arXiv subjects

Jing Guo

Publications and source records attributed to Jing Guo.

At least 199 records · Page 11Linked to original sources

arXiv2009

Projected Performance Advantage of Multilayer Graphene Nanoribbon as Transistor Channel Material

The performance limits of the multilayer graphene nanoribbon (GNR) field-effect transistor (FET) are assessed and compared to those of monolayer GNR FET and carbon nanotube (CNT) FET. The results show that with a thin high-k gate insulator and reduced interlayer coupling, multilayer GNR FET can significantly outperform its CNT counterpart with a similar gate and bandgap in terms of the ballistic on-current. In the presence of optical phonon scattering, which has a short mean free path in the graphene-derived nanostructures, the advantage of the multilayer GNRFET is even more significant. The simulation results indicate multilayer GNRs with incommensurate non-AB stacking and weak interlayer coupling are the best candidate for high performance GNR FETs.

cond-mat.mes-hall↗

arXiv2009

Computational Study of Tunneling Transistor Based on Graphene Nanoribbon

Tunneling field-effect transistors (FETs) have been intensely explored recently due to its potential to address power concerns in nanoelectronics. The recently discovered graphene nanoribbon (GNR) is ideal for tunneling FETs due to its symmetric bandstructure, light effective mass, and monolayer-thin body. In this work, we examine the device physics of p-i-n GNR tunneling FETs using atomistic quantum transport simulations. The important role of the edge bond relaxation in the device characteristics is identified. The device, however, has ambipolar I-V characteristics, which are not preferred for digital electronics applications. We suggest that using either an asymmetric source-drain doping or a properly designed gate underlap can effectively suppress the ambipolar I-V. A subthreshold slope of 14mV/dec and a significantly improved on-off ratio can be obtained by the p-i-n GNR tunneling FETs.

cond-mat.mtrl-sci↗

arXiv2009

Modeling edge effects in Graphene Nanoribbon Field-effect Transistors with real and mode space methods

A computationally efficient mode space simulation method for atomistic simulation of a graphene nanoribbon field-effect transistor in the ballistic limits is developed. The proposed simulation scheme, which solves the nonequilibrium Green's function coupled with a three dimensional Poisson equation, is based on the atomistic Hamiltonian in a decoupled mode space. The mode space approach, which only treats a few modes (subbands), significantly reduces the simulation time. Additionally, the edge bond relaxation and the third nearest neighbor effects are also included in the quantum transport solver. Simulation examples show that, mode space approach can significantly decrease the simulation cost by about an order of magnitude, yet the results are still accurate. This article also demonstrates that the effects of edge bond relaxation and third nearest neighbor significantly influence the transistor's performance and are necessary to be included in the modeling.

cond-mat.mtrl-sci↗

arXiv2008

Diameter-Dependent Electron Mobility of InAs Nanowires

Temperature-dependent I-V and C-V spectroscopy of single InAs nanowire field-effect transistors were utilized to directly shed light on the intrinsic electron transport properties as a function of nanowire radius. From C-V characterizations, the densities of thermally-activated fixed charges and trap states on the surface of untreated (i.e., without any surface functionalization) nanowires are investigated while enabling the accurate measurement of the gate oxide capacitance; therefore, leading to the direct assessment of the field-effect mobility for electrons. The field-effect mobility is found to monotonically decrease as the radius is reduced to sub-10 nm, with the low temperature transport data clearly highlighting the drastic impact of the surface roughness scattering on the mobility degradation for miniaturized nanowires. More generally, the approach presented here may serve as a versatile and powerful platform for in-depth characterization of nanoscale, electronic materials.

cond-mat.mtrl-sci↗

arXiv2008

Performance Comparison of Graphene Nanoribbon FETs with Schottky Contacts and Doped Reservoirs

