Design, Modeling and Analysis of Non-classical Field Effect Transistors

Transistor scaling following per Moore\u27s Law slows down its pace when entering into nanometer regime where short channel effects (SCEs), including threshold voltage fluctuation, increased leakage current and mobility degradation, become pronounced in the traditional planar silicon MOSFET. In addition, as the demand of diversified functionalities rises, conventional silicon technologies cannot satisfy all non-digital applications requirements because of restrictions that stem from the fundamental material properties. Therefore, novel device materials and structures are desirable to fuel further evolution of semiconductor technologies. In this dissertation, I have proposed innovative device structures and addressed design considerations of those non-classical field effect transistors for digital, analog/RF and power applications with projected benefits. Considering device process difficulties and the dramatic fabrication cost, application-oriented device design and optimization are performed through device physics analysis and TCAD modeling methodology to develop design guidelines utilizing transistor\u27s improved characteristics toward application-specific circuit performance enhancement. Results support proposed device design methodologies that will allow development of novel transistors capable of overcoming limitation of planar nanoscale MOSFETs. In this work, both silicon and III-V compound devices are designed, optimized and characterized for digital and non-digital applications through calibrated 2-D and 3-D TCAD simulation. For digital functionalities, silicon and InGaAs MOSFETs have been investigated. Optimized 3-D silicon-on-insulator (SOI) and body-on-insulator (BOI) FinFETs are simulated to demonstrate their impact on the performance of volatile memory SRAM module with consideration of self-heating effects. Comprehensive simulation results suggest that the current drivability degradation due to increased device temperature is modest for both devices and corresponding digital circuits. However, SOI FinFET is recommended for the design of low voltage operation digital modules because of its faster AC response and better SCEs management than the BOI structure. The FinFET concept is also applied to the non-volatile memory cell at 22 nm technology node for low voltage operation with suppressed SCEs. In addition to the silicon technology, our TCAD estimation based on upper projections show that the InGaAs FinFET, with superior mobility and improved interface conditions, achieve tremendous drive current boost and aggressively suppressed SCEs and thereby a strong contender for low-power high-performance applications over the silicon counterpart. For non-digital functionalities, multi-fin FETs and GaN HEMT have been studied. Mixed-mode simulations along with developed optimization guidelines establish the realistic application potential of underlap design of silicon multi-Fin FETs for analog/RF operation. The device with underlap design shows compromised current drivability but improve analog intrinsic gain and high frequency performance. To investigate the potential of the novel N-polar GaN material, for the first time, I have provided calibrated TCAD modeling of E-mode N-polar GaN single-channel HEMT. In this work, I have also proposed a novel E-mode dual-channel hybrid MIS-HEMT showing greatly enhanced current carrying capability. The impact of GaN layer scaling has been investigated through extensive TCAD simulations and demonstrated techniques for device optimization

Comparison of Silicon-on-Insulator and Body-on-Insulator FinFET Based Digital Circuits with Consideration on Self-Heating Effects

In summary, we have compared SOI and BOI FinFET device characteristics and the performance of digital circuits designed with those devices. For low voltage supply, SHE is modest in both devices and during digital circuit operations. SOI FinFET CMOS inverter and SRAM cell characteristics are very similar to BOI ones. Considering the lesser fabrication complexity, SOI FinFET thus would be more preferable than BOI FinFET for the design of low voltage digital circuits

Pre-Peak Deformation and Damage Features of Sandstone under Cyclic Loading

In this paper, several sandstone specimens are prepared and subjected to uniaxial compression and cyclic loading. For each specimen, the loading segment of the stress-strain curve was fitted, and the peak slope of this segment was taken as the elastic modulus of the specimen in that cycle. It is learned that, under cyclic loading, the elastic modulus of each specimen increased with the growing number of load cycles, and tended to be stable; meanwhile, strain hardening was observed on all specimens. Moreover, the specimens are similar in corresponding stress, although varied in corresponding strain. In the same cycle, the tangent modulus of the loading phase was smaller than that of the unloading phase under the same stress. Finally, the damage variables of sandstone specimens under cyclic loading were defined from the angle of energy, revealing that the damage variables had logarithmic growth with the load cycles in the later stage

Hysteresis Characteristics of Brittle Rock Deformation under Constant Load Cyclic Loading and Unloading

This paper mainly explores the deformation characteristics of limestone specimens under constant load cyclic loading. For limestone specimens under uniaxial compression, the stress-strain curve can be divided into three stages: compaction stage, elastic stage and sudden failure stage. Under cyclic loading, the hysteresis loop on the stress-strain curve is long and thin, taking the shape of "toothpicks". The axial strain and radial strain both change with the stress amplitude and cycle number, but in different variation patterns. There is a stress amplitude "threshold" for radial deformation, indicating that the radial deformation is more sensitive to stress amplitude than the axial deformation. It is calculated that the incremental deformation between peaks includes both plastic deformation and the deformation recoverable after unloading, and the recoverable deformation is positively correlated with the load amplitude of the cyclic loading

Giant magnetic quantum oscillations in the thermal conductivity of TaAs: Indications of chiral zero sound

Charge transport of topological semimetals has been in the focus of intensive investigations because of their non-trivial band topology. Heat transport of these materials, on the other hand, is largely unexplored and remains elusive. Here we report on an observation of unprecedented, giant magnetic quantum oscillations of thermal conductivity in the prototypical Weyl semimetal TaAs. The oscillations are antiphase with the quantum oscillating electronic density of states of a Weyl pocket, and their amplitudes amount to two orders of magnitude of the estimation based on the Wiedemann-Franz law. Our analyses show that all the conventional heat-transport mechanisms through diffusions of propagating electrons, phonons and electron-hole bipolar excitations, are far inadequate to account for these phenomena. Taking further experimental facts that the parallel field configuration favors much higher magneto-thermal conductivity, we propose that the newly proposed chiral zero sound provides a reasonable explanation to these exotic phenomena. More work focusing on other topological semimetals along the same line is badly called for.Comment: 15 pages, 5 figure

Teachers’ Perceptions of Online Teaching Do Not Differ across Disciplines: A Survey

Since the outbreak of COVID-19, online teaching has been widely practiced. Ensuring the quality and efficiency of online teaching has become an important research topic. Teachers’ views of online teaching directly affect the quality of instruction. The study aimed to understand whether there are differences in the basic perceptions of online teaching among teachers in different disciplines. Through a web-based questionnaire, the researchers surveyed 198 teachers from different disciplines about their perceptions of online teaching. The research method was a convergent mixed-method design. SPSS 22.0 was used to analyze quantitative data, and qualitative data were analyzed using NVivo 11. The results showed significant differences in the attitudes of teachers to adopt online teaching as the norm in different disciplines. Social science teachers preferred online education not to be the norm, while natural science teachers preferred online education to be the norm. In addition, there was little difference in the perceptions of online teaching among teachers of different disciplines. Most of them pointed out the problem of interactive communication in online teaching and gave suggestions about it. Online teaching has value, but there is still much room for improvement. It is necessary to strengthen the construction of facilities for online education, consider the characteristics of disciplines, and train teachers in teaching methods, learning psychology, and technology