In this thesis, we have used a combination of physical analysis, numerical simulation and experimental work to identify and overcome some of the main challenges in AlGaN/GaN high electron mobility transistors (HEMTs) for high frequency applications. In spite of their excellent material properties, GaN-based HEMTs are still below the theoretical predictions in their high frequency performance. If the frequency performance could be improved, the superior breakdown characteristics of nitride semiconductors would make these devices the best option for power amplifiers at any frequency. To achieve this goal, we have first identified some critical parameters that limit the high frequency performance of AlGaN/GaN HEMTs and then we have demonstrated several new technologies to increase the performance. Some of these technologies include advanced drain delay engineering, charge control in the channel and new N-face GaN HEMTs. Although more work is needed in the future to combine all these new technologies, the initial results are extremely promising.

In spite of the great progress in performance achieved during the last few years, GaN high electron mobility transistors (HEMTs) still have several important issues to be solved for millimeter-wave (30 ~ 300 GHz) applications. One of the key challenges is to improve its high frequency characteristics. In this thesis, we particularly focus on fT and fma, two of the most important figures of merit in frequency performance of GaN HEMTs and investigate them both analytically and experimentally. Based on an improved physical understanding and new process technologies, we aim to demonstrate the state-of-the-art high frequency performance of GaN HEMTs. To maximize fmax, parasitic components in the device (Ri, R, Rg, Cgd, and go) are carefully minimized and the optimized 60-nm AlGaN/GaN HEMT shows a very high fmax of 300 GHz. The lower-than-expected fT observed in many AlGaN/GaN HEMTs is attributed to a significant drop of the intrinsic transconductance at high frequency (RF gm) with respect to the intrinsic DC g. (called RF gm-collapse). By suppressing RF gm-collapse and harmoniously scaling the device, a record fT of 225 GHz is achieved in the 55-nm AlGaN/GaN HEMT. Another important challenge for the wide adoption of GaN devices is to develop suitable technology to integrate these GaN transistors with Si(100) electronics. In this thesis, we demonstrate a new technology to integrate, for the first time, GaN HEMTs and Si(100) MOSFETs on the same chip. This integration enables the development of hybrid circuits that take advantage of the high-frequency and power capability of GaN and the unsurpassed circuit scalability and complexity of Si electronics.

This work has arisen out of the strong demand for a superior power-added efficiency (PAE) of AlGaN/GaN high electron mobility transistor (HEMT) high-power amplifiers (HPAs) that are part of any advanced wireless multifunctional RF-system with limited prime energy. Different concepts and approaches on device and design level for PAE improvements are analyzed, e.g. structural and layout changes of the GaN transistor and advanced circuit design techniques for PAE improvements of GaN HEMT HPAs.

This book demonstrates to readers why Gallium Nitride (GaN) transistors have a superior performance as compared to the already mature Silicon technology. The new GaN-based transistors here described enable both high frequency and high efficiency power conversion, leading to smaller and more efficient power systems. Coverage includes i) GaN substrates and device physics; ii) innovative GaN -transistors structure (lateral and vertical); iii) reliability and robustness of GaN-power transistors; iv) impact of parasitic on GaN based power conversion, v) new power converter architectures and vi) GaN in switched mode power conversion. Provides single-source reference to Gallium Nitride (GaN)-based technologies, from the material level to circuit level, both for power conversions architectures and switched mode power amplifiers; Demonstrates how GaN is a superior technology for switching devices, enabling both high frequency, high efficiency and lower cost power conversion; Enables design of smaller, cheaper and more efficient power supplies.

Abstract: In this thesis, we have investigated and overcome the major limiting factors of intrinsic and parasitic parameters in III-Nitride high electron mobility transistors for high frequency performance with a combined study of simulations and experimental work. The high frequency RF performance of these transistors is severely degraded when short-channel effects, and parasitic resistances are present. To investigate short-channel effects, first, we have developed a simulation model for Ga-polar and N-polar HEMT structures, and found the main reasons for these degradations are drain-induced barrier-lowering and space-charge-limited current injection. To mitigate this, a strong electrostatic back-barrier structure from N-polar orientation is suggested. Secondly, the effect of quantum displacement in GaN HEMTs were investigated using both simulation and experimental measurements. It was discovered that the quantum displacement in a highly scaled device can give more than 2x change in the gate-source capacitance between two different orientations. Therefore it is imperative that the device design for such highly scaled devices consider quantum displacement effects in order to avoid short-channel effects. Another limiting factor from extrinsic elements is the parasitic resistances, especially from contact resistance. To achieve low-resistance non-alloyed Ohmic contacts, we developed two new process technologies that included an inserted graphene layer between metal and AlGaN, and a graded n+ AlGaN Ohmic layer. Both approaches utilized the current path where no barrier exists. In this work, we set the record low contact resistance of 0.049 Ohm mm for Ga-polar technology using the graded AlGaN scheme. The most crucial factor for improving high frequency performance for III-Nitride HEMTs is the saturation of the effective electron velocity. We have modeled the velocity saturation in GaN channel based on LO phonon emission, and explain the phenomena of rapidly decreasing behavior transconductance. This model was also applied to 2-D device simulation where it was possible to obtain the DC and RF characteristics matching to the experimental results. To overcome the fast reduction in gm and fT, we have introduced a polarization graded channel in AlGaN/GaN HEMT and graded AlGaN HFET to tailor the charge profile, and demonstrated a flat gm profile for the first time in field-effect-transistor structure. The high frequency performance of this engineered channel was measured from a highly scaled graded AlGaN HFET with advanced process technology including contact layer regrowth and e-beam lithography. Although the flat gm improved the linearity of fT and fmax, they do not stay flat due to an increasing capacitance profile in a regular 2 X 50 micrometre device. With further scaling of the device width, we obtained an increasing gm profile which can compensate the increment of capacitance, and finally we demonstrated flat fT and fmax profile which has been unseen in any field-effect-transistors. All these results shown in this thesis can contribute to further improvements in high frequency and high power performance of III-Nitride HEMTs for the future RF application beyond mm-Wave frequency.

This book focusses on III-V high electron mobility transistors (HEMTs) including basic physics, material used, fabrications details, modeling, simulation, and other important aspects. It initiates by describing principle of operation, material systems and material technologies followed by description of the structure, I-V characteristics, modeling of DC and RF parameters of AlGaN/GaN HEMTs. The book also provides information about source/drain engineering, gate engineering and channel engineering techniques used to improve the DC-RF and breakdown performance of HEMTs. Finally, the book also highlights the importance of metal oxide semiconductor high electron mobility transistors (MOS-HEMT). Key Features Combines III-As/P/N HEMTs with reliability and current status in single volume Includes AC/DC modelling and (sub)millimeter wave devices with reliability analysis Covers all theoretical and experimental aspects of HEMTs Discusses AlGaN/GaN transistors Presents DC, RF and breakdown characteristics of HEMTs on various material systems using graphs and plots

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