矽基板高功率之氮化鎵高電子遷移率電晶體與蕭特基二極體設計與製作

Design and Fabrication of High Power GaN-Based HEMTs and Schottky Barrier Diodes on Silicon Substrates

楊瑞銘

指導教授 ： 徐碩鴻

國立清華大學/電機資訊學院/電子工程研究所/碩士(2012年)

摘要

氮化鎵的寬能隙、高臨界電壓以及高電子飽和速度的材料特性使之在功率元件的應用上相較於矽材料有相當大的優勢，而在近幾年來，建立於矽基板上之氮化鋁鎵/氮化鎵高電子遷移率電晶體與蕭特基二極體已被廣泛研究與討論，其中，因緩衝層的品質好壞而造成的漏電流更為此領域的重要議題之一。然而，對於功率元件的應用而言，過大的漏電流容易造成能源的浪費且將會嚴重降低系統運轉的效率。這之間已有研究指出元件的歐姆接觸製程除了降低接觸電阻值以減少功率消耗外，還要同時考慮歐姆合金尖端鑽入所造成的緩衝層漏電流。故本論文研究著重在使用不同的歐姆接觸金屬堆疊與不同的快速熱退火的溫度來實現歐姆接觸的最佳化，並同時觀察對緩衝層漏電流的影響。此外，我們將此最佳化後歐姆接觸的金屬堆疊實施在高電子遷移率電晶體與蕭特基二極體上，並觀察其對元件影響。在高電子遷移率電晶體的設計上，不同形狀的元件結構被採用來觀察不同的歐姆接觸金屬堆疊對崩潰耐壓的影響，而這些形狀則為指叉狀、矩形結構和大尺寸元件。在蕭特基二極體元件研究上，我們利用矽擴散進蕭基特與歐姆電極下的氮化鋁鎵/氮化鎵中，成功達到將開啟電壓從1.4 V 降低至1.27 V 與接觸電阻並且同時解決歐姆合金尖端鑽入的問題，並同時改善其崩潰電壓。

關鍵字

氮化鎵 ； 高電子遷移率電晶體 ； 蕭特基二極體

並列摘要

Gallium Nitride has generated significant interests for high-voltage applications due to its superior material properties such as wide bandgap, high critical electric field, high electron saturation velocity, and good thermal stability. These properties offer several potential advantages over silicon-based devices. High performance AlGaN/GaN high-electron mobility transistors (HEMTs) and Schottky Barrier Diodes (SBDs) have been realized on the silicon substrate for high-power applications in recent years, while one issue remains for these devices is the relatively high leakage current due to the quality of the buffer layer. Such a large leakage current causes serious off-state loss in the power supply and reduces the efficiency of the system. Optimization of the ohmic contacts requires not only to lowering the contact resistance to reduce power loss, but also needs to consider the buffer leakage due to ohmic metal spikes. This thesis focuses on the optimization of ohmic contacts of GaN HEMTs by using different metal stacks and rapid thermal annealing temperatures, while monitoring the buffer leakage at the same time. In addition, the impacts of using different ohmic contact alloys on HEMTs and SBDs are also analyzed. In the study of HEMTs, several layouts including one-finger, square-gate devices, and large scale devices also are used to investigate the impacts of alloyed ohmic contact on breakdown voltage such as off-state leakage. It is also observed that the turn-on voltage of the Si-diffused devices is shifted from 1.4 V to 1.27 V by diffusing silicon atoms into AlGaN/GaN layer underneath the Schottky electrode. The proposed Si-diffused ohmic contact can achieve both low contact resistance and smooth surface morphology, because the silicon atoms underneath ohmic electrode can prevent metal spikes originating from alloyed ohmic contact and improve breakdown voltage

參考文獻

[1] O. Ambacher, “Growth and applications of group III-nitrides,” Journal of Physics D (Applied Physics), vol. 31, pp. 2653-2710, 1998.

[2] B. J. Baliga, “Power Semiconductors Devices,” Boston: PWS, pp. 28-29, 1996.

[3] M. Willander et al., “Silicon carbide and diamond for high temperature devices applications,” Journal of materials science: materials in electronics, pp. 1-25, 2006.

[4] B. Gelmont et al., “Monte Carlo Simulation of Electron Transport in Gallium Nitride,” J. Appl. Phys., vol. 74, no. 3, pp. 1818-1821, 1993.

[5] A. N. Bright et al., “Correlation of contact resistance with microstructure for Au/Ni/Al/Ti/AlGaN/GaN ohmic contacts using transmission electron microscopy,” J. Appl. Phys., vol. 89, no. 6, pp. 3144-3150, 2001.