One way to increase the efficiency is using multiple connections. Sun spectrum is divided into several parts that each layer of solar cell absorbs a part of the sun's spectrum. The InGaN material has a direct band gap of 0.7 to 3.4 eV with high absorption coefficient and good electron mobility, and is a suitable material for multi-junctions cells. Firstly, in this thesis three solar cells In0.4Ga0.6N, In0.68Ga0.32N and GaAs are simulated. In0.68Ga0.32N with a band gap of 1.3 eV absorbs the wavelengths of less than 950 nm of the Sun spectrum, In0.4Ga0.6N solar cell with a band gap of 2 eV absorbs the wavelengths of less than 620 nm, and GaAs with an energy of 1.42 eV absorbs the wavelengths of less than 870 nm. In order to reduce the surface recombination, window layer and BSF are used on the sides of the absorber layer. These layers are interconnected by a tunnel junction. The parasitic junctions are due to the placing of different solar cell layers in series, which causes the current to flow in the opposite direction of the cell current. This junction causes the current to flow in the desired direction. Because the connections are in series, the short-circuit current of the multi-junction solar cells is equal to the minimum short-circuit current of the different layers and its voltage is equal to the sum of the voltages of all layers. In the final section, two structures for two-junction cells are presented. The first structure is In0.4Ga0.6N / GaAs with GaAs tunnel junnction with a thickness of 2 nm, in which the open-circuit voltage of 2.52 V and the efficiency of 30.64% is obtained. The second structure is In0.4Ga0.6N / In0.68Ga0.32N, in which the GaAs tunnel junction has a thickness of 25 nm and the window layers and BSF contain InGaP, InAlGaP, and AlGaAs materials, has reached open-circuit voltage of 3.05V and efficiency of 36.81%.

 Various methods have been proposed to improve the performance of the CdTe solar cells, including grading the energy band- gap of the absorber layer. In this study in order to grading the energy band- gap of absorber layer in the CdTe cell, at the first stage Cd_(1-x) Zn_x Telayer is replaced by CdTe and by changing the value of x it can change the energy gap of absorber layer. by changing the density of Zn from the initial value of 0 to 40%, the energy gap of the cell is increased toward the back of the connection. therfore the efficiency of primary cell ZnO/CdS/CdTe is increased from an initial value of 17.62% to 19.35%. In the next stage a Cd_0.6 Zn_0.4 Te layer with thickness of 0.1 μm is added to the primary structure of ZnO/CdS/CdTe and the efficiency of the cell reaches to 20.22%. In this structure, due to the creation of conduction band offset in the intersection of CdTe/CdZnTe, the transmission of minority carriers to the front of the connection prevented and the possibility of recombination in the CdZnTe layer increases. To eliminate this problem the same ten layers of Cd_(1-x) Zn_x Tewith Xmax=40 %are placed between CdTe and Cd_0.6 Zn_0.4 Te layers. In this situation energy band is corrected and gathering the carriers in the connections improves. In the structure of CdTe cells, according to its affinity of 4.28ev and energy gap of 1.42ev, for making an ohm connection, a metal with the work function greater than or equal to 5.7 should be used in the cell. Because metal with this work function is not available, metals that have the greatest work function can be used in the CdTe cells structure. The result is that in the structure of CdTe cells, there is always a Schottky connection, and negative effect of this connection must be reduced, which is accomplished in this study.

IMPATT Diode or Impact Ionization Transit Time Diode is one of the most powerful semiconductor Devices in MW and millimeter wave generation and widely use in amplifiers and high-power oscillation systems. In this study, first the basis of IMPATT diode has been investigated, then we compare the diode characterization based on different semiconductor materials. It will be shown, the semiconductors with bigger Band Gap like Phosphor and Gallium Arsenide have better result on output power and also better efficiency. Finally by using of Silvaco, a 94GHZ SI IMPATT diode is simulated and devices characters has been investigated on different temperatures. From the obtained results, Breakdown voltage will be increased from -19.4v to -20.8v on the temperature range from 300K to 450K. Also by using Silvaco Smart Spice, The negative Resistance chart is obtained for the simulated Diode. The negative resistance chart on AC part, confirm the 180 degree phase difference between AC Voltage and current.

