Dmitri E. Nikonov

Analysis of graphene nanoribbons as a channel material for field-effect transistors

Electronic properties of graphene (carbon) nanoribbons are studied and compared to those of carbon nanotubes. The nanoribbons are found to have qualitatively similar electron band structure which depends on chirality but with a significantly narrower band gap. The low- and high-field mobilities of the nanoribbons are evaluated and found to be higher than those of carbon nanotubes for the same unit cell but lower at matched band gap or carrier concentration. Due to the inverse relationship between mobility and band gap, it is concluded that graphene nanoribbons operated as field-effect transistors must have band gaps <0.5eV to achieve mobilities significantly higher than those of silicon and thus may be better suited for low power applications.

Overview of Beyond-CMOS Devices and a Uniform Methodology for Their Benchmarking

Multiple logic devices are presently under study within the Nanoelectronic Research Initiative (NRI) to carry the development of integrated circuits beyond the complementary metal-oxide-semiconductor (CMOS) roadmap. Structure and operational principles of these devices are described. Theories used for benchmarking these devices are overviewed, and a general methodology is described for consistent estimates of the circuit area, switching time, and energy. The results of the comparison of the NRI logic devices using these benchmarks are presented.

Performance Projections for Ballistic Graphene Nanoribbon Field-Effect Transistors

The upper limit performance potential of ballistic carbon nanoribbon MOSFETs (CNR MOSFETs) is examined. Calculation of the bandstructure of nanoribbons using a single pz-orbital tight-binding method and evaluation of the current-voltage characteristics of a nanoribbon MOSFET were used in a semiclassical ballistic model. The authors find that semiconducting ribbons a few nanometers in width behave electronically in a manner similar to carbon nanotubes, achieving similar ON-current performance. The calculations show that semiconducting CNR transistors can be candidates for high-mobility digital switches, with the potential to outperform the silicon MOSFET. Although wide ribbons have small bandgaps, which would increase subthreshold leakage due to band to band tunneling, their ON-current capabilities could still be attractive for certain applications

Performance Comparison Between p-i-n Tunneling Transistors and Conventional MOSFETs

In this paper, we present a detailed performance comparison between conventional n-i-n MOSFET transistors and tunneling field-effect transistors (TFETs) based on the p-i-n geometry, using semiconducting carbon nanotubes as the model channel material. Quantum-transport simulations are performed using the nonequilibrium Greens function formalism considering realistic phonon-scattering and band-to-band tunneling mechanisms. Simulations show that TFETs have a smaller quantum capacitance at most gate biases. Despite lower on-current, they can switch faster in a range of on/off-current ratios. Switching energy for TFETs is observed to be fundamentally smaller than that for MOSFETs, leading to lower dynamic power dissipation. Furthermore, the beneficial features of TFETs are retained with different bandgap materials. These reasons suggest that the p-i-n TFET is well suited for low-power applications.

Modeling of Nanoscale Devices

We aim to provide engineers with an introduction to the nonequilibrium Greens function (NEGF) approach, which is a powerful conceptual tool and a practical analysis method to treat nanoscale electronic devices with quantum mechanical and atomistic effects. We first review the basis for the traditional, semiclassical description of carriers that has served device engineers for more than 50 years. We then describe why this traditional approach loses validity at the nanoscale. Next, we describe semiclassical ballistic transport and the Landauer-Buttiker approach to phase-coherent quantum transport. Realistic devices include interactions that break quantum mechanical phase and also cause energy relaxation. As a result, transport in nanodevices is between diffusive and phase coherent. We introduce the NEGF approach, which can be used to model devices all the way from ballistic to diffusive limits. This is followed by a summary of equations that are used to model a large class of structures such as nanotransistors, carbon nanotubes, and nanowires. Applications of the NEGF method in the ballistic and scattering limits to silicon nanotransistors are discussed.