We present an atomistic 3D simulation study of the performance of graphene nanoribbon (GNR) Schottky barrier (SB) FETs and transistors with doped reservoirs (MOSFETs) by means of the self-consistent solution of the Poisson and Schrodinger equations within the non-equilibrium Green's function (NEGF) formalism. Ideal MOSFETs show slightly better electrical performance, for both digital and THz applications. The impact of non-idealities on device performance has been investigated, taking into account the presence of single vacancy, edge roughness and ionized impurities along the channel. In general, MOSFETs show more robust characteristics than SBFETs. Edge roughness and single vacancy defect largely affect performance of both device types.

cond-mat.mes-hall↗

arXiv2008

Room Temperature All Semiconducting sub-10nm Graphene Nanoribbon Field-Effect Transistors

Sub-10nm wide graphene nanoribbon field-effect transistors (GNRFETs) are studied systematically. All sub-10nm GNRs afforded semiconducting FETs without exception, with Ion/Ioff ratio up to 10^6 and on-state current density as high as ~2000uA/um. We estimated carrier mobility ~200cm2/Vs and scattering mean free path ~10nm in sub-10nm GNRs. Scattering mechanisms by edges, acoustic phonon and defects are discussed. The sub-10nm GNRFETs are comparable to small diameter (d<=~1.2nm) carbon nanotube FETs with Pd contacts in on-state current density and Ion/Ioff ratio, but have the advantage of producing all-semiconducting devices.

cond-mat.mes-hall↗

arXiv2007

Effect of edge roughness in graphene nanoribbon transistors

The effects of edge irregularity and mixed edge shapes on the characteristics of graphene nanoribbon transistors are examined by self-consistent atomistic simulations based on the non-equilibrium Green's function formalism. The minimal leakage current increases due to the localized states induced in the band gap, and the on-current decreases due to smaller quantum transmission and the self-consistent electrostatic effect in general. Although the ratio between the on-current and minimal leakage current decreases, the transistor still switches even in the presence of edge roughness. The variation between devices, however, can be large, especially for a short channel length.

cond-mat.mes-hall↗

arXiv2007

A Computational Study of Vertical Partial Gate Carbon Nanotube FETs

A vertical partial gate carbon nanotube (CNT) field-effect transistor (FET), which is amenable to the vertical CNT growth process and offers the potential for a parallel CNT array channel, is simulated using a self-consistent atomistic approach. We show that the underlap between the gate and the bottom electrode (required for isolation between electrodes) is advantageous for transistor operation because it suppresses ambipolar conduction. A vertical CNTFET with a gate length that covers only 1/6 of the channel length has a much smaller minimum leakage current than one without underlap, while maintaining comparable on current. Both n-type and p-type transistor operations with balanced performance metrics can be achieved on a single partial gate FET by using proper bias schemes. Even with a gate underlap, it is demonstrated that increasing the CNT diameter still leads to a simultaneous increase of on current and minimum leakage current. Along with a partial gate, the simulated transistor features a significant amount of air between the surface of the channel CNT and the gate insulator, as is caused by the vertical CNT growth process. Filing this pore with a high-k insulator is shown to have the potential to decrease the on current, due to electrostatic phenomena at the source-channel contact.

cond-mat.mes-hall↗

arXiv2007

Scaling Behaviors of Graphene Nanoribbon FETs: A Three Dimensional Quantum Simulation Study

The scaling behaviors of graphene nanoribbon (GNR) Schottky barrier field-effect transistors (SBFETs) are studied by solving the non-equilibrium Green's function (NEGF) transport equation in an atomistic basis set self-consistently with a three-dimensional Poisson equation. The armchair edge GNR channel shares similarities with a zigzag CNT, but it has a different geometry and quantum confinement boundary condition in the transverse direction. The results indicate that the I-V characteristics are ambipolar and strongly depend on the GNR width because the bandgap of the GNR is approximately inversely proportional to its width, which agrees with recent experiments. A multiple gate geometry improves immunity to short channel effects, but it offers smaller improvement than it does for Si MOSFETs in terms of the on-current and transconductance. Reducing the oxide thickness is more useful for improving transistor performance than using a high-k gate insulator. Significant increase of the minimal leakage current is observed when the channel length is scaled below 10nm because the small effective mass facilitates strong source-drain tunneling. The GNRFET, therefore, does not promise extending the ultimate scaling limit of Si MOSFETs. The intrinsic switching speed of a GNR SBFET, however, is several times faster than that of Si MOSFETs, which could lead to promising high speed electronics applications, where the large leakage of GNR SBFETs is of less concern.

