyFiED project contributes to this transition, aiming to develop an innovative and holistic methodological approach for energy-efficient district renovation and deliver three large scale demonstration cases in the cities of Lund (Sweden), Laguna de Duero (Spain) and Soma (Turkey). CITyFiED methodology consists of several phases that ease the decision-making tasks towards the district renovation, considering the energy efficiency as the main pillar and local authorities as clients. For the case of Torrelago district (Spain) the intervention consists of a set of energy conservative measures including the facąde retrofitting of 143.025 m2 of living space in 31 twelve-storey buildings; the renovation of the district heating network with a new biomass thermal plant; the integration of renewable energy sources, including a micro-cogeneration system, and the installation of individual smart meters. After the renovation action, one-year monitoring campaign is ongoing. The CITyFiED monitoring platform will collect information from the energy systems and deliver environmental, technical, economic and social key performance indicators by March 2019. At the end of the project the achievement of the predefined goals will be verified: Up to 36 % of energy saving and 3,429 tons-CO2/yr emissions saving covering the 59,4 % of the energy consumption with renewable sources.The research and results presented in this paper evolve from activities related to the CITyFiED project, which has received funding from the European Commission under the Grant Agreement no. 609129. This article is the result of cooperative research work of many experts from various countries and we would like to gratefully acknowledge the rest of the CITyFiED partners

Monte Carlo Analysis of the Dynamic Behavior of InAlAs/InGaAs Velocity Modulation Transistors: A Geometrical Optimization

The influence of the geometry on the dynamic behavior of InAlAs/InGaAs velocity modulation transistors is analyzed by means of a Monte Carlo simulator in order to optimize the performance of this new type of transistor. In velocity modulation transistors, based on the topology of a double-gate high electron mobility transistor, the source and drain electrodes are connected by two channels with different mobilities, and electrons are transferred between both of them by changing the gate voltages in differential mode. Consequently, the drain current is modulated while keeping the total carrier density constant, thus in principle avoiding capacitance charging/discharging delays. However, the low values taken by the transconductance, as well as the high capacitance between the two gates in differential-mode operation, lead to a deficient dynamic performance. This behavior can be geometrically optimized by increasing the mobility difference between the two channels, by increasing the channel width and, mainly, by reducing the gate length, with a higher immunity to short channel effects than the traditional architectures.ROOTHz (FP7-243845

Monte Carlo study of kink effect in isolated-gate InAs/AlSb high electron mobility transistors

A semiclassical two-dimensional ensemble Monte Carlo simulator is used to perform a physical analysis of the kink effect in InAs/AlSb high electron mobility transistors (HEMTs). Kink effect, this is, an anomalous increase in the drain current I-D when increasing the drain-to-source voltage V-DS, leads to a reduction in the gain and a rise in the level of noise, thus limiting the utility of these devices for microwave applications. Due to the small band gap of InAs, InAs/AlSb HEMTs are very susceptible to suffer from impact ionization processes, with the subsequent hole transport through the structure, both implicated in the kink effect. The results indicate that, when V-DS is high enough for the onset of impact ionization, holes thus generated tend to pile up in the buffer (at the gate-drain side) due to the valence-band energy barrier between the buffer and the channel. Due to this accumulation of positive charge the channel is further opened and I-D increases, leading to the kink effect in the I-V characteristics and eventually to the device electrical breakdown. The understanding of this phenomenon provides useful information for the development of kink-effect-free InAs/AlSb HEMTs

Kink effect and noise performance in isolated-gate InAs/AlSb High Electron Mobility Transistors

Kink effect can spoil the otherwise excellent low-noise performance of InAs/AlSb HEMTs. It has its origin in the pile up of holes (generated by impact ionization) taking place mainly at the drain side of the buffer, which leads to a reduction of the gate-induced channel depletion and results in a drain current enhancement. Our results indicate that the generation of holes by impact ionization and their further recombination lead to fluctuations in the charge of the hole pile up, which provoke an important increase of the drain-current noise, even when the kink effect is hardly perceptible in the output characteristics.ROOTHz (FP7-243845

