Finding energy sources to satisfy the world's growing demand is one of society's foremost challenges for the next half-century. The challenge in converting sunlight to electricity via photovoltaic solar cells is dramatically reducing $/watt of delivered solar electricity. In this context the sun trackers are such devices for efficiency improvement. The diurnal and seasonal movement of earth affects the radiation intensity on the solar systems. Sun-trackers move the solar systems to compensate for these motions, keeping the best orientation relative to the sun. Although using sun-tracker is not essential, its use can boost the collected energy 10–100% in different periods of time and geographical conditions. However, it is not recommended to use tracking system for small solar panels because of high energy losses in the driving systems. It is found that the power consumption by tracking device is 2–3% of the increased energy. In this paper different types of sun-tracking systems are reviewed and their cons and pros are discussed. The most efficient and popular sun-tracking device was found to be in the form of polar-axis and azimuth/elevation types.

A review of principle and sun-tracking methods for maximizing solar systems output

Resonance conditions for a substrate-superstrate printed antenna geometry which allow for large antenna gain are presented. Asymptotic formulas for gain, beamwidth, and bandwidth are given, and the bandwidth limitation of the method is discussed. The method is extended to produce narrow patterns about the horizon, and directive patterns at two different angles.

Gain enhancement methods for printed circuit antennas

Nanofabrication by electron beam lithography and its applications

Random Matrix Methods for Wireless Communications: Estimation

A novel design of a transmission window within the absorption band of a circuit analog absorber, named as frequency-selective rasorber (FSR), is presented. Based on an equivalent circuit model, the conditions are formulated to produce a passband with small insertion loss and to reduce the reflection at frequencies below and above the passband in the meanwhile. Simple design guidelines of our proposed FSR are then developed. With loaded lumped elements, the arrays of square-loop and cross-dipole are combined to realize its implementation. It is shown through measurements that an insertion loss of 0.68 dB can be obtained at 4.42 GHz and the fractional bandwidth for at least 10 dB reflection reduction within the lower and upper frequency bands is 92.3% under the normal incidence. A good agreement between simulated and measured results validates our design.

Frequency-Selective Rasorber Based on Square-Loop and Cross-Dipole Arrays

Temperature-dependent properties of organic-on- inorganic Ag/p-CuPc/n-GaAs/Ag photoelectric cell

A simple photo-voltaic tracking system

In this paper, heterojunctions were fabricated by employing p-type Si and thin films of poly-N-epoxipropylcarbazole (PEPC) doped with tetracyanoquinodimethane (TCNQ). The PEPC films were grown on Si wafers at room temperature but with different gravity (g) conditions:-1, 123, 277, and 1107g. Current-voltage (I-V) characteristics of the grown hybrid structures were evaluated as a function temperature (T) ranging from 20/spl deg/C to 60/spl deg/C. It was found that all samples are p-p isotype heterojunctions and the junctions fabricated at a high value of g, i.e., at 277 and 1107 g, showed reversible rectifying properties as a function of device temperature. Whereas the behavior of devices fabricated at 123 and 1 g were rectifying at room temperature, but became almost nonconductive after treating the samples at 60/spl deg/C. Rectification ratio, threshold voltage, reverse saturation current, and junction resistance of the fabricated junctions were evaluated at different temperatures. At T=60/spl deg/C, the devices grown at 1107 g exhibited rectification ratio less than unity which may be attributed to the switching of the depletion at the interface. This has been explained by assuming the generation of carriers are at elevated temperatures in the organic film, and their subsequent emission from the organic to the inorganic side of the heterojunction.

Temperature-dependent I-V characteristics of organic-inorganic heterojunction diodes

Cryptography plays a vital role for ensuring secure communication between multiple entities. In many contemporary studies, researchers contributed towards identifying best cryptography mechanisms in terms of their performance results. Selection of cryptographic technique according to a particular context is a big question; to answer this question, many existing studies have claimed that technique selection is purely dependent on desired quality attributes such as efficiency and security. It has been identified that existing reviews are either focused only towards symmetric or asymmetric encryption types. Another limitation is found that a criterion for performance comparisons only covers common parameters. In this paper, we have evaluated the performance of different symmetric and asymmetric algorithms by covering multiple parameters such as encryption/decryption time, key generation time and file size. For evaluation purpose, we have performed simulations in a sample context in which multiple cryptography algorithms have been compared. Simulation results are visualized in a way that clearly depicts which algorithm is most suitable while achieving a particular quality attribute.

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Cryptography: A Comparative Analysis for Modern Techniques

A comparison of nine different nonlinear I–V models for the simulation of submicron GaAs MESFET dc characteristics has been made. Drain-to-source current, Ids as a function of gate-to-source, Vgs and drain-to-source, Vds voltages has been simulated and then compared with experimental data. To determine the accuracy of a model, root-mean-square (RMS) errors were calculated. The lowest RMS error was observed for Ahmed model whereas it was highest for Statz model. An ideal Schottky barrier junction free from interface states has been assumed in these models and thus their applications in device simulation are limited. To simulate output characteristics of a GaAs MESFET having finite density of sates at Schottky barrier, Ahmed model has been modified. It has been demonstrated that the proposed model is a comprehensive one and can simulate the device characteristics, with significant improved accuracy, for varying Schottky barrier conditions.

Muhammad Mansoor Ahmed

Papers

Temperature-dependent properties of organic-on- inorganic Ag/p-CuPc/n-GaAs/Ag photoelectric cell

A simple photo-voltaic tracking system

Temperature-dependent I-V characteristics of organic-inorganic heterojunction diodes

Cryptography: A Comparative Analysis for Modern Techniques

A comprehensive four parameters I–V model for GaAs MESFET output characteristics