Farseem M. Mohammedy

Prospect of decentralized hybrid power generation in Bangladesh using biomass, solar PV & wind

Limited fossil fuels will soon not be enough to meet up the energy-shortage in Bangladesh and therefore enhanced attention must be given to utilize renewable energy sources. Solar and wind are two prominent renewable energy sources in Bangladesh. We propose an optimum decentralized solar PV-wind-biomass-fossil fuel based hybrid power system to meet up the energy-demand of a typical community in an environment-friendly way. The potential of the decentralized hybrid system in Bangladesh is estimated utilizing Geospatial toolkit, NASA SSE solar radiation data and HOMER optimization software. Financial viability of the system for Bangladesh is also assessed utilizing a proposed decentralized hybrid system using HOMER for Rangpur which has unique high solar (4.75 kWh/m2/day) and high wind (over 2m/s wind-speed throughout the year with 250W/m2 power density) supply in Bangladesh. The annual electricity generation of the proposed system is almost 21.55 MWh and its cost of energy is less in Rangpur (minimum 0.246

Analytical modeling and simulation of subthreshold behavior of dual material gate (DMG) Al 0.7 Ga 0.3 Sb/InAs HEMT

/kWh) relative to other areas due to its rich green energy resources. The proposed system, simulated for four different fuel-combinations, is most cost-effective but most detrimental to environment if the generator is run with natural gas. On the other hand, Co-firing with biomass results in doubling the cost but reduces pollutant-generation to almost 50%.

Analysis of zero-bias resistance area product for InGaSb PIN Photodiodes

In this work, a two dimensional (2-D) analytical model and simulation of dual material gate (DMG) Al0.7Ga0.3Sb/InAs High Electron Mobility Transistor (HEMT) has been presented. Superiority of DMG over single material gate (SMG) structure for this kind of HEMTs has been highlighted. Simulation results show the suppression of short channel effects (SCEs) and improvement of carrier transport efficiency. Material with lower work function works as a screening gate while material with higher work function works as a control gate and thus suppresses the short channel effects. The electric field also becomes more uniform and so the carrier transport efficiency improves. This work shows the variation of channel potential and electric field with channel length for both double material gate (DMG) and single material gate (SMG) Al0.7Ga0.3Sb/InAs HEMT. All the results have been verified using ATLAS device simulator. So this work shows a simple model to effectively analyze the sub-threshold behavior of DMG Al0.7Ga0.3Sb/InAs HEMTs.

Efficiency enhancement and reflectance reduction of thin film silicon plasmonic solar cells

This paper represents a model of zero-bias resistance area product of InGaSb PIN photodiodes grown on InGaSb metamorphic layer through the analysis of surface leakage and bulk current components of these photodiodes. The model is further developed by considering effects of dislocation density in the diodes. Different optoelectronic properties of the material are extracted by fitting the obtained models with experimental data, such as the values of electron and hole diffusion lengths (L<inf>n</inf> and L<inf>p</inf>), surface recombination velocity inside the material and at the exposed mesa edges (S<inf>n</inf> and s) are found to be 70.34 µm, 6.44 µm, 2.2584×10² cm/s and 4.1195×10⁵ cm/s, respectively. The extracted dislocation density is 6.67×10⁸ cm⁻² which is nearly close to the measured value of ∼2–5×10⁸ cm⁻² for this type of photodiodes.

Comparative study of subthreshold characteristics of different antimonide-based and nitride-based dual material gate (DMG) HEMTs

Nano-particles have the potential to couple incident sunlight into localized modes, reducing reflection and increasing absorption, thereby significantly improving the performance of thin film solar cells. In this article, a plasmonic solar cell has been studied and modeled using TiO2 as a space layer on the top of the silicon solar cell and Indium nano-particles for its plasmonic scattering. Finite difference time domain (FDTD) simulation has been performed to investigate the role of various thickness of the space layer and it has been verified with the experimental results. From the study, the optical reflectance and EQE responses for 15nm, 28nm, 59.5nm and 70nm thick TiO2 space layer has been demonstrated and an enhanced performance for the thickness of 59.5nm TiO2 has been noted compared with the other thickness.

