Yeasir Arafat

Flicker level mitigation of weak grid connected wind turbine with DFIG by injecting optimum reactive power using STATCOM

Now-a-days, power system cannot be thought of as a standalone system, rather it is integrated with numerous renewable power sources such as wind energy, solar energy etc. With the development of high capacity electric power generation from wind energy, large number of wind turbines are being installed and connected to power systems. Consequently, stoic operation of wind turbine system is very important for power system stability. Flicker is an inevitable but undesired behaviour especially in wind turbine connected weak grid where the penetration level of wind power is significant high so as to affect the power system operation and control. This is mainly caused by fluctuations in the output power due to wind speed variations, the wind gradient and the tower shadow effect. This conspicuous phenomenon is a limiting factor for integrating wind turbines into weak grid network. This paper studies the effect of injecting optimum reactive power using Static Synchronous Compensator (STATCOM) on flicker mitigation of weak grid connected wind turbine with Doubly Fed Induction Generator (DFIG). Details of performance appraisal with inter parameter dependency has also been demonstrated convincingly.

Technical and financial aspects of solar PV system for city dwellers of Bangladesh where green energy installation is mandatory to get utility power supply

Acute power crisis in Bangladesh stresses on the necessity of alternative sources of power generation in short, mid and long term basis. If solar panels be the alternative short term power source, then per unit production cost is a huge barrier in making this technology popular among the mass. But, urban house owners are under a mandatory condition of installing solar panels on their rooftop to get new electricity connection from power utilities. In such a situation, a solar energy rate has been proposed that will cover the solar electricity production cost, alternately called the cost recovery scheme. This rate is near about the maximum per unit rate of energy the government purchases from quick rental plants. Also, different types of solar modules have been compared. This can be a technical support for the city dwellers to who wants to buy solar panels. In this paper, a typical 6 storied building has been surveyed for the calculation of annual average electricity consumption and selected for solar PV design utilizing the unused rooftop of the building.

Base transit time of a Heterojunction Bipolar Transistor with Gaussian doped base

Base transit time for an npn SiGe HBT is calculated assuming Gaussian doped base and a generalized trapezoidal Ge profile considering field dependent mobility in excess to doping dependent mobility and diffusivity. Band-gap narrowing (BGN) due to heavy doping, due to presence of Ge and due to change in the density of states (DOS) are also considered. For presence of Ge, a different saturation velocity has been used. Ge profile variation has been incorporated by a single parameter. Base transit time of SiGe HBT is calculated and computed for different Ge contents. The calculated base transit time shows similar variations as found in literature for BJT.

Dependency of Heterojunction Transistors' speed on various physical parameters: A comparative study of SiGe & AlGaAs HBTs

It is very essential to find out how the speed of a Heterojunction Bipolar Transistor (HBT) depends on different physical parameters of the transistor as the device has become indispensable in modern ultrafast circuits. As the speed of an HBT is a very strong function of its base transit time, here we investigate its dependency on minority carrier injection, base width, base emitter voltage, peak base doping concentration and slope of base doping for an AlGaAs HBT. The analytical model of base transit time of this AlGaAs HBT is based on SIGe model as found in literature. Comparison of base transit time obtained from similar simulation for SIGe and AlGaAs HBT has been presented in this work.

Effect of high k-dielectric as gate oxide on short channel effects of junction-less transistor

In this work, short channel effects such as drain induced barrier lowering, sub-threshold swing, on-current and off-current and on-off ratio have been analyzed for different oxide materials as gate oxide for double gate junction-less transistors by using device simulator. Just like inversion mode MOSFETS, junction-less transistors show better short channel effects when high-k dielectrics are used.

Parameters optimization of a heterojunction thin film solar cell to improve the conversion efficiency

Thin Film Solar Cell is a technology with great potential because of its low material consumption and relatively high efficiency. In this paper several techniques have been taken into consideration which can improve the efficiency of thin film solar cell like CIGS and ZnTeO. Simulations have been performed to optimize the parametric values that can greatly influence the efficiency of thin film solar cell. Effect of change of cell width, transmittance, recombination lifetime etc. on the conversion efficiency has been studied and optimized in MATLAB. It has been found that thin film solar cells with larger cell width can provide better efficiency only when the minority carrier mobility of the cell material crosses a critical value. If the mobility of the cell material is below this critical value, the efficiency of the cell decreases with increase in cell width.

Analytical modeling of base transit time for a Si 1−y Ge y heterojunction bipolar transistor

An analytical model for base transit time of an exponentially doped base npn Si1−yGey HBT has been developed. The model is valid in all levels of injection before the onset of Kirk effect. In this analysis, bandgap-narrowing effect, high-injection effect, carrier velocity saturation at the base edge of the base collector junction and doping dependent mobility are incorporated. It is found that base transit time depends on the Ge profiles in the base. The increase of Ge content for the same profile results in a decrease of transit time. Results of this work are compared with results available in literature.

SRAM NBTI mitigation with predictable activity based dynamic stress-recovery timing

Negative Bias Temperature Instability (NBTI) is one of the major reliability issues for nano-scale semi-conductor devices. With the continuous device scaling, NBTI effect is becoming more and more severe for lower technology node, causing life-time degradation. Several solutions have been proposed to develop device life-time by interrupting interface trap generation in Si-SiO2 interface. Implementing periodic stress and relaxation mode by applying periodic data flipping of the SRAM memory cells is one of the most effective ways of NBTI effect mitigation. Life-time degradation of a device depends on the ratio of stress and recovery timing for a certain time period. In this paper, a dynamic stress-recovery timing pattern for periodic flipping based on SRAM activity has been proposed and concluded that with the proposed stress-recovery timing pattern, NBTI degradation in each PMOS device of the SRAM array has further been mitigated.

Thickness optimization and composition grading effect in heterojunction CIGS Solar Cell

Thin Film Solar Cell is a technology which has a great potential as it has low material consumption and relatively high efficiency to others. Among thin film solar cells CIGS is a promising one. But due to scarcity of gallium and indium its industrial production can be affected. Here in this paper via simulation (using device simulator) we have tried to achieve good acceptable efficiency using very thin layer of CIGS absorber layer with the help of grading and appropriate gallium composition and by optimizing emitter efficiency which is close to the efficiency of CIGS cells having thick absorber layer.

Estimating the transit time of an Al y Ga 1−y As HBT in the Gaussian doped base region for low to moderately high level of injection

For a Gaussian doped graded base AlGaAs Hetero-junction Bipolar Transistor (HBT), base transit time (BTT) has been determined using a suitable analytical model. Here the authors have investigated the dependency of base transit time on different physical and input parameters such as the collector current density, base emitter voltage etc. Electric field dependent carrier mobility also has been included into the analytic model and found that the base transit time worsens compared to the value found without taken the field dependent mobility into account.