Sharif Digital Repository

We present an explicit solution of carrier and field distributions in abrupt PN junctions under equilibrium. An accurate logarithmic numerical method is implemented and results are compared to the analytical solutions. Analysis of results shows reasonable agreement with numerical solution as well as the depletion layer approximation. We discuss extensions to the asymmetric junctions. Approximate relations for differential capacitance C-V and current-voltage I-V characteristics are also found under non-zero external bias  

The efficiency of Mn luminescence is enhanced in low-thickness ZnS:Mn layers. While various hypotheses can be considered to explain the phenomenon, it seems more reasonable to consider the luminescence enhancement effect a result of the formation of dot-like regions with higher local Mn concentration  

We present an edge emitting laser structure employing silicon carbide polymers only. In this structure, 3C-SiC well embedded in 6H-SiC barriers are used as the active region, which is sandwiched between the 6H-SiC mirror at the top and bottom of structure. The basic design goal is to use only silicon carbide polymers and decrease the threshold current and stable optical wavelength of the lasers with silicon carbide polymers. This paper provides key results of the device characteristics, including the light power versus electrical current and the optical wavelength versus current  

In the present work, a new structure of the strained quantum-well (QW) laser diode is designed and simulated. In this structure, the active region consists of tow 6H-SiC barrier and 3C-SiC quantum well (QW)which is sandwiched between two layers of 6H-SiC that can be interpreted in terms of a type-II heterostructure character and a built-in electric field due to the pyroel ectricity of 6H using an industrial-based numerical simulator. The basic design goal was to decrease the threshold current by using only silicon carbide polymers. We could obtain a working model at stable optical wavelength of 0.83μm. This paper provides key results of the device characteristics, including the light power... 

In this paper, the effect of radial structure on the performance of a linear-lateral GaAs high power photoconductive semiconductor switch (PCSS) is investigated. For this purpose a three-dimensional device modeling is used to model the optically initiated GaAs switch. In this simulation a p-type device with carbon as shallow acceptor is compensated by deep donor EL2 level as a trap level. The PCSS device is designed in a back-triggered, radially symmetric switch structure which extends the blocking voltage by reducing the peak electric field near the electrodes. Device modeling was performed and the effect of different trap concentrations on dark I-V characteristics has been investigated. In... 

In the present paper, we investigated the various elements of heat sources within a silicon carbide polymers (6H-SiC and 3C-SiC) semiconductor laser. The device employs 3C-SiC quantum well (QW) which is sandwiched between two layers of 6H-SiC as cladding regions that can be interpreted in terms of a type-II heterostructure character and a built-in electric field due to the pyroelectricity of 6H using a numerical simulator. The basic design goal was the study of the various elements of heat sources, including the Joule heat power, the Peltier-Thomson heat power and the recombination heat power  

We report a study on chalcopyrite solar cells fabricated by low-cost, nonvacuum, and selenization-free methods. Superstrate-type CuInS2 (CIS) thin-film solar cells were prepared by sequential ink deposition. The CIS film was formed from a stable low-carbon ink, which was synthesized at low temperature (<120 °C). The CIS nanoparticle ink was prepared with n-butylamine and acetic acid as the solvent and stabilizer, respectively. The viscous and stable ink that formed through the dispersion of the final nanoparticles in N,N-dimethylformamide (DMF) could be deposited readily onto the substrate. The major features of the obtained ink are the small amount of impurity phases and negligible carbon... 

Nanostructured aluminum titanate (AT) was synthesized by the sol-gel method. The calcination temperature was selected as a processing variable to obtain different phase ratios of AT and its parent phases i.e. TiO2 and Al2O3. The main aim behind the present work was to cast light on the photocatalytic performance of AT-based nanocomposites and evaluate their capability to serve as a novel photocatalyst. X-ray diffraction (XRD) analysis and field-emission scanning electron microscopy (FE-SEM) studies were conducted to evaluate the microstructure-sensitive properties of the synthesized powders. Diffuse reflectance spectroscopy (DRS) and the methylene blue degradation test were carried out to... 

Using a model reduction method, a formula for the ideality factor of a pn junction as a function of the diffusion and generation terms of its reverse current is derived. Using this formula a method for separate computation of these two currents for a pn junction is presented. The validity of the method is investigated using computer simulations for an assumed diode with known ideality factor and total leakage current. Experimental results for two commercially available diodes validate the proposed technique  

In this paper, the effect of two-dimensional electron plasma formed on the surface of a semiconductor on the refraction phase of an optical beam is investigated. Both the density and effective mass of the surface state electrons can be controlled by a transverse electric field. While the surface density of electrons varies slowly with the applied field, their effective mass changes instantaneously. Therefore, a high bandwidth for the phase modulation can be obtained. It is expected that the device operates in the far-infrared region with a semiconductor as the base material. GaAs is one of the best candidates having both large dynamic range, and large ratio of free electron to the crystal... 

