Development of Large-area GaN High Electron Mobility Transistors and the Applications to Power Electronics

氮化鎵高電子遷移率電晶體具備高崩潰電壓、高輸出功率與低導通電阻等優良特性，將可取代現今已接近物理極限的矽材功率元件，廣泛應用於高壓電子元件以及高頻高效率電源轉換系統中，例如驅動電動車產品的馬達系統，藉以提升能源使用的效率。然而，由於氮化鋁鎵/氮化鎵異質介面形成大量二維電子氣，傳統氮化鎵高電子遷移率電晶體為空乏型操作。基於電路安全性與電路設計複雜度的考量，空乏型操作的特性將限制其在電源轉換上的應用。 本論文利用 p 型氮化鎵覆蓋層實現增強型操作，並探討製程流程、蝕刻深度與退火溫度對於臨界電壓、操作電流與崩潰電壓之影響。本文中的增強型氮化鎵高電子遷移率電晶體在16微米的閘汲極距離下，可達成 1630伏特的高崩潰電壓操作。與傳統氮化鎵高電子遷移率電晶體相較， p型氮化鎵覆蓋層的存在賦予閘極金屬更多樣性的選擇。本文實現三種不同閘極金屬的增強型氮化鎵高電子遷移率電晶體，探討閘極金屬/p型氮化鎵覆蓋層介面所形成的蕭特基二極體之位障對元件直流特性的影響，並提供不同應用下適當閘極金屬選擇的參考依據。 本論文同時也開發大面積多指型功率元件，並以兩種方式達成操作電流高於6安培之總寬度為30毫米的增強型功率元件。其一為利用空乏型氮化鎵高電子遷移率電晶體串接矽功率元件，並以SO-8形式封裝此 cascode架構；另一則為使用p型氮化鎵覆蓋層單晶片增強型架構。Silicon is the most widely used material for power devices. However, Si-based power devices are approaching their material limits. Gallium nitride (GaN) high electron mobility transistors (HEMTs) have great potential for high-frequency and high-power applications because of their excellent characteristics, such as high breakdown voltage, high output power and low on-resistance. Nonetheless, the inherent normally-on behavior excludes GaN HEMTs from most power electronic applications for reduced circuit complexity and fail-safe operation. In this thesis, p-GaN cap layer was utilized to raise the conduction band energy underneath the gate contact in order to achieve enhancement-mode (E-mode) operation. The effects of p-GaN etching depth and alloy temperature on the characteristics of the E-mode GaN HEMTs were investigated in order to improve the device performances. By adjusting the process methods, a noticeably high breakdown voltage of 1630V was achieved for the p-GaN cap HEMTs with LGD = 16μm. Although Ni/Au is commonly used as a gate metal for the commercial GaN HEMTs, many metals can be chosen as the gate contact metal of p-GaN cap HEMTs because most of the metals have adequate work function difference comparing to p-GaN. In this thesis, different gate metals including Ni/Au-, Ti/Au-, and Mo/Ti/Au-gate GaN HEMTs were demonstrated to study the impacts of gate metals on the device performances, such as VTHs, saturated output currents, and breakdown voltages of GaN HEMTs. Compared to Ni/Au-gate HEMTs, the devices with a Mo/Ti/Au gate can improve 32% of the breakdown voltage with a trade-off of reducing 11% of operating current at LGD = 6um. Moreover, multi-finger large-area power devices were demonstrated. Two kinds of methods were employed to realize E-mode power devices whose operating currents are higher than 6A. One is using a Si MOSFET in series to drive the depletion-mode (D-mode) GaN HEMTs, the other one is using p-GaN cap E-mode HEMTs.口試委員會審定書 i 誌謝 ii 中文摘要 iii ABSTRACT iv CONTENTS vi LIST OF FIGURES ix LIST OF TABLES xiv Chapter 1 Introduction 1 1.1 GaN Applications Overview 1 1.2 AlGaN/GaN HEMTs 3 1.3 E-mode AlGaN/GaN HEMTs 4 Chapter 2 Development of Small-area p-GaN Cap AlGaN/GaN HEMTs 7 2.1 Introduction 7 2.2 Development of D-mode AlGaN/GaN HEMTs 7 2.2.1 Device Structure Design 7 2.2.2 Fabrication 8 2.3 Development of p-GaN cap AlGaN/GaN HEMTs 10 2.3.1 Device Structure Design 10 2.3.2 Fabrication 11 2.4 Small-area p-GaN cap AlGaN/GaN HEMTs performed by Process A 14 2.4.1 Fabrication of Process A 14 2.4.2 Performance of Process A 14 2.5 Small-area p-GaN cap AlGaN/GaN HEMTs performed by Process B 18 2.5.1 Fabrication of Process B 18 2.5.2 Performance of Process B 19 2.6 The Impact of Gate Metals on Performance of Devices 23 2.6.1 Introduction 23 2.6.2 Device Fabrication 24 2.6.3 Transfer Curves 26 2.6.4 ID-VDS Curves 29 2.6.5 Reverse Conduction Mode 30 2.6.6 Breakdown Voltage 32 2.7 Summary 32 Chapter 3 Development of Large-Area GaN HEMTs 35 3.1 Introduction 35 3.2 Comparison of Different Passivation Layers on Devices 35 3.2.1 Effects of Passivation Layers 35 3.2.2 Fabrication 36 3.2.3 Discussion 37 3.2.4 Summary 40 3.3 1st Edition Large-Area D-mode AlGaN/GaN HEMTs 40 3.3.1 Device Structure Design 40 3.3.2 Fabrication 43 3.3.3 Bare Device Performance 43 3.3.4 Summary 44 3.4 2nd Edition Large-Area D-mode AlGaN/GaN HEMTs 46 3.4.1 Device Structure Design 46 3.4.2 Bare Device Performance 47 3.4.3 Summary 48 3.5 3rd Edition Large-Area D-mode AlGaN/GaN HEMTs 49 3.5.1 Device Structure Design 49 3.5.2 Bare Device Performance 49 3.5.3 Summary 52 3.6 Large-Area E-mode AlGaN/GaN HEMTs 52 Chapter 4 Package 55 4.1 Introduction 55 4.2 Cascode Structure 56 4.3 Package Types 57 4.3.1 COB 57 4.3.2 SOIC 57 4.4 Performance of Package 57 4.4.1 Si MOSFET 57 4.4.2 COB 59 4.4.3 SO-8 62 4.5 Summary 65 Chapter 5 Conclusion 66 REFERENCE 6