Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to one-dimensional materials, it is relatively easy to fabricate complex structures from them. The most widely studied two-dimensional material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained silicon films or requires high voltages. Although single layers of MoS(2) have a large intrinsic bandgap of 1.8 eV (ref. 16), previously reported mobilities in the 0.5-3 cm(2) V(-1) s(-1) range are too low for practical devices. Here, we use a halfnium oxide gate dielectric to demonstrate a room-temperature single-layer MoS(2) mobility of at least 200 cm(2) V(-1) s(-1), similar to that of graphene nanoribbons, and demonstrate transistors with room-temperature current on/off ratios of 1 × 10(8) and ultralow standby power dissipation. Because monolayer MoS(2) has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS(2) could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.

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Single-layer MoS2 transistors

Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

Black phosphorus field-effect transistors

We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Band parameters for III–V compound semiconductors and their alloys

We provide a broad review of fundamental electronic properties of two-dimensional graphene with the emphasis on density and temperature dependent carrier transport in doped or gated graphene structures. A salient feature of our review is a critical comparison between carrier transport in graphene and in two-dimensional semiconductor systems (e.g. heterostructures, quantum wells, inversion layers) so that the unique features of graphene electronic properties arising from its gap- less, massless, chiral Dirac spectrum are highlighted. Experiment and theory as well as quantum and semi-classical transport are discussed in a synergistic manner in order to provide a unified and comprehensive perspective. Although the emphasis of the review is on those aspects of graphene transport where reasonable consensus exists in the literature, open questions are discussed as well. Various physical mechanisms controlling transport are described in depth including long- range charged impurity scattering, screening, short-range defect scattering, phonon scattering, many-body effects, Klein tunneling, minimum conductivity at the Dirac point, electron-hole puddle formation, p-n junctions, localization, percolation, quantum-classical crossover, midgap states, quantum Hall effects, and other phenomena.

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Electronic transport in two-dimensional graphene

We simulate a quantum point contact (QPC). Using spin-density-fimctional theory (SDFT), we obtain a spin-dependent energy barrier structure in the QPC that may help explain anomalies observed in experiment.

Spin Filtering Effects in a Quantum Point Contact

We study theoretically the effect of few elastic scatterers on electron transport in the ballistic regime. In
laterally confined structures (quasi 1-d or 2-d), resonant transmission peaks occur when total electron
energy equals any miniband energy. Unit transmission probability is approached when the scattering defect
is small, and far from other scatterers, even if the scatterer is strong enough to decrease significantly the
conductance away from resonance. Both numerical and analytical methods are used. Resonances occur for
all shapes of confining potential.

Electron-waveguide transmission resonance at a defect

The sensitivity of the optical interband transition energies with respect to the band offsets is investigated theoretically for various potential profiles that can be simulated by thin layers of semiconductors. Guided by simple physical arguments we point out which potential profiles are favorable to reveal accurate values for the band offsets. A many-band envelope-function model is used to confirm our predictions quantitatively for GaAs/Ga1−xAlxAs multi-quantum-wells.

Multi-quantum-wells to reveal the band offsets at semiconductor interfaces

High electron mobility transistors (HEMT) [1] have become important devices for high fre-quency and low noise applications [2]. Recently the use of In-rich InGaAs on InP-based pseudomorphic HEMTs (pHEMT) has resulted in remarkable sub-mm wave ampli-fiers above 300 GHz [3]. Due to the growing interest in scaling limitations inherent to this technology, we have performed a systematic study of these devices to better un-derstand the parameters responsible for affecting device behaviour and key to achieving optimal high-frequency performance. In the next section, we briefly discuss the details of the particle-based simulator used in this work and then de-scribe the two-dimensional (2D) device structure that is used in our simulations. The section following provides simulation results and a discussion and comparison of these with measured experimental data. Finally, we offer some conclusions about the scaling of this technology and point out some of the limitations that may arise from cer-tain approximations made in our study.

Hot electron effects in ultra‐short gate length InAs/InAlAs HEMTs

Here we report simulation results for high-frequency, high-power state-of-the-art GaN High Electron Mobility Transistors (HEMTs), using a full band Cellular Monte Carlo simulator, which includes the full details of the band structure and the phonon spectra, in order to study the RF performance of the new promising technology available nowadays.



A complete characterization of an InGaN back – barrier device has been performed using experimental data to calibrate the few adjustable parameters of the simulator. Furthermore, a study on a device structure based on the N-face configuration was performed. Finally, threading dislocation effects on HEMT device transport properties were investigated (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

David K. Ferry

Papers

Spin Filtering Effects in a Quantum Point Contact

Electron-waveguide transmission resonance at a defect

Multi-quantum-wells to reveal the band offsets at semiconductor interfaces

Hot electron effects in ultra‐short gate length InAs/InAlAs HEMTs

RF and DC characterization of state‐of‐the‐art GaN HEMT devices through cellular Monte Carlo simulations