Sharif Digital Repository

In this paper, we present an envelope function analysis technique for the design of the emission spectra of a white quantum-well light-emitting diode (QWLED). The nano- metric heterostructure that we are dealing with is a multiple QW, consisting of periods of three single QWs with various well thicknesses. With the aid of 6 × 6 Luttinger Hamiltonian, we employ the combination of two methods, k · p perturbation and the transfer matrix method, to acquire the electron and hole wave functions numerically. The envelope function approximation was considered to obtain these wave functions for a special basis set. While adjacent valence sub-bands have been determined approximately, the conduction... 

Large signal behavior of the Laser Diode is analyzed with the aim of finding a qualitative relationship between modulation amplitude and the change in the shape of its transfer function. It is found that, at quasi-static conditions, increasing the amplitude of the modulating signal smoothes out the original piecewise-linear transfer function of the diode. It further shifts the now smoothed knee point to lower DC currents. As the frequency increases, however, these effects become less and less pronounced. In addition, the transfer function becomes rather wavy due to the failure of current density to reach the saturation level necessary for adequate stimulated emission. At even higher... 

In this paper, a special probe is designed using an array of photodiodes and a 960 nm LED for recording the wrist photoplethysmogram (PPG). The design is based on Monte-Carlo simulation of light propagation in tissues. Two series of experiments were carried-out: normal breathing and deep breathing. In both experiments, finger and wrist PPG were simultaneously recorded from the index finger. In the first series of experiment, signals from 9 subjects were recorded and the correlation coefficient for the raw signals (AC+DC), the AC and DC components of wrist and finger PPG were (62.5% ± 12.1%), (91.2% ± 6.9%) and (61% ± 13.4%) respectively. In the second series of experiment (deep breathing),... 

This paper proposes a graphene nanoribbon field effect transistor (GNRFET) consisting of pitched semiconducting GNRs as the channels that are connected to the metallic graphene source/drain in a seamless fashion. We obtained the diagrams for frequency bandwidths, step time responses, and Nyquist stability for the seamless pitched GNRFET (SP-GNRFET) with a channel having 100 pitched GNRs at 10 nm pitch in the common source configuration with various dimensions of the GNRs. The aforementioned diagrams were also obtained for the pitched carbon nanotube field effect transistor (CNTFET) with a channel having 100 pitched CNTs at 10 nm pitch in the common source configuration with various... 

We prepared porous silicon samples coated by continuous palladium layer in electroless process. Scanning electron microscopy (SEM) showed cauliflower-shape Pd clusters on the surface. I-V curves of Schottky like Pd/porous Si samples were measured in air and in hydrogen. These measurements showed a metal-interface-semiconductor configuration rather than an ideal Schottky diode. Variations of the electrical current in the presence of diluted hydrogen at room temperature revealed that the samples can sense hydrogen in a wide range of concentration (100-40,000 ppm) without any saturation behavior. Hydrogen sensing properties of these samples were investigated at room temperature for a duration... 

In recent years, quantum dots (QDs) have been widely used in upcoming nanotechnology-based solar cells, light-emitting diodes and even bioimaging, due to their tunable optical properties and excellent quantum yields. But, such nanostructures are currently constituted by heavy elements which can threat the human health and living environment. Hence, in this work, the in vivo effects of CdTe nanocrystals (NCs) (as one of the promising QDs) on spermatozoa of male mice and subsequently on fertility of female mice were investigated, for the first time. To do this, CdTe NCs were synthesized through an environment-friendly (aqueous-based solution) method. The sperm cells presented a high potential... 

The effect of thermal noise and avalanche photodiode (APD) noise in a code division multiple access (CDMA) system using generalized optical orthogonal codes (GOOC) are investigated for the first time. The modulations used are pulse position modulation (PPM) and overlapping PPM (OPPM). Double optical hardlimiter receiver structure is selected because of its efficient performance. We show that, in a system with all mentioned noise sources, known parameter values, and a bit error rate of 10-7, GOOC codes with cross correlation (λ) equal to 2 support 20 more users compared to strict OOC. We have also analyzed the spectral efficiency of the GOOC OPPM system and compared it with GOOC PPM. The... 

We report the design, fabrication and error analysis of a sun sensor array composed of six photodiodes. The sensor estimates the direction of the sun using a linear least squares method. The performance of the sensor is deteriorated by three major sources: fabrication errors, scattered environmental light, and inexact modeling of photodiodes. Using a calibration procedure and modeling the uniform component of the environmental light, we mitigate the first two errors and significantly reduce root mean squared error from 2.63° to 0.83°. For a Field of View (FOV) of 110°, the maximum estimation error also drops from 3.8° to 1.6°. Through exact mathematical modeling of photodiodes, we... 

