ESD on GaN-based LEDs: An analysis based on dynamic electroluminescence measurements and current waveforms

This work presents an extensive study on the effects of Electrostatic Discharges (ESD) on state-of-the-art GaN based LEDs, based on optical and electrical measurements carried out during the ESD events. ESD events were simulated through a Transmission Line Pulser (TLP) which generates voltage pulses with a duration of 100 ns and increasing amplitude: during each pulse, spatially-resolved electroluminescence measurements were carried out through an high speed EMCCD camera. These measurements allowed to identify the chip region where the discharge is localized and the change in the damaged area induced by consecutive ESD events. Also the current and voltage waveforms at the LED terminal were monitored during the tests; this analysis provided important information about modifications the impedance of the devices. The analysis was carried out on different types of commercially available low-power GaN-based LEDs with several differences in the manufacturing technology. Thanks to these tests we have identified two different failure behaviours during a destructive ESD event, clearly related to the different defects in the semiconductor lattice and to structure of the chip.     

Ensuring accuracy in optical and electrical measurement of ultra-bright LEDs during reliability test

In order to accurately estimate the lifetime of Light Emitting Diodes (LEDs), accelerated stress tests have to be performed, and measurements done throughout the tests should be free of artifact and repeatable to ensure accurate observations of the degradation of the LEDs. In this work, measurements artifact arising from DC biasing and PWM settings were first presented. Electrical measurements consistency was then studied. Optimized setting for accurate measurement is obtained using Response Surface Method with Central Composite Design, and the setting is verified through experiments.     

An investigation of the electrical and optical properties of DC electroluminescent ZnS:Mn, Cu-powder panels

Electrical and optical measurements on ZnS : Mn, Cu-dc electroluminescent (DCEL) powder panels are reported. It is shown that current conduction in these devices depends upon avalanche breakdown in a thin insulating Cu-depleted layer of phosphor of thickness up to approximately 1 µm. However, it is concluded that the overall current-voltage characteristics of a panel are determined by the various components external to this layer. That the current density throughout this layer is highly nonuniform is deduced and the consequences of this result are discussed in terms of the effect on saturation brightness. A new model to explain the details of the observed electrical behavior of DCEL powder panels is introduced and compared with previously proposed models. Finally the various failure modes observed in these devices are identified and their elimination is discussed.     

Mismatch robustness and security of chaotic optical communications based on injection-locking chaos synchronization

Mismatch robustness and its effects on security of chaotic optical communication system based on injection-locking chaos synchronization are studied numerically by establishing the corresponding SIMULINK model. Unlike previous studies, we focus on the communication relating issues when parameter mismatches are considered. The mismatch robustness of generalized synchronization is discussed firstly in terms of cross-correlation coefficient and synchronization error. Decoding performances as well as the effects of message strength are examined for both with and without mismatch cases. Effects of injection strength on system decoding performances are also investigated by examining the chaos-pass filtering effect. Finally, a modified decoding scheme is brought forward to improve the system decoding capability. Results show that the system under consideration exhibits unconspicuous difference in both synchronization and decoding characteristics when large parameter mismatches are considered. The system based on injection-locking chaos synchronization is inappropriate for the applications where high transmission security is necessary.     

两种高温老化方式对功率白光LED光热参数的影响

测试和比较了大功率白光LED在高温耐电(HTCD)和高温存储(HTS)两种老化条件的光热性能变化。结果表明,在HTCD老化下,光通量衰减达到40～60%;而HTS下的衰减只有10～14%。这说明,电流应力对LED的寿命影响比较大。利用热阻瞬态响应法测试和结构函数理论分析两种高温老化条件下LED的热特性,结果表明,在HTCD老化下LED热阻的变化较HTS更为明显,并且热阻变化大多体现在导热Ag胶层。这主要是由于高温条件下电流应力引起Ag颗粒的空间分布不均,使粘结界面产生空隙导致热阻发生不同程度的改变。     

光电信号转换处理中的CPLD应用

利用CPLD作为硬件设计平台,在某些芯片对光电转换中的信号进行处理前,对芯片进行初始化配置及其他逻辑控制.仿真显示该方法可以实现所需时序,在顺利通过所设计的测试方案后,最终将相关VHDL代码下载到光电转换板卡中用于实际应用.结果表明,设计完全满足系统要求.     

