VTechWorks
VTechWorks provides global access to Virginia Tech scholarship, including journal articles, books, theses, dissertations, conference papers, slide presentations, technical reports, working papers, administrative documents, videos, images, and more by faculty, students, and staff. Faculty can deposit items to VTechWorks from Elements, including journal articles covered by the University open access policy. Email vtechworks@vt.edu for help.
Open Access Policy
Virginia Tech's open access policy enables researchers to deposit the accepted version of scholarly articles with no embargo.
Theses and Dissertations
Virginia Tech was first in the world to require ETDs in 1997, and continues to add scans of older theses and dissertations.
Open Textbooks
More than 40 freely available and openly licensed textbooks are among our most downloaded items.
Communities in VTechWorks
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Recent Submissions
Advancing Sustainability in Data Centers: Evaluation of Hybrid Air/Liquid Cooling Schemes for IT payload using Sea Water
Latif, Imran; Ashraf, Muhammad Mubashar; Haider, Umaima; Reeves, Gemma; Untaroiu, Alexandrina; Coelho, Fabio; Browne, Denis (IEEE, 2024-12)
The growth in cloud computing, big data, AI and -performance computing (HPC) necessitate the deployment of additional data centers (DC's) with high energy demands. The unprecedented increase in the Thermal Design Power (TDP) of the computing chips will require innovative cooling techniques. Furthermore, DC's are increasingly limited in their ability to add powerful GPU servers by power capacity constraints. As cooling energy use accounts for up to 40% of DC energy consumption, creative cooling solutions are urgently needed to allow deployment of additional servers, enhance sustainability and increase energy efficiency of DC's. The information in this study is provided from Start Campus' Sines facility supported by Alfa Laval for the heat exchanger and CO2 emission calculations.The study evaluates the performance and sustainability impact of various data center cooling strategies including an air-only deployment and a subsequent hybrid air/water cooling solution all utilizing sea water as the cooling source. We evaluate scenarios from 3MW to 15+1MW of IT load in 3MW increments which correspond to the size of heat exchangers used in the Start Campus' modular system design. This study also evaluates the CO2 emissions compared to a conventional chiller system for all the presented scenarios. Results indicate that the effective use of the sea water cooled system combined with liquid cooled systems improve the efficiency of the DC, plays a role in decreasing the CO2 emissions and supports in achieving sustainability goals.
Enhancing the User Experience and Decision-Making of Green Building Certification Resources through Usability Testing Methods
Jefferson, Dwayne Ernest (Virginia Tech, 2025-01-16)
Multi-Kilovolt Gallium Nitride Power Transistors
Guo, Yijin (Virginia Tech, 2025-01-16)
Power semiconductor device, as a significant contributor to power electronics industry, plays an indispensable role in energy conversion applications including electric vehicles, data centers, consumer electronics, power grids, etc. The evolution of power semiconductor materials has progressed from traditional silicon (Si) to wide-bandgap materials including silicon carbide (SiC) and gallium nitride (GaN). Benefitting from the wide bandgap, high electron mobility and good thermal conductivity of GaN, GaN-based power devices can achieve fast switching speed, high breakdown voltage, and small on-resistance. They have been deployed in numerous power electronics applications, outperforming the Si and SiC counterparts. Nevertheless, despite their inherent advantages, the commercialization of GaN devices, particularly high-electron-mobility transistors (HEMTs), has predominantly been confined to the low-voltage domain of typically below 650 volts. This limitation blocks GaN HEMTs for medium- and high-voltage applications such as electric vehicles, renewable energy processing, and power grids, which have a total market size over USD$15 billion.
The challenge for GaN HEMTs to reach high-voltage applications arises primarily from the highly non-uniform electric field (E-field) distribution within the device structure, predisposing the device to premature breakdown and limiting its operational voltage range. Consequently, the quest for higher voltage capabilities in GaN HEMTs requires the fundamental understanding and effective mitigation of this non-uniform E-field distribution.
In this work, the p-type GaN-based Reduced Surface Field (RESURF) structure is proposed to balance the net charge in the two-dimensional-electron gas (2DEG) channel in GaN HEMT. This design enables a uniform distribution of E-field, enabling the voltage upscaling in GaN HEMT up to 10,000 V (i.e., 10 kV), which is the milestone voltage class in unipolar power devices for high-power applications.
The first part of this thesis introduces the history, background and mechanism of power semiconductor devices and provides solid reasons for GaN as a competitive participant in power electronics industry. It covers a basic introduction about GaN HEMT devices and their commercialization status and states the challenges GaN HEMTs are facing when dealing with mass production. An innovative RESURF structure is introduced to overcome the existing trade-off between on-resistance and breakdown voltage, and to achieve superior overall performance that would be beneficial for GaN HEMT to upscale the voltage classes.
Secondly, the development of a 10 kV unidirectional GaN HEMTs is discussed in detail. An optimized fabrication process flow, including etching, metal deposition, contact formation and dielectric passivation, is established. The RESURF structure is formed through a two-step chlorine-based etching process, with an innovative introduction of sulfur hexafluoride (SF6) that enables a self-termination etch stop onto the AlGaN surface without damage to the 2DEG channel beneath. A controlled slow etch recipe has been developed as well, aiming for large-scale manufacturing with improved yields. A detailed analysis of the on-state and off-state I-V characterization of devices with various RESURF thickness and length provides an insight into the device breakdown mechanism, which has been verified with physics-based technology computer-aided design (TCAD) simulation.
