Nadia H. Rafat

Theoretical Study of Metal-Insulator-Metal Tunneling Diode Figures of Merit

The performance of metal-insulator-metal (MIM) diodes is investigated. In this paper, we derive an analytical model that uses the Transfer Matrix Method based on the Airy Functions (AF-TMM) for the tunneling transmission probability through any number of insulating layers. The fast-computing AF-TMM simulator results show a complete matching with the numerical Non-Equilibrium Greens Function and a reasonable matching with previously published experimental results. This study shows the effect of the work function difference and insulator thickness on MIM diode performance. The advantage of using two insulator layers on enhancing the diode responsivity, resistance, and nonlinearity is also investigated.

The limiting efficiency of band gap graded solar cells

Two fundamental mechanisms limit the maximum attainable efficiency of solar cells, namely the radiative recombination and Auger recombination. We show in this paper that proper band gap grading of the solar cell localizes the Auger recombination around the metallurgical junction. Two beneficial effects result from this Auger recombination localization; first the cell is less sensitive to the surface conditions, and second, the previous estimates for the limiting efficiency of solar cells by Shockley, Tiedje, and Green are revised upwardly. We calculate the optimum bandgap grading profile for several real material systems, including GaInAsP lattice matched to InP, and a-SiGe on a-Si substrate.

Complete band gaps of phononic crystal plates with square rods.

Much of previous work has been devoted in studying complete band gaps for bulk phononic crystal (PC). In this paper, we theoretically investigate the existence and widths of these gaps for PC plates. We focus our attention on steel rods of square cross sectional area embedded in epoxy matrix. The equations for calculating the dispersion relation for square rods in a square or a triangular lattice have been derived. Our analysis is based on super cell plane wave expansion (SC-PWE) method. The influence of inclusions filling factor and plate thickness on the existence and width of the phononic band gaps has been discussed. Our calculations show that there is a certain filling factor (f=0.55) below which arrangement of square rods in a triangular lattice is superior to the arrangement in a square lattice. A comparison between square and circular cross sectional rods reveals that the former has superior normalized gap width than the latter in case of a square lattice. This situation is switched in case of a triangular lattice. Moreover, a maximum normalized gap width of 0.7 can be achieved for PC plate of square rods embedded in a square lattice and having height 90% of the lattice constant.

Dipole Nantennas Terminated by Traveling Wave Rectifiers for Ambient Thermal Energy Harvesting

In this paper, rectennas formed from nanodipole antennas terminated by plasmonic metal-insulator-metal (MIM) travelling wave transmission line rectifiers are developed for ambient thermal energy harvesting at 30 THz. The transmission lines are formed from two strips coupled either vertically or laterally. A systematic design approach is presented, that shows how different components can be integrated with each other with maximum radiation receiving nantenna efficiency, maximum coupling efficiency between nantenna and rectifier, and maximum MIM diode rectifier efficiency. The tunneling current of the rectifier is calculated using the transfer matrix method and the nonequilibrium Greens function. A detailed parametric study of the coupled strips plasmonic transmission lines is presented and thoroughly discussed. The overall efficiencies of the proposed travelling wave rectennas are fully expressed and compared.

Modeling of a wide band pass optical filter based on 1D ternary dielectric?metallic?dielectric photonic crystals

We suggest ternary structures of dielectric?metal?dielectric photonic structures to be used as optical band pass filters. We theoretically study and evaluate the optical properties of these one-dimensional structures. ZnSe/metal/LiF and SiC/metal/SiO2 with three metals, namely silver, gold and copper, are suggested as the optical filters. We calculated the reflectance and the transmittance of electromagnetic waves (EMW) out of these structures using the transfer matrix method. These calculations take place for the case of normal and oblique incidences using actual measured values for the indices of refraction. Our calculations show that such photonic crystal (PC) filters have a well shaped pass band in the visible range and block efficiently ultraviolet and infrared EMW. Our results show that PCs having silver as the metal layer are preferred to those having gold and copper because of the high transmittance in the visible range. A SiC/Ag/SiO2 filter shows better performance than a ZnSe/Ag/LiF one from the transmittance and the shape of the band points of view. Such a filter shows a well shaped wide band over the range of incident angles from 0? to 80?.

Enhanced Model of Conductive Filament-Based Memristor via Including Trapezoidal Electron Tunneling Barrier Effect

Memristors exhibit very promising features such as nonvolatility and small area. Several types of memristors have been developed in the last decade using different materials along with physical models explaining their behaviors. In this paper, we modify a previously published model to account for a trapezoidal electron tunneling barrier rather than a zero field or constant potential barrier. The model is verified against experimental data showing better agreement. We then perform a study to find out the effect of different memristors parameters on its I-V characteristics and how to shape the characteristics to fit the applications. Finally, we provide a SPICE model which takes into account the tunneling capacitance and clarify that any fabricated memristor has, inherently, a memcapacitor in parallel. The dominant element may be the memristor or the memcapacitor depending on the frequency of operation.

Analytical modeling of the radial pn junction nanowire solar cells

In photovoltaic solar cells, radial p-n junctions have been considered a very promising structure to improve the carrier collection efficiency and accordingly the conversion efficiency. In the present study, the semiconductor equations, namely Poissons and continuity equations for a cylindrical p-n junction solar cell, have been solved analytically. The analytical model is based on Greens function theory to calculate the current density, open circuit voltage, fill factor, and conversion efficiency. The model has been used to simulate p-n and p-i-n silicon radial solar cells. The validity and accuracy of the present simulator were confirmed through a comparison with previously published experimental and numerical reports.

Analytical study of carrier photogeneration and photocurrent in thin silicon solar cells with specular and diffuse back reflection

An analytical study of the effect of back reflection in thin silicon solar cells implementing a practical (nonideal) diffuse reflector at the back side is presented. Both the diffuse and specular reflection components are considered. The reflection properties of the back reflector are described by means of three parameters, which are interpreted in terms of a simple physical model. An exact expression for the generation profile resulting from specular and diffuse reflections is derived and its dependence on the reflector parameters is studied. A mechanism through which specular reflection can significantly enhance light trapping in the case of relatively weak diffuse reflection is proposed. The photocurrent boost doe to both types of reflections acting simultaneously is estimated.

Characterization of MIM diodes based on Nb/ Nb 2 O 5

MIM diodes based on Nb/Nb2O5 are analyzed. The tunneling probability calculation is based on the transfer matrix method and the non equilibrium greens function. Contour plots are presented showing the effect of the oxide thickness and the work function difference between the left metal electrode, Nb and the right metal electrode on the diode resistance, responsivity, and non linearity. Also, the total rectenna efficiency is analyzed for various MIM structures.

An optimum base drift field for a p-n junction solar cell

The concept of an optimum uniform drift field in the base region of a p-n junction single-crystal solar cell is presented. A comprehensive analytical treatment is given which takes into account the effect of variations of the material parameters, due to the doping profile which produces the field, on the cell performance. The field improves the cell performance while the associated variation of material parameters degrades it. These two opposing factors lead to an optimum field for each cell design. Calculations based on this treatment are presented for the cell parameters versus field. The results confirm the concept of an optimum field.