A. James Clark School of Engineering

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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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    Challenges of Overcoming Defects in Wide Bandgap Semiconductor Power Electronics
    (MDPI, 2021-12-22) Setera, Brett; Christou, Aristos
    The role of crystal defects in wide bandgap semiconductors and dielectrics under extreme environments (high temperature, high electric and magnetic fields, intense radiation, and mechanical stresses) found in power electronics is reviewed. Understanding defects requires real-time in situ material characterization during material synthesis and when the material is subjected to extreme environmental stress. Wide bandgap semiconductor devices are reviewed from the point of view of the role of defects and their impact on performance. It is shown that the reduction of defects represents a fundamental breakthrough that will enable wide bandgap (WBG) semiconductors to reach full potential. The main emphasis of the present review is to understand defect dynamics in WBG semiconductor bulk and at interfaces during the material synthesis and when subjected to extreme environments. High-brightness X-rays from synchrotron sources and advanced electron microscopy techniques are used for atomic-level material probing to understand and optimize the genesis and movement of crystal defects during material synthesis and extreme environmental stress. Strongly linked multi-scale modeling provides a deeper understanding of defect formation and defect dynamics in extreme environments.
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    Novel Materials and Structures for Wide and Ultra-Wide Bandgap Semiconductor Switches
    (2018) Shahin, David Issa; Christou, Aristos; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Semiconductor power switches are necessary for the deployment of next-generation electrical systems, including renewable energy generators, electric vehicle drivetrains, and high-power communications systems. Current silicon-based technologies are limited by insufficient blocking voltages due to bandgap limitations and processing-induced defects, undesirably high on-state resistances due to gate charge trapping at poorly understood dielectric/semiconductor interfaces, and limited reliability due to electrical and thermal failure under aggressive operating conditions. As such, new materials and device architectures are required to achieve previously unattained power, efficiency, and reliability. This dissertation identifies and investigates material candidates and demonstrates their incorporation into new device architectures for power switches. Wide bandgap (WBG) semiconductors such as GaN, and ultra-wide bandgap (UWBG) semiconductors such as Ga2O3 and diamond are employed to address the previously stated limitations. Gate charge trapping in these systems is addressed through use of high-k dielectrics not previously employed for WBG and UWBG switches. ZrO2 and HfO2 dielectrics are presented as candidates for dielectric and interface charge tuning on GaN and Ga2O3, thereby allowing the possibility of threshold voltage manipulation and normally-off behavior in WBG and UWBG switches. Fabrication technologies for WBG and UWBG switches are also reported. Normally-on and -off AlGaN/GaN MOS-HEMTs with threshold voltages between -3 to +4 V are demonstrated through a combination of ZrO2 dielectric selection and AlGaN recess etching. Design and processing for normally-off vertical GaN MOSFETs are also developed, with emphasis on critical challenges in fabricating these devices. Additionally, the fabrication and stability of hydrogen-terminated diamond switches with Al2O3 surface transfer dopants are reported. Finally, new materials and processes for improved electrical and thermal stability in power switches are demonstrated. TiN is presented as a reliable gate electrode for AlGaN/GaN HEMTs, imparting superior resistance to reverse gate bias electrical stress and temperatures up to 800 °C that otherwise destroyed conventional Ni/Au-gated HEMTs. A novel process for plasma-free selective area etching of nanocrystalline diamond heat spreading films is also presented, which promises to avoid plasma damage to the underlying semiconductor and enables etching of diamond films along features inaccessible to a typical plasma-based process.
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    INVESTIGATION OF RELIABILITY IN GALLIUM NITRIDE HIGH ELECTRON MOBILITY TRANSISTORS USING EQUIVALENT CIRCUIT MODELS FOR USE IN HIGH POWER, HIGH FREQUENCY MICROWAVE AMPLIFIERS
    (2010) Huebschman, Benjamin David; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Gallium Nitride (GaN) is beginning to emerge as an alternative to the Gallium Arsenide in high power, high frequency microwave communications. Other novel semiconductors show potential at higher frequency applications. The largest obstacles to GaN emerging as the dominant microwave semiconductor are the issue of cost, which could be reduced through volume, and question of reliability. A new approach to the analysis of reliability has been developed based on the periodic generation of equivalent circuit models while a device is stressed in a manner that is similar to performance likely to be seen during commercial operation. Care was made in this research to ensure that the stress measurements used to induce degradation are as close as possible to those that would degrade a device in real world applications. Equivalent circuit models (ECM) can be used to simulate a device in computer aided design (CAD) software, but these models also provide a picture of the physical properties within the device at a specific point in time. The periodic generation of ECMs allows the researcher to understand the physical changes in the device over time by performing non-destructive electronic measurements. By analyzing the changes in device performance, the physical mechanism of device degradation can be determined. A system was developed to induce degradation and perform measurements of sufficient detail to produce a large signal ECM. Software for producing the ECM was also created. The changes in the ECM were analyzed to diagnose the physical changes in the device under test (DUT) and to identify a method of degradation. The information acquired from this system can be used to improve the device manufacturing process at the foundry. It can also be used to incorporate device degradation into the operation of systems.