The third part of this work demonstrates a 3.3-kV monolithic bidirectional switch (MBDS), which a novel device concept that can significantly simplify the circuit design in alternative current (AC) power conversion. A symmetrical p-GaN junction termination extension (JTE) design is proposed for electric field management, and the lateral conduction of this GaN-based MBDS enables a state-of-the-art high-voltage bidirectional switch with low on-resistance, achieving considerable performance advantage compared to the conventional bidirectional switch implemented by discrete devices.
In summary, this research work covers the design, fabrication, characterization, simulation, and breakdown mechanism analysis of GaN-based unidirectional and bidirectional transistors for multi-kilovolt power conversion applications. The extended p-GaN configuration (RESURF for unidirectional devices and JTE for bidirectional devices) offers a spatially-distributed E-field management, enhancing the breakdown voltage scaling capability of GaN HEMTs to exploit the full material advantages of GaN.
Data-driven Algorithms for Critical Detection Problems: From Healthcare to Cybersecurity Defenses
Song, Wenjia (Virginia Tech, 2025-01-16)
Machine learning and data-driven approaches have been widely applied to critical detection problems, but their performance is often hindered by data-related challenges. This dissertation seeks to address three key challenges: data imbalance, scarcity of high-quality labels, and excessive data processing requirements, through studies in healthcare and cybersecurity.
We study healthcare problems with imbalanced clinical datasets that lead to performance disparities across prediction classes and demographic groups. We systematically evaluate these disparities and propose a Double Prioritized (DP) bias correction method that significantly improves the model performance for underrepresented groups and reduces biases. Cyber threats, such as ransomware and advanced persistent threats (APTs), have presented growing threats in recent years. Existing ransomware defenses often rely on black-box models trained on unverified traces, providing limited interpretability. To address the scarcity of reliably labeled training data, we experimentally profile runtime ransomware behaviors of real-world samples and identify core patterns, enabling explainable and trustworthy detection. For APT detection, the large size of system audit logs hinders real-time detection. We introduce Madeline, a lightweight system that efficiently processes voluminous logs with compact representations, overcoming real-time detection bottlenecks.
These contributions provide deployable and effective solutions, offering insights for future research within and beyond the fields of healthcare and cybersecurity.
Cariño, Standards, and the Reinforcement Loop: How Mathematics and Science Teachers Integrate Engineering-Based Learning in the Borderlands at an All-Girls STEAM School
Benitez, Ramon (Virginia Tech, 2025-01-16)
This study investigated the pedagogical and cultural factors influencing how teachers at a STEAM school integrated engineering into mathematics and science for a majority Latina population. The study was based on the guiding proposition: Teachers at a STEAM school in the Borderlands are uniquely positioned to integrate engineering into mathematics and science, and they play a significant role in broadening the participation of Latinas. This instrumental, embedded case study focused on understanding the pedagogical and cultural factors that influenced how teachers at a science, technology, engineering, arts, and mathematics (STEAM) school integrated engineering, using data from artifact packages and interviews. As a key step toward broadening the participation of Latinas, the study examined how teachers employed engineering-based pedagogies (such as project- and problem-based learning) alongside instructional strategies that supported the cultural backgrounds of Latina students in their mathematics and science courses.
The teacher participants were selected from an urban preparatory academy along the southwest border of the United States (U.S.). Mathematics and science teachers were considered embedded units within the case study. Participating teachers gathered curricular artifacts to create a "scoop" package that represented their instruction of engineering in the context of their courses. These artifacts were annotated and used in artifact-based interviews, which served as a primary data source where pairs of participants discussed their past instructional decisions and the factors that influenced how they integrated engineering for a majority Latina population.
This study focused on three key factors influencing teachers' instructional choices: state standards, the reinforcement loop, and Cariño. Teachers emphasized using content standards to backward-design engineering-based activities that supported other subjects rather than focusing solely on engineering. Their reform-oriented approach guided this integration. The reinforcement loop, supported by school structures, enhanced teachers' motivation, confidence, and teaching practices, fostering engineering-based learning. Teachers described this loop as a routinized process that also encouraged student engagement through effective classroom management. Ultimately, the five factors were interconnected through the teachers' Cariño (authentic care and respect). Cariño extended beyond simple affection or kindness; it required recognizing students as whole individuals with unique personal and cultural identities. Teachers in the study emphasized that this care had to address students' emotional, psychological, and academic well-being, creating an environment where students felt respected, valued, and a sense of belonging. This comprehensive approach to care was a hallmark of Cariño in education.
This study's limitations include its focus on a single STEAM-centric school with ideal conditions for integrating engineering into core subjects, limiting generalizability to schools with different resources, larger class sizes, or non-STEAM focuses. It also overlooked student and family perspectives, despite the critical role of family support in shaping engagement. Methodological challenges, such as unclear artifact submission guidance and disruptions due to teacher schedules, further impacted data collection. Transferability depends on understanding unique enabling factors, such as Cariño, a pedagogy of authentic care emphasizing students' academic and community success. While effective in this setting, its broader applicability to diverse schools with varying dynamics remains uncertain. Future research should explore diverse schools, investigate family involvement in STEAM education, and address methodological gaps to better capture cultural practices and teacher perspectives, ultimately broadening the reach of engineering-based learning.