Showing papers on "Quantum capacitance published in 2007"

Carrier statistics and quantum capacitance of graphene sheets and ribbons

Abstract: In this work, fundamental results for carrier statistics in graphene two-dimensional sheets and nanoscale ribbons are derived. Though the behavior of intrinsic carrier densities in two-dimennsional graphene sheets is found to differ drastically from traditional semiconductors, very narrow (sub-10nm) ribbons are found to be similar to traditional narrow-gap semiconductors. The quantum capacitance, an important parameter in the electrostatic design of devices, is derived for both two-dimensional graphene sheets and nanoribbons.

Gate electrostatics and quantum capacitance of graphene nanoribbons.

Abstract: Capacitance-voltage (C-V) characteristics are important for understanding fundamental electronic structures and device applications of nanomaterials. The C-V characteristics of graphene nanoribbons (GNRs) are examined using self-consistent atomistic simulations. The results indicate strong dependence of the GNR C-V characteristics on the edge shape. For zigzag edge GNRs, highly nonuniform charge distribution in the transverse direction due to edge states lowers the gate capacitance considerably, and the self-consistent electrostatic potential significantly alters the band structure and carrier velocity. For an armchair edge GNR, the quantum capacitance is a factor of 2 smaller than its corresponding zigzag carbon nanotube, and a multiple gate geometry is less beneficial for transistor applications. Magnetic field results in pronounced oscillations on C-V characteristics.

Analytical Models for the Insight Into the Use of Alternative Channel Materials in Ballistic nano-MOSFETs

Abstract: This paper presents new analytical derivations for the ballistic current of n-MOSFETs as a function of the transport direction, of the properties of the channel material, and of the technological parameters. The main purpose of the analytical expressions is to provide an insight into the optimization of the transistors with alternative channel materials. Our results simply explain why, for a given two-dimensional (2-D) density of states, an elliptic 2-D minimum can provide a current larger than a circular minimum if the best transport direction is selected. Furthermore, we analytically show that the use of channel materials with very small transport masses implies a tradeoff between the electron velocity and the gate drive capacitance, because of the finite capacitance of the inversion layer. This latter effect should be seriously considered in the context of the aggressive scaling of the equivalent oxide thickness enforced by the introduction of high-K dielectrics and multigate MOSFETs

Quantum inductance and negative electrochemical capacitance at finite frequency in a two-plate quantum capacitor

Abstract: We report on theoretical investigations of frequency-dependent quantum capacitance. It is found that at finite frequency, a quantum capacitor can be characterized by a classical $RLC$ circuit with three parameters: a static electrochemical capacitance, a charge relaxation resistance, and a quantum inductance. The quantum inductance is proportional to the characteristic time scale of electron dynamics, and due to its existence, the time-dependent current can accumulate a phase delay and lags behind the applied ac voltage, leading to a negative effective capacitance.

Carrier Statistics and Quantum Capacitance of Graphene Sheets and Ribbons

Abstract: In this work, fundamental results for carrier statistics in graphene 2-dimensional sheets and nanoscale ribbons are derived. Though the behavior of intrinsic carrier densities in 2d graphene sheets is found to differ drastically from traditional semiconductors, very narrow (sub-10 nm) ribbons are found to be similar to traditional narrow-gap semiconductors. The quantum capacitance, an important parameter in the electrostatic design of devices, is derived for both 2d graphene sheets and nanoribbons.

Atomistic Modeling of Gate-All-Around Si-Nanowire Field-Effect Transistors

Abstract: We report atomistic simulations of the transport properties of Si-nanowire (SiNW) field-effect transistors. Results have been obtained within a self-consistent approach based on the nonequilibrium Green's function (NEGF) scheme in the density functional theory framework. We analyze in detail the operation of an ultrascaled SiNW channel device and study the characteristics and transfer characteristics behavior of the device while varying several parameters including doping, gate and oxide lengths, and temperature. We focus our attention to the quantum capacitance of the SiNW and show that a well-tempered device design can be accomplished in this regime by choosing suitable doping profiles and gate contact parameters.

