GaN基凹槽栅MOSFET器件设计与制备技术研究

发布时间：2019-02-25 12:59

【摘要】：AlGaN/GaN异质结由于极化效应在异质结界面靠近GaN侧产生了高浓度、高电子迁移率的二维电子气导电沟道,使得AlGaN/GaN HEMT器件具有导通电阻小、开关速度快、正向导通饱和电流密度大等特点,在器件应用中占据较大优势,因此得到广泛关注和研究。然而常规HEMT器件为常开型器件,器件实际应用中需要负压驱动,驱动电路设计难度大、成本增加,且负压驱动电路不具备失效保护功能,使得系统安全性降低。因此,如何实现高性能的增强型GaN电力电子器件成为目前研究的热点。本文采用凹槽栅MOSFET器件结构实现GaN增强型器件。凹槽栅结构实现增强型器件的机理是通过改变栅极下方势垒层的厚度来减弱栅槽异质结的极化效应,耗尽栅槽下方沟道的二维电子气,完成阈值电压调控,从而实现增强型器件。本文首先通过仿真软件研究了栅槽刻蚀深度T对器件阈值电压的影响,为后续实验提供理论基础。在仿真理论分析的基础上进行了AlGaN/GaN凹槽栅MOSFET器件的实验研究。实验中出现凹槽栅MOSFET器件阈值电压为负值的非正常现象,测试器件的C-V特性发现在Al_2O_3与GaN界面存在大量固定正电荷。通过第一性原理计算分析得出固定正电荷的产生是由于栅槽刻蚀产生大量Ga的悬挂键,且Ga-O键的结合能小于Ga-N键,栅介质Al_2O_3中的O原子容易替代GaN中的N原子形成Ga-O键,Ga-O键与Ga的悬挂键均显正电特性。固定正电荷在栅极下方吸引等量电子聚集,使得凹槽栅器件阈值电压为负值。为了降低栅介质Al_2O_3与GaN界面固定正电荷的电荷密度,实现器件阈值电压调控,在优化实验中引入了N2氛围中的栅介质后退火工艺。N离子Al_2O_3/GaN界面热处理可有效恢复Ga-N键,降低了Al_2O_3与Ga N界面正电荷密度,实现了器件阈值电压7.1V的大范围调控。界面固定正电荷密度的降低也使得沟道散射机制减弱,增强型器件沟道载流子迁移率增大,最终得到阈值电压7.6V,导通电阻19.5Ω.mm,饱和漏极电流355mA/mm,击穿电压1050V@Lgd=20μm的凹槽栅增强型MOSFET器件,是目前报道的阈值电压大于5V器件中漏极饱和电流密度最大的AlGaN/GaN增强型器件。
[Abstract]:Due to polarization effect of AlGaN/GaN heterojunction, high concentration and high electron mobility of 2-D electron gas conduction channel are produced near the GaN side of the heterojunction surface, which makes AlGaN/GaN HEMT devices have low on-resistance and fast switching speed. Because of its high saturation current density, forward conduction has been paid more and more attention because of its advantages in device applications. However, conventional HEMT devices are normally open devices, which require negative voltage drive in practical application. The design of drive circuit is difficult, the cost is increased, and the negative voltage drive circuit does not have the function of failure protection, which makes the security of the system lower. Therefore, how to achieve high-performance enhanced GaN power electronic devices has become a hot research topic. In this paper, the grooved gate MOSFET device structure is used to realize the GaN enhanced device. The mechanism of the enhanced device with grooved gate structure is to weaken the polarization effect of the heterojunction by changing the thickness of the barrier layer below the gate, to exhaust the two-dimensional electron gas in the channel under the gate groove, and to complete the threshold voltage regulation. Thus, the enhanced device is realized. In this paper, the influence of gate groove etching depth T on the threshold voltage of the device is studied by means of simulation software, which provides a theoretical basis for subsequent experiments. Based on the theoretical analysis of simulation, the experimental study of AlGaN/GaN grooved gate MOSFET device is carried out. The abnormal phenomenon of negative threshold voltage of grooved gate MOSFET device is found in the experiment. The C-V characteristic of the device is measured and it is found that there is a large number of fixed positive charges at the interface between Al_2O_3 and GaN. Through first-principle calculation and analysis, it is found that the generation of fixed positive charge is due to a large number of Ga hanging bonds produced by gate groove etching, and the binding energy of Ga-O bond is less than that of Ga-N bond. The O atom in the gate dielectric Al_2O_3 can easily replace the N atom in GaN to form the Ga-O bond. The Ga-O bond and the hanging bond of Ga both exhibit positive electrical properties. The fixed positive charge attracts equal amount of electrons under the gate so that the threshold voltage of the grooved gate device is negative. In order to reduce the charge density of fixed positive charge at the interface between Al_2O_3 and GaN, the threshold voltage of the device can be controlled. In the optimization experiment, the post-annealing process of gate dielectric in N2 atmosphere was introduced. The N ion Al_2O_3/GaN interface heat treatment can effectively restore the Ga-N bond and reduce the positive charge density between Al_2O_3 and Ga N. The threshold voltage of 7.1V is controlled in a wide range. The decrease of the fixed positive charge density at the interface also weakens the channel scattering mechanism and increases the channel carrier mobility of the enhanced devices. Finally, the threshold voltage of 7.6V and the on-resistance of 19.5 惟路mm, saturated drain current of 355mA-1 / mm are obtained. The Groove Gate enhanced MOSFET device with breakdown voltage of 1050V@Lgd=20 渭 m is reported to be the largest drain saturation current density enhanced AlGaN/GaN device in which the threshold voltage is larger than 5V.
【学位授予单位】：电子科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TN386

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