George S. Kliros

Analytical modeling of uniaxial strain effects on the performance of double-gate graphene nanoribbon field-effect transistors

AbstractThe effects of uniaxial tensile strain on the ultimate performance of a dual-gated graphene nanoribbon field-effect transistor (GNR-FET) are studied using a fully analytical model based on effective mass approximation and semiclassical ballistic transport. The model incorporates the effects of edge bond relaxation and third nearest neighbor (3NN) interaction. To calculate the performance metrics of GNR-FETs, analytical expressions are used for the charge density, quantum capacitance, and drain current as functions of both gate and drain voltages. It is found that the current under a fixed bias can change several times with applied uniaxial strain and these changes are strongly related to strain-induced changes in both band gap and effective mass of the GNR. Intrinsic switching delay time, cutoff frequency, and Ion/Ioff ratio are also calculated for various uniaxial strain values. The results indicate that the variation in both cutoff frequency and Ion/Ioff ratio versus applied tensile strain inversely corresponds to that of the band gap and effective mass. Although a significant high frequency and switching performance can be achieved by uniaxial strain engineering, tradeoff issues should be carefully considered.

Quantum capacitance of bilayer graphene

We present a simple phenomenological model for the quantum capacitance of bilayer graphene. Quantum capacitance is calculated from the broadened density of states taking into account electron-hole puddles and possible finite lifetime of electronic states through a Gaussian broadening distribution. The obtained results are in agreement with many features recently observed in quantum capacitance measurements on gated bilayer graphene. The temperature dependence of quantum capacitance is also investigated.

Modeling of carrier density and quantum capacitance in graphene nanoribbon FETs

Gate voltage control of carrier density and quantum capacitance is an important step for understanding the device physics and assessing the performance of nanoscale transistors. In this paper, we present a simple phenomenological model for the carrier density and quantum capacitance of graphene nanoribbon field-effect transistors as functions of gate voltage, Fermi level position and temperature. Quantum capacitance is calculated from the broadened density of states incorporating the presence of electron-hole puddles and possible finite lifetime of electronic states through a Gaussian broadening distribution. Thin gate-insulators of high-κ dielectric constant are used in our calculations in order to approach the quantum capacitance limit.

Scaling effects on the gate capacitance of graphene nanoribbon transistors

Scaling effects on the gate capacitance of graphene nanoribbon field-effect transistors (GNRFETs) are studied by means of a semi-analytical model. The influence of nanoribbon width, gateinsulator thickness and dielectric constant scaling on the capacitance - voltage characteristics is explored. Gate capacitance has non-monotonic behavior with ripples for thin and high-k gate-insulators. However, beyond the quantum capacitance limit, the ripples are suppressed and smooth monotonic characteristics are obtained.

Dielectric‐EBG covered conical antenna for UWB applications

Purpose – The purpose of this paper is to investigate of the ultra‐wide band (UWB) characteristics of a conical antenna covered by an electromagnetic band‐gap (EBG) structure composed of alternating high‐ and low‐permittivity dielectric spherical shells.Design/methodology/approach – A finite difference time domain in spherical coordinates is implemented in order to characterize the antennas performance and waveform fidelity in case an UWB pulse is used. The method of projected effective permittivity is used in order to treat accurately the dielectric interfaces between the dissimilar spherical shells.Findings – The design achieves a very wide impedance bandwidth above 5.5 GHz and presents UWB radiation characteristics and high average gain over the whole bandwidth. The radiation patterns are monopole‐like and their frequency dependence is small in the whole UWB frequency band. A time domain study has shown that the antenna distorts the excitation pulse in a moderate way.Originality/value – In this paper,...

Effect of uniaxial strain on the current-voltage characteristics of graphene nanoribbon field-effect transistors

We present a simulation study on the current-voltage characteristics of a dual-gated Graphene Nanoribbon Field Effect Transistor (GNR-FET) when its channel is under uniaxial tensile strain. Our study uses a fully analytical model based on effective mass approximation and semiclassical ballistic transport. The model incorporates the effects of edge bond relaxation and third nearest neighbor (3NN) interaction. It is found that the current under a fixed bias can change several times with applied uniaxial strain and these changes are strongly related to strain induced changes in both band gap and effective mass of the GNR. Furthermore, other characteristics as transconductance, gate capacitance and cutoff frequency are also calculated for various strain values.

Quantum capacitance of MIS structures based on diluted magnetic semiconductors

Quantum capacitance has an important role in nanoscale device modelling. In the present paper, we investigate the quantum magneto-capacitance of metal-insulator-semiconductor (MIS) structures based on diluted magnetic semiconductors (DMS) in the presence of Rashba spin-orbit interaction (SOI). Typical beating patterns with well defined node-positions in the oscillating quantum capacitance are observed. A simple relation that predicts the positions of nodes in the beating patterns is obtained. The interplay between the giant Zeeman splitting (including s-d exchange interaction) and the Rashba SOI, is discussed.

Design of an ultra-thin bandwidth-optimized metamaterial absorber for EMC applications

We present a simple design and simulation of an ultra-thin metamaterial (MTM) absorber that operates around 11.5 GHz. The advantages of the absorber are the small overall thickness (~λ/70), the absence of a metallic back plate and its wide-angle good performance. More than two absorbing layers can be stacked to achieve nearly perfect absorbance. Moreover, by exploiting the scalability property of MTMs, a 2×2 array of unit cells is designed to form an extended unit cell that exhibits an enhanced bandwidth performance with FWHM of 6.57%. The designed microwave absorber can be utilized for EMC applications.

Thermoelectric performance of electrons in ballistic graphene ribbons

We present an analytical model based on linear response theory and the Landauer formalism in order to calculate the electrical conductivity, thermopower and thermal conductivity of ballistic graphene ribbons and clarify both the temperature and Fermi-level dependences. The electronic figure of merit ZTel as an upper limit of the thermoelectric efficiency of short and wide graphene ribbons is investigated over a broad range of temperatures.

Propagation characteristics of thermally diffused expanded core fibers with complex refractive index profiles

In this paper, we investigate the propagation characteristics of thermally diffused expanded core (TEC) fibers with complex refractive index using the numerical Galerkins method. Complex modal profiles and propagation constants for TEC fibers of different expanded core radii are calculated. The imaginary part of the electric field results in wavefront distortion showing that the power flows out of or into the doped region according to the sign of the imaginary part of the refractive index. The gain or loss as a function of both mode field radius and operating wavelength is calculated numerically. Moreover, an approximate formula for the loss or gain coefficient of these fibers is given and the obtained approximate results agree well with those from Galerkins method applied to the complex refractive index fiber.