Candidate: Demin Yin
Title: Theoretical Investigation of Contact Effects on the Performance of 2D-Material Nanotransistors
Date: July 16, 2019
Time: 10:00 AM
Place: EIT 3145
Supervisor(s): Yoon, Young Ki
Two-dimensional (2D) materials have attracted significant attention for electronic device applications since the graphene transistor was demonstrated. 2D materials not only exhibit good mobility and suitable bandgap, but also provide great opportunities for flexible and transparent device applications. However, fabrication of high performance 2D transistors is limited by various factors, including contact properties. In this study, some promising 2D materials, such as black phosphorus (BP) and molybdenum disulfide (MoS2), and their applications in next-generation electronics are studied. In particular, simulation methods for ohmic and Schottky contact 2D-material field-effect transistors (FETs) are discussed in details. Simulation settings in non-equilibrium Green’s function (NEGF) and boundary conditions in the Poisson’s equation are specified respectively. A quantum transport simulator is built to explore the device performance particularly for different types of contacts.
First, the effects of contact resistance (Rc) to high-frequency performance limit of BP FETs are studied using self-consistent quantum simulations. Detailed comparison between intrinsic and extrinsic cut-off frequency (fT) and unity power gain frequency (fmax) are made. Unlike zero-bandgap graphene devices, semiconducting BP FETs exhibit clear saturation behaviors, which is critical for high fmax. It is shown that near THz frequency range of fT and fmax are highly promising for high-frequency applications, which is possible with an aggressive channel length scaling (Lch » 10 nm) along with state-of-the-art Rc. Our benchmarking against the experimental data indicates that there still exists large room for optimization for Rc.
Temperature responses of 2D-material FETs have been reported, which can be categorized into two different trends. To understand this, a model based on the effective mass approximation is utilized, building comprehensive understanding of low-temperature current-voltage measurements of multilayer MoS2 thin-film-transistors (TFTs). Our model suggests the different temperature responses with Schottky and ohmic contacts can result from various aspects such as Schottky barrier height and barrier thickness. We also investigated the distinct device-to-device low-temperature responses in multilayer MoS2 TFTs. Our comprehensive theoretical and experimental studies provide a systematic scheme for the contact properties in 2D material based FETs.
Lastly, future works are suggested. The geometry of the contact and the disorders at the metal-semiconductor interface can be critical factors determining the overall device performance.
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