Electrical Transport Mechanisms in Ultra-Thin Field Effect Transistors: A Comprehensive Analysis of Material, Device Structure, and External Influences

Abstract

Over the past several decades, the relentless miniaturization of transistors has driven remarkable advancements, reducing energy consumption per switching event and significantly boosting the computational power of integrated circuits. Device architectures have evolved from Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) to Fully Depleted Silicon-On-Insulator (FDSOI) devices, Fin Field-Effect Transistors (FinFETs), and, most recently, Gate-All-Around (GAA) transistors. These innovations have been crucial in pushing the limits of physical scaling, enabling continued performance improvements even as fundamental physical limits are approached. However, as silicon technology nears these limitations, the traditional scaling approach outlined by Moore’s Law is coming to an end. This has shifted attention toward alternative materials capable of supporting further miniaturization. Among these, two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) and graphene nanoribbons have emerged as promising candidates for the next generation of ultra-scaled transistors. Unlike conventional bulk silicon, 2D materials—particularly TMDs—demonstrate exceptional electrical properties even at atomic thicknesses. They offer high charge carrier mobility while maintaining substantial electrostatic gate control over the channel, which is crucial for mitigating short-channel effects and reducing power leakage in extremely scaled devices. Recent research has focused on large-scale synthesis, contact engineering, and the integration of high-k dielectrics in 2D FETs, contributing to major advancements in the field. Despite these advancements, TMD-based devices continue to display poorly understood characteristics, including Fermi level pinning, unintentional doping, and ambipolar behavior, which are intricately linked and observed in both n-type and p-type TMD-based FETs. However, conventional interpretations have explained these complex properties through a single factor rather than addressing them comprehensively, which has limited precise understanding of their true nature. This thesis begins by addressing the limitations of existing Schottky barrier extraction methods that often emphasize the strong Fermi level pinning effect in ultra-thin TMD FETs. In these channels, the conduction mechanism transitions from thermionic emission to tunneling current as the gate electric field increases, and the Schottky barrier is typically extracted during this transition point. However, under varying temperature conditions, the flat band shift can cause an overestimation of the barrier height. To correct for these temperature-dependent inaccuracies, a refined Schottky barrier extraction model has been developed, providing more accurate barrier height values for 2D FETs (Chapter 2). We further explore the Schottky barrier effect in ultra-thin body FETs, where the depletion region is constrained by the physical limitation of the channel thickness. Theoretically, the Schottky barrier in ultra-thin bodies leads to a redistribution of charge due to the energy difference between the semiconductor and the metal. This energy difference pulls charge carriers into the ultra-thin channel, creating an accumulation region near the electrode. This effect helps explain why high work-function metals, such as Pd and Ni, can result in high on-current, particularly when deposition-induced defects are minimized (Chapter 3). Based on these studies, we focused more closely on the body effect, which can have a greater impact than the interpretation of contact resistance caused by strong Fermi level pinning, particularly in cases where the body effect persists regardless of channel length and could potentially be extracted alongside contact resistance (e.g., through methods like the TLM). In ultra-thin bodies, where the surface-to-volume ratio is significantly reduced, making them highly sensitive to external conditions, we theoretically predicted the current changes that occur due to the body effect. Additionally, in our experiments, we confirmed the existence of additional resistance attributed to substrate effects. We also predicted that when the body effect is present, trap-limited conduction could occur, where current flows through traps in the depletion region. This was confirmed through electrical analysis, validating the role of traps in influencing the conduction mechanism under these conditions (Chapter 4). This naturally leads to the development of a unified trap-limited conduction model, which integrates Poole-Frenkel (P-F) emission and trap-limited space charge-limited conduction (T- SCLC) within the Shockley-Read-Hall (SRH) framework. The difference in total trap-limited conduction between pristine MoS2 and Nb-substituted p-doped MoS2 was found to be approximately two orders of magnitude. Previous models lacked the capability to comprehensively analyze both high-injection and low-injection regimes. The new model provides a thorough understanding of how trapped and free carriers interact dynamically under electric fields, particularly in cases of low-injection and high-injection conditions (Chapter 5). Based on the previously mentioned evidence, we further investigated in detail how dielectric materials influence the electrical properties of TMD channels. We designed three modes using h-BN as a dielectric spacer: (1) Direct-contact mode, where the channel is in direct contact with the dielectric material; (2) Proximity mode, where a 3 nm h-BN spacer was introduced to allow charge transport but minimize orbital mixing and strain from the dielectric surface; and (3) Noncontact mode, where a 10 nm h-BN spacer was used to fully prevent any interaction between the channel and the substrate. By comparing the results from these modes, we demonstrated the precise effects of the substrate on the channel properties (Chapter 6). This thesis explores the interplay between material properties and device architectures in ultra-thin TMD-based FETs, focusing on the significant impact of the body effect, trap-limited conduction, and dielectric interactions on device performance. Through a combination of experimental and theoretical analyses, we proposed comprehensive optimization strategies to address challenges such as high contact resistance and complex conduction mechanisms in 2D semiconductor devices. By addressing unresolved questions and offering insights into these complex properties, this study provides a pathway to further improve the performance of TMD-based devices.