【作者】 龚欣；

【导师】 郝跃；

【作者基本信息】 西安电子科技大学 ， 微电子学与固体电子学， 2007， 博士

【摘要】 宽禁带GaN材料由于其优良的电特性和材料特性在大功率微波和毫米波器件应用领域引起了广泛关注。本文主要关注氮化物基异质结双极晶体管(HBT)及其相关技术。由于Ⅲ-V族氮化物HBT优良的器件线性度和高温高功率特性,它被认为是未来的一种理想器件。虽然GaN材料在高频高功率应用领域很有优势,但是无论从理论上还是实验上,都还需要克服一系列难题。目前对GaN基电子器件特别是GaN HBT的研究还处于初级阶段,器件工作的很多机理还没有定论。本文正是在这种背景下,针对目前GaN HBT研究中存在的问题从理论和实验两个方面进行了探索。论文首先介绍了异质结双极晶体管的基本理论。详细描述了半导体异质结的能带图、空间电荷区的电势电场分布及其载流子的输运,采用热电子发射和扩散模型相结合的思想推导了非平衡突变异质结界面处准费米能级的断续,以及适用于异质结的Shockley边界条件。并在此基础上给出了理想突变异质结及异质结双极晶体管的伏安特性。器件模拟为器件设计指明了方向,对器件的研制具有非常重要的指导作用。因此,论文在模拟器Atlas上搭建了GaN HBT器件模拟平台,对GaN HBT器件进行了数值模拟。研究了极化效应和发射区渐变对GaN HBT器件特性的影响。结果显示：对于Ga面向上的材料,极化效应对器件特性影响不大,而发射区组分渐变可以消除导带尖峰,提高电子的注入效率。详细讨论了GaN HBT纵向设计(发射区、基区和集电区设计)中要注意的问题。GaN器件不同于硅半导体器件,它的材料层的掺杂都是原位的,因此对于GaN器件来说,材料生长是关键环节。本文详细介绍了GaN HBT材料生长的工艺流程,对所生长出的材料特别是p型GaN材料进行了微分析。结果表明：我们生长的p型GaN材料有较好的结晶质量和电特性,Ga(Mg)/N比为1.106,缺陷密度约为3×1018cm-2。GaN HBT制作中,已刻蚀p型GaN表面金属接触的非欧姆特性是严重影响器件特性的因素。论文针对此问题重点对刻蚀表面的p型欧姆接触进行了研究。方面优化刻蚀条件获取低刻蚀损伤的p型基区刻蚀表面;另一方面还摸索了修复并降低刻蚀损伤的方法。论文对研制GaN HBT的核心问题进行研究。给出了GaN HBT的器件版图,进行了CB结二极管的制作和特性分析,二极管的导通电压约为4V,反向击穿电压大于50V；给出了GaN HBT器件的详细工艺流程,对GaN HBT器件进行了试制,并对其进行了测试分析。分析表明,p型基区未能很好的掺杂、基区太薄以及工艺中的过刻蚀、Mg的记忆效应及高温下的扩散会造成器件IC-VCE曲线呈现电阻特性。此外,利用本文建立的器件仿真平台,论文还对SiCOI (SiC On Insulator) MESFET器件进行了研究,提出了一种可以大大提高器件击穿电压的新型器件结构——介质槽隔离结构,并对其进行了优化,发现多台阶介质槽阻断结构是一种更好的器件结构,可以在不影响器件其他特性如频率特性的基础上,大大提高器件的击穿电压。更多还原

【Abstract】 The wide-bandgap GaN material system has attracted much attention for high power microwave and millimeter wave device applications due to superior electronic and material properties. In this paper, nitride-based heterojunction bipolar transistor (HBT) and related techniques are discussed. BecauseⅢ-Ⅴnitride HBT has better device linearity, high temperature and high power properties, it has been considered as an ideal device in the future. Although GaN material is an excellent candidate for high frequency and high power applications, there are a series of difficulties to be conquered in both aspects of theory and experiment. At present the study of GaN-based devices is in the primary phase, especially for GaN HBTs, many mechanisms of device operating haven’t last word. Under the condition, these problems, which exists in the research of GaN HBT are discussed from the view of theory and experiment.First, the paper introduces the basic theories of heterojunction bipolar transistor. The bandgap structures, the distribution of potential and electric field of space-charge area and the transportation of currents are described in detail.Using the combination of Thermionic Emission Theory and Diffusion Theory, we calculate the discontinuity in quasi-fermi levels at the interface of the heterojunction and the Shockley Boundary Conditions which are for heterojunction. Then theⅠ-Ⅴcharacteristics of ideal abrupt heterojunctions and heterojunction bipolar transistors is given.The device simulation offers the direction for the device design, which is very important in the process of device development. So the GaN HBT is numerical simulated. The influences brought by the polarization effect and graded emitter are studied. The results show that polarization effect has a little affection to the characteristics of Ga-face terminated device, while the graded emitter can eliminate the peak of conduction band and improve the inject efficiency. Then we discuss the problems to which attention should be paid in the vertical design (emitter, base and collector design).GaN-based devices are different from Si’. To the former, the growth of material is a very important process since they are in-situ doped. The paper introduces the growth process flow of GaN HBT material in detail, and analyzes these materials, especially the p type GaN material. It has been resulted that the p-GaN has good crystal quality and electrical characteristic. The ratio of Ga(Mg) to N is 1.106. The defect density is about 3×1018cm-2.In the fabrication of GaN HBT, the non-ohmic contact to etched p-GaN is a big hurdle. To this point, the contact to etched p-GaN is discussed. Firstly, we optimize the etching conditions to reduce the etch damage as low as possible; and secondly, we also discuss the methods of repairing and reducing the etching damage.The paper discusses the kernel problems in the fabrication of GaN HBT. The device layouts, the fabrication and characteristic analysis of CB junction are also shown. The turn-on voltage is about 4V, and the breakdown voltage is above 50V. Besides, this paper gives the pprocess flow of GaN HBT. We trial-produce the GaN HBT device and test the characteristics. It is shown that the poor doping of base, too thin base and over-etching in the process, Mg memory effect and diffusion under high temperature lead to theⅠC-ⅠCE curves show resistance characteristic.Moreover, characteristic of SiCOI (SiC On Insulator) MESFET is also investigated using the device simulation platform set up in this paper. A new type of SiCOI MESFET device structure, SiCOI MESFET with dielectric groove isolation, is presented, and the device structure is optimized. We see SiCOI MESFET with multi-step dielectric groove isolation is a better structure, of which a large decrease of saturated drain current and transconductance doesn’t occur while a large increase of the breakdown voltage can be achieved.更多还原