Transport properties of unrestricted carriers in bridge-channel MoS2 field-effect transistors

Rethinking the Characterization of Nanoscale Field-Effect Transistors: A Universal Approach

Standard methods for calculating transport parameters in nanoscale field-effect transistors (FETs), namely carrier concentration and mobility, require a linear connection between the gate voltage and channel conductance; however, this is often not the case. One reason often overlooked is that shifts in chemical and electric potential can partially compensate each other, commonly referred to as quantum capacitance. In nanoscale FETs, capacitance is often unmeasurable and an analytical formula is required, which assumes the conducting channel as metallic and common methods of determining threshold voltage no longer couple properly into transport equations. As present and future FET structures become smaller and have increased channel-gate coupling, this issue will render standard methods impossible to use. This work discusses the validity of common methods of characterization for nanoscale FETs, develops a universal model to determine transport properties by only measuring the threshold voltage of an FET and presents a new parameter to easily classify FETs as either quantum capacitance-limited or metallic approximated charge transport. Also considered in this work is electrical hysteresis from trap states and, in combination with the proposed universal model, novel techniques are introduced to measure and remove the errors associated with these effects often ignored in literature.

Locally Gated SnS2/hBN Thin Film Transistors with a Broadband Photoresponse

Next-generation flexible and transparent electronics demand newer materials with superior characteristics. Tin dichalcogenides, Sn(S,Se)₂, are layered crystal materials that show promise for implementation in flexible electronics and optoelectronics. They have band gap energies that are dependent on their atomic layer number and selenium content. A variety of studies has focused in particular on tin disulfide (SnS₂) channel transistors with conventional silicon substrates. However, the effort of interchanging the gate dielectric by utilizing high-quality hexagonal boron nitride (hBN) still remains. In this work, the hBN coupled SnS₂ thin film transistors are demonstrated with bottom-gated device configuration. The electrical transport characteristics of the SnS₂ channel transistor present a high current on/off ratio, reaching as high as 10⁵ and a ten-fold enhancement in subthreshold swing compared to a high-κ dielectric covered device. We also demonstrate the spectral photoresponsivity from ultraviolet to infrared in a multi-layered SnS₂ phototransistor. The device architecture is suitable to promote diverse studied on flexible and transparent thin film transistors for further applications.

Electrical and photovoltaic properties of residue-free MoS2 thin films by liquid exfoliation method

Molybdenum disulfide (MoS₂) film fabricated by a liquid exfoliation method has significant potential for various applications, because of its advantages of mass production and low-temperature processes. In this study, residue-free MoS₂ thin films were formed during the liquid exfoliation process and their electrical properties were characterized with an interdigitated electrode. Then, the MoS₂ film thickness could be controlled by centrifuge condition in the range of 20 ∼ 40 nm, and its carrier concentration and mobility were measured at about 7.36 × 10¹⁶ cm^-3 and 4.67 cm² V^-1 s^-1, respectively. Detailed analysis on the films was done by atomic force microscopy, Raman spectroscopy, and high-resolution transmission electron microscopy measurements for verifying the film quality. For application of the photovoltaic device, a Au/MoS₂/silicon/In junction structure was fabricated, which then showed power conversion efficiency of 1.01% under illumination of 100 mW cm^-2.

Selective control of electron and hole tunneling in 2D assembly

Recent discoveries in the field of two-dimensional (2D) materials have led to the demonstration of exotic devices. Although they have new potential applications in electronics, thermally activated transport over a metal/semiconductor barrier sets physical subthermionic limitations. The challenge of realizing an innovative transistor geometry that exploits this concern remains. A new class of 2D assembly (namely, "carristor") with a configuration similar to the metal-insulator-semiconductor structure is introduced in this work. Superior functionalities, such as a current rectification ratio of up to 400,000 and a switching ratio of higher than 10⁶ at room temperature, are realized by quantum-mechanical tunneling of majority and minority carriers across the barrier. These carristors have a potential application as the fundamental building block of low-power consumption electronics.