Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of atomic-scale thickness, direct bandgap, strong spin–orbit coupling and favourable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examined and their properties are discussed, with particular attention to their charge density wave, superconductive and topological phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties. Two-dimensional transition metal dichalcogenides (TMDCs) exhibit attractive electronic and mechanical properties. In this Review, the charge density wave, superconductive and topological phases of TMCDs are discussed, along with their synthesis and applications in devices with enhanced mobility and with the use of strain engineering to improve their properties.

2D transition metal dichalcogenides

The compelling demand for higher performance and lower power consumption in electronic systems is the main driving force of the electronics industry's quest for devices and/or architectures based on new materials. Here, we provide a review of electronic devices based on two-dimensional materials, outlining their potential as a technological option beyond scaled complementary metal-oxide-semiconductor switches. We focus on the performance limits and advantages of these materials and associated technologies, when exploited for both digital and analog applications, focusing on the main figures of merit needed to meet industry requirements. We also discuss the use of two-dimensional materials as an enabling factor for flexible electronics and provide our perspectives on future developments.

Electronics based on two-dimensional materials

Physical Review B

International Technology Roadmap for Semiconductors 2003の要求清浄度について － シリコンウエハ表面と雰囲気環境に要求される清浄度, 分析方法の現状について －

In the quest for higher performance, the dimensions of field-effect transistors (FETs) continue to decrease. However, the reduction in size of FETs comprising 3D semiconductors is limited by the rate at which heat, generated from static power, is dissipated. The increase in static power and the leakage of current between the source and drain electrodes that causes this increase, are referred to as short-channel effects. In FETs with channels made from 2D semiconductors, leakage current is almost eliminated because all electrons are confined in atomically thin channels and, hence, are uniformly influenced by the gate voltage. In this Review, we provide a mathematical framework to evaluate the performance of FETs and describe the challenges for improving the performances of short-channel FETs in relation to the properties of 2D materials, including graphene, transition metal dichalcogenides, phosphorene and silicene. We also describe tunnelling FETs that possess extremely low-power switching behaviour and explain how they can be realized using heterostructures of 2D semiconductors. Field-effect transistors (FETs) with semiconducting channels made from 2D materials are known to have fewer problems with short-channel effects than devices comprising 3D semiconductors. In this Review, a mathematical framework to evaluate the performance of FETs is outlined with a focus on the properties of 2D materials, such as graphene, transition metal dichalcogenides, phosphorene and silicene.

Two-dimensional semiconductors for transistors

Monolayer graphene exhibits exceptional electronic and mechanical properties, making it a very promising material for nanoelectromechanical devices. Here, we conclusively demonstrate the piezoresistive effect in graphene in a nanoelectromechanical membrane configuration that provides direct electrical readout of pressure to strain transduction. This makes it highly relevant for an important class of nanoelectromechanical system (NEMS) transducers. This demonstration is consistent with our simulations and previously reported gauge factors and simulation values. The membrane in our experiment acts as a strain gauge independent of crystallographic orientation and allows for aggressive size scalability. When compared with conventional pressure sensors, the sensors have orders of magnitude higher sensitivity per unit area.

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Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes

Inverters based on uniaxially tensile strained Si (sSi) nanowire (NW) tunneling field-effect transistors (TFETs) are fabricated. Tilted dopant implantation using the gate as a shadow mask allows self-aligned formation of p-i-n TFETs. The steep junctions formed by dopant segregation at low temperatures improve the band-to-band tunneling, resulting in higher on-currents of n- and p-TFETs of > 10 μA/μm at VDS=0.5 V. The subthreshold slope for n-channel TFETs reaches a minimum value of 30 mV/dec, and is <; 60 mV/dec over one order of magnitude of drain current. The first sSi NW complementary TFET inverters show sharp transitions and fairly high static gain even at very lowVDD=0.2 V. The first transient response analysis of the inverters shows clear output voltage overshoots and a fall time of 2 ns at VDD=1.0 V.

Inverters With Strained Si Nanowire Complementary Tunnel Field-Effect Transistors

In this paper, we have extensively investigated the silicon thickness dependence of the low field electron mobility in ultrathin silicon-on-insulator (UT-SOI) MOSFETs operated both in single- and in double-gate mode. A physically based model including all the scattering mechanisms that are known to be most relevant in bulk MOSFETs has been extended and applied to SOI structures. A systematic comparison with the measurements shows that the experimental mobility dependence on the silicon thickness (T/sub SI/) cannot be quantitatively explained within the transport picture that seems adequate for bulk transistors. In an attempt to improve the agreement with the experiments, we have critically rediscussed our model for the phonon scattering and developed a model for the scattering induced by the T/sub SI/ fluctuations. Our results suggest that the importance of the surface optical (SO) phonons could be significantly enhanced in UT-SOI MOSFETs with respect to bulk transistors. Furthermore, both the SO phonon and the T/sub SI/ fluctuation scattering are remarkably enhanced with reducing T/sub SI/, so that they could help explain the experimental mobility behavior.

Physically based modeling of low field electron mobility in ultrathin single- and double-gate SOI n-MOSFETs

In this paper, and in Part II, Monte Carlo (MC) simulations including quantum corrections to the potential and calibrated scattering models are used to study electronic transport in bulk and double-gate silicon-on-insulator MOSFETs with L/sub G/ down to 14-nm designed according to the 2003 International Technology Roadmap for Semiconductors. Simulations with and without scattering are used to assess the influence of quasi-ballistic transport on the MOSFET on-current. We analyze in detail the flux of back-scattered carriers. The role of scattering in different parts of the device is clarified and the MC results are compared to simple models for quasi-ballistic transport presented in the literature.

Understanding quasi-ballistic transport in nano-MOSFETs: part I-scattering in the channel and in the drain

1. Introduction 2. Bulk semiconductors and the semi-classical model 3. Quantum confined inversion layers 4. Carrier scattering in silicon MOS transistors 5. The Boltzmann transport equation 6. The Monte Carlo method for the Boltzmann transport equation 7. Simulation of bulk and SOI silicon MOSFETs 8. MOS transistors with arbitrary crystal orientation 9. MOS transistors with strained silicon channels 10. MOS transistors with alternative materials Appendix A. Mathematical definitions and properties Appendix B. Integrals and transformations over a finite area A Appendix C. Calculation of the equi-energy lines with the k-p model Appendix D. Matrix elements beyond the envelope function approximation Appendix E. Charge density produced by a perturbation potential.

David Esseni

Papers

Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes

Inverters With Strained Si Nanowire Complementary Tunnel Field-Effect Transistors

Physically based modeling of low field electron mobility in ultrathin single- and double-gate SOI n-MOSFETs

Understanding quasi-ballistic transport in nano-MOSFETs: part I-scattering in the channel and in the drain

Nanoscale MOS Transistors: Semi-Classical Transport and Applications