Quantum capacitance

About: Quantum capacitance is a research topic. Over the lifetime, 954 publications have been published within this topic receiving 24165 citations.

An exactly solvable model for the graphene transistor in the quantum capacitance limit

Abstract: We explore the ultimate behavior of the graphene transistor in the quantum capacitance limit. The quantum capacitance formulation allows for an exactly solvable model, and the ideal assumptions provide an upper bound on performance, including peak currents of 1 mA/μm with mobilities as low as 2000 cm2/V s for channel length of 1 μm, as well as linearly increasing transconductance not observed in conventional transistors. A negative differential resistance is predicted under certain conditions, with a maximum peak-to-valley-current ratio of 4. Finally, the effects of oxide scaling are elucidated and the oxide capacitances required for quantum capacitance limited behavior are quantified.

Adsorption of metal atoms on silicene: stability and quantum capacitance of silicene-based electrode materials.

Abstract: We explore the adsorption stability and quantum capacitance of transition metal atoms on silicene based on first-principles calculations. Silicene with a buckled atomic layer has a high surface/volume ratio and silicene-based materials are expected to have potential applications for supercapacitors. We find that the most favorable adsorption sites on pristine silicene are valley sites for Al and Ti, and hollow sites for Ag, Cu and Au, respectively. Among all these systems with the doping of metal atoms, silicene is modulated to possess a quasi-metallic characteristic, accompanied by an appreciable electron transfer and the formation of defect states near the Fermi level. Due to the low density of states near the Fermi level, the quantum capacitance of pristine silicene has been limited. By the doping of metal atoms, especially Ti atoms, with the introduction of localized defect states near the Fermi level, quantum capacitance is found to be enhanced significantly. In addition, the quantum capacitance is found to increase monotonically following the increase of doping concentrations.

Contact-induced charge contributions to non-local spin transport measurements in Co/MgO/graphene devices

Abstract: Recently, it has been shown that oxide barriers in graphene-based non-local spin-valve structures can be the bottleneck for spin transport. The barriers may cause spin dephasing during or right after electrical spin injection which limits spin transport parameters such as the spin lifetime of the whole device. An important task is to characterize the quality of the oxide barriers of both spin injection and detection contacts in a fabricated device. To address this issue, we discuss the influence of spatially inhomogeneous oxide barriers and especially conducting pinholes within the barriers on the background signal in non-local measurements of graphene/MgO/Co spin-valve devices. By both simulations and reference measurements on devices with non-ferromagnetic electrodes, we demonstrate that the background signal can be caused by an inhomogeneous current flow through the oxide barriers. As a main result, we demonstrate the existence of charge accumulation besides the actual spin accumulation signal in non-local voltage measurements, which can be explained by a redistribution of charge carriers by a perpendicular magnetic field similar to the classical Hall effect. Furthermore, we present systematic studies of the phase of the low frequency non-local ac voltage signal which is measured in non-local spin measurements when applying ac lock-in techniques. This phase has so far widely been neglected in the analysis of non-local spin transport. We demonstrate that this phase is another hallmark of the homogeneity of the MgO spin injection and detection barriers. We link backgate dependent changes of the phase to the interplay between the capacitance of the oxide barrier and the quantum capacitance of graphene.

The Quantum and Classical Capacitance Limits of InSb and InAs Nanowire FETs

Abstract: A comparison of nanowire FETs (NWFETs) of identical geometries but operating in two different regimes, namely, the quantum capacitance (QC) and classical capacitance (CC) regimes, is presented n-type InSb and InAs NWFETs up to ~50 nm in diameter operate in the QC limit (QCL), and the corresponding p-type NWFETs operate in the CC limit Drive currents at a fixed gate overdrive for the n- and p-type devices are found to be well matched Nevertheless, the p-type devices have twice the delay times, half the intrinsic cutoff frequencies, twice the power-delay products, and four to five times the energy-delay products of the n-type devices, assuming transport is ballistic Analytical expressions are derived for the QC, the current, the charge, the power-delay product, the energy-delay product, the gate delay time, and the cutoff frequency for a single-moded device operating in the QCL The expressions for the power-delay product, energy-delay product, and the cutoff frequency are fundamental limits for such devices

Multiscale Analysis for Field-Effect Penetration through Two-Dimensional Materials

Abstract: Gate-tunable two-dimensional (2D) materials-based quantum capacitors (QCs) and van der Waals heterostructures involve tuning transport or optoelectronic characteristics by the field effect. Recent studies have attributed the observed gate-tunable characteristics to the change of the Fermi level in the first 2D layer adjacent to the dielectrics, whereas the penetration of the field effect through the one-molecule-thick material is often ignored or oversimplified. Here, we present a multiscale theoretical approach that combines first-principles electronic structure calculations and the Poisson–Boltzmann equation methods to model penetration of the field effect through graphene in a metal–oxide–graphene–semiconductor (MOGS) QC, including quantifying the degree of “transparency” for graphene two-dimensional electron gas (2DEG) to an electric displacement field. We find that the space charge density in the semiconductor layer can be modulated by gating in a nonlinear manner, forming an accumulation or inversio...