In this thesis we study and simulate silicon carbide-based UV detectors with Schottky junction based on nickel silicide. In this thesis the structure, function and improvement of the plasmon absorption based detectorsis are reviewed. In order to simulate the structure, the models and parameters necessary for the silicon carbide detectors are extracted. One of the main problems of the detector is low absorption coefficient compared to the other materials (such as GaN). Waxing rate of Silicon carbide layers absorption coefficient are investigated. FDTD numerical method to study their impact is mployed. It has been shown that an increase in the epi-layer’s impurity density and spectral response will decrease the optical response. In addition we have shown that for the epi-layer with density greater than 1016 cm-3 the Schottky junction leakage current saturation occurs. Increasing the thickness of the epi-layer leads to a decrease in the optical response and also reduces the spectral response, so that with a thickness of 50 micron, the spectral response will be zero. In each of the mentioned cases, the proposed effects are physically justified and have provided solutions to implement these cases. In the two-dimensional simulation of electrical analysis, the detector responsivity has been studied as a parameter to evaluate it. In analyzing the electric field distribution, the optical generation rate and current-voltage and capacitance-voltage characteristic curves are investigated. In optical analysis of the ultraviolet detector, the surface plasmon resonance simulation using FDTD software (package Lumerical) is used. In this part of the thesis, nanoparticles of aluminum, gold, silver and copper with different array and different geometric shapes to achieve the highest absorption, are studied. Simulation results show that for nano-ring thickness equals to one nanometer, the UV spectrum absorption (wavelength smaller than 250 nm) compared to the non-nanoparticle state is increased. Moreover, the highest absorption of 4H-SiC layer with nano-ring aluminum nanoparticles is occurred at the outer ring diameter of 11 nm. The highest absorption of 4H-SiC layer for aluminum nano-ribbon nanoparticles is occurred at the nano-ribbon height equal to one nanometer. By increasing the nano-ribbonheight, the absorption rate will be lower than that without nanoparticles state at low wavelengths

Electron avalanche detectors are the new generation of avalanche detectors in which impact ionization phenomena occurs just by electron. Recently because of items such as gain and sound the attentions to this avalanche has been increased. In the ideal conditions under local model by increasing gain excess noise factor we will be near to 2. the first materials that it’s electron avalanche detectors behavior was been discovered was HgCdTe. this material encounters by challenges like dark current that was due small band gap and growth processes. In the recent years the researches shows that avalanche made by InAs has characteristics like as electron avalanche decorators. Having larger band gap This material has litter dark current in comparing with HgCdTe. furthermore composition of group III-V has easier building processing and low costings by band gap 3.5eV has good detectors between 1550 to 3500wave length. In this research after proposing the basically concepts of detectors and the gain mechanism in electron avalanche detectors, the assibilation principals by Silvaco, powerful software has been introduced then the effect of simulation of one InAs detectors compared with valid reference and ultimately the gain behavior examined by changing some parameters and improved gain compared with valid references.

In this thesis, an all optical 4-channel demultiplexer has been proposed using 2D photonic crystal based resonant cavities. The fundamental structure is composed of dielectric rods immersed in air. The lattice constant, The radius of rods and the dielectric refractive index are 690nm, 204nm and 3.4 respectively. By employing resonant cavity and removing one rod and changing the radius of rod at both sides of the cavity, the proposed demultiplexer can separate 4 optical channels at 1544.3nm, 1549.6nm, 1554nm and 1558.4nm . The average transmission efficiency, quality factor and band width are 99.75%, 2185.75 and 0.7nm, respectively.

