Browsing by Author "Costa, J."
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- Applications for a-Si:H TFTs: modelling and simulationPublication . Lourenço, P.; Fantoni, Alessandro; Fernandes, M.; Costa, J.; Vieira, ManuelaHydrogenated amorphous silicon thin film transistors have been used as switching elements in liquid crystal displays and large area matrix addressed sensor arrays. Later, these devices have also been used as analogue active elements in organic light emitting diode displays. However, this technology suffers from bias induced meta-stability. This issue introduces both threshold voltage and subthreshold slope shifts over time when gate bias is applied. Such instabilities jeopardize long term performance of circuits that rely on these components. Nevertheless, hydrogenated amorphous silicon thin film transistors present an exponential transfer characteristic when operating on subthreshold region and their typical power consumption is under 1 µW. This low power characteristic makes these devices ideally suited for low power electronic design. This work demonstrates, through transient analysis of a wellestablished simulation model for hydrogenated amorphous silicon, the viability of thin film transistors technology to perform both analogue and digital functions. Hence, these structures may be used in both application fields. To this end, two different sets of analyses have been conducted with hydrogenated amorphous silicon based thin film transistors. The first set considers a driving circuit for an active matrix of organic light emitting diodes, biased in a way to minimize the “memory effect” (increasing shift on threshold voltage) due to long term operation. The second set of analyses were conducted upon the implementation of complementary output universal gates, namely NOR/OR and XNOR/XOR elements.
- A co-simulation approach to the wheel–rail contact with flexible railway trackPublication . Antunes, P.; Magalhães, H.; Ambrósio, J.; Pombo, João; Costa, J.The standard approach to railway vehicle dynamic analysis includes running the vehicle multibody models in rigid railway tracks. The wheel–rail contact, independently of the rolling contact model used, is either handled online or via lookup tables. This traditional approach disregards the coupling effects between the railway vehicle dynamics and the railway track flexibility. In this work the assumption of rigidity of the railway track is relaxed and a finite element model of the complete track, i.e. rails, pads, sleepers, ballast and infrastructure, is used to represent the track geometry and flexibility. A rail–wheel contact model that evaluates the contact conditions and forces is used online. The dynamics of the railway vehicle is described using a multibody methodology while the track structure is described using a finite element approach. Due to the fact that not only the multibody and the finite element dynamic analysis use different integration algorithms but also because the vehicle and track models are simulated in different, codes a co-simulation procedure is proposed and demonstrated to address the coupled dynamics of the system. This approach allows us to analyze the vehicle dynamics in a flexible track with a general geometry modeled with finite elements, i.e. including curvature, cant, vertical slopes and irregularities, which is another novel contribution. The methodology proposed in this work is demonstrated in na application, in which the railway vehicle–track interaction shows the influence of the vehicle dynamics on the track dynamics and vice versa.
- Lithographic mask defects mitigation on a multimode interference structurePublication . Lourenço, P.; Fantoni, Alessandro; Costa, J.; Vieira, ManuelaOver the last decades, the lithographic technology has greatly contributed for the confirmation of Moore's law in the semiconductor industry. Key developments in lithography such as the operational wavelength decreasing, together with a performance increase in lens and imaging technology, enabled the reduction of cost per function in integrated circuits technology. In this work, the impact of lithographic defects introduced by the manufacturing process is analyzed through simulations and two mitigation techniques are presented. These perturbations are a consequence of the limited lithographic mask resolution reflected on deviations from the geometry of the ideal device. For this purpose, the Beam Propagation and Finite Differences Time Domain methods have been used to simulate a multi-mode interference structure based on silicon nitride. The structure is affected by random perturbations and the obtained results revealed a strong dependence between mask resolution, and imbalance and power loss. Two strategies have been followed concerning the mitigation of power loss and imbalance: - Access waveguides tapering and adjustable power splitting ratios through the electro-optic effect. Both strategies revealed results that indicate an improvement on device's performance. However, once built, the former is a static design that favors indiscriminately all propagating modes in the multimode section. In the latter, finer tuning capabilities targeting different propagating modes may be enabled by dynamic compensation of power loss and imbalance, when in a closed loop control architecture. Such a control architecture may operate by sampling the output waveguides, extracting the error signal and, finally, negatively feeding it back to the electro - optic effect system, hence improving imbalance and power loss.