cond-mat.mes-hall↗

arXiv2007

Gate Electrostatics and Quantum Capacitance of Graphene Nanoribbons

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.

cond-mat.mes-hall↗

arXiv2007

Electrically driven thermal light emission from individual single-walled carbon nanotubes

Light emission from nanostructures exhibits rich quantum effects and has broad applications. Single-walled carbon nanotubes (SWNTs) are one-dimensional (1D) metals or semiconductors, in which large number of electronic states in a narrow range of energies, known as van Hove singularities, can lead to strong spectral transitions. Photoluminescence and electroluminescence involving interband transitions and excitons have been observed in semiconducting SWNTs, but are not expected in metallic tubes due to non-radiative relaxations. Here, we show that in the negative differential conductance regime, a suspended quasi-metallic SWNT (QM-SWNT) emits light due to joule-heating, displaying strong peaks in the visible and infrared corresponding to interband transitions. This is a result of thermal light emission in 1D, in stark contrast with featureless blackbody-like emission observed in large bundles of SWNTs or multi-walled nanotubes. This allows for probing of the electronic temperature and non-equilibrium hot optical phonons in joule-heated QM-SWNTs.

cond-mat.mes-hall↗

arXiv2006

Effect of Phonon Scattering on Intrinsic Delay and Cut-Off Frequency of Carbon Nanotube FETs

The effect of phonon scattering on the intrinsic delay and cut-off frequency of Schottky barrier carbon nanotube (CNT) FETs is examined by self-consistently solving the Poisson equation and the Schrodinger equation using the non-equilibrium Greens function (NEGF) formalism. Carriers are mostly scattered by optical and zone boundary phonons beyond the beginning of the channel. We show that the scattering has a small direct effect on the DC on-current of the CNTFET, but it results in significant pile-up of charge and degradation of average carrier velocity. Due to charge pile-up and random walks of carriers, the intrinsic gate capacitance and delay significantly increase, and the intrinsic cut-off frequency decreases. The results are important for assessing the performance potential of CNTFETs for radio-frequency (RF) electronics and digital electronics applications.

cond-mat.mes-hall↗

arXiv2005

High Performance N-Type Carbon Nanotube Field Effect Transistors with Chemically Doped Contacts

Short channel (~80 nm) n-type single-walled carbon nanotube (SWNT) field-effect transistors (FETs) with potassium (K) doped source and drain regions and high-k gate dielectrics (ALD HfO2) are obtained. For nanotubes with diameter ~ 1.6 nm and bandgap ~ 0.55 eV, we obtain n-MOSFET-like devices exhibiting high on-currents due to chemically suppressed Schottky barriers at the contacts, subthreshold swing of 70mV/decade, negligible ambipolar conduction and high on/off ratios up to 10^6 at a bias voltage of 0.5V. The results compare favorably with the state-of-the-art silicon n-MOSFETs and demonstrate the potential of SWNTs for future complementary electronics. The effects of doping level on the electrical characteristics of the nanotube devices are discussed.

cond-mat.mtrl-sci↗

arXiv2004

Self-Aligned Ballistic Molecular Transistors and Electrically Parallel Nanotube Arrays

Carbon nanotube field-effect transistors with structures and properties near the scaling limit with short (down to 50 nm) channels, self aligned geometries, palladium electrodes with low contact resistance and high-k dielectric gate insulators are realized. Electrical transport in these miniature transistors is near ballistic up to high biases at both room and low temperatures. Atomic layer deposited (ALD) high-k films interact with nanotube sidewalls via van der Waals interactions without causing weak localization at 4 K. New fundamental understanding of ballistic transport, optical phonon scattering and potential interfacial scattering mechanisms in nanotubes are obtained.

