钝化与场板结构对AlGaN/GaN HEMT电流崩塌的影响

由于AlGaN/GaN HEMT的几何结构以及很强的极化效应,柵漏区域的电场很大,以至于电子可以从柵隧穿到AlGaN表面.隧穿的电子在表面累积,导致柵下耗尽区的电子向漏端延伸,从而引起漏极电流的下降.文中采用应力测试方法,研究了未钝化、钝化以及场板三种结构的AlGaN/GaN HEMT 的电流崩塌程度.实验结果表明,钝化隔断了电子从柵隧穿到AlGaN表面的通道,场板结构能够有效降低柵边缘电场,均减少了电子从柵隧穿到表面陷阱的几率,从而使虚柵的作用减弱,有效地抑制了电流崩塌效应.     

应力条件下AIGaN／GaN HEMT电流崩塌效应研究

采用应力测试方法，获得了AlGaN／GaN HEMT漏极电流随时间的变化关系。实验结果表叫，应力导致漏极电流崩塌56．2％：不同电压应力条件下，只要所加时间足够长和电压足够人，相同栅压的电流崩塌程度都近似相等；漏极电流恢复时间与大小分别为34．5＋σVGS与σ（VGS-VT）（2-β）。研究表明，栅-漏间表面态捕获的电子使得表面电势发生变化，引起沟道中二维电子气浓度降低，从而导致电流崩塌效应的产生。此结论可提用于AlGaN／GaNHEMT器件电流崩塌效应进一步的理论探索和器件研究。     

栅脉冲下AlGaN/GaN HEMT电流崩塌效应研究

源于AlGaN/GaNHEMT器件的大量测试分析发现,栅脉冲条件下漏极电流比直流情况下减小了47%;随着信号频率的改变,漏极电流按μnCoxW[α (0.13 0.64f)VGS (0.13 0.32 f)VGS2](VGS-Vth)2/L的规律变化;脉冲信号宽度对漏电流崩塌影响较小.基于实验结果的理论分析认为,电子从栅极注入到栅漏之间并被表面态所俘获,在沟道中形成增加的耗尽层,使得沟道二维电子气浓度减小,从而导致形成电流崩塌效应的主要原因之一.该结论有助于AlGaN HEMT器件脉冲条件下电流崩塌效应理论解释和器件应用.     

AlGaN/GaN HFET电流崩塌效应研究

电流崩塌效应是限制AlGaN/GaN HFET高输出功率特性的一个重要因素,文中从器件研制的角度研究了AlGaN/GaN HFET的电流崩塌效应.研究结果表明,采用传统的化合物半导体器件细栅工艺制作的器件,栅边缘易发生钻蚀效应,SiN层出现钻蚀区域,器件电流崩塌明显;采用ICP刻蚀SiN后,AlGaN表面产生损伤,电流崩塌效应进一步增强;采用电子束直写方式和SiN钝化,电子未对AlGaN层产生损伤,电流崩塌参量小于20%;采用场板结构,SiN层增加了表面态俘获电子的释放通道.电流崩塌效应得到进一步抑制,减小到小于10%.     

凹槽栅场板结构的AlGaN/GaN HEMT特性分析

研究了无场调制板结构、有场调制板结构但无凹槽栅、结合场调制板结构和凹槽栅工艺三种AlGaN/GaN HEMT的动态I-V特性和微波特性,认为场调制板结构和凹槽栅工艺可以有效改善AlGaN/GaN HEMT器件沟道内电场分布,显著减小电流崩塌现象,提高器件的微波输出功率特性.利用此技术研制的1mm栅宽AlGaN/GaNHEMT输出功率大于10W.     

凹槽栅场板结构的AlGaN/GaN HEMT特性分析

研究了无场调制板结构、有场调制板结构但无凹槽栅、结合场调制板结构和凹槽栅工艺三种AlGaN/GaN HEMT的动态I-V特性和微波特性,认为场调制板结构和凹槽栅工艺可以有效改善AlGaN/GaN HEMT器件沟道内电场分布,显著减小电流崩塌现象,提高器件的微波输出功率特性.利用此技术研制的1mm栅宽AlGaN/GaNHEMT输出功率大于10W.     

