聚合物发光二极管中可逆的不稳定行为

研究了存在于聚合物发光二极管(PLED)中的一种可逆的"负阻"现象及短期衰退行为.当加在PLED上的正向偏压大于10V之后,其电流和发光强度将在某偏压下出现突然的转折,即电流或光强骤增而器件上压降减小."负阻"现象将随测量次数的增加而逐渐消失.采用CCD摄像头摄取发光象素上发光的变化情况的图像,发现光强的突变与电流的突变是相对应的.对以不同极性脉冲偏置观察发光光强的短期衰退情况时发现,反向偏置有助于抑制正向的发光衰退行为.我们初步认为这些现象可能与PLED中存在的缺陷态及其上电荷的填充状况有关.     

聚合物发光器件中输运特性的模拟分析

对聚合物发光二极管 I- V特性的测量发现 ,被测器件内存在着类似于某些无机器件中的负阻现象和“迟滞回线”状场致漂移的伏安特性 .模拟分析表明 ,一种反向势垒的存在及其击穿 ,应是引起负阻现象的原因 .缺陷态的存在及其电荷填充的变化 ,是导致 I- V特性曲线随偏压扫描方向变化的主要原因 .而低场下的接触性能决定着发光二级管载流子的输运性质 :若为非欧姆接触 ,则 I- V曲线可用 F- N隧穿模型来描述 ;若为欧姆接触 ,则应用陷阱电荷限制电流 (TCL)模型来描述     

聚合物发光器件中输运特性的模拟分析

对聚合物发光二极管I-V特性的测量发现,被测器件内存在着类似于某些无机器件中的负阻现象和"迟滞回线”状场致漂移的伏安特性.模拟分析表明,一种反向势垒的存在及其击穿,应是引起负阻现象的原因.缺陷态的存在及其电荷填充的变化,是导致I-V特性曲线随偏压扫描方向变化的主要原因.而低场下的接触性能决定着发光二级管载流子的输运性质:若为非欧姆接触,则I-V曲线可用F-N隧穿模型来描述;若为欧姆接触,则应用陷阱电荷限制电流(TCL)模型来描述.     

隧道二极管及其微波放大和振荡(上)

一、引言隧道二极管是一种新型的半导体器件,它的工作机理是依赖于电子穿透窄p-n结禁带的量子隧道机构而完成的.这种高掺杂的突变型p-n结二极管,在施以反向直流偏压时电流很快地增加;在小的正向直流偏压下具有微分负阻.其伏安特性曲线如图1所示.利用负阻效应可以组成放大器、振荡器、变频器、     

(AlGa)As/GaAs DH激光器低温负阻的研究

在室温至 77K范围内测量了 AlGaAs/GaAs DH激光器的正向伏安特性.发现有些激光器在低温下具有负阻的特性. 采用类 Schottky模型对DH激光器中的N-Al_(0.3)Ga_(0.7)As/n-GaAs异质结的电流传输特性进行分析,认为负阻现象是由于N型(AlGa)As层掺杂浓度偏低,N-n异质结在低温DH激光器正向偏置较大时,电子从n区隧道击穿N-n异质结势垒,或空穴从P型有源区注入N区引起的.此外N-AlGaAs层的施主杂质Sn的电离能很大也对低温负阻有贡献.     

基于MEMS技术新型硅磁敏三极管负阻-振荡特性

介绍了-种新型硅磁电负阻-振荡器件--S型负阻-振荡硅磁敏三极管.该器件是基于MEMS技术在p型高阻单晶硅片上制作的具有立体结构的新型磁电转换器件,采用KOH各向异性腐蚀技术实现发射区及引线的制作.实验结果表明,集电极电流随外加磁场的变化而变化;在基极注入电流一定时,出现集电极电流受外加偏压VCE调制的负阻-振荡特性,且集电极电流振荡随外加磁场而变化.对该器件负阻-振荡特性的形成机理进行了讨论,结果表明,在集电区n+π结和基区与π区形成的p+π结均处于反偏条件下,当π区满足雪崩倍增效应产生的条件时,该磁敏三极管伏-安特性曲线中的Vp+x偏压相对应的基极注入条件下的集电极电流出现S型负阻-振荡特性.在发射极和基极间的n+π结和p+π结附近存在的大量深能级杂质将对负阻-振荡特性进行调制.     