 GaAs material is one of the best choices for converting solar energy to electrical energy. Due to high constant absorption, direct energy band gap and also close to the optimum value of band gap, is one of the best semiconductor materials in space applications. At the beginning of this thesis GaAs/GaAsP solar cell simulated using the SILVACO software and after studies on thickness and doping concentration of layers a desired solar cell with characteristics V_oc=〖0.9〗^v ,J_sc=29.44〖mA/cm〗^2 ,FF=86.5 and Efficiency=23.01% obtained. Structure problems eliminated by adding a grading layer of p and n types. The number of layers and doping concentration of graded region, plays an important role in performance of solar cell. Efficiency of solar cell improved by using 10 thin p-grading layer with 10nm thickness and 2e18 cm-3 doping concentration. therefore, due to the establishment of an extra electric field, across the graded region, the minority carrier collection in this structure improved and this leading to an increasment in solar conversion efficiency. Characteristics of new structure were V_oc=〖1.05〗^v ,J_sc=29.5〖mA/cm〗^2 ,FF=87.64, Efficiency = 27.17%. Then efficiency increasement in solar conversion was 4.16%. Afterward, adding a GaAs and GaAsP tunnel junction to structure had been studied and resulting no increasment in solar conversion efficiency.

Radiation exposure of the solar cell at a given time interval will reduce the solar cell output power. The impact of cosmic rays is one of the major environmental concerns of space solar cells. In This thesis,by the introduction of the modeling of the effects of space radiation on the solar cell in SILVACO software the effects of cosmic radiation on the solar cellare numerically simulated. The effect of electron and proton radiation on the basic parameters of GaAs solar cells single link simulation and simulation results are presented. Our simulation done by AM0G decentralized solar light spectrum at 300 Kelvin degrees.Simulation results show that by increasing the proton intensity radiation,carrier’s recombination and removed absorbed photons will be increased and hence energy conversion efficiency and output power are decreased. By increasing proton collision energy on the surface of solar cellsat fixed intensity, proton away from sensitive cells and will be less likely to influence its performance. So increasing the proton radiation energy can increase cell output.

 Thin film GaAs solar cell is one of the best solutions for converting solar energy to electricity. This cells have exhibited some remarkable characteristics such ashigh absorption coefficient and direct band gap near the optimized amount(1.42eV) . For these reasons, GaAS cells show great promise for space applications and there is now considerable interest in developing these cells for high efficiency. In this thesis, first the structure of basic GaAs/AlGaAs solar cell is simulated by SILVACO and voltage-current curve and it’s important parameters is obtained.Then effect of changing the impurity density of window, emitter and substrate layers on short circuit current, open circuit voltage , fill factor and efficiency is considered and optimum impurities for mentioned layers are determined.In the simulation of basic GaAs/AlGaAscell at 300 K,Voc=1.0096V, Jsc=28.046 , FF=87.057 and efficiency 24.64% and in simulation of proposed cell, Voc=1.005V, Jsc=31.2 , FF=87.32 and efficiency 24.64% are obtained, which improved by 2.73 to the basic cell. In next stage the influence of temperature on proposed cell is studied. Little reduction in performance of proposed cell until 338(_^°)K is observed. Therefore, this cell is suitabale for using in warm climate zones.

 Single solar cells can be described by the so-called one-diode equivalent circuit model. The same model can also be applied to describe tandem solar cells, modules, and photovoltaic arrays. Based on this model, several authors have investigated individual module parameters for identical subcells, and how they are related to their subcell parameters.In This analysis the photo current (Iph) calculated by effective model, and dual junction solar cells simulated by SILVACO software (ATLAS environment).The output current of simulated cells will be analyzed by photo spectrum emission. Another parameters of simulated cell like as effect of density, tunneling width, changes in cell material composition and recombination also analyzed by this software. a 1.45 A/cm2 cell current (with 0.02 µm tunneling width) can be achieved by apply changes to simulated cell material composition. This show 0.39 A/cm2 increase in current for same cell with different material and 1.06 A/cm2 of current.Increase the tunneling width by 0.05 µm decreases the cell current to 0.83A/cm2 . then increasing the tunneling width decreases the current and not preferable .Analysis show that a cell with 0.398 Gmol/cm3 density in tunneling have on acceptable response and increasing the density to 4.85Gmol/cm3 increase the output current of cell to suitable range