应力条件下AlGaN/GaNHEMT电流崩塌效应研究

采用应力测试方法,获得了AlGaN/GaN HEMT漏极电流随时间的变化关系.实验结果表明,应力导致漏极电流崩塌56.2%;不同电压应力条件下,只要所加时间足够长和电压足够大,相同栅压的电流崩塌程度都近似相等;漏极电流恢复时间与大小分别为34.5 αVGS与α(VGS-VT)(2-βt).研究表明,栅-漏间表面态捕获的电子使得表面电势发生变化,引起沟道中二维电子气浓度降低,从而导致电流崩塌效应的产生.此结论可望用于AlGaN/GaN HEMT器件电流崩塌效应进一步的理论探索和器件研究.     

GaN HFET沟道热电子隧穿电流崩塌模型

研究了GaN HFET中沟道热电子隧穿到表面态及表面态电子跃迁到表面导带两种跃迁过程及其激活能.从沟道热电子隧穿过程出发,提出了新的电流崩塌微观模型.用该微观模型解释了光离化谱、DLTS、瞬态电流及电流崩塌等各类实验现象.研究了各种异质结构的不同电流崩塌特性,在此基础上讨论了无电流崩塌器件的优化设计.     

GaN HFET沟道热电子隧穿电流崩塌模型

研究了GaN HFET中沟道热电子隧穿到表面态及表面态电子跃迁到表面导带两种跃迁过程及其激活能.从沟道热电子隧穿过程出发,提出了新的电流崩塌微观模型.用该微观模型解释了光离化谱、DLTS、瞬态电流及电流崩塌等各类实验现象.研究了各种异质结构的不同电流崩塌特性,在此基础上讨论了无电流崩塌器件的优化设计.     

AlGaN/GaN HEMT场板结构对与表面态相关的延迟特性的影响

为了研究AlGaN/GaN HEMT器件中场板结构对电流崩塌效应影响的物理机理,我们运用数值模拟的方法研究了场板结构和与表面态相关的栅延迟现象之间的关系。模拟显示场板结构可以显著地抑制电流崩塌效应的强度,但是对延迟时间没有影响。场板结构可以增大AlGaN表面附近的空穴积累,导致表面态的离化率增大从而抑制器件的电流崩塌。     

A study on current collapse in AlGaN/GaN HEMTs induced by bias stress

Drain current collapse in AlGaN/GaN HEMTs has been studied systematically by applying bias stress to the device. The collapse was suppressed by light illumination with energy smaller than the bandgap. The position dependence of the light illumination and the measurement of series source and drain resistances revealed that the collapse was caused by the surface states between the gate and drain electrodes, which captured electrons injected from the gate. The current collapse was eliminated by the passivation of the device surface with Si/sub 3/N/sub 4/ film.     

应力对GaN HEMT器件电流崩塌的影响

电流崩塌是目前GaN HEMT微波功率器件中最严重的问题之一,国内外都有研究,但尚无统一结论。通过实验,研究了GaN HEMT器件电流崩塌现象。研究表明,不同电应力条件下,导致漏电流崩塌和最大跨导下降的物理机制不同,在大电场应力下,主要物理机制是栅隧穿电子填充表面态;而在热电子应力下,是沟道热电子填充界面态。     

PI/Cr钝化对提高AlGaN/GaN HEMTs 击穿电压的研究

A novel AlGaN/GaN high electric mobility transistor（HEMT） with polyimide（PI）/chromium（Cr） as thepassivationlayerisproposedforenhancingbreakdownvoltageanditsDCperformanceisalsoinvestigated.The Cr nanoparticles firstly introduced in PI thin films by the co-evaporation can be used to increase the permittivity of PI film. The high-permittivity PI/Cr passivation acting as field plate can suppress the fringing electric field peak at the drain-side edge of the gate electrode. This mechanism is demonstrated in accord with measured results. The experimental results show that in comparison with the AlGaN/GaN HEMTs without passivation, the breakdown voltage of HEMTs with the PI/Cr composite thin films can be significantly improved, from 122 to 248 V.     