GaN基绿光LED芯片电流加速失效机理研究

采用电流加速的电应力老化方法研究GaN基绿光 LED芯片的失效机理。LED芯片在经过60 mA 电流老化424 h后,其发光效率总体趋势都是随老化时间增加而减小 ,但是小测量电流相比于大测量电 流的发光效率衰减程度更为明显。同时,在正向偏压下电流电压曲线基本没有变化,而反向 偏压下的反向 电流随老化时间的增加而快速增加。笔者认为在电应力老化作用下,随老化时间增加,有源 区的缺陷能级 增多,在正向偏压下,缺陷能级起到一个有效陷阱的作用,增加了载流子的寿命,降低了辐 射复合的几率, 使得发光效率降低,但是并没有减小正向偏压下的电流,而反向偏压时,缺陷能级起到了一 个漏电通道的作用,使得反向电流增大。     

基于MEMS技术新型硅磁敏三极管负阻-振荡特性

介绍了一种新型硅磁电负阻振荡器件——S型负阻-振荡硅磁敏三极管．该器件是基于MEMS技术在p型高阻单晶硅片上制作的具有立体结构的新型磁电转换器件，采用KOH各向异性腐蚀技术实现发射区及引线的制作．实验结果表明，集电极电流随外加磁场的变化而变化；在基极注入电流一定时，出现集电极电流受外加偏压VCE调制的负阻-振荡特性，且集电极电流振荡随外加磁场而变化．对该器件负阻振荡特性的形成机理进行了讨论，结果表明，在集电区n+π结和基区与π区形成的p+π结均处于反偏条件下，当π区满足雪崩倍增效应产生的条件时，该磁敏三极管伏-安特性曲线中的Vp+π偏压相对应的基极注入条件下的集电极电流出现S型负阻-振荡特性．在发射极和基极间的n+π结和p+π结附近存在的大量深能级杂质将对负阻-振荡特性进行调制．     

体效应对超深亚微米SOI器件总剂量效应的影响

研究体偏置效应对超深亚微米绝缘体上硅（SOI,Silicon-on-insulator）器件总剂量效应的影响.在TG偏置下,辐照130nm PD（部分耗尽,partially depleted）SOI NMOSFET（N型金属-氧化物半导体场效应晶体管,n-type Metal-Oxide-Semiconductor Field-Effect Transistor）器件,监测辐照前后在不同体偏压下器件的电学参数.短沟道器件受到总剂量辐照影响更敏感,且宽长比越大,辐射导致的器件损伤亦更大.在辐射一定剂量后,部分耗尽器件将转变为全耗尽器件,并且可以观察到辐射诱导的耦合效应.对于10μm/0.35μm的器件,辐照后出现了明显的阈值电压漂移和大的泄漏电流.辐照前体偏压为负时的转移特性曲线相比于体电压为零时发生了正向漂移.当体电压Vb=-1.1V时部分耗尽器件变为全耗尽器件,|Vb|的继续增加无法导致耗尽区宽度的继续增加,说明体区负偏压已经无法实现耗尽区宽度的调制,因此器件的转移特性曲线也没有出现类似辐照前的正向漂移.     

调幅光注入下半导体激光器光强调制特性的实验研究

在受注入光调制的半导体激光器 (LD)光强调制实验中 ,当LD的偏置电流置于某些值时 ,观察到其输出光强不随注入光强改变的实验现象。实验还观察到输出光强的幅度随偏置电流的增加而呈周期性变化。理论分析表明这是由于激活区内载流子消耗、带间载流子吸收综合作用的结果 ,而这些效应与腔内的模式选择、注入电流产生的热效应等过程有关。     

电磁脉冲对太阳电池的损伤效应研究

针对电磁脉冲对太阳电池造成的损伤效应,通过器件-电路联合仿真的方法,在建立硅太阳电池单元器件模型的基础上,分析研究了在阶跃脉冲电压的注入下,太阳电池性能的退化情况.结果表明:在电压幅值一定时,上升时间存在一临界值,当上升时间大于这一临界值时,太阳电池的性能退化是可恢复的,小于这一临界值时,太阳电池短路电流和开路电压将迅速衰减,填充因子也随之减小,电池部分失效甚至全部毁坏.在上升时间一定时,不同幅值的电压都会使得电池的短路电流和开路电压下降,但相对于开路电压,短路电流的下降并不显著,当电压幅值大于某一值时,开路电压下降趋于加快,随着电压幅值的不断增加,太阳电池将被击穿,从而完全失效.     