Variable capacitance mechanisms in carbon nanotubes

Abstract: This paper demonstrates that semiconducting carbon nanotubes can be used to implement a room temperature capacitance tunable via an external bias, i.e., a varactor, which is a key element in any communication system working up to a few terahertz. The paper describes the implementation of two types of varactors based, respectively, on a single gated nanotube and on a biased array of carbon nanotubes. In the former case, the varactor is the density-of-states dependent quantum capacitance that can be tuned via a gate voltage due to the shift of the Fermi level. In the latter case, the varactor consists of a selectively biased brushlike carbon nanotube array with a capacitance tuned via attractive and repulsive electrostatic forces between different nanotubes of the array.

DFT Modeling of Bulk-Modulated Carbon Nanotube Field-Effect Transistors

Abstract: We report density-functional theory (DFT) atomistic simulations of the nonequilibrium transport properties of carbon nanotube (CNT) field-effect transistors (FETs). Results have been obtained within a self-consistent approach based on the nonequilibrium Green's functions (NEGF) scheme. We show that, as the current modulation mechanism is based on the local screening properties of the nanotube channel, a completely new, negative quantum capacitance regime can be entered by the device. We show how a well-tempered device design can be accomplished in this regime by choosing suitable doping profiles and gate contact parameters. At the same time, we detail the fundamental physical mechanisms underlying the bulk-switching operation, including them in a very practical and accurate model, whose parameters can be easily controlled in order to improve the device performance. The dependence of the nanotube screening properties on the temperature is finally explained by means of a self-consistent temperature analysis

Modelling of multiwall carbon nanotube transmission lines

Abstract: A hybrid transmission line/quantum-mechanical model is proposed for the analysis of multiwall carbon nanotubes in the radio frequency range. The developed model applies to multiwall carbon nanotubes having high number of shells, and diameter ranging from about 50 nm to 150 nm, like the ones growing inside alumina membranes. Numerical calculations are performed in order to assess the influence of the p.u.l. kinetic inductance and of the p.u.l. quantum capacitance on the propagation characteristics.

Analytical Model of Carbon Nanotube Electrostatics: Density of States, Effective Mass, Carrier Density, and Quantum Capacitance

Abstract: Fundamental electrostatic properties of achiral carbon nanotubes (CNTs) are derived analytically. These include an exact derivation of the density of states within the nearest neighbor tight-binding formalism, the group velocity, and the effective mass. In addition, the non-degenerate equilibrium carrier density, and quantum capacitance are presented analytically. The quantum capacitance is used to provide a low energy C-V theory for a top-gated CNT device with good correlation to experimental data.

Capacitance–voltage characteristics and switching time of double barrier resonant tunneling diode fabricated with epi-Si and γ-Al2O3

Abstract: Capacitance-voltage and conductance-voltage characteristics of resonant tunneling diodes fabricated with epitaxial Si/ γ -Al 2 O 3 heterostructure have been studied at room temperature. The capacitance-voltage characteristics of this structure show a large capacitance peak near the resonant tunneling bias. This capacitance peak is considered as quantum capacitance originated from the charge storage in the quantum well of the structure during tunneling process. Capacitance-voltage characteristics also were studied at different frequencies to understand the charge storage mechanism in the resonant tunneling diode structure. Resonant tunneling diodes with different barrier thicknesses were studied and tremendous improvement in the NDR characteristics was observed. A maximum peak-to-valley current ratio of 248 was obtained at room temperature. Using this capacitance value switching time and maximum operational frequency of the RTD structure were determined.

High density integrated capacitors using multi-walled carbon nanotubes

Abstract: The development of high density integrated capacitors is crucial for the implementation of high performance mixed-signal integrated circuits. In this paper, we propose three possible high density integrated capacitor configurations based on multi-walled carbon nanotubes (MWCNT). We develop an RLC model for the MWCNT-based capacitor configurations and examine the design trade-off between capacitance per area and losses due to parasitic resistance and inductance. The results indicate that the proposed MWCNT based capacitor configurations can potentially offer orders of magnitude larger capacitance per area and comparable quality factors to traditional metal-based integrated capacitors.

