Mode space approach for tight-binding transport simulations in graphene nanoribbon field-effect transistors including phonon scattering

In this paper, we present a mode space method for atomistic non-equilibrium Green's function simulations of armchair graphene nanoribbon FETs that includes electron-phonon scattering. With reference to both conventional and tunnel FET structures, we show that, in the ideal case of a smooth electrostatic potential, the modes can be decoupled in different groups without any loss of accuracy. Thus, inter-subband scattering due to electron-phonon interactions is properly accounted for, while the overall simulation time considerably improves with respect to real-space, with a speed-up factor of 40 for a 1.5-nm-wide device. Such factor increases with the square of the device width. We also discuss the accuracy of two commonly used approximations of the scattering self-energies: the neglect of the off-diagonal entries in the mode-space expressions and the neglect of the Hermitian part of the retarded self-energy. While the latter is an acceptable approximation in most bias conditions, the former is somewhat inaccurate when the device is in the off-state and optical phonon scattering is essential in determining the current via band-to-band tunneling. Finally, we show that, in the presence of a disordered potential, a coupled mode space approach is necessary, but the results are still accurate compared to the real-space solution.Comment: 10 pages, 12 figures. Copyright (2013) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physic

Boosting the voltage gain of graphene FETs through a differential amplifier scheme with positive feedback

We study a possible circuit solution to overcome the problem of low voltage gain of short-channel graphene FETs. The circuit consists of a fully differential amplifier with a load made of a cross-coupled transistor pair. Starting from the device characteristics obtained from self-consistent ballistic quantum transport simulations, we explore the circuit parameter space and evaluate the amplifier performance in terms of dc voltage gain and voltage gain bandwidth. We show that the dc gain can be effectively improved by the negative differential resistance provided by the cross-coupled pair. Contact resistance is the main obstacle to achieving gain bandwidth products in the terahertz range. Limitations of the proposed amplifier are identified with its poor linearity and relatively large Miller capacitance.Comment: 19 pages, 10 figure

Contact-induced negative differential resistance in short-channel graphene FETs

In this work, we clarify the physical mechanism for the phenomenon of negative output differential resistance (NDR) in short-channel graphene FETs (GFETs) through non-equilibrium Green's function (NEGF) simulations and a simpler semianalytical ballistic model that captures the essential physics. This NDR phenomenon is due to a transport mode bottleneck effect induced by the graphene Dirac point in the different device regions, including the contacts. NDR is found to occur only when the gate biasing produces an n-p-n or p-n-p polarity configuration along the channel, for both positive and negative drain-source voltage sweep. In addition, we also explore the impact on the NDR effect of contact-induced energy broadening in the source and drain regions and a finite contact resistance.Comment: 7 pages, 9 figure

Salvage procedure after sternochondroplasty for pectus excavatum.

Vacuum-Assisted Closure in association to flap reconstruction is a well-established approach for treating complicated wounds. Case presentation: The authors present a case of soft-tissue breakdown with hardware exposure in a patient treated with strenochondroplasty for pectus excavatum. VAC therapy was applied in association to multiple debridement procedures and final flap closure. This was achieved without the need for hardware removal, thus maintaining adequate skeletal fixation. To our knowledge this approach has not yet been described into the literature after sternochondroplasty procedure. Conclusion Aggressive debridement and VAC therapy before final closure may represent a new, conservative method for managing surgical complications after sternochondroplasty procedures

Semianalytical quantum model for graphene field-effect transistors

We develop a semianalytical model for monolayer graphene field-effect transistors in the ballistic limit. Two types of devices are considered: in the first device, the source and drain regions are doped by charge transfer with Schottky contacts, while, in the second device, the source and drain regions are doped electrostatically by a back gate. The model captures two important effects that influence the operation of both devices: (i) the finite density of states in the source and drain regions, which limits the number of states available for transport and can be responsible for negative output differential resistance effects, and (ii) quantum tunneling across the potential steps at the source-channel and drain-channel interfaces. By comparison with a self-consistent non-equilibrium Green's function solver, we show that our model provides very accurate results for both types of devices, in the bias region of quasi-saturation as well as in that of negative differential resistance.Comment: 10 pages, 14 figure

The New Transverse-Facial Artery Musculomucosal Flap for Intraoral Reconstructions

With the bilateral progression of the 2 pedicled flaps, we were able to successfully restore both form and function of the soft palate, with a single-stage straightforward procedure, preserving at the same time the natural course of the facial arteries. Both flaps healed uneventfully. Six days after operation, the patient was placed on a liquid diet with no velopharyngeal insufficiency. This is to our knowledge the first extensive palatal reconstruction carried out with intraoral flaps onl

Transesophageal echocardiography in orthotopic liver transplantation: a comprehensive intraoperative monitoring tool

Intraoperative transesophageal echocardiography is a minimally invasive monitoring tool that can provide real-time visual information on ventricular function and hemodynamic volume status in patients undergoing liver transplantation. The American Association for the Study of Liver Diseases states that transesophageal echocardiography should be used in all liver transplant candidates in order to assess chamber sizes, hypertrophy, systolic and diastolic function, valvular function, and left ventricle outflow tract obstruction. However, intraoperative transesophageal echocardiography can be used to â\u80\u9cvisualizeâ\u80\u9d other organs too; thanks to its proximity and access to multiple acoustic windows: liver, lung, spleen, and kidney. Although only limited scientific evidence exists promoting this comprehensive use, we describe the feasibility of TEE in the setting of liver transplantation: it is a highly valuable tool, not only as a cardiovascular monitoring, but also as a tool to evaluate lungs and pleural spaces, to assess hepatic vein blood flow and inferior vena cava anastomosis and patency, i.e., in cases of modified surgical techniques. The aim of this case series is to add our own experience of TEE as a comprehensive intraoperative monitoring tool in the field of orthotopic liver transplantation (and major liver resection) to the literature

Graphene Base Transistors: A Simulation Study of DC and Small-Signal Operation

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A New Approach in the Management of Triangular Fossa Auricular Defects: The Posterior Auricular Artery Perforator Antihelix-Conchal Flap (PAAP Flap).

Background: The reconstruction of extensive scapha or triangular fossa defects to the ear still represents a challenge considering the lack of local simple surgical procedures. Objective: To present a new perforator flap which is a modification of the Elsahy and Ladocsi chondrocutaneous rotation flaps. Patients and Methods: Ten patients, in whom the oncological resection to the triangular fossa did not include cartilage, but skin and perichondrium. Defects no larger than 3 cm in diameter were treated. The dissection of the flap was carried out from lateral to medial on the anterior subperichondral plane. The flap was thus raised without the cartilage component but with the anterior perichondrium branched with the perforating vessels. Results: All flaps survived and treated patients presented good aesthetic results. Conclusions: We think that this procedure represents a valuable option to be considered in reconstructing well selected composite defects to upper ear