An 8-Gb/s optical backplane bus based on microchannelinterconnects: design, fabrication, and performance measurements

We describe the characteristics of a microchannel-based optical backplane including signal-to-noise ratio (SNR), interconnect distances, and data transfer rates. The backplane employs 250 μm-spacing two-dimensional (2-D) vertical cavity surface emitting lasers (VCSELs) and a microlens array to implement 500 μm-, 750 μm-, and 1-mm optical beam arrays. By integrating the transmitter and a multiplexed polymeric hologram as a deflector/beam-splitter for the guided-wave optical backplane, the result demonstrates a multibus line architecture and its high-speed characteristics. Maximum interconnect distances of 6 cm and 14 cm can be achieved to satisfy 10^-12 bit error rate (BER) using 2×2 beams of 500 μm- and 1 mm-spacing array devices. The total data transfer rate of the developed backplane has shown 8 Gb/s from eye diagram measurements     

An investigation on the influence of electromagnetic interference induced in conducting wire of universal LEDs

Electromagnetic Interference (EMI) has a detrimental effect upon the performance of Optical-Fiber Communication (OFC) systems. The present study considers the case where EMI is induced in a conducting wire (CW), and derives equations to establish the influence of the induced EMI on GaP and GaAsP Light-Emitting Diodes (LEDs). These equations are then verified experimentally. The results indicate that the degree of influence of the EMI upon both LED devices depends upon the interference power, the interference frequency, the induced power, the input resistance of the device, the inverse saturation current, and the ideal factor of the LED. Moreover, it is found that the induced interference current increases with an increasing interference frequency and that the EMI has a greater influence on devices with a lower input impedance. The theoretical results are found to be in good agreement with the experimental data.     

A comparison of gamma-induced degradation and forward bias-induced degradation in GaP:Zn,0 LEDs

Both the effects of gamma irradiation and high-current forward bias on GaP:Zn,0 light emitting diodes (LEDs) have been studied by measuring constant current 76K electroluminescence (EL) spectra below 1000 nm, total light output at room temperature, and capacitance transients using the technique of deep level transient spectroscopy (DLTS). The LEDs were divided into two sets; a control set which was subjected only to forward bias, and a set exposed to gamma irradiation and forward bias. The results indicate that gamma-induced light output degradation occurs through a different mechanism than forward bias-induced degradation of the control LEDs. Gamma irradiation, which has no effect on the Zn-O concentration, degrades the light output through the introduction of competing non-radiative recombination centers which reduce the minority carrier lifetime. In contrast, forward bias-induced light output degradation is primarily due to a reduction in Zn-O concentration through the apparent recombination-enhanced interaction of Zn-O centers with impurities rather than defects. This work was supported in part by the Air Force Weapons Laboratory (AFWL) Kirtland AFB, Albuquerque, NM under P. O. 77-027, and in part by the U. S. Department of Energy (DOE) under contract number AT(29-1)789.     

Humidity robustness for high voltage power modules: Limiting mechanisms and improvement of lifetime

During the last years applications for power modules with harsh environmental conditions have gained increasing importance. Static humidity tests with T = 85 °C and RH = 85% with high voltage have established as basic testing method for the humidity robustness of semiconductor power modules. The present work is showing the improvement of humidity robustness with respect to the established test methods. Electrical performance monitoring and analytic approaches to prove the results with respect to established work on the field of humidity testing.     

Comparative study of AlGaN/GaN HEMTs robustness versus buffer design variations by applying Electroluminescence and electrical measurements

By means of step stressing tests on AlGaN/GaN HEMTs the robustness properties of devices fabricated on wafers with different buffer designs have been compared to each other (standard UID GaN buffer and UID Al_0.05Ga_0.95N back-barrier in combination with GaN channel layer). The devices with GaN buffer showed an abrupt increase of gate leakage current after reaching drain bias values in the range of 30 V while devices with Al_0.05Ga_0.95N back-barrier did not show any degradation up to 120 V drain bias. All DC-Step-Stress tests have been accompanied by Electroluminescence (EL) analysis and electrical characterization techniques before, during and after stress. It has been shown that EL at forward and reverse bias conditions can be used as an indicator of potential device degradation. Devices comprising an AlGaN back-barrier design demonstrated superior robustness.     

基于半导体光放大器的组播实验研究

提出了一种基于半导体光放大器(SOA)的组播实验方案,首先利用谱展宽和谱切片得到多波长脉冲源,将其作为探测光并与信号光共同注入到SOA中,利用SOA的交叉增益调制(XGM)效应,信号光与每一个波长的脉冲光发生作用,在SOA输出端通过可调光带通滤波器(TBDF)得到每一波长的变换后信号,实现光信号的组播过程。在实验中,10 Gb/s的归零(RZ)码作为信号光,而由多波长脉冲源产生的4波长脉冲光作为探测光,在探测光波长附近分别得到了与编码信号光逻辑取"非"的信号。     

Incident angle of parallel light measurements based on optical vernier principle

The incident angle of a parallel light beam is hard to measure accurately under the requirement of both high precision and large measurement range. A solution based on the optical vernier principle is presented, and a corresponding measuring device is given. Compared with the former method, the suggested apparatus realizes high accuracy in a larger measurement range. The experiments on the designed apparatus show a high precision of better than 0.02° in a measurement range of ±64° and the experimental results are in accordance with analytical results in theory. The invented apparatus can be widely used in many technical areas such as aerospace, precision measurement and automatic control.     