 Abstract :
Organic materials to reduce energy consumption and production costs in manufacturing solar cells were used. With the arrival of these polymeric materials to the semiconductor industry a new generation of solar cells were formed. Organic solar cells despite its many advantages, have low optical conversion efficiency and short lifetime.Organic-inorganic solar cells was the solution to overcome these problems. In this study, an organic-inorganic a-Si:H/PCPDTBT:PCBM solar cell was simulated using drift-diffusion and Koster’s models. Excellent agreement between simulation and experimental results obtained. The effects of organic cell active layer’s thickness was measured on its output parameters and the optimal thickness of the active layer was determined to increase the short-circuit current density. Finally, the optimal organic cell used in the organic-inorganic hybrid cell’s structure. As a result, the short circuit current density and the conversion efficiency improved.

Due to having of high efficiency and high radiation resistance against of high energy particles, InGaP/GaAs based solar cell has been attracted more attention among of multijunction solar cells. In this thesis, first the structure of simple GaAs cell is simulated by silvaco and voltage-current curve and it’s Important parameters is obtained. Again, in order to improving of this structure by adding some layers and new materials to simple GaAs cell, including of BSF layer and window layer, voltage-current curve and it’s parameters is obtained. In continue, we simulated GaInP cell separately and obtained it’s voltage-current curve. Then, structure of InGaP/GaAs based solar cell under AM0 is analyzed and in addition to voltage-current curve, open-circuit voltage, short-circuit current, fill-factor, and efficiency values is obtained. In this stage cell’s efficiency reached to 24.46%. In according to importance of solar cell’s efficiency, in next section in order to improving InGaP/GaAs cell we used AlInGaP in cell’s structure instead of GaAs emitter. In this way, changing of impurity density and thickness of bottom cell’s layers is studied that improve of impurity density and layer’s thicknesses caused to increased open-circuit voltage and efficiency. Base on simulation results, cell’s efficiency reached to 33.72% in this stage. Then by applying of some changes in structure of top cell, we also improved short-circuit current and therefore we improved impurity density of top structure layers of cell. By combining results and according to the obtained results from simulation, finally cell’s efficiency increased to 36.532% and appropriate structure for InGaP/GaAs was obtained.

Thin film InP solar cell is one of the best solutions for converting solar energy to electricity. This cells have exhibited some remarkable characteristics such ashigh absorption coefficient ( about ), direct band gap, high radiation resistance and low annealing temperature (about 〖100〗^o C). For these reasons, InP cells show great promise for space applications and there is now considerable interest in developing these cells for high efficiency. In this study, 1-D drift-diffusion model has been used for simulation. Drift-diffusion equations are solved by gummel numerical method, using MATLAB and Silvaco softwares. At first, the method of transferring carriers produced in this type of cells and factors limiting the efficiency of the cell are studied here. Since, the distance between the back contact and the space charge is small in this group of cells due to low width of absorber layer; therefore, much light reaches to the back contact, and recombination at this point is the main mechanism of losses in the cell. Thus, some solutions for moving carriers far from back contact of thecells such as addingwindow and BSF layers to the basic cell and grading the band gap of window layer have been investigated.In the simulation of basic InP cell at 300 K with E_g=1.35 eV, the parameters Voc=0.853V, Jsc=33.51 , FF=82.18 and efficiency 23.51% are obtained ,while the simulation of the proposed cell indicated that , Voc=1.019V, Jsc=34.43 , FF=86.88 and efficiency 30.5% can be attained, showing an improwment of 6.99% of efficiency with respect to the basic cell.