Showing papers on "Quantum capacitance published in 2006"

Quantitative theory of nanowire and nanotube antenna performance

Abstract: We present quantitative predictions of the performance of nanotubes and nanowires as antennas, including the radiation resistance, the input reactance and resistance, and antenna efficiency, as a function of frequency and nanotube length. Particular attention is paid to the quantum capacitance and kinetic inductance. We develop models for both far-field antenna patterns as well as near-field antenna-to-antenna coupling. In so doing, we also develop a circuit model for a transmission line made of two parallel nanotubes, which has applications for nanointerconnect technology. Finally, we derive an analog of Hallen's integral equation appropriate for single-walled carbon nanotube antennas

Measurement of the quantum capacitance of interacting electrons in carbon nanotubes

Abstract: The electronic capacitance of a one-dimensional system such as a carbon nanotube is a thermodynamic quantity that contains fundamental information about the ground state1. It is composed of an electrostatic component describing the interactions between electrons and their correlations, and a kinetic term given by the electronic density of states. Here, we use a field-effect transistor geometry to obtain the first direct capacitance measurement of individual carbon nanotubes, as a function of the carrier density. Our measurements detect the electrostatic part of the capacitance as well as the quantum corrections arising from the electronic density of states. We identify the van-Hove singularities that correspond to the one-dimensional electron and hole sub-bands and show that the measured capacitance exhibits clear electron–hole symmetry. Finally, our measurements suggest the existence of a negative capacitance, which has recently been predicted to exist in one dimension as a result of interactions between electrons2,3,4.

Electrochemistry at single-walled carbon nanotubes: the role of band structure and quantum capacitance.

Abstract: We present a theoretical description of the kinetics of electrochemical charge transfer at single-walled carbon nanotube (SWNT) electrodes, explicitly taking into account the SWNT electronic band structure. SWNTs have a distinct and low density of electronic states (DOS), as expressed by a small value of the quantum capacitance. We show that this greatly affects the alignment and occupation of electronic states in voltammetric experiments and thus the electrode kinetics. We model electrochemistry at metallic and semiconducting SWNTs as well as at graphene by applying the Gerischer−Marcus model of electron transfer kinetics. We predict that the semiconducting or metallic SWNT band structure and its distinct van Hove singularities can be resolved in voltammetry, in a manner analogous to scanning tunneling spectroscopy. Consequently, SWNTs of different atomic structure yield different rate constants due to structure-dependent variations in the DOS. Interestingly, the rate of charge transfer does not necessar...

Comparison of performance limits for carbon nanoribbon and carbon nanotube transistors

Abstract: Carbon-based nanostructures promise near ballistic transport and are being intensively explored for device applications. In this letter, the performance limits of carbon nanoribbon (CNR) field-effect transistors (FETs) and carbon nanotube (CNT) FETs are compared. The ballistic channel conductance and the quantum capacitance of the CNRFET are about a factor of 2 smaller than those of the CNTFET because of the different valley degeneracy factors for CNRs and CNTs. The intrinsic speed of the CNRFET is faster due to a larger average carrier injection velocity. The gate capacitance plays an important role in determining which transistor delivers a larger on current.

A numerical Schrödinger–Poisson solver for radially symmetric nanowire core–shell structures

Abstract: We present here a general purpose numerical Schrodinger–Poisson solver for radially symmetric nanowire core–shell structures for electronic and optoelectronic applications. The solver provides self-consistent solutions of the Schrodinger equation and the Poisson equation in cylindrical coordinates, for nanowire core–shell structures with radial compositional variation. Quantized energy levels as well as their associated electron wavefunctions and populations can be obtained from the solutions. Individual equation solvers were verified by comparison with scenarios where analytical results exist; verification of the self-consistent solution process was done by comparing results in the large radius limit with numerical solutions for a rectangular slab structure. We apply this solver to compute the charge/capacitance–voltage characteristics for a nanowire field effect device with wrap-around gate. It is shown that quantum confinement and the low dimensionality can give rise to, for representative nanowire FETs considered, ∼30% reduction in gate capacitance compared to the classically predicted value, and is ∼1/3 of the geometrical barrier limited capacitance.