The effect of grain boundary shunt currents on mis solar cell performance

The degradation of solar cells by grain boundaries can take any of three forms: recombination of minority carriers, forward current due to recombination in the space charge region of the junction or Schottky barrier, or forward currents due to shunting. There is no doubt that minority carrier recombination occurs and degrades the short circuit current. There seems little doubt that grain boundaries also degrade the solar cell open circuit voltage, but whether the degradation is due to recombination in the space charge region or due to shunting is not clear. To date most attention has been paid to space charge recombination. There is data, however, that shunt currents can flow, especially in doped regions of the grain boundaries with the conductivity along the grain boundary estimated from that data to be 10⁻¹² to 10⁻⁵ mhos/square. We will present an analysis assuming such grain boundary conductivities and show that the predictions of such a shunt model are in agreement with experiment. Specifically the shunt current is predicted to increase exponentially as qV/2kT where V is the forward bias, the shunt current significantly lowers the open circuit voltage, and it has negligible effect on the short circuit current.     

一种支持OLED亮度补偿的电流型PWM像素驱动电路

OLED(organic light-emitting diode,OLED)微显示器长时间工作在高对比度、高亮度的状态下,OLED像素衰退不一致,发光亮度衰退也不一致,会产生残影现象。因此,提出了一种改进的电流型PWM像素驱动电路,保持了对OLED像素衰退补偿效果,同时可以读出OLED阳极电压,计算得到OLED衰退信息,以便于对OLED亮度衰退进行有效的补偿。文章中分析了改进的电流型PWM驱动电路结构,及其对OLED衰退补偿和亮度补偿的原理。通过模拟仿真,得到几个影响OLED衰退补偿效果的关键参数。当OLED像素衰退电阻R_oled小于40 MΩ时,该电流型PWM驱动电路电流衰退度与传统2T1C驱动电路相比,只为其衰退度的50%。     

High voltage degradation of GaN High Electron Mobility Transistors on silicon substrate

We have electrically stressed GaN High Electron Mobility Transistors on Si substrate at high voltages. We observe a pattern of device degradation that differs markedly from previous reports in GaN-on-SiC HEMTs. Similarly to these devices, the gate leakage current of GaN-on-Si HEMTs increases by several orders of magnitude at a certain critical voltage and this increase is irreversible. However, in contrast with devices on SiC, the critical voltage varies substantially across the wafer, even over short distances, with values as high as 75 V being observed. In addition, for voltages below the critical voltage, we observe a prominent degradation in the drain current and the source and drain resistances, something not observed in devices on SiC. This degradation is almost completely recoverable under UV illumination. We attribute these results to the high mismatch that exists between GaN and Si that leads to a large concentration of electrically active traps and a lower and non-uniform initial strain in the AlGaN barrier. This is evidenced by observed correlations between threshold voltage and maximum drain current in fresh devices and their corresponding critical voltages.     

Methanol treatment on low-conductive PEDOT:PSS to enhance the PLED's performance

In order to achieve high device efficiency in blue-emitting polymer light-emitting diodes (PLED) and white-emitting PLED, low-conductive PEDOT:PSS Clevios™ P CH 8000 (8000) has been used to replace widely adopted high-conductive PEDOT:PSS Clevios™ P Al 4083 (4083). In a blue PLED with poly (dibenzothiophene-S,S-dioxide-co-9,9-dioctyl-2,7-fluorene) (PF-FSO) as the emission layer, though the 8000 device has a higher efficiency and a lower turn-on voltage than the 4083 device, the 8000 device exhibits low peak luminance, sharp efficiency roll-off, and permanent degradation of the emission material. By analyzing the hole-only device and the X-ray photoelectron spectroscopy spectra, it's revealed that the insulating PSS layer on 8000 surface is responsible for the inefficient hole injection. A simple methanol treatment on the 8000 surface effectively removes the redundant PSS. Without the insulating PSS layer, the hole injection becomes efficient which extends the recombination zone. As the result, the methanol-treated 8000 device not only retains low turn-on voltage and the high device efficiency of the untreated 8000 device, but also achieves the peak luminance, mild efficiency roll-off, and device stability of the 4083 device.     