A carbon nanotube capacitor structure

Abstract: In this abstract, we present additional details on a new capacitor, CNCAP (carbon nanotube capacitor), compare it to existing integrated circuit capacitor technologies, and address its manufacturability. Properties of metallic, single wall CNTs (large surface area and relatively low resistance) allow for the creation of a very high density capacitor structure. In the CNCAP, each CNT electrode is surrounded by four CNTs connected to the opposing electrode. This structure allows for a potential capacitance/area greater than 100 fF/mum2. The electrical model of two parallel metallic, single wall, CNTs. The principle scattering mechanism in a CNT is due to acoustic phonons (under low bias) and the ideal CNT resistance is based on its length R = (h/4e2)(1+L/lambdaacc) where lambdaacc is the mean free path. The inductance, L, of the CNTs is taken as 4.07 nH/mum, which corresponds to the kinetic inductance. The quantum capacitance per unit length (CQ) of the CNTs is 388 aF/mum.

Gate Electrostatics and Quantum Capacitance of Graphene Nanoribbons

Abstract: Capacitance-voltage (C-V) characteristics are important for understanding fundamental electronic structures and device applications of nanomaterials. The C-V characteristics of graphene nanoribbons (GNRs) are examined using self-consistent atomistic simulations. The results indicate strong dependence of the GNR C-V characteristics on the edge shape. For zigzag edge GNRs, highly non-uniform charge distribution in the transverse direction due to edge states lowers the gate capacitance considerably, and the self-consistent electrostatic potential significantly alters the band structure and carrier velocity. For an armchair edge GNR, the quantum capacitance is a factor of 2 smaller than its corresponding zigzag carbon nanotube, and a multiple gate geometry is less beneficial for transistor applications. Magnetic field results in pronounced oscillations on C-V characteristics.

Negativity of the excess noise in a quantum wire capacitively coupled to a gate

Abstract: The electrical current noise of a quantum wire is expected to increase with increasing applied voltage. We show that this intuition can be wrong. Specifically, we consider a single channel quantum wire with impurities and with a capacitive coupling to nearby metallic gates and find that its excess noise, defined as the change in the noise caused by the finite voltage, can be negative at zero temperature. This feature is present both for large (c ≫ cq) and small (c ≪ cq) capacitive coupling, where c is the geometrical and cq the quantum capacitance of the wire. In particular, for c ≫ cq, negativity of the excess noise can occur at finite frequency when the transmission coefficients are

Shunt quantum capacitance induced source switching in an electron Y-branch switch

Abstract: Switching between two electron sources can be realized classically by two complementary operating gates. We have studied the switching between two branches serving as electron sources in a Y-branched nanojunction controlled by one gate. By sweeping the voltage at the in-plane gate we observed switching between the electron sources. The source switching deviates from the classical source and drain switching and is related to a shunt quantum capacitance.

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

Abstract: A low complexity computational model of the current-voltage characteristics for graphene nano-ribbon (GNR) field effect transistors (FET), able to simulate a hundred of points in few seconds using a PC, is presented. For quantum capacitance controlled devices, self-consistent calculations of the electrostatic potential can be skipped. Instead, analytical closed-form electrostatic potential from Laplace's equation yields accurate results compared with that obtained by self-consistent Non-Equilibrium Green's Functions (NEGF) method. The model includes both tunnelling 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.

Modeling and analysis of intrinsic gate capacitance for carbon nanotube array based devices considering variation in screening effect and diameter

Abstract: An accurate analytical model for intrinsic gate capacitance of carbon nanotube array based back-gated FET is proposed. The model accounts for electrostatic capacitive coupling between nanotubes as well as screening effect for any given number of nanotubes, their diameter, and pitch. Analysis using this model shows that as the number of nanotubes increases, although the overall electrostatic capacitance per nanotube decreases by up to 50%, the electrostatic capacitance becomes the dominant factor over the quantum capacitance. Furthermore, applicability of the proposed model in presence of practical fabrication process variations such as variation in diameter of carbon nanotubes is demonstrated using a probabilistic approach.

