Browsing by Subject "Transistor"
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Item CVD MoS₂ for high speed devices and circuits(2018-05-03) Sanne, Atresh Murlidhar; Banerjee, Sanjay; Akinwande, Deji; Register, Leonard F; Sreenivasan, S.V.; Rao, RajeshTwo-dimensional layered materials (2DLMs) have been widely studied as a potential alternative to the complementary metal-oxide semiconducting field-effect transistor (CMOS FET) "switch." The atomically thin body of 2DLMs lends itself to improved electrostatic gate control, leading to a suppression of the short channel effects which limit the scalability of CMOS devices. While many experiments have examined 2DLMs as a low power solution for aggressively scaled digital devices, their feasibility study for use in high speed radio frequency (RF) devices and circuits is still in its infancy. Current technological trends such as the Internet of Things (IoT) and 5G communication have increased the demand for novel high speed devices to serve next-generation circuits and systems. Graphene, as a 2DLM, has garnered significant interest for its use in high speed radio frequency (RF) devices and circuits. A carrier mobility greater than 10,000 cm²/Vs, ambipolar transport, and excellent thermomechanical stability has afforded graphene cutoff frequencies greater than 400 GHz. Multi-transistor integrated circuits, including a fully integrated RF receiver have been demonstrated using graphene. However, graphene poses a limitation in high speed operation in that the Dirac cone band structure results in a zero bandgap, leading to semi-metallic transport behavior. As a result, graphene field-effect transistors (GFETs) exhibit a low ION/IOFF ratio and non-saturating output behavior. This translates to FETs showing reduced power and voltage gains, hindering the realization of high performance amplifiers, mixers, and other RF circuit elements. Another class of 2DLMs has generated renewed interest for its potential to replace silicon as the next-generation CMOS "switch." Transition metal dichalcogenides (TMDs) is a family of 2DLMs with the general chemical formula MX₂ (M = metal, X = chalcogen). Of the class of TMDs, molybdenum disulfide (MoS₂) is of special interest. With its thickness-dependent electronic properties, MoS₂ has been considered for applications in the fields of opto-electronics, flexible electronics, spintronics, and coupled electro-mechanics. Its single layer direct bandgap of ~1.8 eV allows for high ION/IOFF metal-oxide semiconducting FETs. More relevant for RF applications, theoretical studies predict MoS₂ can afford saturation velocities greater than 3×10⁶ cm/s. While the mobility of MoS₂ is lower than that of graphene, the intrinsic bandgap in MoS₂ has shown voltage gains, A [subscript v] = g [subscript m] /g [subscript ds], greater than 30. Thus far, most of the studies of graphene and MoS₂ have utilized crystalline exfoliated layers, which provide a convenient high quality source of material for laboratory experiments. However, for industrial scale applications, the mechanical cleavage process is not scalable and, thus far, there have been few studies on large area chemical vapor deposited (CVD) MoS₂ RF FETs. In this dissertation, the initial efforts to utilize CVD MoS₂ for RF FETs are presented. The RF figures-of-merit transit frequency, fT, and maximum frequency of oscillation are measured for CVD MoS₂. The effects of different substrates and superstrates on MoS₂ are investigated. In order to improve the cutoff frequencies, a combination of channel length scaling and device geometry modifications are applied. Simple RF circuits are demonstrated experimentally using CVD MoS₂ FETs. Additionally larger circuit building blocks are simulated using experimental data. The goal of this work is to provide a baseline of the RF performance achievable using CVD MoS₂. Hopefully, this work will motivate future studies directing MoS₂ towards industrial electronic applications.Item Electronic properties and electron-electron interaction effects in transition metal dichalcogenides(2018-08-09) Larentis, Stefano; Tutuc, Emanuel, 1974-; Banerjee, Sanjay K; MacDonald, Allan H; Register, Leonard F; Shi, LiTransition metal dichalcogenides (TMDs) are a new class of two-dimensional layered materials characterized by a MX₂ chemical formula, where M (X) stands for a transition metal (chalcogen). MoS₂, MoSe₂ and MoTe₂ are semiconducting TMDs, which at the monolayer limit possess