Resistive sensors composed of SnO2 and CuO, are known to be highly efficient in detection of detrimental H2S gas in terms of sensitivity, selectivity and speed. Recently, dependency of electrical response of the sensor toward H2S gas concentration has been related to the selective mechanism (formation of CuS) by a theoretical model. Another important factor in design of gas sensors is the working temperature which so far has not been explicitly explained for H2S sensing process of SnO2-CuO system. In present study, origin of this temperature for SnO2-CuO thin bilayer based on the selective mechanism has been theoretically interpreted. For this purpose, Poisson, Laplace and continuity... 

Rectifiers are an integral part of power harvesting systems. In this paper, the literature on RF power rectifiers is surveyed, starting from the well-known voltage doubler. Effects of using low turn-on voltage devices on forward and reverse losses, and therefore, on conversion efficiency, is discussed. Samples of rectifiers with external devices, such as Schottky diodes are presented. Idea of external Vth cancellation through a rechargeable battery, self Vth cancellation, and floating gate transistors with charge injection onto the gates are demonstrated. Then, standard bridge rectifier and its modified versions, including Vth cancellation technique, are explained. Using low voltage devices... 

Unidirectional current flow is at the heart of modern electronics, which has been conceived by making p–n junctions or Schottky barriers between different kinds of materials. Within such elements, however, synthesis of thin film lateral heterostructures has so far remained challenging. Here, a one-step simple synthesis of p-type, n-type, and metallic lateral heterostructures using bipolar electrodeposition (BPE) technique is reported. Molybdenum oxides and sulfides with gradient of oxygen and sulfur are deposited at a metallic substrate. A lateral heterostructure is achieved with electrical properties that change from p- to n-type semiconductor and then to metal by moving in the plane of the... 

Visible light communication (VLC) uses LED lights for simultaneous illumination and data transmission. The throughput of a VLC network is highly dependent on the location of VLC access points (AP). In this paper, we propose a novel optimization framework for approximately optimal VLC AP placement which takes into account the stationary distribution of users for the first time. This distribution is the result of the mobility pattern of users in an indoor environment. The proposed method is independent of the resource allocation algorithm and eliminates the need for exhaustive simulations to find the optimal location for VLC APs. Simulation results show a significant improvement in the... 

In this paper, a reconfigurable patch antenna with Circular Polarization (CP) diversity with theoretical discussion and verification is proposed for fifth generation (5G) of mobile communication systems. The proposed antenna contains two PIN diodes, which are correctly placed on the ground plane to obtain polarization diversity. By switching between two ON/OFF modes in the PIN diodes, the proposed antenna can support either the RHCP mode or the LHCP mode. An antenna with the well-matched impedance bandwidths (S11 ≤ −10 dB) of 2.5 GHz (27 ∼ 29.5 GHz) and 3 GHz (36 ∼ 39 GHz) and the dual-band 3-dB axial ratios of 6% (27.3–29 GHz) and 8.4% (35–38.2 GHz) operates in both the RHCP and LHCP modes.... 

In this work, a simple method to prepare white light emissive diodes based on quantum dot (QD) colloidal solutions using mixed carbon QDs (CQDs) and CsPbI3 perovskite (PQDs) is reported. The right combination generates emission across the entire visible spectrum upon ultraviolet excitation. The white light emission of the final films provides high and stable color rendering index of 92%, tuning the chromaticity coordinates of the emission through the applied voltage. By varying the CsPbI3 QD concentration, a mixture is obtained that emits the “warm”, “neutral”, and “cold” white light sought for many indoor lighting applications, or to approximate the visible region of the solar spectrum,... 

In this paper, a novel structure is presented in order to decrease the polarization charges of quantum wells. The main purpose of this design is to make electron and hole wavefunctions closer to each other and to increase overlap integral following an increase of radiative recombination rates and internal quantum efficiency. Furthermore, carriers will be increased and become more balanced and identical which leads to an increase in efficiency of light-emitting diodes. The improvement of radiative recombination rates is studied in new structures. Energy bands diagram, carriers density, current density–voltage, and power density–current density are used to demonstrate the superior performance... 

In this paper, a novel structure is presented in order to decrease the polarization charges of quantum wells. The main purpose of this design is to make electron and hole wavefunctions closer to each other and to increase overlap integral following an increase of radiative recombination rates and internal quantum efficiency. Furthermore, carriers will be increased and become more balanced and identical which leads to an increase in efficiency of light-emitting diodes. The improvement of radiative recombination rates is studied in new structures. Energy bands diagram, carriers density, current density–voltage, and power density–current density are used to demonstrate the superior performance...