Effects of physical parameters on subthreshold characteristics of nitride and antimonide-based double material gate (DMG) HEMTs

As MOSFET technology reaching its limits, new devices like HEMTs (high electron mobility transistors) are gradually gaining more interests. HEMTs have high mobility due to reduce scattering for the spatially separated doped region. But as their dimensions reach the nano-scale region, different SCEs (short channel effects) and carrier transport inefficiency creep in. To solve these problems a new structure, DMG (dual material gate) HEMT has been proposed. In this paper we have shown comparison of subthreshold characteristics among different antimonide-based DMG HEMTs and between antimonide-based and nitride-based DMG HEMTs. We have found that as the band-offsets in the heterojunction become deeper, the effect of using DMG over SMG (single material gate) reduces in case of antimonide-based HEMTs. Our work also shows that DMG structure is more effective in reducing SCEs for antimonide-based HEMTs than nitride-based HEMTs, while nitride-based DMG HEMTs give more efficient carrier transport than its antimonide-based counterpart.

Modeling of a Type-II antimonide based superlattice for novel optical switching Applications

In this papar we have varied the physical parameters of double material gate (DMG) HEMT and noted the change in subthreshold characteristics. The physical parameters are: control-gate length, barrier layer thickness and the work function difference between two gate materials. A semi-classical analytical model has been used to determine the channel potential and electric field. One nitride-based and three antimonide-based formations have been used in this work and their results have been compared. We have found that nitride-based DMG HEMTs are less sensitive to process variation of gate fabrication compared to antimonide-based DMG HEMTs. It has been seen that variation in barrier layer thickness affects nitride-based HEMTs more strongly than antimonide-based DMG HEMTs, while the effect of change in workfunction difference is more prominent in antimonide-based HEMTs. Among the antimonide-based HEMTs, it have been seen that the formation with deep conduction band discontinuity is more susceptible to change in control-gate length than formations with shallow conduction band discontinuity. But effects of barrier layer thickness change and change in workfunction difference are more noticeable in shallow conduction band formations, making these formations more effective as double material gate (DMG) structures.

Analysis of optical properties of InGaSb PIN photdiodes

In recent times, the Type-II InAs/GaSb Superlatttice has been opted as a viable replacement for HgCdTe based photodetectors as the band structure of these devices can be tailored. Significant progress has been made and ongoing research is being conducted in the growth and characterization of these devices. We present the model of such a device with experimentally verified dimensions and parameters. The Transfer Matrix Method (TMM) has been adopted to represent the wave function solution under zero bias and non-zero bias respectively. Cutoff wavelength of 10μm range was achieved. These devices have the added advantage of tunability with respect to well width and bias voltages and have attractive applications in optical switching.

Resonant cavity enhanced photodetectors: Theory, design and modeling

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP

Photovoltaic characteristics and non-idealities of thin film silicon plasmonic solar cells

Abstract Photodetector is one of key component in optoelectronic integrated circuits (OEICs). Photodetectors are extensively used in optical communication systems, optical interconnections, and biomedical imaging, and they typically operate from visible to near-infrared wavelength. For most applications, one or more of the following performance characteristics including high-sensitivity or quantum efficiency, high-speed, low noise, high dynamic range may be required. However, in optimizing the design of photodetectors, there is a key performance trade-off between quantum efficiency or sensitivity, and speed. To overcome this trade-off and simultaneously obtain high speed and high sensitivity, resonant cavity enhanced photodetectors (RCE-PDs) are used. Here, we discuss, in detail, various RCE-PD structures with an emphasis on theory, design, modeling and performance characteristics. Important research results are summarized and ideas on how to improve the design of RCE-PDs are presented. The time and frequency response, important for high-speed or high bit rate applications, are discussed from the perspective of detectors in real applications. For optimized design of OEICs, circuit models are indispensable. Therefore, we discuss circuit models for RCE-PDs, including the effects of parasitic elements on time response characteristics as well as device design optimization. The various materials combinations that have been used for RCE-PDs as well as different types of photodetectors are summarized. Finally, the rapidly emerging, high-performance RCE quantum dot photodetectors for mid-infrared applications are introduced.