bandgaps >1 eV, rendering them attractive as possible channel material for scaled transistors. The bandstructures of monolayers feature coupled spin and valley degrees of freedom, thanks to large spin-orbit interaction, and large effective masses (m*), suggesting that electron-electron interaction effects are expected to be important in these semiconductors. In this dissertation we discuss the fabrication and electrical characterization of TMD-based electronic devices, with a focus on their electronic properties, including scattering mechanisms contributing to the mobility, carriers' effective mass, band offset in heterostructures, electronic compressibility, and spin susceptibility. We begin studying the four-point field-effect mobilities of few-layers MoS₂, MoSe₂ and MoTe₂ field effect transistors (FETs), in top-contact, bottom-gate architectures. Using hexagonal boron-nitride dielectrics, we fabricate FETs with an improved bottom-contact, dual-gate architecture to probe transport at low temperatures in monolayer MoS₂, and mono- and bilayer MoSe₂. From conductivity and carrier density measurements we determine the Hall mobility, which shows strong temperature dependence, consistent with phonon scattering, and saturates at low temperatures because of impurity scattering. High mobility MoSe₂ samples probed in perpendicular magnetic field, at low temperatures show Shubnikov-de Haas oscillations. Using magnetotransport we probe carriers in spin split bands at the K point in the conduction band and extract their m* = 0.8m [subscript e]; m [subscript e] is the bare electron mass. Quantum Hall states emerging at either odd or even filling factors are explained by a density dependent, interaction enhanced Zeeman splitting. Gated graphene-MoS₂ heterostructures reveal a saturating electron branch conductivity at the onset of MoS₂ population. Magnetotransport measurements probe the graphene electron density, which saturates and decreases as MoS₂ populates, a finding associated with the negative compressibility of MoS₂ electrons, modeled by a decreasing chemical potential, where many-body contributions dominate. Using a multi-gate architecture in monolayer MoTe₂ FETs, that allows for independent contact resistance and threshold voltage tuning, we integrate reconfigurable n- and p-FETs, and demonstrate a complementary inverter.Item III-V MOSFETs from planar to 3D(2013-08) Xue, Fei, active 2013; Lee, Jack Chung-YeungSi complementary metal-oxide-semiconductor (CMOS) technology has been prospered through continuously scaling of its feature size. As scaling is approaching its physical limitations, new materials and device structures are expected. High electron mobility III-V materials are attractive as alternative channel materials for future post-Si CMOS applications due to their outstanding transport property. High-k dielectrics/metal gate stack was applied to reduced gate leakage current and thus lower the power dissipation. Combining their benefits, great efforts have been devoted to explore III-V/high-k/metal metal-oxide-semiconductor field-effect-transistors (MOSFETs). The main challenges for III-V MOSFETs include interface issues of high-k/III-V, source and drain contact, silicon integration and reliability. A comprehensive study on III-V MOSFETs has been presented here focusing on three areas: 1) III-V/high-k/metal gate stack: material and electrical properties of various high-k dielectrics on III-V substrates have been systematically examined; 2) device architecture: device structures from planar surface channel MOSFETs and buried channel quantum well FETs (QWFETs) to 3D gate-wrapped-around FETs (GWAFETs) and tunneling FETs (TFETs) have been designed and analyzed; 3) fabrication process: process flow has been set up and optimized to build scaled planar and 3D devices with feature size down to 40nm. Potential of high performances have been demonstrated using novel III-V/high-k devices. Effective channel mobility was significantly improved by applying buried channel QWFET structure. Short channel effect control for sub-100nm devices was enhanced by shrinking gate dielectrics, reducing channel thickness and moving from 2D planar to 3D GWAFET structure. InGaAs TFETs have also been developed for ultra-low power application. This research work demonstrates that III-V/high-k/metal MOSFETs with superior device performances are promising candidates for future ultimately scaled logic devices.Item Nanoscale graphene