Dincer Unluer

Diluted chirality dependence in edge rough graphene nanoribbon field-effect transistors

We investigate the role of various structural nonidealities on the performance of armchair-edge graphene nanoribbon field effect transistors (GNRFETs). Our results show that edge roughness dilutes the chirality dependence often predicted by theory but absent experimentally. Instead, GNRs are classifiable into wide (semimetallic) versus narrow (semiconducting) strips, defining thereby the building blocks for wide-narrow-wide all-graphene devices and interconnects. Small bandgaps limit drain bias at the expense of band-to-band tunneling in GNRFETs. We outline the relation between device performance metrics and nonidealities such as width modulation, width dislocations and surface step, and nonideality parameters such as roughness amplitude and correlation length.

Body-coupled communication for body sensor networks

Body sensor networks (BSNs) offer a wealth of opportunities for precise, accurate, continuous, and non-invasive sensing of physiological phenomena, but their unique operating environment, the body-area, poses unique technical challenges. Popular communications solutions that utilize 2.4 GHz radio transmission suffer from significant and highly variable path loss in this setting. To compensate for such loss, radio transceivers often transmit at power levels at or above 1 mW -- a reality that limits battery life. We propose the use of body-coupled communication to address this issue, as it presents several distinct advantages over existing solutions, namely: reduced power consumption, minimal interference, and increased privacy. In this paper, we demonstrate a 23 MHz body-coupled channel that supports reliable data transfer with an average received power of 30 dBm over a 2.4 GHz radio frequency link. This scheme reduces power needed for transmission and increases battery life by up to 100%, while maintaining a favorable environment for application-specific quality of service requirements. Finally, we propose a system-level hardware architecture and explore its implications on BSN infrastructure.

Monolithically Patterned Wide–Narrow–Wide All-Graphene Devices

We investigate theoretically the performance advantages of all-graphene nanoribbon field-effect transistors (GNRFETs) whose channel and source/drain (contact) regions are patterned monolithically from a 2-D single sheet of graphene. In our simulated devices, the source/drain and interconnect regions are composed of wide GNR sections that are semimetallic, while the channel regions consist of narrow GNR sections that open semiconducting bandgaps. Our simulation employs a fully atomistic model of the device, contact, and interfacial regions using tight-binding theory. The electronic structures are coupled with a self-consistent 3-D Poissons equation to capture the nontrivial contact electrostatics, along with a quantum kinetic formulation of transport based on nonequilibrium Greens functions. Although we only consider a specific device geometry, our results establish several general performance advantages of such monolithic devices (besides those related to fabrication and patterning), namely, the improved electrostatics, suppressed short-channel effects, and Ohmic contacts at the narrow-to-wide interfaces.

Graphene devices, interconnect and circuits — challenges and opportunities

Graphene has recently emerged as a serious contender for the post Silicon era. Graphene NanoRibbon (GNR) devices have similar performance characteristics to Carbon Nano Tube (CNT) ones. However, lithographic patterning methods applied to graphene can avoid the degree of chirality control and alignment issues typical of CNTs, and GNR devices and GNR interconnect can in principle be seamlessly obtained by patterning single graphene sheets, thus leading to monolithically device-interconnect structures. Electrically doped GNR devices in series and in parallel can be used for creating complex GNR FET digital circuits. There are also several important challenges facing the graphene “brave new world,” but many of the difficulties hopefully will have tractable solutions. This paper examines the topic of GNR FET circuit design from a bottom-up theoretical perspective, starting with GNR device and interconnect modeling and simulation, while trying to reconcile theory with some recent experimental results.

Electronic ratchet: A non-equilibrium, low power switch

In this paper, we present a novel computing paradigm using a non-equilibrium electronic ratchet which is capable of driving current in the absence of an applied drain bias. By using a time varying, spatially asymmetric potential, we demonstrate that it is possible to create a net current from drift-diffusion processes of charge carriers. This is especially useful in reducing static dissipation encountered in conventional logic circuits. In addition, since the electronic ratchet acts as voltage-controlled current source, we find that the dynamic dissipation associated with charging/discharging of load capacitors is also decreased. Furthermore, we show that the ratchet device is naturally amenable to a dissipation reduction technique known as adiabatic clocking. Because of the unique charging mechanism of the ratchet, timing constraints on logic inputs—an important drawback of conventional adiabatic circuits—are not needed to achieve adiabatic computation.

Computing With Nonequilibrium Ratchets

Electronic ratchets transduce local spatial asymmetries into directed currents in the absence of a global drain bias by rectifying temporal signals that reside far from the thermal equilibrium. We show that the absence of a drain bias can provide distinct energy advantages for computation, specifically, reducing static dissipation in a logic circuit. Since the ratchet functions as a gate voltage-controlled current source, it also potentially reduces the dynamic dissipation associated with charging/discharging capacitors. In addition, the unique charging mechanism eliminates timing-related constraints on logic inputs, in principle allowing for adiabatic charging. We calculate the ratchet currents in classical and quantum limits, and show how a sequence of ratchets can be cascaded to realize universal Boolean logic.