A Quantum–Mechanical View on the Capacitance of a Silicon p-n Junction

Abstract: We have calculated the capacitance of a silicon p-n junction from a self-consistent solution to the effective-mass Schroumldinger and Poisson equations. Although the p-n product and the charge distribution deviate strongly from the semiclassical calculations, the quantum mechanically calculated capacitance of the silicon p-n junction differs only weakly from the semiclassical result. We show that the deviation from the semiclassical result can be approximated as band-gap narrowing in the quasi-neutral regions due to the exchange-energy term in the effective-mass Schroumldinger equation

Measurement of impedance of individual carbon nanotubes

Abstract: We describe impedance measurements of individual single wall carbon nanotubes (SWNTs) in the frequency range of 40 Hz to 100 MHz. The tubes were assembled on the active channel of field effect transistor (FET) structures from aqueous suspension using dielectrophoresis. The FET channels were made by using photo-lithography. We utilized a resistance-capacitance (RC) lumped element circuit model to describe the observed impedance of the tubes and the corresponding contact resistance. At the low frequency limit the impedance is frequency independent and equivalent to the real resistance. In the high frequency range we observe a sharp conductor-insulator transition at a crossover frequency, above which the circuit response becomes capacitive. The extracted SWNT capacitance, CSWNT, of about 4 10-14 F/mum, is independent on the total real resistance, however the CSWNT value is larger than that theoretically predicted quantum capacitance of a perfect tube. Our observations also indicate that the damping frequency is lower than the theoretically predicted in SWNTs.

A performance analysis for single-walled metallic Carbon Nanotubes as global and intermediate on-chip interconnects

Abstract: This paper analyzes several delay estimates for metallic carbon nanotubes (CNT) as interconnects of very large scale integrated (VLSI) chips. A study of the 2005 edition of the international technology roadmap (ITRS) [1] for global/intermediate interconnects is presented to highlight the significant issues encountered with the projected performance of copper/aluminum interconnects till 2020. Then a worst case performance analysis of metallic CNTs is presented and compared versus copper interconnects. It is evaluated that the RC delay of CNTs does not meet the future RC delay requirements of on-chip intermediate and global wires.

Quantum Capacitance Measurement for SWNT FET with Thin ALD High-k Dielectric

Abstract: The thin conformal HfO2 dielectric, which could provide very large geometric top-gate-capacitance Cgg comparable to SWNT quantum capacitance Cq, and the capacitance measurement technique developed by us, which could reduce the background capacitance down to C0 ~30aF, are two key promising factors for us to study the quantum capacitance of the SWNT FET. We successfully got the pronounced oscillating peaks in the Cq vs. top-gate VTG, which will be very usefully for us to better characterize the performance of the SWNT FETs and to further study the low-dimensional electronic structure.

Quantum Capacitance Spectroscopy of Single Nanotube Molecules

Abstract: The electronic structures of single-walled carbon nanotubes (SWNT) are stemmed from graphene and sensitivity to diameter, chirality and family types Probing non-linear electronic effects due to trigonal anisotropy in graphene and daughter SWNTs is fundamental and remains challenging Here, we use quantum capacitance spectroscopy to determine the first and second subband van Hove singularity positions of semiconducting SWNTs and reveal deviations from linear theory in a family dependent manner, resulted from trigonal warping in graphene Quantum capacitance spectra further reveal the shapes and electron-hole effective-masses of various subbands of a SWNT, amplify the chirality and family dependence of trigonal distortion and thus enables structural identification of a general SWNT up to large diameter without limitations of optical spectroscopy Single-molecule capacitance spectroscopy provides a powerful approach to probe the electronic structures of graphene, SWNTs and other low-dimensional systems It also enables the first revelation of carrier-mobility in SWNTs of known chirality