for RF circuits and systems(2013-08) Parrish, Kristen Nguyen; Akinwande, DejiIncreased challenges in CMOS scaling have motivated the development of alternatives to silicon circuit technologies, including graphene transistor development. In this work, we present a circuit simulator model for graphene FETs, developed to both fit measured data and predict new behaviors, motivating future research. The model is implemented in Agilent ADS, a circuit level simulator that is commonly used for non-standard transistor technologies, for use with parameter variation analyses, as well as easy integration with CMOS design kits. We present conclusions drawn from the model, including analyses on the effects of contact resistance and oxide scaling. We have also derived a quantum-capacitance limited model, used to intuit intrinsic behaviors of graphene transistors, as well as outline upper bounds on performance. Additionally, the ideal frequency doubler has been examined and compared with graphene, and performance limits for graphene frequency multipliers are elucidated. Performance as a demodulator is also discussed. We leverage this advancement in modeling research to advance circuit- and system-level research using graphene transistor technology. We first explore the development of a GHz planar carbon antenna for use on an RF frontend. This research is further developed in work towards the first standalone carbon radio on flexible plastics. A front end receiver, comprised of an integrated carbon antenna, transmission lines, and a graphene transistor for demodulation, are all fabricated onto one plastic substrate, to be interfaced with speakers for a full radio demo. This complete system will motivate further research on graphene-on-plastic systems.Item Solid electrolyte substrates for two-dimensional transition metal dichalcogenide growth, transistors and circuits(2021-08-13) Alam, Md Hasibul; Akinwande, Deji; Banerjee, Sanjay K; Dodabalapur, Ananth; Incorvia, Jean Anne; Lai, KejiThe high surface charge carrier densities, accumulated by the electrostatic gating of two-dimensional (2D) materials with ionic liquids (ILs), have often been exploited in 2D transistors and devices. However, the intrinsic liquid nature, sensitivity to humidity, and the stress induced in frozen liquids prevent them from forming an ideal platform for electrostatic gating and surface probe techniques. This dissertation reports a lithium-ion (Li-ion) solid electrolyte substrate (or simply Li-ion glass) alternative to ILs, by demonstrating its application in high-performance transistors and circuits using 2D transition metal dichalcogenide (TMD). The back-gated n-type MoS² and p-type WSe² transistors resulted in sub-threshold values approaching the ideal limit of 60 mV/dec while maintaining a high ON/OFF ratio (> 10⁶) and a complementary inverter amplifier gain of 34 under a 1 V supply, the highest among comparable solid-state amplifiers. Microscopic studies using microwave impedance microscopy clearly show a uniform and homogeneous channel formation, indicating a smooth interface between the TMD and the underlying electrolytic substrate. This dissertation also reports the direct growth of few-layer (3-4L) single-crystal MoS² on lithium-ion solid electrolyte substrate by chemical vapor deposition (CVD) and demonstrates efficient gate control in the as-grown crystal via electrolytic gating. The gating efficiency of the transistors fabricated on the as-grown crystals, and back-gated by the solid electrolyte, are comparable to the devices with exfoliated and transferred material with an additional gain in mobility value. Field-effect mobility in the range of 42-49 cm²V⁻¹s⁻¹ with current densities as high as 120 μA/μm with 0.5 μm channel length has been achieved, as expected from devices free from material transfer-related damage and impurity. This CVD growth method can potentially be extended for other 2D TMDs to realize high-mobility transistors and study intrinsic device properties. To sum up, the dissertation demonstrates solid electrolytes as an ideal platform for 2D TMD synthesis, advanced thin-film transistors, and circuits, otherwise difficult to achieve with liquid electrolytes. The results, therefore, further establish solid electrolytes as a promising alternative to ILs for surface science experiments and advanced thin-film devices. Finally, based on the work presented in this dissertation, some future research directions have been proposed