Slow transients observed in AlGaN/GaN HFETs: effects of SiN/sub x/ passivation and UV illumination

Very slow drain current and surface potential transients have been observed in AlGaN/GaN heterostructure field effect transistors that are subjected to high bias stress. Simultaneous measurements of drain current and surface potential indicate that large change in surface potential after stress is responsible for the reduction in drain current in these devices. Measurements of surface potential profile from the gate edge toward the drain as a function of time indicate that surface potential changes occur mostly near the gate. It is proposed that the surface potential changes are caused by electrons which tunnel from the gate under high bias stress and get trapped at the surface states near the gate. Passivation of the surface with SiN/sub x/ reduces the transient magnitudes to a large extent. This correlates with a large improvement in microwave power performance in these devices after passivation. UV illumination of these devices totally eliminates the drain current and surface potential transients.     

C波段0.75mm AlGaN/GaN功率器件

研制并测试了以蓝宝石作衬底的10×75μm×0.8μm AlGaN/GaN微波器件,采用等离子增强气相化学沉积的方法生长了250nm的Si3N4形成钝化层,直流特性从0.56A/mm上升到0.66A/mm,跨导从158mS/mm增为170mS/mm,截止频率由10.7GHz增大到13.7GHz,同时在4GHz下,Vds＝25V, Vgs＝-2.5V,输出功率由0.90W增至1.79W,输出功率密度达到2.4W/mm. 钝化有效地改善了器件的输出特性,减小和消除了表面寄生栅对器件的影响.     

掺杂AlGaN/GaN HEMT电流崩塌效应研究

对不同掺杂浓度AlGaN/GaN HEMTs施加直流偏置应力,研究掺杂AlGaN/GaN HEMTs电流崩塌效应.实验表明,掺杂AlGaN势垒层对器件电流崩塌效应有明显的抑制作用,随着掺杂浓度增加,掺杂对电流崩塌效应的抑制作用越显著.这是因为对于掺杂AlGaN/GaN HEMT,表面态俘获电子将耗尽掺杂AlGaN层,从而能对2DEG起屏蔽作用.AlGaN体内杂质电离后留下正电荷也能进一步屏蔽表面态对沟道2DEG的影响.     

Effect of the surface and barrier defects on the AlGaN/GaN HEMT low-frequency noise performance

We report on the effect of Si/sub 3/N/sub 4/ passivation of the surface of AlGaN/GaN transistors on low-frequency noise performance. Low-frequency noise measurements were performed on the device before and after the passivation by a Si/sub 3/N/sub 4/ film. A lower level of the low-frequency noise was observed from the device after the passivation. The passivation layer improved high-frequency, large-signal device performance, but introduced parasitic leakage current from the gate. A lower level of flicker noise is explained by the fact that noise is mostly originated from the fluctuation of sheet charge and mobility in the ungated region of the device due to the defects on the surface and in the barrier of the unpassivated device. Passivation eliminates part of the defects and higher leakage current increases the number of electrons on the surface and in the vicinity of the barrier defects, lowering the contribution to the low-frequency noise according to Hooge's law.     

Hydrodynamic simulation of surface traps in the AlGaN/GaN HEMT

Computer simulation of an AlGaN/GaN HEMT is carried out using commercially available software DESSIS. Traps located at the top of the AlGaN layer have been identified as being the primary source of electrons in the AlGaN/GaN HEMT. Recent experiments have focused on their role in HEMT performance with regard to the virtual gate effect and current collapse. In this work, analysis is carried out on these devices through the development of two different models designed to describe the 2DEG formation. Simulation of these models using the hydrodynamic model, which takes into account heating of the electrons, has been carried out to provide a more detailed understanding of the role of surface traps.     

微波功率AlGaN/GaN HFET的二维能带和异质结构设计

在综述微波功率AlGaN/GaN HFET技术发展趋势基础上,提出了二维异质结能带优化设计的新课题.从自洽求解薛定谔方程和泊松方程出发研究了利用异质界面上的极化电荷来剪裁异质结能带.用极化电荷设计近矩形前势垒能增大高能热电子的隧穿势垒宽度,抑制电流崩塌.背势垒中的极化电荷强化了沟道阱的量子限制,减弱了沟道中的强场峰,能提高击穿电压和抑制电流崩塌.薄势垒层中的极化电荷强化了沟道阱的结构,降低势垒高度后能产生高密度的电子气.优化设计二维异质结构能抑制沟道中的强场峰和电流崩塌,提高击穿电压和大漏压下的输出功率.