Ferney A. Chaves

Explicit Analytical Charge and Capacitance Models of Undoped Double-Gate MOSFETs

An analytical, explicit, and continuous-charge model for undoped symmetrical double-gate (DG) MOSFETs is presented. This charge model allows obtaining analytical expressions of all total capacitances. The model is based on a unified-charge-control model derived from Poissons equation and is valid from below to well above threshold, showing a smooth transition between the different regimes. The drain current, charge, and capacitances are written as continuous explicit functions of the applied bias. We obtained very good agreement between the calculated capacitance characteristics and 2-D numerical device simulations, for different silicon film thicknesses.

A simple drain current model for Schottky-barrier carbon nanotube field effect transistors

We report on a new computational model to efficiently simulate carbon nanotube-based field effect transistors (CNT-FET). In the model, a central region is formed by a semiconducting nanotube that acts as the conducting channel, surrounded by a thin oxide layer and a metal gate electrode. At both ends of the semiconducting channel, two semi-infinite metallic reservoirs act as source and drain contacts. The current–voltage characteristics are computed using the Landauer formalism, including the effect of the Schottky barrier physics. The main operational regimes of the CNT-FET are described, including thermionic and tunnel current components, capturing ambipolar conduction, multichannel ballistic transport and electrostatics dominated by the nanotube capacitance. The calculations are successfully compared to results given by more sophisticated methods based on non-equilibrium Greens function formalism (NEGF).

Tunable Graphene–GaSe Dual Heterojunction Device

A field-effect device based on dual graphene-GaSe heterojunctions is demonstrated. Monolayer graphene is used as electrodes on a GaSe channel to form two opposing Schottky diodes controllable by local top gates. The device exhibits strong rectification with tunable threshold voltage. Detailed theoretical modeling is used to explain the device operation and to distinguish the differences compared to a single diode.

Physical model of the contact resistivity of metal-graphene junctions

While graphene-based technology shows great promise for a variety of electronic applications, including radio-frequency devices, the resistance of the metal-graphene contact is a technological bottleneck for the realization of viable graphene electronics. One of the most important factors in determining the resistance of a metal-graphene junction is the contact resistivity. Despite the large number of experimental works that exist in the literature measuring the contact resistivity, a simple model of it is still lacking. In this paper, we present a comprehensive physical model for the contact resistivity of these junctions, based on the Bardeen Transfer Hamiltonian method. This model unveils the role played by different electrical and physical parameters in determining the specific contact resistivity, such as the chemical potential of interaction, the work metal-graphene function difference, and the insulator thickness between the metal and graphene. In addition, our model reveals that the contact resist...

A physics-based model of gate-tunable metal–graphene contact resistance benchmarked against experimental data

The metal-graphene contact resistance is a technological bottleneck for the realization of viable graphene based electronics. We report a useful model to find the gate tunable components of this resistance determined by the sequential tunneling of carriers between the 3D-metal and 2D-graphene underneath followed by Klein tunneling to the graphene in the channel. This model quantifies the intrinsic factors that control that resistance, including the effect of unintended chemical doping. Our results agree with experimental results for several metals.

Impact of graphene polycrystallinity on the performance of graphene field-effect transistors

We have used a multi-scale physics-based model to predict how the grain size and different grain boundary morphologies of polycrystalline graphene will impact the performance metrics of graphene field-effect transistors. We show that polycrystallinity has a negative impact on the transconductance, which translates to a severe degradation of the maximum and cutoff frequencies. On the other hand, polycrystallinity has a positive impact on current saturation, and a negligible effect on the intrinsic gain. These results reveal the complex role played by graphene grain boundaries and can be used to guide the further development and optimization of graphene-based electronic devices.

Accurate Calculation of Gate Tunneling Current in Double-Gate and Single-Gate SOI MOSFETs Through Gate Dielectric Stacks

Recently, a new generation of MOSFETs, called multigate transistors, has emerged with geometries that will allow the downscaling and continuing enhancement of computer performance into next decade. The low dimensions in these nanoscale transistors exhibit increasing quantum effects, which are no longer negligible. Gate tunneling current is one of such effects that should be efficiently modeled. In this paper, an accurate description of tunneling in ultrathin body double-gate and single-gate MOSFET devices through layers of high- κ dielectrics, which relies on the precise determination of quasi-bound states, is developed. For this purpose, the perfectly matched layer method is embedded in each iteration of a 1-D Schrödinger-Poisson solver by introducing a complex stretched coordinate which allows applying artificial absorbing layers in the boundaries.

Errata for “Explicit Analytical Charge and Capacitance Models of Undoped Double-Gate MOSFETs” [Jul 07 1718-1724]

In the above titled paper (ibid., vol. 54, no. 7, pp. 1718-1724, Jul 07), an error was found in Equation (9). The correct equation is presented here.

Electrostatics of two-dimensional lateral junctions

The increasing technological control of two-dimensional (2D) materials has allowed the demonstration of 2D lateral junctions exhibiting unique properties that might serve as the basis for a new generation of 2D electronic and optoelectronic devices. Notably, the chemically doped MoS2 homojunction, the WSe2-MoS2 monolayer and MoS2 monolayer/multilayer heterojunctions, have been demonstrated. Here we report the investigation of 2D lateral junction electrostatics, which differs from the bulk case because of the weaker screening, producing a much longer transition region between the space-charge region and the quasi-neutral region, making inappropriate the use of the complete-depletion region approximation. For such a purpose we have developed a method based on the conformal mapping technique to solve the 2D electrostatics, widely applicable to every kind of junctions, giving accurate results for even large asymmetric charge distribution scenarios.

Photoresponse of Graphene-Gated Graphene-GaSe Heterojunction Devices

Because of their extraordinary physical properties, low-dimensional materials including graphene and gallium selenide (GaSe) are promising for future electronic and optoelectronic applications, particularly in transparent-flexible photodetectors. Currently, the photodetectors working at the near-infrared spectral range are highly indispensable in optical communications. However, the current photodetector architectures are typically complex, and it is normally difficult to control the architecture parameters. Here, we report graphene–GaSe heterojunction-based field-effect transistors with broadband photodetection from 730–1550 nm. Chemical-vapor-deposited graphene was employed as transparent gate and contact electrodes with tunable resistance, which enables effective photocurrent generation in the heterojunctions. The photoresponsivity was shown from 10 to 0.05 mA/W in the near-infrared region under the gate control. To understand behavior of the transistor, we analyzed the results via simulation performed using a model for the gate-tunable graphene–semiconductor heterojunction where possible Fermi level pinning effect is considered.