An investigation of electrical and structural properties of Ni-germanosilicided Schottky diode

Ni-germanosilicided Schottky barrier diode has been fabricated by annealing the deposited Ni film on strained-Si and characterized electrically in the temperature range of 125 K–300 K. The chemical phases and morphology of the germanosilicided films were studied by using scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The Schottky barrier height (_b), ideality factor (n) and interface state density (D_it) have been determined from the current–voltage (I–V) and capacitance–voltage (C–V) characteristics. The current–voltage characteristics have also been simulated using SEMICAD device simulator to model the Schottky junction. An interfacial layer and a series resistance were included in the diode model to achieve a better agreement with the experimental data. It has been found that the barrier height values extracted from the I–V and C–V characteristics are different, indicating the existence of an in-homogeneous Schottky interface. Results are also compared with bulk-Si Schottky diode processed in the same run. The variation of electrical properties between the strained- and bulk-Si Schottky diodes has been attributed to the presence of out-diffused Ge at the interface.     

基于FBG传感系统的可调光滤波器非线性研究

采用光纤布拉格光栅(FBG)传感系统研究压电陶瓷(PZT)驱动法布里-珀罗(F-P)型可调谐光滤波器(TOF)的非线性特性。基于多光束干涉理论建立了TOF的非线性模型,推导了透射带波长和自由光谱范围(FSR)对驱动电压的非线性响应;基于FBG传感系统测试了F-P型TOF的波长非线性,并采用多项式拟合对其进行描述,实测F-P型TOF波长的非线性误差最大为1.006 nm;基于F-P型TOF的非线性模型,研究了其波长定位误差,并提出采用参考光栅的方法降低波长定位误差。实验表明,F-P型TOF的波长随机误差可由73～81 pm降至12 pm以下。     

Effect of the fabrication conditions of SiGe LEDs on their luminescence and electrical properties

SiGe-based n⁺–p–p⁺ light-emitting diodes (LEDs) with heavily doped layers fabricated by the diffusion (of boron and phosphorus) and CVD (chemical-vapor deposition of polycrystalline silicon layers doped with boron and phosphorus) techniques are studied. The electroluminescence spectra of both kinds of LEDs are identical, but the emission intensity of CVD diodes is ～20 times lower. The reverse and forward currents in the CVD diodes are substantially higher than those in diffusion-grown diodes. The poorer luminescence and electrical properties of the CVD diodes are due to the formation of defects at the interface between the emitter and base layers.     

多颗粒LED封装警示灯热特性数值与实验研究

针对一款新型LED旋转警示车灯,选取了三个不同的关键参数(灯柱半径r,散热底座半径R及散热底座高度H),基于有限元理论及热电偶测量法,研究了多颗粒LED封装灯具结构的热特性。结果表明:最高温度点处于LED颗粒的中心处,最高温度可达67.31℃(仿真时),64.95℃(实验时)。散热底座半径对结构的最高温度影响最大。可以通过散热底座的设计来提高LED颗粒的热可靠性及车灯的使用寿命;亦为LED灯具结构设计提供了设计基础。     

An optical filter based on carrier nonlinearities for optical RFchannelizing and spectrum analysis

We experimentally demonstrate filtering based on the finite time response of the interband carrier nonlinearities in a direct bandgap semiconductor. The filter is implemented in a mixed strain polarization insensitive multiple-quantum-well, broad-area semiconductor amplifier. The key to the filter implementation is the separation of a four-wave mixing generated sideband from the linearly amplified inputs based on spatial filtering. Both spectrum analysis and channelizing of RF modulated optical carriers are demonstrated     

Obtaining of SmS based semiconducting material and investigation of its electrical properties

Semiconductor samarium monosulfide polycrystals obtained by reaction between samarium trihydride (SmH₃) and its sesquisulfide were studied. The temperature, baric and frequency dependences of the resistivity and structural features of the samples were investigated. It is shown that the value of X-ray coherent scattering region is extremely small for SmS samples, 320 Å; critical pressure of semiconductor-metal phase transition is higher than in the samples, obtained by other methods, 0.88 GPa; the temperature dependence of the resistivity has metallic behavior. Hopping mechanism of electron transport was found. All these features are explained by more defective structure of the polycrystalline SmS samples.