隨著無線通訊技術市場的擴張以及自動駕駛汽車的演進，高頻元件如氮化鋁鎵/氮化鎵高電子遷移率電晶體受到越來越多的關注。由於其優異的特性如寬能隙和高電子遷移率，使此元件非常適合用在高功率和高頻率的操作下，其異質接面所產生的大量二維電子氣更提供元件大電流、低阻抗的特性; 然而，在缺少低價氮化鎵基板的情況下，此電晶體需磊晶在其他基板材料上。因為低組值矽基板的低廉價格和容易製作成大面積晶圓之特性，使其在眾多材料選擇中占有極大優勢; 不過矽基板仍然有許多困難需要克服，如低崩潰電場、晶格不匹配和高頻漏電流。目前已經有許多研究致力於克服這些瓶頸，而其中一項就是局部矽基板移除技術。
在此研究中，我們探討氮化鋁鎵/氮化鎵高電子遷移率電晶體在局部移除矽基板之後的特性。藉由霍爾量測和拉曼量測，研究發現材料在移除矽基板之後並不會有負面影響; 然而，因為移除良好導熱性的矽基板，電晶體遇到更嚴重的熱效應導致電流下降。接著我們探討電子陷阱的捕獲效應，結果指出挖除基板後能夠大幅抑制電流崩塌效應。
下一步我們研究電晶體在高頻下的特性。元件在移除基板後的順向增益電壓呈現一個特殊的情形: 挖除基板的電晶體在低頻下有較差的順向增益電壓，但在高頻下卻有較好的順向增益電壓，這是因為低頻時挖除基板的電晶體受到較嚴重的熱效應導致增益降低，但在高頻時因為移除基板漏流，可取得較好的順向增益電壓。我們也觀察到挖除基板的電晶體能取得較高的電流增益截止頻率和功率增益截止頻率，雜訊指數也能被大幅抑制。
從此篇研究結果，我們證實矽基板移除技術是同時獲得良好高頻特性和低價格兩項優勢的有效方法。

With the upsurge of wireless communication market like autonomous vehicle (self-driving car) and fifth-generation wireless systems (5G). Gallium nitride high electron mobility transistors (GaN HEMTs) have gained more focus in recent years. Due to the outstanding material properties including wide-band-gap and high electron mobility, GaN HEMTs are suitable for high power and microwave system. The two dimensional electron gas (2DEG) formed in heterojunction ensures the large operating output current and low ON-resistance of the device.
In absence of low-cost GaN bulk substrate, GaN is grown on various kinds of substrates. LR Si is the most attractive one due to low cost and large size availability, which is crucial in terms of commercialization. However, devices fabricated on LR Si have many drawbacks, such as lattice mismatch, low breakdown voltage and high absorption of RF signal. To overcome the obstacles, local Si removal is considered as a promising approach.
In this thesis, we aim to investigate electrical properties of conventional AlGaN/GaN/Si HEMTs after local Si removal. With Hall measurement and Raman spectroscopy analysis, material properties of epi structure are confirmed to remain unchanged after Si removal. However, device with Si removal incur severe self-heating effect since air possesses lower thermal conductivity compared to Si. We also investigate trapping effect and conclude that current collapse can be suppressed effectively after Si removal due to decrease of buffer tapping.
Furthermore, impact of substrate removal on microwave performance is studied. Our results show that lower transconductance (gm) caused by self-heating effect may degrades forward voltage gain S21 at low frequency. However, input signal couples with conductive substrate at high frequency has larger impact on S21 than self-heating effect, which results in larger S21 at high frequency after Si removal. Current gain cut-off frequency and power gain cut-off frequency are also improved since higher power efficiency can be attained. Moreover, substrate leakage current has longer response time compared to channel current due lower carrier mobility, which means it is not in phase with output signal and acts as a noise source. The measurement results indicate that noise figure can be greatly reduced after Si removal.
From earlier discussion, we conclude that Si removal is an effective method to increase microwave performance of HEMT on LR-Si, which provides a cost-effective solution to maintain RF performance.

致謝 i
摘要 ii
Abstract iii
Contents v
List of Figures vii
Chapter 1 Introduction 1
1.1 GaN Applications Overview 1
1.2 AlGaN/GaN HEMTs 2
1.3 Comparison of Substrate materials 4
1.4 Phenomenon of Current Collapse 5
1.5 Microwave loss of GaN-on-Si HEMTs 8
1.6 Thesis Outline 9
Chapter 2 Electrical properties of AlGaN/GaN/Si HEMTs with local Si removal 10
2.1 Introduction 10
2.2 Fabrication process of AlGaN/GaN/Si HEMTs with local Si removal 11
2.3 I-V characteristics and material properties 13
2.4 Current collapse phenomenon characterization 17
2.4.1 Measurement setup 17
2.4.2 Dynamic characteristics 20
2.4.3 Mechanism of current collapse suppression 25
2.5 Summary 27
Chapter 3 Microwave performance characterization after Si removal 28
3.1 Introduction 28
3.2 Microwave performance 30
3.2.1 Device parameters 30
3.2.2 Scattering parameters 30
3.2.3 Current gain cut-off frequency (fT) and power gain cut-off frequency (fmax) 34
3.2.4 Noise figure 37
3.3 Small signal circuit model 40
3.4 Summary 42
Chapter 4 Conclusion 43
Reference 45

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