Pingjuan Niu

Enhancing the Light Extraction Efficiency of GaN-based LEDs

GaN-based light-emitting diode (LED) has been widely used in recent years, and tremendous progress has been achieved in GaN-based semiconductor materials and relevant process. However, owing to the large refractive index contrast between GaN-based semiconductor materials and air, light can be easily totally internally reflected at the semiconductor/air interface, and the critical angle for light to escape from the semiconductor is small. Therefore, the light extraction efficiency for GaN-based LED is still low and needs improving. Some of the leading approaches to enhance light extraction efficiency of GaN-based LED such as surface texturing or roughening, omnidirectional reflectors, photonic crystals, laser liftoff, transparent electrode, patterned substrate and so on are introduced in detail. For each approach, how the variation in device structure or material improves the light extraction efficiency is analyzed thoroughly. At last, some of mentioned approaches that are promising are evaluated and viewed briefly.

Design of greenhouse monitoring and control system based on LED lighting

LED light source module is developed according to the design of light distribution of RRGB chip based on the demand of plants for light, temperature and humidity. Several light source modules constitute LED array planar light source module. Monitoring and control of the system is realized through the combination of upper computer based on LabVIEW and controller. The controller conducts real-time detection on photon flux density (PFD) of red, green and blue bands, temperature and humidity and displays the detection information on the interface of upper computer. Pulse width modulation (PWM) signal is adjusted dynamically through the controller so as to make PFD value of plants under lighting remain constant, thus realizing intelligent fill lighting of the lighting system in combination with ambient light. Meanwhile, the system also realizes intelligent adjustment of temperature and humidity threshold range. This intelligent fill lighting system meets the requirement of plants for fill lighting environment and greatly saving energies through the setting of different PFD values and temperature and humidity ranges of red, green and blue bands by upper computer and the completion of environmental control by lower computer according to the setting based on different fill lighting demands of different plants or the same plant in different growth stages for PFD and light quality ratio.

High-Power LED Photoelectrothermal Analysis Based on Backpropagation Artificial Neural Networks

As an electroluminescentdevice, the coupling relationship between light-emitting diode (LED) input currents, optical power, and LED junction temperature is a complicated multiphysics process. In this paper, a simplified LED photoelectron-thermal (PET) model by artificial neural network (ANN), which can translatemultiphysics field issue into a singlephysics field problem, ismentioned to study the coupling relationship. In the first, an LED lumens, optical power, and electric power at different temperatures are monitored in a temperature controlling integrating sphere. Then, a backpropagation (BP) ANN is trained by these data to construct an LED Photo-Electron-Thermal (PET) relationship. In addition, LED luminaire thermal analyzing is performed using a finite-element method on the outputs of the BP ANN. Finally, the advantage of this method in terms of saving computing resources and computing time is analyzed by comparing the degrees of freedom in different models. The result shows that at least 6.7 times computing resource are saved by this method, which will reduce the LED thermal management system analyzing time greatly. Finally, the application and extension of this model are discussed.

Investigation on high-efficiency Ga0.51In0.49P/In0.01Ga0.99As/Ge triple-junction solar cells for space applications

Ga0.51In0.49P/In0.01Ga0.99As/Ge triple-junction solar cells for space applications were grown on 4 inch Ge substrates by metal organic chemical vapor deposition methods. The triple-junction solar cells were obtained by optimizing the subcell structure, showing a high open-circuit voltage of 2.77 V and a high conversion efficiency of 31% with 30.15 cm2 area under the AM0 spectrum at 25 °C. In addition, the In0.01Ga0.99As middle subcell structure was focused by optimizing in order to improve the anti radiation ability of triple-junction solar cells, and the remaining factor of conversion efficiency for middle subcell structure was enhanced from 84% to 92%. Finally, the remaining factor of external quantum efficiency for triple-junction solar cells was increased from 80% to 85.5%.

Photoluminescence and thermal stability of Mn 2+ -doped CdSe/CdS/ZnS quantum dots

Colloidal Manganese-doped cadmium selenium quantum dots (Mn-doped CdSe QDs) and Mn-doped CdSe/CdS/ZnS core/multishell QDs were successfully synthesized via non-trioctylphosphine (non-TOP) green route. The TEM image reveals that the CdSe QDs are almost spherical in shape and have a good monodispersion. The optical properties of Mn-doped CdSe QDs were investigatedusing ultraviolet-visible and photoluminescence spectroscopy. The absorption spectra exhibit a red-shift with increasing QDs size, which is attributed to the quantum confinement effect. The PL spectra exhibit a first blue-shift to a later red-shiftwith increasing QDs size, which is attributed to the combined effect of Mn-doping and quantum confinement effect. Quantum confinement allows tuning of the CdSebandgap energy across the Mnexcited-state energies. Temperature-dependent (300–500 K) PL data suggest that Mn-doped CdSe/CdS/ZnS core/multishell QDs with enhanced thermal stabilities could be realized by blocking the nonradiative recombination centers with thick CdS/ZnS shells.

