Exploiting Ambipolarity in Graphene Field-Effect Transistors for Novel Designs on High-Frequency Analog Electronics.

Exploiting ambipolar electrical conductivity based on graphene field-effect transistors has raised enormous interest for high-frequency (HF) analog electronics. Controlling the device polarity, by biasing the graphene transistor around the vertex of the V-shaped transfer curve, enables to redesign and highly simplify conventional analog circuits, and simultaneously to seek for multifunctionalities, especially in the HF domain. This study presents new insights for the design of different HF applications such as power amplifiers, mixers, frequency multipliers, phase shifters, and modulators that specifically leverage the inherent ambipolarity of graphene-based transistors.

Compact Modeling of Two-Dimensional Field-Effect Biosensors.

A compact model able to predict the electrical read-out of field-effect biosensors based on two-dimensional (2D) semiconductors is introduced. It comprises the analytical description of the electrostatics including the charge density in the 2D semiconductor, the site-binding modeling of the barrier oxide surface charge, and the Stern layer plus an ion-permeable membrane, all coupled with the carrier transport inside the biosensor and solved by making use of the Donnan potential inside the ion-permeable membrane formed by charged macromolecules. This electrostatics and transport description account for the main surface-related physical and chemical processes that impact the biosensor electrical performance, including the transport along the low-dimensional channel in the diffusive regime, electrolyte screening, and the impact of biological charges. The model is implemented in Verilog-A and can be employed on standard circuit design tools. The theoretical predictions obtained with the model are validated against measurements of a MoS2 field-effect biosensor for streptavidin detection showing excellent agreement in all operation regimes and leading the way for the circuit-level simulation of biosensors based on 2D semiconductors.

Compact Modeling Technology for the Simulation of Integrated Circuits Based on Graphene Field-Effect Transistors.

The progress made toward the definition of a modular compact modeling technology for graphene field-effect transistors (GFETs) that enables the electrical analysis of arbitrary GFET-based integrated circuits is reported. A set of primary models embracing the main physical principles defines the ideal GFET response under DC, transient (time domain), AC (frequency domain), and noise (frequency domain) analysis. Another set of secondary models accounts for the GFET non-idealities, such as extrinsic-, short-channel-, trapping/detrapping-, self-heating-, and non-quasi static-effects, which can have a significant impact under static and/or dynamic operation. At both device and circuit levels, significant consistency is demonstrated between the simulation output and experimental data for relevant operating conditions. Additionally, a perspective of the challenges during the scale up of the GFET modeling technology toward higher technology readiness levels while drawing a collaborative scenario among fabrication technology groups, modeling groups, and circuit designers, is provided.

Does carrier velocity saturation help to enhance f max in graphene field-effect transistors?

It has been argued that current saturation in graphene field-effect transistors (GFETs) is needed to get optimal maximum oscillation frequency (f max). This paper investigates whether velocity saturation can help to get better current saturation and if that correlates with enhanced f max. We have fabricated 500 nm GFETs with high extrinsic f max (37 GHz), and later simulated with a drift-diffusion model augmented with the relevant factors that influence carrier velocity, namely: short-channel electrostatics, saturation velocity effect, graphene/dielectric interface traps, and self-heating effects. Crucially, the model provides microscopic details of channel parameters such as carrier concentration, drift and saturation velocities, allowing us to correlate the observed macroscopic behavior with the local magnitudes. When biasing the GFET so all carriers in the channel are of the same sign resulting in highly concentrated unipolar channel, we find that the larger the drain bias is, both closer the carrier velocity to its saturation value and the higher the f max are. However, the highest f max can be achieved at biases where there exists a depletion of carriers near source or drain. In such a situation, the highest f max is not found in the velocity saturation regime, but where carrier velocity is far below its saturated value and the contribution of the diffusion mechanism to the current is comparable to the drift mechanism. The position and magnitude of the highest f max depend on the carrier concentration and total velocity, which are interdependent and are also affected by the self-heating. Importantly, this effect was found to severely limit radio-frequency performance, reducing the highest f max from ∼60 to ∼40 GHz.

GFET Asymmetric Transfer Response Analysis through Access Region Resistances.

Graphene-based devices are planned to augment the functionality of Si and III-V based technology in radio-frequency (RF) electronics. The expectations in designing graphene field-effect transistors (GFETs) with enhanced RF performance have attracted significant experimental efforts, mainly concentrated on achieving high mobility samples. However, little attention has been paid, so far, to the role of the access regions in these devices. Here, we analyse in detail, via numerical simulations, how the GFET transfer response is severely impacted by these regions, showing that they play a significant role in the asymmetric saturated behaviour commonly observed in GFETs. We also investigate how the modulation of the access region conductivity (i.e., by the influence of a back gate) and the presence of imperfections in the graphene layer (e.g., charge puddles) affects the transfer response. The analysis is extended to assess the application of GFETs for RF applications, by evaluating their cut-off frequency.