Atomistic deconstruction of clear performance advantages of a monolithically patterned wide-narrow-wide all-graphene FET

Experiments show that wide GNRs are all metallic while ultra thin ribbons (<10 nm in width) are all semiconducting [1] and the bandgap increases as the width get narrower. This leads us to design an all a graphene nanoribbon field effect transistor (GNRFET) and atomistically simulate its transport properties. We expect this system to benefit from the unique 2-D electrostatics of the source-drain regions and the covalent bonding at the contact-channel interfaces. Our device model is unique compared to standard treatments of all graphene nanoribbons with side gates [2], because our drain/source contacts and the channel are patterned from a single graphene sheet template. Also our top gate geometry is superior compared to the side gates that act through fringing fields and are susceptible to tunneling due to the absence of an interfacing oxide film. Simulations were conducted using orthogonal tight-binding (TB) and the bandstructures coupled with non-equilibrium Greens Function [3] (NEGF)-based quantum transport formalism. The simulated geometry includes a metallic gate approximately three times wider than the channel placed 1nm above it. A wide, finite, and grounded substrate is placed 3nm below the channel region to control the device I–V characteristics. We used HfO2 (k=16) as a high-k top-gate dielectric and SiO2 (k=3.9) as the substrate dielectric with grounded substrate contact.

Physics-based GNRFET compact model for digital circuit design

Graphene has attracted significant interest as a possible candidate for future transistors because of its high carrier mobility and current density [1]. The biggest setback of intrinsic graphene in digital applications is the absence of a bandgap, needed for digital logic to distinguish between high and low current states. Experiments demonstrated the opening of a bandgap by either applying an interlayer electrical field on bilayer graphene [2], or quantum confinement in narrow graphene nanoribbons (GNRs) (<10 nm in width) [3], which ushered in the design of wide-narrow-wide GNR field-effect transistors (GNRFET) [4]. However, such a bandgap comes at the expense of mobility [5]. Goal of this paper is to show the architectural ramifications of small bandgap graphene, using physics based compact model benchmarked with experiment.

Graphene Nanoribbons: From Chemistry to Circuits

The Y-chart is a powerful tool for understanding the relationship between various views (behavioral, structural, and physical) of a system, at different levels of abstraction, from high-level, architecture and circuits, to low-level, devices and materials. We thus use the Y-chart adapted for graphene to guide the chapter and explore the relationship among the various views and levels of abstraction. We start with the innermost level, namely, the structural and chemical view. The edge chemistry of patterned graphene nanoribbons (GNRs) lies intermediate between graphene and benzene, and the corresponding strain lifts the degeneracy that otherwise promotes metallicity in bulk graphene. At the same time, roughness at the edges washes out chiral signatures, making the nanoribbon width the principal arbiter of metallicity. The width-dependent conductivity allows the design of a monolithically patterned Wide–Narrow–Wide (WNW) all graphene interconnect-channel heterostructure. In a three-terminal incarnation, this geometry exhibits superior electrostatics, a correspondingly benign short-channel effect and a reduction in the contact Schottky barrier through covalent bonding. However, the small bandgaps make the devices transparent to band-to-band tunneling. Increasing the gap with width confinement (or other ways to break the sublattice symmetry) is projected to reduce the mobility even for very pure samples, through a fundamental asymptotic constraint on the bandstructure. An analogous trade-off, ultimately between error rate (reliability) and delay (switching speed) can be projected to persist for all graphitic derivatives. Proceeding thus to a higher level, a compact model is presented to capture the complex nanoribbon circuits, culminating in inverter characteristics, design metrics, and layout diagrams.

Steep Subthreshold Switching With Nanomechanical FET Relays

We present a physical model for electronic switching in the cantilever-based nanoelectromechanical FETs, focusing on the steepness of its switching curve. We find that the subthreshold swing of the voltage transfer characteristic is governed by two separate considerations. The steepness of the curve is improved beyond the Boltzmann limit when several dipolar charges sitting on the relay move together and amplify the active torque. The steepness is also improved by electrostatic destabilization and pull-in forces that abruptly close the airgap between the tip of the cantilever and the drain, and exponentially enhance the tunnel current. For small sized relays, dipolar and short-range van der Waals sticking forces dominate, while for longer cantilevers the capacitive energy acquires a major role. The individual pull-in and pull-out phases demonstrate a remarkably low subthreshold swing driven by the capacitive forces, sharpened further by dipolar correlation. The sharp switching, however, comes at the expense of a strong hysteresis as the metastable and stable states interchange along the forward and reverse phases of the voltage scan.