cond-mat.other↗

arXiv2004

High-Field, Quasi-Ballistic Transport in Short Carbon Nanotubes

Single walled carbon nanotubes with Pd ohmic contacts and lengths ranging from several microns down to 10 nm are investigated by electron transport experiments and theory. The mean free path (mfp) for acoustic phonon scattering is estimated to be lap~300 nm, and that for optical phonon scattering is lop~15 nm. Transport through very short (~10 nm) nanotubes is free of significant acoustic and optical phonon scattering and thus ballistic and quasi-ballistic at the low and high bias voltage limits respectively. High currents of up to 70 uA can flow through a short nanotube. Possible mechanisms for the eventual electrical breakdown of short nanotubes at high fields are discussed. The results presented here have important implications to high performance nanotube transistors and interconnects.

cond-mat.mes-hall↗

arXiv2003

Towards Multi-Scale Modeling of Carbon Nanotube Transistors

Multiscale simulation approaches are needed in order to address scientific and technological questions in the rapidly developing field of carbon nanotube electronics. In this paper, we describe an effort underway to develop a comprehensive capability for multiscale simulation of carbon nanotube electronics. We focus in this paper on one element of that hierarchy, the simulation of ballistic CNTFETs by self-consistently solving the Poisson and Schrodinger equations using the non-equilibrium Greens function (NEGF) formalism. The NEGF transport equation is solved at two levels: i) a semi-empirical atomistic level using the pz orbitals of carbon atoms as the basis, and ii) an atomistic mode space approach, which only treats a few subbands in the tube-circumferential direction while retaining an atomistic grid along the carrier transport direction. Simulation examples show that these approaches describe quantum transport effects in nanotube transistors. The paper concludes with a brief discussion of how these semi-empirical device level simulations can be connected to ab initio, continuum, and circuit level simulations in the multi-scale hierarchy.

cond-mat.mes-hall↗

arXiv2003

Carbon Nanotube Field-Effect Transistors With Integrated Ohmic Contacts and High-k Gate Dielectrics

High performance enhancement mode semiconducting carbon nanotube field-effect transistors (CNTFETs) are obtained by combining ohmic metal-tube contacts, high dielectric constant HfO2 films as gate insulators, and electrostatically doped nanotube segments as source/drain electrodes. The combination of these elements affords high ON currents, subthreshold swings of ~ 70-80 mV/decade, and allows for low OFF currents and suppressed ambipolar conduction. The doped source and drain approach resembles that of MOSFETs and can impart excellent OFF states to nanotube FETs under aggressive vertical scaling. This presents an important advantage over devices with metal source/drain, or devices commonly referred to as Schottky barrier FETs.

cond-mat.mes-hall↗

arXiv2003

Predicted Performance Advantages of Carbon Nanotube Transistors with Doped Nanotubes as Source/Drain

Most carbon nanotube field-effect transistors (CNTFETs) directly attach metal source/drain contacts to an intrinsic nanotube channel. When the gate oxide thickness is reduced, such transistors display strong ambipolar conduction, even when the Schottky barrier for electrons (or for holes) is zero. The resulting leakage current, which increases exponentially with the drain voltage, constrains the potential applications of such devices. In this paper, we use numerical simulations to show that if CNT based metal-oxide-semiconductor (MOS) FETs can be achieved by using heavily doped CNT sections as source and drain, ambipolar conduction will be suppressed, leakage current will be reduced, and the scaling limit imposed by source-drain tunneling will be extended. By eliminating the Schottky barrier between the source and channel, the transistor will be capable of delivering more on-current. The leakage current of such devices will be controlled by the full bandgap of CNTs (instead of half of the bandgap for SB CNTFETs) and band-to-band tunneling. These factors will depend on the diameter of nanotubes and the power supply voltage.

cond-mat.mes-hall↗