Scaling of graphene field-effect transistors supported on hexagonal boron nitride: radio-frequency stability as a limiting factor.

The quality of graphene in nanodevices has increased hugely thanks to the use of hexagonal boron nitride as a supporting layer. This paper studies to which extent hBN together with channel length scaling can be exploited in graphene field-effect transistors (GFETs) to get a competitive radio-frequency (RF) performance. Carrier mobility and saturation velocity were obtained from an ensemble Monte Carlo simulator that accounted for the relevant scattering mechanisms (intrinsic phonons, scattering with impurities and defects, etc). This information is fed into a self-consistent simulator, which solves the drift-diffusion equation coupled with the two-dimensional Poisson's equation to take full account of short channel effects. Simulated GFET characteristics were benchmarked against experimental data from our fabricated devices. Our simulations show that scalability is supposed to bring to RF performance an improvement that is, however, highly limited by instability. Despite the possibility of a lower performance, a careful choice of the bias point can avoid instability. Nevertheless, maximum oscillation frequencies are still achievable in the THz region for channel lengths of a few hundreds of nanometers.

Programa para ayudar a estudiantes de tercer año a integrar el contenido y la forma en la realización de la historia clínica / A program to help third year students to integrate content and form in the clinical history

Introducción. Las habilidades de comunicación se han considerado como importantes e independientes para ser adquiridas por los estudiantes; sin embargo, representan una parte fundamental de la consulta. Objetivo. Describir un programa formativo de integración de habilidades de anamnesis y comunicación para estudiantes de tercero y analizar la correlación entre ambos dominios. Sujetos y métodos. Estudio observacional descriptivo. Emplazamiento: Aula de Habilidades de la Facultad de Medicina de Córdoba. Muestra: 158 estudiantes de tercero de medicina. Intervenciones: se ha desarrollado un programa interactivo en habilidades de comunicación y realización de anamnesis; un evaluador independiente valoró el desarrollo de ambas durante la entrevista clínica de los encuentros video grabados con una paciente simulada utilizando una herramienta ad hoc. Resultados. Duración media de cada entrevista: 5:56 min (IC 95% = 5:38-6:15 min), con un máximo de 10 min. Puntuación media global de la escala de valoración: 56,2 (máx. 104) (IC 95% = 54,20-58,19); habilidades de relación médico paciente puntuación media: 25,58 (máx. 40) (IC 95% = 24,7-26,5); anamnesis biomédica: 30,61 (máx. 64) (IC 95% = 29,20-32,03). Existe una correlación positiva entre el tiempo usado y la puntuación global obtenida en la escala de valoración (coeficiente de Pearson = 0,65; p < 0,01) y entre los apartados biomédicos y comunicación (coeficiente de Pearson = 0,44; p < 0,01). Conclusiones. El programa fue factible; las habilidades en comunicación que mejor se llevaron a cabo fueron: reactividad, uso del lenguaje no verbal, recibimiento y cierre de entrevista. El tiempo es una variable importante para desarrollar las habilidades aprendidas y la mayoría de los estudiantes no agotan el disponible. Los estudiantes que mejor abordan los aspectos relacionales también consiguen más información en la anamnesis (AU)

Introduction. Communication skills are considered as important and independent capacity to be acquired by students, however, are a fundamental part of the consultation. Aim. To describe a formative program for third year students with the objective of integrating anamnesis and communication skills in an integrating way and evaluate the correlation between them both. Subjects and methods. Observational descriptive study. Location: Skills Classroom in the Medicine Faculty of Cordoba. Subjects: 158 3rd year medicine students. Interventions: an interactive program in communications skills in clinical stories performances has been developed. An independent evaluator valued the communication and anamnesis skills during the clinical interview of the recorded meetings with simulated patient using a tool ad hoc. Results. Average interview time: 5:56 min (95% CI = 5:38-6:15 min) (maximum 10 min). Average global score rating scale: 56.2 (maximum 104) (95% CI = 54.20-58.19); skills of doctor-patient average score: 25.58 (maximum 40) (95% CI = 24.7-26.5), and biomedical anamnesis: 30.61 (maximum 64) (95% CI = 29.20-32.03). A positive correlation exists between with the time used by students and the obtained punctuation in the rating scale (Pearson coefficient = 0.65; p < 0.01), just like in the biomedical and communication sections (Pearson coefficient = 0.44; p < 0.01). Conclusions. The program was feasible, in which the communications skills had the best performance: reactivity, nonverbal language, reception and closing interview; against a not well performed biomedical anamnesis. Time is an important variable to develop the skills learnt and the students uses less than the maximum. The group of students with the best psychosocial aspects, got more details of anamnesis in their interviews (AU)