Showing papers on "Quantum capacitance published in 2008"

RF performance of top-gated, zero-bandgap graphene field-effect transistors

Abstract: We present the first experimental high-frequency measurements of graphene field-effect transistors (GFETs), demonstrating an fT of 14.7 GHz for a 500-nm-length device. We also present detailed measurement and analysis of velocity saturation in GFETs, demonstrating the potential for velocities approaching 108 cm/sec and the effect of an ambipolar channel on current-voltage characteristics.

Single carbon nanotube transistor at GHz frequency

Abstract: We report on microwave operation of top-gated single carbon nanotube transistors. From transmission measurements in the 0.1−1.6 GHz range, we deduce device transconductance gm and gate−nanotube capacitance Cg of micro- and nanometric devices. A large and frequency-independent gm ∼ 20 μS is observed on short devices, which meets the best dc results. The capacitance per unit gate length of 60 aF/μm is typical of top gates on a conventional oxide with e ∼ 10. This value is a factor of 3−5 below the nanotube quantum capacitance which, according to recent simulations, favors high transit frequencies fT = gm/2πCg. For our smallest devices, we find a large fT ∼ 50 GHz with no evidence of saturation in length dependence.

Outperforming the Conventional Scaling Rules in the Quantum-Capacitance Limit

Abstract: We present a study on the scaling behavior of field- effect transistors in the quantum-capacitance limit (QCL). It will be shown that a significant performance improvement in terms of the power delay product can be obtained in devices scaled toward the QCL. As a result, nanowires or nanotubes exhibiting a 1-D transport are a premier choice as active channel materials for transistor devices since the QCL can be attained in such systems.

Mobility Extraction and Quantum Capacitance Impact in High Performance Graphene Field-effect Transistor Devices

Abstract: The field-effect mobility of graphene devices is discussed. We argue that the graphene ballistic mean free path can only be extracted by taking into account both, the electrical characteristics and the channel length dependent mobility. In doing so we find a ballistic mean free path of 300nm at room-temperature for a carrier concentration of ~1e12/cm2 and that a substantial series resistance of around 300ohmum has to be taken into account. Furthermore, we demonstrate first quantum capacitance measurements on single-layer graphene devices.

Mobility extraction and quantum capacitance impact in high performance graphene field-effect transistor devices

Abstract: The field-effect mobility of graphene devices is discussed. We argue that the graphene ballistic mean free path, Lball can only be extracted by taking into account both, the electrical characteristics and the channel length dependent mobility. In doing so we find a ballistic mean free path of Lball=300plusmn100 nm at room-temperature for a carrier concentration of ~1012 cm-2 and that a substantial series resistance of around 300 Omega mum has to be taken into account. Furthermore, we demonstrate first quantum capacitance measurements on single-layer graphene devices.

Bandstructure Effects in Silicon Nanowire Hole Transport

Abstract: Bandstructure effects in p-channel MOS (PMOS) transport of strongly quantized silicon nanowire FETs in various transport orientations are examined. A 20-band sp3d5s* spin-orbit (SO) coupled atomistic tight-binding model coupled to a self-consistent Poisson solver is used for the valence-band dispersion calculation. A ballistic FET model is used to evaluate the capacitance and current-voltage characteristics. The dispersion shapes and curvatures are strong functions of device size, lattice orientation, and bias, and cannot be described within the effective mass approximation. The anisotropy of the confinement mass in the different quantization directions can cause the charge to preferably accumulate in the (110) and then on the (112) rather than on (100) surfaces, leading to significant differences in the charge distributions for different wire orientations. The total gate capacitance of the nanowire FET devices is, however, very similar for all wires in all the investigated transport orientations ([100], [110], [111]), and is degraded from the oxide capacitance by ~30%. The [111] and then the [110] oriented nanowires indicate highest carrier velocities and better on-current performance compared to [100] wires. The dispersion features and quantization behavior, although a complicated function of physical and electrostatic confinement, can be explained at first order by looking at the anisotropic shape of the heavy-hole valence band.

