Low Dimension Materials for Extend/Beyond CMOS Applications
Abstract
2-dimensional layered materials, transition metal dichalcogenides (TMDs) are considered promising candidates for the post-Si era. They have attracted wide attention from both fundamental and applied research communities, owing to their unique material properties and potential applications including logic and memory devices. The key reason that TMD materials are excellent channel material choices for logic devices is that their inherent layered-structures allowing uniform thickness control with atomic-level precision and can reach the ultimate monolayer thickness limit of ~ 0.7nm, which is desirable for the implementation of ultra-scaled devices to maintain excellent electrostatic control. In this thesis, I will first focus on a unique contact gating on TMD-based Schottky-barrier transistors and discuss in great detail the impacts of current injection from source/drain contacts to channel regions in a typical back-gated structure. I will reveal a commonly observed overestimation of TMD mobility when substantially large contact resistance has a strong gate dependence, and suggest that an appropriate method is necessary for precise mobility extraction. For extending CMOS beyond Si, various extrinsic doping schemes are investigated to achieve a much-reduced contact resistance (RC) on tungsten diselenide (WSe2) which is associated with the Schottky-barrier height. Devices implemented on ultra-thin gate dielectric are then explored on tungsten disulfide (WS2) with various TMD body thicknesses (TCH), with an excellent off-state behavior and on-state performance being achieved on properly selected TCH based on our statistics. The observed ultra-low RC and record-high on-state current at a much-scaled overdrive (VOV) highlights the advantages and necessities of exploiting ultra-thin gate dielectric on TMD-based electronics. The second part of the thesis focuses on beyond CMOS applications including reconfigurable logics and tunneling field-effect transistors (TFET). Owing to ultra-thin TCH in low dimensional materials, the triple-gate design allows efficient electrostatic control on WSe2 and carbon nanotube (CNT) by creating various doping types and concentrations within different channel segments. Therefore, a single triple-gate device can operate as a metal oxide semiconductor field-effect transistor (MOSFET), a TFET, or a diode depending on the choice of the gate biases. In the TFET mode, a steep subthreshold slope less than 60 mV/dec has been achieved in the CNT devices at room temperature. To extend the applications from device to circuits, the capability of modulating the device on-state performance is utilized in a WSe2CMOSbased inverter and static random access memory (SRAM) design to improve the read and write stability and achieve a low voltage operation.
Degree
Ph.D.
Advisors
Chen, Purdue University.
Subject Area
Computer science|Electrical engineering
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