Wen-Chung Chang

A User-Friendly Web Content Management System

This study applies two open source projects, namely PHP and MySQL, to create a powerful, user-friendly Web-based system for creating, maintaining and publishing a database comprising news information in the form of text and graphical images. The proposed system is specifically designed to enable non-technical users with no knowledge of computer programming, graphic imaging tools, or markup languages such as HTML to add new material to a Website or to modify the existing contents as and when required in an intuitive and real-time fashion. As a result, a technically competent Website management team is not required, and thus the cost and flexibility of the Website management process is significantly improved.

Effects of Giant Optical Anisotropy in R-plane GaN/AlGaN Quantum Wells by Valence Band Mixing

Investigation of optical anisotropy spectra in the R-plane (i. e., the [1012]-oriented layer plane) of GaN/Al0.2Ga0.8N quantum wells with different widths is studied. The optical matrix elements in the wurtzite quantum wells are calculated using the k·p finite difference scheme. The calculations show that the valence band mixing effect produces giant in-plane optical anisotropy in [1012]-oriented GaN/Al0.2Ga0.8N quantum wells with a narrow width. The nature of the in-plane optical anisotropy is found to be dependent on the well width. Specifically, it is found that the anisotropy changes from x′-polarization to y′-polarization as the well width increases. DOI: 10.2529/PIERS060801054904

On the improvement of solar cell based photovoltaic charger system

Due to the energy crisis, renewable energy sources have been deeply concerned as possible solutions to remain resources on the earth. Among these energy sources, solar energy, a free conversion, non-polluted and inexhaustible energy source, has been used to generate electricity for decades. However, the efficiency of solar cell is so far still low and how to make the photovoltaic chargers achieve the maximum of efficiency becomes worthwhile to research further. This paper focuses on the analyses of loading effects between photovoltaic (hereunder as PV) array module and pattern for series of batteries

Study on Solar Materials with New Solar Cell Structure Using Surface Selective Etching and Periodical Barrier Technology

This study presents a simple technique for improving the power conversion efficiency of a AlGaAs-GaAs based solar cell. A traditional III-V semiconductor solar cell heterojunction structure, i.e. n-GaAs/n-AlGaAs/n-GaAs/i-GaAs/p-GaAs is used. The top n-GaAs/n-AlGaAs structure is responsible for the selective etching stop layer. The selective etched surface barriers associated with polymer gratings with different aspect ratios are produced on solar cell by using the photolithography and Micro Electro Mechanical Systems (MEMS) techniques. A reflective-type diffraction optical grating is fabricated on the surface of the solar cell to redirect the incident light reflected from the solar cell back onto the solar cell surface. The experimental results show that the addition of the optical grating increases the open circuit voltage, Voc, from 4.51 V to 4.73V and improves the maximum output voltage, Vm, from 4.12V to 4.32V. From inspection of surface reflectivity , the average reflectivity is also found to be 13.7% down to 9.9 %.

UV LEDs with Low Dislocation GaN Buffer Layers Grown by MOCVD

New MOCVD grown UV (ultra-violet) LEDs using low dislocation density GaN buffer layers on sapphire have been studied. Two different LED characteristics of GaN substrates, i.e. 5um-thick and 20um-thick buffer layers, on sapphire are compared with each other. The enhanced LED characteristics show ~29.5% reduction in current-voltage resistance, ~8.5% reduction in turn-on voltage and output power saturation at higher current. Better GaN buffer quality and heat dissipation due to the lower defect density are believed to the enhanced reason.

GaAs/InGaAs Optoelectronic Switch for Triple-Logic Applications

A new GaAs/InGaAs triangular barrier optoelectronic switch combined with tri-state characteristic is fabricated and demonstrated. Two GaAs/InGaAs barriers are employed to provide potential barriers for electron thermionic emission and hole confinement, respectively. Applying a sufficient DC voltage to this device, a double S-shaped negative differential resistance (NDR) phenomenon with nearly equal switching voltage difference is appeared at room temperature. This unique NDR property can be introduced to triple stable regions into the device circuit design. Based on a proper circuit design with suitable load line, the studied device has potential for triple-logic applications.

High-Quality Polysilicon Thin Film Recrystallization by Laser Annealing

Amorphous silicon (a:Si) recrystalized to poly-silicon (poly:Si) in different gas environments by excimer laser annealing (ELA) is studied. Variations of threshold laser power for the generation of surface ablation in pure N2 gas and the mixture of N2:98% and O2:2% environments are also investigated, respectively. From experiments, it is found the combination of N2:98% and O2:2% gas can enhance the threshold laser power from 320mJ/cm2 to 390mJ/cm2 for the suppressing of surface ablation phenomenon. In the condition of average grain over 0.25um, the process window (i.e. laser power for processing ability) is 30mJ/cm2 for pure N2 only, but is 50mJ/cm2 for the combination of N2:98% and O2:2%.

Ultrasonic Welding Driver with Class-E Inverter Design

A robust circuit design using matching technology to design the ultrasonic welding transducer driver with zero voltage switching is proposed. The feedback output voltage is used to control the oscillator frequency to achieve the self-tracking function. Experimental results exhibit that the Class-E inverter circuit can be effectively and stably applied on the high power ultrasonic welding system.

Automatic Frequency-Tracking Control for Ultrasound Welding System

A new frequency-tracking control method to catch the optimal working frequency for the high power ultrasonic welding system is proposed. In a high power ultrasonic resonant system, the induced high temperature will change the working frequency. Therefore, the proposed control method to track the optimal working frequency becomes very attractive and important. This control method is practically implemented by a FPGA chip which basically includes two logic circuits. One logic circuit is to find the optimal working frequency automatically and the other one is to adjust the working frequency by detecting the working current simultaneously. Experimental results exhibit the new method can effectively control and track frequency for high power ultrasonic welding system.

Electrokinetic Instability Induced Valveless Multi-Switching in a Microfluidic Chip

This work presents the experimental investigation with electrokinetic flow mixing for bio-analytical chip applications. By DC electrokinetic instability induced technique, we design a 5x5 microfluidic device which possesses microfluidic sample handling in flow multi-switching. The device not only control single sample flows into different outlet considered but also the multi-sample injection into specific outlet ports. Experimental results indicate that the sample flow could be electrokintcally pre-focusing to a narrow stream and then guided into a desired outlet port and successfully control devices of the voltage in the microfluidic chip.