Device reliability study of AlGaN/GaN high electron mobility transistors under high gate and channel electric fields via low frequency noise spectroscopy

A set of different short term stress conditions are applied to AlGaN/GaN high electron mobility transistors and changes in the electronic behaviour of the gate stack and channel region are investigated by simultaneous gate and drain current low frequency noise measurements. Permanent degradation of gate current noise is observed during high gate reverse bias stress which is linked to defect creation in the gate edges. In the channel region a permanent degradation of drain noise is observed after a relatively high drain voltage stress in the ON-state. This is attributed to an increase in the trap density at the AlGaN/GaN interface under the gated part of the channel. It was found that self-heating alone does not cause any permanent degradation to the channel or gate stack. OFF-state stress also does not affect the gate stack or the channel.     

Efficient polymer light-emitting diodes with ZnO nanoparticles and interpretation of observed sub-bandgap turn-on phenomenon

Low-temperature liquid-phase synthesized ZnO nanoparticles (NPs) are used to realize highly-efficient polymer light-emitting diodes (PLEDs) containing a poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) emissive layer. It is shown that a 77-nm-thick ZnO electron transport layer (ETL) increases the maximum luminance and current efficiency of the PLED from ∼450 to ∼13,100 cd/m² and 0.55–4.7 cd/A, respectively. In addition, the ZnO layer shifts the peak in the electroluminescence (EL) spectrum from ∼535 nm for a PLED with a CsN₃ layer or no NP layer to ∼585 nm. After ruling out the possibility of ZnO-defect-related emissions and micro-cavity or interference effects, it is argued that the sub-bandgap turn-on behavior of the PLED with a ZnO ETL is the result of emission zone transition and enhanced hole injection rate near the anode, based on the trap-filled space-charge limited conduction (SCLC) phenomenon in the current density-voltage curves and the current-dependent EL spectra.     

Achieving Low-Voltage Operation of Intrinsically-Stretchable Organic Light-Emitting Diodes

A minimum operating voltage of intrinsically-stretchable organic light-emitting diodes (ISOLEDs) is always required for practical applications. However, the lack of protocols for the lamination complicates the task of attaining a reliable ISOLED without inducing degradation. Here, a solvent-vapor-assisted lamination (SVAL) method to reinforce the cathode interface is presented; this process lowers the operation voltage and increases the stretchability of ISOLEDs. Achieving a uniform contact and strong adhesion at the interface is the key to attaining reliable lamination. A cold-pressing (CP) treatment is applied first to reduce the surface roughness of silver nanowires before the surface embedding process. A subsequent solvent vapor treatment before the lamination partially solvated the surface of the active layer with an increase in the segmental motion of polymer chains, which substantially increases the interfacial adhesion after lamination. The combination of CP and SVAL treatments considerably reduces threshold voltage V_th (i.e., voltage at which current shows an abrupt increase for light-emission) from 6.7 to 2.7 V. The ISOLED also exhibts excellent mechanical stretchability, with no significant change in luminance under 30% strain. This study can assist in the development of practical applications of intrinsically-stretchable optoelectronic devices.     

Hot electron degradation effects in 0.14 μm AlInAs/GaInAs/InP HEMTs

We report on hot electron stress measurements on 0.14 μm MOCVD grown AlInAs/GaInAs/InP HEMTs. The stress measurements increase the drain-source current and hence induce a temporary negative shift in the threshold voltage in unpassivated HEMTs. A permanent negative shift in the threshold voltage has been obtained in passivated HEMTs. The observed degradation (temporary/permanent) is due to the storage of positive charges (created by the impact ionization in the channel) in the Schottky AlInAs layer (temporary) or at the interface of semiconductor-passivation layer (permanent). For a given drain-source bias, a shift in the threshold voltage is larger in the gate-source bias region where the device has a maximum transconductance value.