Monte Carlo study of kink effect in Isolated-Gate InAs/AlSb High Electron Mobility Transistors

A semiclassical 2D ensemble Monte Carlo simulator is used to perform a physical microscopic analysis of kink effect in InAs/AlSb High Electron Mobility Transistors (HEMTs). Due to the small bandgap of InAs, these devices are very susceptible to suffer impact ionization processes, with the subsequent hole transport through the structure, both implicated in the kink effect. The results indicate that, for high enough VDS, holes generated by impact ionization tend to pile up in the buffer (at the gate-drain side) due to the valence-band energy barrier between the buffer and the channel. Due to this accumulation of positive charge the channel is further opened and ID increases, leading to the kink effect in the I-V characteristics and even to the device breakdown. The microscopic understanding of this phenomenon provides useful information for a design optimization of kink-effect-free InAs HEMTs.ROOTHz (FP7-243845

Monte Carlo Analysis of Impact Ionization in Isolated-Gate InAs/AlSb High Electron Mobility Transistors

We perform a physical analysis of the kink effect in InAs/AlSb high electron mobility transistors by means of a semiclassical 2D ensemble Monte Carlo simulator. Due to the small bandgap of InAs, InAs/AlSb high electron mobility transistors are very susceptible to suffer from impact ionization processes, with the subsequent hole transport through the structure, both implicated in the kink effect. When the drain-to-source voltage VDS is high enough for the onset of impact ionization, holes generated tend to pile up at the gate-drain side of the buffer. This occurs due to the valence-band energy barrier between the buffer and the channel. Because of this accumulation of positive charge, the channel is further opened and the drain current ID increases, leading to the kink effect in the I–V characteristics.ROOTHz (FP7-243845

Technological Parameters and Edge Fringing Capacitance in GaN Schottky Barrier Diodes: Monte Carlo Simulations

[EN]Schottky barrier diodes (SBDs) with realistic geometries have been studied by means of a 2-D ensemble Monte Carlo simulator. The non-linearity of the Capacitance-Voltage (C-V) characteristic is the most important parameter for optimizing SBDs as frequency multipliers. In this paper, by changing the values of several technological parameters, we analyze their influence on the edge fringing capacitance in a GaN SBD. We have found that the parameters related with the dielectric used for the passivation and the lateral extension of the epilayer significantly affect the fringing capacitance, thus increasing the value of the total capacitance above the ideal one.Spanish MINECO and FEDER under Project TEC2017-83910-R and Junta de Castilla y León and FEDER under Project SA254P1

On the Practical Limitations for the Generation of Gunn Oscillations in Highly Doped GaN Diodes

Planar Gunn diodes based on doped GaN active layers with different geometries have been fabricated and characterized. Gunn oscillations have not been observed due to the catastrophic breakdown of the diodes for applied voltages around 20-25 V, much below the bias theoretically needed for the onset of Gunn oscillations. The breakdown of the diodes has been analyzed by pulsed I-V measurements at low temperature, and it has been observed to be almost independent of the geometry of the channels, thus allowing to discard self-heating effects as the origin of the device burning. The other possible mechanism for the device failure is impact-ionization avalanche due to the high electric fields present at the anode corner of the isolating trenches. However, Monte Carlo simulations using the typical value of the intervalley energy separation of GaN, ε_(1-2)=2.2 eV, show that impact ionization mechanisms are not significant for the voltages for which the experimental failure is observed. But recent experiments showed that ε_(1-2) is lower, around 0.9 eV. This lower intervalley separation leads to a much lower threshold voltage for the Gunn oscillations, not far from the experimental breakdown. Therefore, we attribute the devices failure to an avalanche process just when Gunn domains start to form, since they increase the population of electrons at the high electric field region, thus strongly enhancing impact ionization mechanisms which lead to the diode failure