Single Carbon Nanotube Transistor at GHz Frequency

Abstract: We report on microwave operation of top-gated single carbon nanotube transistors. From transmission measurements in the 0.1-1.6 GHz range, we deduce device transconductance gm and gate-nanotube capacitance Cg of micro- and nanometric devices. A large and frequency-independent gm of about 20 microSiemens is observed on short devices, which meets the best dc results. The capacitance per unit gate length of 60 aF/micrometer is typical of top gates on a conventional oxide with a dielectric constant equal to 10. This value is a factor of 3-5 below the nanotube quantum capacitance which, according to recent simulations, favors high transit frequencies . For our smallest devices, we find a large transit frequency equal to 50 GHz with no evidence of saturation in length dependence.

Computational Model of Edge Effects in Graphene Nanoribbon Transistors

Abstract: We present a semi-analytical model incorporating the effects of edge bond relaxation, the third nearest neighbor interactions, and edge scattering in graphene nanoribbon field-effect transistors (GNRFETs) with armchair-edge GNR (AGNR) channels. Unlike carbon nanotubes (CNTs) which do not have edges, the existence of edges in the AGNRs has a significant effect on the quantum capacitance and ballistic I-V characteristics of GNRFETs. For an AGNR with an index of m=3p, the band gap decreases and the ON current increases whereas for an AGNR with an index of m=3p+1, the quantum capacitance increases and the ON current decreases. The effect of edge scattering, which reduces the ON current, is also included in the model.

Analytical ballistic theory of carbon nanotube transistors: Experimental validation, device physics, parameter extraction, and performance projection

Abstract: We developed a fully analytical ballistic theory of carbon nanotube field effect transistors enabled by the development of an analytical surface potential capturing the temperature dependence and gate and quantum capacitance electrostatics. The analytical ballistic theory is compared to the experimental results of a ballistic transistor with good agreement. The validated analytical theory enables intuitive circuit design, provides techniques for parameter extraction of the bandgap and surface potential, and elucidates on the device physics of drain optical phonon scattering and its role in reducing the linear conductance and intrinsic gain of the transistor. Furthermore, a threshold voltage definition is proposed reflecting the bandgap-diameter dependence. Projections for key analog and digital performances are discussed.

A current-voltage model for Schottky-barrier graphene-based transistors.

Abstract: A low complexity computational model of the current-voltage characteristics for graphene nanoribbon (GNR) field effect transistors (FET), being able to simulate a hundred points in a few seconds using a personal computer, is presented. For quantum capacitance controlled devices, self-consistent calculations of the electrostatic potential can be skipped. Instead, an analytical closed-form electrostatic potential from Laplace's equation yields accurate results compared with that obtained by the self-consistent non-equilibrium Green's functions (NEGF) method. The model includes both tunneling current through the Schottky barrier (SB) at the contact interfaces and thermionic current above the barrier, properly capturing the effect of arbitrary physical and electrical parameters.

Compact Modeling of Ballistic Nanowire MOSFETs

Abstract: Nanowire MOSFETs attract attention due to the probable high performance and the excellent controllability of device current. We present a compact model of ballistic nanowire MOSFET that aids our understanding of physics and the overall properties of the device. The relationship between the gate overdrive and the carrier density is derived and combined with the current expression to yield the current-voltage (I-V) characteristics. The subthreshold characteristics and the short channel effect are also discussed. The effects of the quantum capacitance on device characteristics are analyzed. The low-temperature expression is also derived, and the relation to quantum conductance is discussed. The I-V characteristics are numerically evaluated and examined, employing a reported subband model. The drain- and gate-bias dependences of device current are shown, and the effects of the quantum capacitance and conductance on these characteristics are indicated.

Performance of $n$ -Type InSb and InAs Nanowire Field-Effect Transistors

Abstract: The electronic structure of highly scaled InSb and InAs nanowire (NW) field-effect transistors (FETs) is calculated with an eight-band kmiddotp model. Cross sections of 4 nm or less result in bandgaps of 0.8 eV or more. For these cross sections, all devices are single moded and operate in the quantum capacitance limit. Analytical expressions for the transconductance, cutoff frequency, and gate delay time are presented and compared to numerical results. The dependence of the intrinsic cutoff frequency on drive current is weak, scaling as radic{ID} with values in the 4-7 THz range that are good for RF applications. The gate delay times strongly depend on the drive current, scaling as ID -3/2 with values ranging from 25 to 132 fs which are competitive for digital applications.