Fast transfer of photoinduced electrons and subsequent slow electron-hole recombination in semiconductor heterostructures give rise to long-lived charge separation which is highly desirable for photocatalysis and photovoltaic applications. As a type II heterostructure, CdS/TiO2 nanocomposites extend the absorption edge of the light spectrum to the visible range and demonstrate effective charge separation, resulting in more efficient conversion of solar energy to chemical energy. This improvement in performance is partly explained by the fact that CdS/TiO2 is a type II semiconductor heterostructure and CdS has a smaller energy band gap than UV-active TiO2. Ultrafast transient absorption... 

Two-dimensional semiconductor materials can be combined with conventional silicon-based technology and sort out part of the future challenges in semiconductor technologies due to their novel electrical and optical properties. Here, we exploit the optoelectronics property of the silicon/SnS2 heterojunction and present a new class of backward diodes using a straightforward fabrication method. The results indicate an efficient device with fast photoresponse time (5-10 μs), high photoresponsivity (3740 AW-1), and high quantum efficiency (490%). We discuss device behavior by considering the band-to-band tunneling model and band bending characteristics of the heterostructure. This device structure... 

Promising properties of boron nitride nanomaterials such as their chemical, thermal, and mechanical stability have made them suitable materials in a various range of applications. However, their low electrical conductivity and wide bandgap, particularly in the case of boron nitride quantum dots (BNQDs), have given rise to severe limitations. Efforts on bandgap engineering through doping and surface functionalization have gained little success due to their high thermodynamic stability and inertness. Herein, we present a novel approach to functionalize BNQDs by hydroxyl, methyl, and amine functional groups aiming to adjust the electronic structure. The successful exfoliation is demonstrated by... 

The fabrication of supported noble metal nanocrystals (NCs) with well-controlled morphologies have been attracted considerable interests due to their merits in a wide variety of applications. Photodeposition is a facile and effective method to load metals over semiconductors in a simple slurry reactor under irradiation. By optimizing the photodeposition process, the size, chemical states, and the geometrical distribution of metal NCs have been successfully tuned. However, metal NCs with well-controlled shapes through the photodeposition process have not been reported until now. Here, we report our important advances in the controlled photodeposition process to load regular noble metal NCs.... 

We use the ionic Hubbard model to study the effects of strong correlations on a two-dimensional semiconductor. The spectral gap in the limit where on-site interactions are zero is set by the staggered ionic potential, while in the strong interaction limit it is set by the Hubbard U. Combining mean field solutions of the slave spin and slave rotor methods, we propose two interesting gapped phases in between: (i) the insulating phase before the Mott phase can be viewed as gapping a non-Fermi liquid state of spinons by the staggered ionic potential. The quasi-particles of underlying spinons are orthogonal to physical electrons, giving rise to the 'ARPES-dark' state where the ARPES gap will be... 

Nanoscale Shielded channel transistors are investigated by solving the two-dimensional Poisson equation self-consistently with ballistic quantum transport equations for first time. We present self-consistent solutions of ultrathin body device structures to investigate the effect of electrically shielded channel region which impose charge controlling in the channel region on the characteristics of nanoscale DG-MOSFET. The simulation method is based on Nonequlibrium Green's Function (NEGF). Starting from a basic structure with a gate length of 10 nm, the effect of gate length variation on the performance of the device has been investigated. © 2007 IEEE  

The effect of spin dependent recombination on the transport properties of magnetic semiconductors is investigated theoretically. In particular, for p -type direct band gap semiconductors, a theory based on classic Shockley equations is formulated. In this theory the density of spin and charge has been evaluated analytically by solving the diffusive transport equation and it is shown that the difference between recombination rates affect the lifetimes of spin and charge significantly. It is also demonstrated that a considerable spin charge coupling occur. Application of this theory to pure band to band recombination process is discussed. © 2009 American Institute of Physics  

Theoretical studies on the spin-dependent transmission and current-voltage characteristic in a mesoscopic system, which consists of two semi-infinite ferromagnetic (FM) leads (as source and drain) separated by a typical periodic quantum dot (QD) are presented. The calculations are based on the tight-binding model and transfer matrix method, and investigate the magnetoresistance (MR) and the spin polarization within the Landauer-Büttiker formalism. The spin-dependent transport behavior can be controlled via a gate voltage and an applied bias in the ballistic regime. The numerical results are shown for a periodic polymer chain with nonmagnetic (NM) and FM leads, and also, with two FM leads....