CVD MoS₂ for high speed devices and circuits




Sanne, Atresh Murlidhar

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Two-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.


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