Graphene nano-ribbon (GNR) interconnects: A genuine contender or a delusive dream?

Abstract: This paper presents a comprehensive conductance and delay analysis of graphene nano-ribbon (GNR) interconnects. The conductance model of GNR is derived using a simple tight binding model and the linear response Landauer formula. Several GNR structures are examined, and the conductance among them and other interconnect materials (copper, tungsten and carbon nanotubes) is compared. Impact of different model parameters (mean free path, Fermi level and edge specularity) on the conductance is discussed. An RLC equivalent circuit model is defined to analyze both global and local GNR interconnect delays. The results reveal that till the very end of ITRS'07 roadmap, GNRs cannot match the performance of global level copper or SWCNTs, unless multiple layers along with proper intercalation doping is used and specular nano-ribbon edge is achieved. However, multi-layer zigzag edged GNRs (zz-GNRs) can be comparable to copper at the local level, and can have much better performance than that of tungsten, implying possible application as local interconnects.

Influence of Bandstructure and Channel Structure on the Inversion Layer Capacitance of Silicon and GaAs MOSFETs

Abstract: The influence of MOSFET channel material and channel structure on the inversion layer capacitance is examined. It is well known that the inversion layer capacitance depends strongly on the bandstructure of the channel, but we show that it also depends very strongly on the structure of the channel (e.g., bulk vs. ultrathin body, and the thickness of the body). These results provide some general insights into the channel material and structure tradeoffs that control the inversion layer capacitance-an increasingly important consideration as electrical oxide thicknesses continue to decrease and as new channel materials are considered.

Accurate Intrinsic Gate Capacitance Model for Carbon Nanotube-Array Based FETs Considering Screening Effect

Abstract: In this letter, an accurate semi-analytical model for the intrinsic gate capacitance of carbon nanotube (CN)-array based back-gated field-effect transistors (FETs) is proposed. The model accounts for electrostatic screening effect for any given number of nanotubes, their diameter, pitch, and gate-dielectric thickness. It is shown that screening effect varies significantly not only with the change in density but also with the number of nanotubes and must be considered while modeling the intrinsic gate capacitance of array-based CNFETs for both high-performance and thin-film transistor applications.

Carrier density and quantum capacitance for semiconducting carbon nanotubes

Abstract: A full-band analytical model of the equilibrium carrier density for single-wall semiconducting carbon nanotubes (sCNTs) is presented. The carrier density, which is a fundamental property of all semiconductors, is obtained using a semiempirical method for degenerate positions of the Fermi level and shows good agreement with numerical tight-binding results. The quantum capacitance is subsequently derived from the carrier density and used to develop a C-V model with good agreement with experimental quantum capacitance measurements. An analytical model of the gate coupling function of sCNTs is also reported which relates the internal surface potential with the external applied gate voltage. The diameter temperature and Fermi level dependency, and the essential properties of carbon nanotubes device physics are captured in these analytical equations.

The Gate Leakage Current in Graphene Field-Effect Transistor

Abstract: The unique band structure of graphene makes the gate leakage current in a graphene field-effect transistor (FET) different from that in silicon FET. Theoretical investigation in this letter demonstrates that the Fowler-Nordheim tunneling current (TC) in a graphene FET is different from that in a silicon FET. Numerical calculations show that a higher oxide electric field results in larger TC in a graphene FET than that in a silicon FET. This implies that, to ensure a workable graphene FET, a thicker gate oxide is needed to limit the gate leakage current compared to that for a silicon FET.

Observation of Quantum Capacitance of individual single walled carbon nanotubes

Abstract: We report a measurement on quantum capacitance of individual semiconducting and small band gap SWNTs. The observed quantum capacitance is remarkably smaller than that originating from density of states and it implies a strong electron correlation in SWNTs.

One-Dimensional Nanoelectronic Devices - Towards the Quantum Capacitance Limit

Abstract: We present a study on the scaling of conventional as well as tunneling nanowire/tube field-effect transistors towards the quantum capacitance limit. As it turns out conventional FETs exhibit a scaling benefit in this limit in terms of the power delay product. In addition, the appearance of short channel-like effects due to a parasitics charge pile-up can be avoided. Scaling tFETs towards the QCL allows obtaining regular transistor output characteristics and an on-state performance similar to the performance of cFETs.

