Millimeter-wave GaN high electron mobility transistors and their integration with silicon electronics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references.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.by Jinwook W. Chung.Ph.D

Advanced technologies for improving high frequency performance of AlGaN/GaN high electron mobility transistors

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (leaves 77-80).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.by Jinwook W. Chung.S.M

A Comparative Study of Queue, Delay, and Loss Characteristics of AQM Schemes in QoS-enabled Networks

One of the major component in a QoS-enabled network is active queue management (AQM). Over the last decade numerous AQM schemes have been proposed in the literature. However, much recent work has focused on improving AQM performance through alternate approaches. This study focuses on an unbiased comparative evaluation of the various proposals. The evaluation methodology adopted is the following: we first define the relationship between the terminologies used in this paper, briefly introduce the queue, delay, and loss characteristics-- a subset of network characteristics that can be used to describe the behavior of network entities, and give their mathematical description. Next, we present a method that would be a successful case study based on the NS simulation technique and simulation-based comparisons of AQM schemes chosen, which will help understand how they differ from in terms of per-node queueing information and per-flow end-to-end behavior. Simulation results showed that PI schemes, a feedback-based mechanism, can assist delay sensitive applications to adapt dynamically to underlying network and to stabilize the end-to-end QoS within an acceptable requirement. To understand this attribute and behavior is important for the proper design of queue disciplines, for the provisioning of queues and link capacity, and for choosing parameters in simulation

How to Attract Your Students to Your Games: The Difference of Mediating Effect of Psychic Income on the Relationship between Points of Attachment and Behavioral Attributes

The purpose of the current study was to examine the impact of the points of attachment with the mediating impact of college students’ psychic income on their attendance and word-of-mouth behavioral intentions. A total of 537 samples were collected from college students at a mid-sized southwest university in the United States. A CFA was conducted to evaluate the psychometric properties of the scales. The hypothesized model was tested using a SEM with AMOS 26 software. Additionally, a bootstrapping method was employed to determine the indirect effect of the mediating variable of psychic income on the relationship between points of attachment and behavioral intentions as well as points of attachment and word of mouth. The results reveal that psychic income mediated the proposed relationships

Effects of lower extremity physical activity on shoulder biomechanics and functional recovery in the early phase after arthroscopic rotator cuff repair in male patients: a retrospective study

Surgeons widely use arthroscopic rotator cuff repair (ARCR) to restore biomechanics and function in patients with rotator cuff tears. However, patients show severe pain early after ARCR, and their physical activity level decreases sharply. This study aimed to determine the effect of lower extremity physical activity on shoulder pain, biomechanics, and functional recovery in the early phase after ARCR. This retrospective study included 103 male patients according to the inclusion criteria. We classified subjects into high physical activity (HPA, n = 49) and low physical activity (LPA, n = 54) groups according to the classification criteria. For analysis, we measured the visual analog scale (VAS) score, range of motion (ROM), American Shoulder and Elbow Surgeons (ASES) score, and grip strength preoperatively and six weeks postoperatively. In comparing the HPA and LPA groups at six weeks postoperatively, the VAS score was significantly lower in the HPA group (p < 0.001). The ROM of forward flexion (p = 0.001), abduction (p = 0.005), and external rotation (p = 0.001) of the shoulder was also significantly greater in the HPA group. In particular, the ASES score (p < 0.001) and grip strength (p < 0.001) showed significant improvement only in the HPA group. Patients with higher levels of physical activity after ARCR showed lower subjective pain and faster biomechanical and functional recovery than those with lower levels of physical activity. Therefore, even if the involved shoulder has restrictions during the early phase after ARCR, it is vital to actively recommend physical activity such as walking, stationary cycling, and climbing stairs using the lower extremities

Learning a Sport through Video Gaming: A Mixed-Methods Experimental Study

This study examined the impact of playing a sport video game on learning the sport as well as how the game may influence future intentions of watching or playing the sport. Utilizing American university students with little prior knowledge of cricket, this study employed a mixed-methods pre/post intervention design with randomized experimental (EG; n = 43) and control (CG; n = 46) groups. Results indicated that cricket knowledge significantly increased for the EG pre-test to post-test (p < .05, n^2 = 0.19; particularly regarding cricket rules, terminology, player positions, and field layout), while the CG did not significantly differ. A significant difference was also found between the EG and CG for interest in playing cricket (p < .05, n^2 = 0.9). Qualitative findings supported that video gaming motivated intentions to watch and play cricket. Sport video games can facilitate increased sport knowledge, sport appreciation, and intentions for future physical activity

The stability of graphene band structures against an external periodic perturbation; Na on Graphene

We report that the $\pi$ band of graphene sensitively changes as a function of an external potential induced by Na especially when the potential becomes periodic at low temperature. We have measured the band structures from the graphene layers formed on the 6H-SiC(0001) substrate using angle-resolved photoemission spectroscopy with synchrotron photons. With increasing Na dose, the $\pi$ band appears to be quickly diffused into background at 85 K whereas it becomes significantly enhanced its spectral intensity at room temperature (RT). A new parabolic band centered at $k\sim$1.15 \AA$^{-1}$ also forms near Fermi energy with Na at 85 K while no such a band observed at RT. Such changes in the band structure are found to be reversible with temperature. Analysis based on our first principles calculations suggests that the changes of the $\pi$ band of graphene be mainly driven by the Na-induced potential especially at low temperature where the potential becomes periodic due to the crystallized Na overlayer. The new parabolic band turns to be the $\pi$ band of the underlying buffer layer partially filled by the charge transfer from Na adatoms. The five orders of magnitude increased hopping rate of Na adatoms at RT preventing such a charge transfer explains the absence of the new band at RT.Comment: 6 pages and 6 figure

A Host Protection Framework Against Unauthorized Access for Ensuring Network Survivability

Abstract. Currently, the major focus on the network security is securing individual components as well as preventing unauthorized access to network services. Ironically, Address Resolution Protocol (ARP) poisoning and spoofing techniques can be used to prohibit unauthorized network access and resource modifications. The protecting ARP which relies on hosts caching reply messages can be the primary method in obstructing the misuse of the network. This paper proposes a network service access control framework, which provides a comprehensive, hostby-host perspective on IP (Internet Protocol) over Ethernet networks security. We will also show how this framework can be applied to network elements including detecting, correcting, and preventing security vulnerabilities