Role of quantum capacitance in coupled low-dimensional electron systems

Abstract: We have investigated the charging behavior of a layer of self-assembled InAs quantum dots placed in close vicinity to a two-dimensional electron gas (2DEG). As the gate bias is changed, the number of electrons in each system is altered simultaneously. Based on the quantum capacitance of the involved layers we develop a general model to determine the charging state of coupled low-dimensional systems from capacitance-voltage (CV) spectroscopy. The model is then applied to the special case of a layer of self-assembled quantum dots coupled to a 2DEG. As a complementary method, we have employed Hall voltage measurements. We find that the measurement of the two-dimensional carrier density through lateral transport provides a direct insight into the vertical charging process of the quantum dot system. Six individual charging peaks related to the occupation of the $s$ and $p$ shells of the dots can be resolved. In agreement with results from CV spectroscopy, Coulomb blockade and quantization energies can be extracted. Moreover, the Hall measurement offers a higher peak-to-valley ratio and a better estimate for the number of simultaneously charged dots than the capacitance data.

A Small Signal Equivalent Circuit Model for Resonant Tunnelling Diode

Abstract: We report a resonant tunneling diode (RTD) small signal equivalent circuit model consisting of quantum capacitance and quantum inductance. The model is verified through the actual InAs/In0.53Ga0.47As/AlAs RTD fabricated on an InP substrate. Model parameters are extracted by fitting the equivalent circuit model with ac measurement data in three different regions of RTD current-voltage (I-V) characteristics. The electron lifetime, representing the average time that the carriers remain in the quasibound states during the tunneling process, is also calculated to be 2.09 ps.

Modelling of Chirality-Dependent Current–Voltage Characteristics of Carbon-Nanotube Field-Effect Transistors

Abstract: Current–voltage characteristics of ballistic carbon-nanotube field-effect transistors are characterized with an iterative simulation program. The influence of carbon-nanotube chirality and diameter on the output current is considered. An analytical current–voltage expression under the quantum capacitance limit and low-voltage application is derived. Our simulation results are compared with actual measurement data.

Graphene nanostructures for device applications

Abstract: After an introduction that explains why graphene is in general considered to be promising for electronic applications since a few years, we will discuss several of our own new findings on graphene field-effect transistors. In particular, this article makes three points: 1) The material itself - graphene - offers intrinsic advantages due to reduced scattering in low-dimensional structures. 2) The particular energy dispersion of graphene is a key enabler for operation in a new regime called - the quantum capacitance limit. 3) Scaling of graphene devices is not following the classical, diffusive theory. In detail, we present a simple argument why graphene nanostructures exhibit an intrinsic advantage when considering the gate delay in three-terminal device structures and explain why the possibility to operate in the quantum capacitance C q limit provides additional benefits. We show experimental data on the energy dependence of C q that support our previous statements and show first experimental evidence that scaling of the off-state in graphene transistors follows percolation theory rather than conventional diffusive transport equations.

Low-Temperature DC and RF Measurement and Modelling of InGaAs-InAlAs Resonant Tunneling Diodes down to 15 K

Abstract: The influence of temperature on bias dependent small-signal equivalent circuit components of a resonant tunneling diode (RTD) is investigated from 290 K down to 15 K. The RTD model based on bias dependent parasitic elements and the quantum capacitance as well as the quantum conductance is fitted to both, on-wafer DC and RF s-parameter measurements from 45 MHz to 40 GHz over a bias range of 0 V to 0.80 V. For the full temperature range, good agreement between extracted and measured parameters is shown.

One-dimensional screening effects in bulk-modulated carbon nanotube transistors

Abstract: We report density-functional theory (DFT), atomistic simulations of the screening properties of carbon nanotube (CNT) field-effect transistors (FETs). Our attention has been focused on recently demonstrated [1, 2] bulk-modulated CNTFETs. Our calculations have been intended to explore, at an atomistic level, the effect of the one-dimensional screening properties on the physical mechanisms which govern the channel conductance modulation in these new devices. We find that in order to correctly describe charge response mechanisms in a small-diameter nanotube, a calculation including electron-electron interaction effects is necessary. Many-body effects tied to the attractive nature of the exchange interaction can produce an unconventional charge response which manifests itself in a negative quantum capacitance.

Assessment of Carbon Nanotube FETs for High-Frequency Performance

Abstract: The small size, unusual topography, and technological immaturity of carbon nanotube field-effect transistors (CNFETs) have contributed, no doubt, to the fact that the present record for their measured "frequencyindependent performance" is a modest 23 GHz [1]. For the moment, then, we need to rely on simulations to get a better idea of the high-frequency capability of CNFETs. Any field-effect transistor exhibiting good high-frequency performance is likely to have a high, intrinsic, short-circuit, unity-current-gain, frequency