Quantum capacitance extraction for carbon nanotube interconnects

Abstract: Carbon nanotubes, especially the ones with diameters of the order of a few nanometers exhibit correlated electron transport and are best described using the Tomanaga Luttinger liquid model. Recently the TL model was used to create a convenient transmission line like phenomenological model for carbon nanotubes. In this paper, we have attempted to quantify the quantum capacitances of individual metallic single walled carbon nanotubes and bundles of single walled tubes. Quantum capacitances and values of the interaction parameter `g' are presented for small diameter metallic nanotubes while quantum capacitances are calculated for bundles of carbon nanotubes.

Influence of capacitive effects on the dynamic of a CNTFET by Monte Carlo method

Abstract: We present a study of carbon nanotube field effect transistors (CNTFET) operation and performance using Monte Carlo simulation including phonon scattering. In CNTFETs, operating in the quantum capacitance regime, the low driving electric field in the channel yields a high fraction of ballistic transport. In terms of ballisticity, ION/IOFF ratio and intrinsic delay, the performance of 100 nm-long CNTFET is shown to be as high as that of much smaller Si transistors.

The quantum capacitance limit of high-speed, low-power InSb nanowire field effect transistors

Abstract: The performance metrics of InSb nanowire (NW) FETs are investigated using an analytical 2-band model and a seminumerical ballistic transport model. The first analysis of the diameter dependence of the current, gate delay, power-delay product, and energy-delay product of InSb NW FETs, which operate in the quantum capacitance limit (QCL), are presented. Because of their small density of states, relatively large diameter, les 60 nm, InSb NW FETs operate in the QCL. Both the energy-delay and power-delay products are reduced as the diameter is reduced, and optimum designs are obtained for diameters in the range of 10 - 40 nm. Power-delay product varies from 2times 10-20 J to 68times 10-20 J for all devices with a source Fermi level range of 0.1 - 0.2 eV. The gate delay time for all devices varies from 4 - 16 fs and decreases as the NW diameter increases. These NW FETs provide both ultra-low power switching and high-speed.

Edge chemistry engineering of graphene nanoribbon transistors: A computational study

Abstract: Using the density-functional theory (DFT) simulation and a top-of-the-barrier ballistic transport model, we present a simulation framework for assessing the performance limits of graphene nanoribbon (GNR) FETs with edges terminated by different chemical species. We find significant effects of edge chemistry on the quantum capacitance, carrier injection velocity, channel conductance and balance between the nFET and the pFET of GNRFETs. The H termination is identified to have the largest on current, carrier injection velocity, and the best balance between the nFET and the pFET with typical solid state gating technologies.

Time-Domain Analysis of Carbon Nanotubes

Abstract: Time-domain behavior of carbon nanotubes as the future candidates for interconnects is of great importance. In order to analyze an entire circuit containing carbon nanotube interconnects, we need to have an electrical circuit model for carbon nanotubes as well as the rest of the system. Several studies have demonstrated the extracted electrical elements of carbon nanotubes. At this paper, we're going to perform a comprehensive transient analysis on carbon nanotubes to achieve time-domain parameters like delay, rise time and overshoot. An overall transfer function of carbon nanotubes is obtained analytically. Besides, the time-domain quality metrics as well as relative stability based on the variation of the geometry of carbon nanotubes would be analyzed through simulations.

Modelling of the current-voltage characteristics of a carbon nano tube field effect transistor

Abstract: Working on carbon nanotube field effect transistors (CNTFETs) involving the skill to treat electronic devices at the molecular scale. Nanotubes are being considered as the best candidates for high-speed applications. The charge transport in CNTs is controlled by mobility and saturation velocity. It has also been shown that the high mobility does not always lead to higher carrier velocity. In the high electric field, velocity vectors are changed from randomness to streamline. Velocity approach is applied to the modelling of the current-voltage characteristic of a carbon nanotube field effect transistor. According to the simulation results, in the absence of the quantum capacitance, the short channel effects are arising. However it is foreseeable that if the quantum capacitance takes into consideration, this effect can be improved.