Nonequilibrium Green's Function Treatment of Phonon Scattering in Carbon-Nanotube Transistors

We present a detailed treatment of dissipative quantum transport in carbon-nanotube field-effect transistors (CNT-FETs) using the nonequilibrium Greens function formalism. The effect of phonon scattering on the device characteristics of CNT-FETs is explored using extensive numerical simulation. Both intra- and intervalley scattering mediated by acoustic (AP), optical (OP), and radial-breathing-mode (RBM) phonons are treated. Realistic phonon dispersion calculations are performed using force- constant methods, and electron-phonon coupling is determined through microscopic theory. Specific simulation results are presented for (16,0), (19,0), and (22,0) zigzag CNTFETs, which are in the experimentally useful diameter range. We find that the effect of phonon scattering on device performance has a distinct bias dependence. Up to moderate gate biases, the influence of high-energy OP scattering is suppressed, and the device current is reduced due to elastic backscattering by AP and low-energy RBM phonons. At large gate biases, the current degradation is mainly due to high-energy OP scattering. The influence of both AP and high-energy OP scattering is reduced for larger diameter tubes. The effect of RBM mode, however, is nearly independent of the diameter for the tubes studied here.

Ballistic graphene nanoribbon metal-oxide-semiconductor field-effect transistors: A full real-space quantum transport simulation

A real-space quantum transport simulator for graphene nanoribbon (GNR) metal-oxide-semiconductor field-effect transistors (MOSFETs) has been developed and used to examine the ballistic performance of GNR MOSFETs. This study focuses on the impact of quantum effects on these devices and on the effect of different type of contacts. We found that two-dimensional (2D) semi-infinite graphene contacts produce metal-induced-gap states (MIGS) in the GNR channel. These states enhance quantum tunneling, particularly in short channel devices, they cause Fermi level pinning and degrade the device performance in both the ON-state and OFF-state. Devices with infinitely long contacts having the same width as the channel do not indicate MIGS. Even without MIGS quantum tunneling effects such as band-to-band tunneling still play an important role in the device characteristics and dominate the OFF-state current. This is accurately captured in our nonequilibrium Greens’ function quantum simulations. We show that both narrow (...

Benchmarking of Beyond-CMOS Exploratory Devices for Logic Integrated Circuits

A new benchmarking of beyond-CMOS exploratory devices for logic integrated circuits is presented. It includes new devices with ferroelectric, straintronic, and orbitronic computational state variables. Standby power treatment and memory circuits are included. The set of circuits is extended to sequential logic, including arithmetic logic units. The conclusion that tunneling field-effect transistors are the leading low-power option is reinforced. Ferroelectric transistors may present an attractive option with faster switching delay. Magnetoelectric effects are more energy efficient than spin transfer torque, but the switching speed of magnetization is a limitation. This article enables a better focus on promising beyond-CMOS exploratory devices.

A Three-Terminal Dual-Pillar STT-MRAM for High-Performance Robust Memory Applications

The design of a three-terminal self-aligned dual-pillar (DP) magnetic tunnel junction (MTJ) utilizing both current-induced spin-transfer torque (STT) and magnetic domain-wall motion effects is proposed for high-speed nonvolatile robust memory applications. The choice of a thin tunneling oxide (~0.9 nm) in a write-in port, spatially and electrically separated from a read-out port incorporating a thicker (~1.8 nm) oxide on an extended thin-film multilayer stack, significantly improves the overall cell stability and parametric process yield of a memory array. A dual-bit-line memory architecture incorporating a single-ended voltage-sensing scheme for fast data readout with just one access transistor per cell is also proposed for the first time. The technology-circuit cooptimization of the proposed single-transistor (1T) DP STT magnetic random access memory (MRAM) cell is carried out using effective mass-based transport simulations in nonequilibrium Greens function formalism and accurate micromagnetic simulations involving the Landau-Lifshitz-Gilbert-Slonczewski equation. The proposed DP STT-MRAM bit cell outperforms a state-of-the-art 1T-1MTJ STT-MRAM cell in terms of higher cell tunneling magnetoresistance, simplified memory array architecture with a single supply for read/write, and significantly lower probability of disturb and access failures under parametric process variations with a marginal increase in critical switching current.

Proposal of a Spin Torque Majority Gate Logic

A spin-based logic device is proposed. It is comprised of a common free ferromagnetic layer and four discrete ferromagnetic nanopillars, each containing an independent fixed layer. It has the functionality of a majority gate and is switched via motion of domain walls by spin transfer torque. Validity of its logic operation and a quantitative performance prediction are demonstrated by micromagnetic simulation. It is entirely compatible with complimentary metal-oxide-semiconductor technology.