The Research of Operational Transconductance Amplifier

A topology structure of operational transconductance amplifier is proposed to be devised in this paper. In order to make it have the feature of high-speed, large bandwidth, and high stability, a two-stage cascode structure amplifier is designed by using cascaded single-stage amplifier. And several important parameters are calculated and analyzed theoretically. Finally the designed cascode two-stage amplifier is debugged by using Hspice. And the related parameter values are obtained. The magnification is more than 100dB, unity gain bandwidth is about 635MHz, and output swing is 4V. And the total transconductance of operational transconductance amplifier is 8.8A / V.

Study on temperature characteristic of green photodetector on Si substrate

In the paper, a silicon PN junction photodetector is developed on the basis of n-type single-crystal (100) silicon substrate. The properties of this semiconductor photodetctor depend on the temperature to certain extent. We emphasize on the study on temperature characteristic of the photodetector: Firstly, the temperature behavior of dark current at zero bias voltage and wide temperature range was investigated. Results show that dark current increases exponentially with temperature over room temperature. Secondly, the temperature behavior of photo current at zero bias voltage and wide temperature range was studied. The temperature characteristic is analysesed in the theory and optimized.

An Astable Multivibrator formed by a novel NDRHBT

The earlier astable multivibrator formed by silicon tunnel diode has the disadvantage of low speed and non-modulation. NDRHBT is a novel type of HBT with NDR characteristics and high speed. Its NDR characteristics can be modulated by the base voltage VBE or base current IB. So the astable multivibrator formed by NDRHBT has the advantage of high speed, high frequency, bistability, and frequency modulation by VBE or IB. Thus, it can be applied widely in high frequency oscillation circuits and high speed-digital circuits. In this paper, it is demonstrated that the frequency of the astable multivibrator can be modulated by base voltage VBE. The experimental result shows that the frequency of time interval between two adjacent pulses f1 varies from 7×104Hz down to 2.5×104Hz as VBE changes from 4.5V to 6.5V and exhibits near a linear relationship. So it is can be used as an efficient voltage controlled frequency modulator for pulse signal in high speed digital circuits.

The fabrication of high-brightness and high-power InGaAlP single-side red LED

In this paper, it is reported that the design and fabrication of high-brightness and high-power InGaAlP single-side red LED with electrodes which are interdigitated with the fingers. High-brightness and high-power InGaAlP LED is a new kind of visible light LED developed in recent years, which is driven by large current capacity, high luminous efficiency and excellent heat resistance. It has been used in various fields, such as large area displays, traffic lights, brake lights and so on. As compared with the conventional double-side LED, the single-side LED is more flexible to integrate with other devices and its fabrication is simplified. The size of chip is 1mm2. The fabrication of single-side LED, essentially, is the same as conventional LED, involving photolithography, PECVD SiO2, wet etching, evaporating, lift off and rapid thermal annealing using four masks. To control the widths of mesa and N electrode precisely, the selecting etch technique has been adopted, using HCL: H2O:H2O2 as the InGaAlP etching solution. I-V characteristics, light emission spectrum, luminous flux, luminous intensity and luminous efficiency of this LED have been measured. The characteristics are obtained with turn-on voltage of 1.5V and forward current of 400mA at its forward voltage of 3V. The peak wavelength is 635nm, which corresponds to red light, and the Full Width of Half Maximum is 16.4nm at injection current of 350mA. The luminous intensity is 830 mcd. The color coordinates is x=0.6943, y=0.3056 and the color index is 18.4. So we will conclude that the high-brightness and high-power InGaAlP single-side red LED will become new focus in both scientific research and industrial investment for its wide application.

The design and simulation of same Si-based device used as both LED and PD

A kind of new light emitting diode (LED) based on Si p-n junction forward injection mechanism completely compatible with standard Si-CMOS technology is designed and analyzed, which has higher efficiency than LED based on Si pn junction in reverse bias breakdown mode. At same time according to reversibility of optoelectronic conversion, the same Si-LED can be used as a photodetector (PD). The photoelectric characteristics of this device as both LED and PD are simulated by the commercial software SILVACO. This device is expected to have wide application in next generation optoelectronic integrated circuit (OEIC).