Probing Electronic Properties of Carbon Nanotubes

Abstract: Carbon nanotubes are quasi-one-dimensional objects that have many remarkable electronic properties. In Chapter I, an electrostatic force microscopy technique to probe the local density of states of single-walled carbon nanotubes (SWCNTs) under ambient conditions is described. Coupling the atomic force microscope tip motion with the quantum capacitance of nanotubes enables the van Hove singularities in the one-dimensional density of states to be resolved. We utilized this technique to identify individual semiconducting and metallic tubes, and further to estimate the chiral angle of a nanotube. Moreover, in order to realize a SWCNT interferometer, nanotube loop devices where a self-crossing geometry yields two electron paths that is a possible analog of the optical Sagnac interferometer are fabricated and explored in Chapter II. Scanning gate microscopy reveals for semiconducting devices a 0–50% transmission probability into the loop segment at the junction, which can be controlled by applying back gate voltage, hence shifting the Fermi level of the nanotube. Metallic loop devices having low contact resistance showed a large- scale conductance peak with fast oscillations superposed on it. Possible theoretical explanations including Sagnac-type interference, which takes the velocity difference between left and right movers in to account, and Fabry-Perot-type interference are compared with the experimental observations. In Chapter III, in accordance with increasing demand for developing spin-electronic devices, cobalt-filled multi-walled carbon nanotubes (Co–filled MWCNTs) are first synthesized and imaged by transmission electron microscopy, and also characterized by various spectroscopy tools like X–ray diffraction and energy dispersive X–ray spectrometry. Further, a Co–filled MWCNT device having reproducible switching in magnetoresistance was demonstrated. The last topic, in Chapter IV, covers the effects of a transverse electric field in MWCNT devices, where conductance fluctuations as a function of the transverse electric field were observed. The electric field spacing between the peaks of the fluctuations is in agreement with the theoretical predictions of band structure modulation by transverse electric fields. Future work following our experimental studies is proposed and discussed at the end of each chapter.

Electrostatic capacitance extraction for carbon nanotube interconnects

Abstract: Carbon nanotubes are promising candidates for futuristic nanoelectronic applications due to their excellent properties. In this paper, we present a comprehensive modeling and calculation of electrostatic capacitances for various carbon nanotube systems that can be used to model interconnects in nanotechnology circuits. We provide results for single walled, multiwalled and bundles of single-walled carbon nanotubes as functions of the various design parameters. Numerical computations were performed using the method of moments in conjunction with a Greenpsilas function appropriate for the geometry of the interconnects.

Compact Modeling of Ballistic Nanowire MOSFETs

Abstract: Nanowire MOSFETs attract attention due to the probable high performance and the excellent controllability of device current. We present a compact model of ballistic nanowire MOSFET that aids our understanding of physics and the overall properties of the device. The relationship between the gate overdrive and the carrier density is derived and combined with the current expression to yield the current-voltage (I-V) characteristics. The subthreshold characteristics and the short channel effect are also discussed. The effects of the quantum capacitance on device characteristics are analyzed. The low-temperature expression is also derived, and the relation to quantum conductance is discussed. The I-V characteristics are numerically evaluated and examined, employing a reported subband model. The drain- and gate-bias dependences of device current are shown, and the effects of the quantum capacitance and conductance on these characteristics are indicated.

Analytical Degenerate Carrier Density and Quantum Capacitance for Semiconducting Carbon Nanotubes

Abstract: Analytical equations are developed for the degenerate carrier density and quantum capacitance with good agreement to numerical computation and experimental data. These results lay the foundation for analytical transport and compact modeling of carbon nanotube (CNT) devices, and for evaluating diameter dependence on electrical performance.

A Thin, Vertical, Parallel Plate Capacitor with Multi-Wall Carbon Nanotube Electrodes

Abstract: We propose a new capacitor structure which uses carbon nanotube electrodes and is suitable for use in advanced integrated circuit technologies. Metallic carbon nanotubes have characteristics which make them well suited for capacitor electrodes (low resistance and large surface area per unit volume). We demonstrate that our thin vertical plate carbon nanotube capacitor can exhibit a capacitance per unit area of 175 fF/mum2.