Millimeter-wave (mmWave) and Sub-terahertz (sub-THz) radar imaging and sensing have become major topics in recent years as they offer distinct advantages of non-hazardous radiation, large bandwidth, and high image resolution. Large power is needed to overcome the path loss, increase detection range, and refine image quality. Generating high-power and high-frequency signals is a challenging task. In recent years, gallium nitride (GaN) has emerged as a revolutionary solution against traditional methods with characteristics of high breakdown voltage and large electron saturation velocity under a high electric field. Thus, it holds promising status in generating high-power mmWave and sub-THz signals. This thesis focuses on developing GaN oscillators and signal sources for modern mmWave and sub-THz radar imaging and sensing, bringing out the potential of GaN-based oscillators and solving the accompanying challenges. A K-band cross-coupled oscillator with high power and high frequency relative to the maximum oscillation frequency is proposed using 0.25-µm GaN High Electron Mobility Transistors (HEMTs). A flipped transistor layout is designed and output matching network without buffer amplifiers is proposed to raise the oscillation frequency and obtain high output power (Pout). The proposed design achieves an oscillation frequency of 24.4 GHz which is the highest compared with the works using 0.25-μm GaN HEMT. The maximum Pout achieved is 16 dBm. To lower the phase noise (PN) of the oscillator, a single-transistor local feedback oscillator with a lumped LC reflection resonator at the Ka-band is proposed. A small-signal model of the transistor is developed to accurately predict frequency response under the proposed topology. A lump LC reflection resonator is designed with the local feedback topology to increase the quality factor and successfully lower the PN. This work uses the same 0.25-m GaN technology as the former work but realizes the oscillation at an even higher frequency of 31.3 GHz. A PN of -114 dBc/Hz at 1 MHz and -145 dBc/Hz at 10 MHz is achieved. To further elevate the Pout, a cascode oscillator using 0.15-µm GaN HEMTs is proposed. The cascode configuration improves the Pout, drain efficiency, and tuning range, generating 19.9-dBm measured Pout at 47.8 GHz. Furthermore, sub-THz oscillators and a sub-THz antenna are developed using 0.15-µm GaN HEMTs and gallium arsenide (GaAs) integrated passive device (IPD) technology. A push-push balanced Colpitts oscillator up to 110 GHz is demonstrated by using the parasitic gate-source capacitance as feedback capacitance. The design is further improved using the proposed coupled-line source degeneration to enhance the second harmonic current. In addition, load-pull at the output is conducted to perform maximum power matching for the second harmonic frequency. A simulated Pout of 16 dBm is realized at 99.4 GHz. Finally, a sub-THz antenna based on GaAs IPD is developed for the system-in-package radar system for integration with the GaN oscillators.
In summary, this thesis makes important contributions to the high-power and high-performance oscillators and signal sources based on GaN HEMT technology for modern radar imaging and sensing applications.

Acknowledgements i
Table of Contents ii
List of Figures iv
List of Tables viii
Abstract ix
List of Publications xi
Notations and Abbreviations xiii
Chapter 1. Introduction 1
1.1. Background 1
1.2. Research Motivation 2
1.3. Research Goals 6
1.4. Organization of the Thesis 6
Chapter 2. Literature Review 12
2.1. GaN HEMT Devices 12
2.1.1. Structural configuration 12
2.1.2. Frequency and power 13
2.1.3. Small-signal model 15
2.2. Review of GaN HEMT-Based Oscillators 17
2.2.1. Passive component characteristics 17
2.2.2. Oscillator designs 19
2.2.2.1. Basic high-frequency oscillator topology 19
2.2.2.2. Design consideration: Phase noise 21
2.2.2.3. Design consideration: Oscillation frequency 24
2.2.2.4. Design consideration: Tuning range 26
2.2.2.5. Design consideration: Output power 26
2.3. Summary 27
Chapter 3. K-Band Cross-Coupled Oscillator with High Output Power 36
3.1. Introduction 36
3.2. Introduction of the 0.25-μm GaN HEMT Technology 37
3.3. Topology of the K-Band Cross-Coupled Oscillator 37
3.4. Design of the Cross-Coupled Pair 38
3.5. Design of the Output Matching Network 40
3.6. Experimental Results 41
3.7. Summary 44
Chapter 4. Low-Phase Noise Ka-Band Local Feedback Oscillators 47
4.1. Introduction 47
4.2. Development of the Small-Signal Model 48
4.3. Design of the Common-Gate Local Feedback Oscillator 51
4.4. Design of the Local Feedback Oscillator with Varactor 56
4.5. Experimental Results 58
4.6. Summary 61
Chapter 5. V-band High-Power Cascode Oscillator 64
5.1. Introduction 64
5.2. Design of the Cascode Oscillator 65
5.2.1. Design of the first stage 67
5.2.2. Design of the second stage 70
5.3. Experimental Results 74
5.4. Summary 78
Chapter 6. Sub-THz Push-Push Oscillator Based on 0.15-µm GaN HEMTs 81
6.1. Introduction 82
6.2. Design of the Push-Push Sub-THz Oscillator 82
6.2.1. Push-push balanced Colpitts topology 83
6.2.2. Transistor technology and small-signal modelling 84
6.2.3. Oscillator design and optimization 86
6.2.4. Experimental results 89
6.3. Design of Coupled-Line Source Degeneration Push-Push Oscillator with Second Harmonic Power Matching 92
6.3.1. Design of coupled-line source degeneration 92
6.3.2. Maximum second harmonic extraction 96
6.4. Design of Sub-THz Cavity-Backed Folded Slot Antenna Using GaAs IPD 98
6.4.1. Introduction of the GaAs IPD technology 99
6.4.2. Antenna structure 100
6.4.3. Experimental results 102
6.5. Summary 103
Chapter 7. Conclusions and Future Work 107
7.1. Conclusion of this Thesis 107
7.2. Future Work 109

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Chapter 6:
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Chapter 7:
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