arXiv subjects

Avik W. Ghosh

Publications and source records attributed to Avik W. Ghosh.

At least 73 records · Page 4Linked to original sources

arXiv2015

Transmission Engineering as a route to Subthermal Switching

The physics of low subthreshold devices is interpreted in terms of a gate dependent change in their mode averaged transmission function, in addition to a capacitive shift in their overall mode spectrum. Accordingly, we explore a variety of subthermal switches that alter the bandwidth, bandgap or amplitude of the transmission, and combinations thereof. The Landauer theory for current flow provides a convenient way to derive the subthreshold swing in each case analytically and suggests ways to beat the Boltzmann limit.

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arXiv2015

Modified Dirac Hamiltonian for Efficient Quantum Mechanical Simulations of Micron Sized Devices

Representing massless Dirac fermions on a spatial lattice poses a potential challenge known as the Fermion Doubling problem. Addition of a quadratic term to the Dirac Hamiltonian circumvents this problem. We show that the modified Hamiltonian with the additional term results in a very small Hamiltonian matrix when discretized on a real space square lattice. The resulting Hamiltonian matrix is considerably more efficient for numerical simulations without sacrificing on accuracy and is several orders of magnitude faster than the atomistic tight binding model. Using this Hamiltonian and the Non-Equilibrium Green's Function (NEGF) formalism, we show several transport phenomena in graphene, such as magnetic focusing, chiral tunneling in the ballistic limit and conductivity in the diffusive limit in micron sized graphene devices. The modified Hamiltonian can be used for any system with massless Dirac fermions such as Topological Insulators, opening up a simulation domain that is not readily accessible otherwise.

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arXiv2015

Role of crystal structure and junction morphology on interface thermal conductance

We argue that the relative thermal conductance between interfaces with different morphologies is controlled by crystal structure through $M_{min}/M_c > 1$, the ratio between the {\it minimum mode} count on either side $M_{min}$, and the {\it conserving modes} $M_c$ that preserve phonon momentum transverse to the interface. Junctions with an added homogenous layer, "uniform", and "abrupt" junctions are limited to $M_c$ while junctions with interfacial disorder, "mixed", exploit the expansion of mode spectrum to $M_{min}$. In our studies with cubic crystals, the largest enhancement of conductance from "abrupt" to "mixed" interfaces seems to be correlated with the emergence of voids in the conserving modes, where $M_c = 0$. Such voids typically arise when the interlayer coupling is weakly dispersive, making the bands shift rigidly with momentum. Interfacial mixing also increases alloy scattering, which reduces conductance in opposition with the mode spectrum expansion. Thus the conductance across a "mixed' junction does not always increase relative to that at a "uniform" interface.

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arXiv2015

Quantum transport at the Dirac point: Mapping out the minimum conductivity from pristine to disordered graphene

The phase space for graphene's minimum conductivity $σ_\mathrm{min}$ is mapped out using Landauer theory modified for scattering using Fermi's Golden Rule, as well as the Non-Equilibrium Green's Function (NEGF) simulation with a Monte Carlo sampling over impurity distributions. The resulting `fan diagram' spans the range from ballistic to diffusive over varying aspect ratios ($W/L$), and bears several surprises. {The device aspect ratio determines how much tunneling (between contacts) is allowed and becomes the dominant factor for the evolution of $σ_{min}$ from ballistic to diffusive regime. We find an increasing (for $W/L>1$) or decreasing ($W/L<1$) trend in $σ_{min}$ vs. impurity density, all converging around $128q^2/π^3h\sim 4q^2/h$ at the dirty limit}. In the diffusive limit, the {conductivity} quasi-saturates due to the precise cancellation between the increase in conducting modes from charge puddles vs the reduction in average transmission from scattering at the Dirac Point. In the clean ballistic limit, the calculated conductivity of the lowest mode shows a surprising absence of Fabry-Pérot oscillations, unlike other materials including bilayer graphene. We argue that the lack of oscillations even at low temperature is a signature of Klein tunneling.

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arXiv2015

Subthermal switching with nanomechanical relays

We present a physical model for electronic switching in cantilever based nano-electro-mechanical field effect transistors, 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 ability of the charges to correlate together through dipolar interactions and amplify the active torque, versus the active pull-in forces that drive an abrupt phase transition and close the air gap between the tip of the cantilever and the drain. 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 strong hysteresis as the metastable and stable states interchange along the forward and reverse phases of the voltage scan.

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arXiv2015

Enhancing phonon flow through 1D interfaces by Impedance Matching

We extend concepts from microwave engineering to thermal interfaces and explore the principles of impedance matching in 1D. The extension is based on the generalization of acoustic impedance to non linear dispersions using the contact broadening matrix $Γ(ω)$, extracted from the phonon self energy. For a single junction, we find that for coherent and incoherent phonons the optimal thermal conductance occurs when the matching $Γ(ω)$ equals the Geometric Mean (GM) of the contact broadenings. This criteria favors the transmission of both low and high frequency phonons by requiring that (1) the low frequency acoustic impedance of the junction matches that of the two contacts by minimizing the sum of interfacial resistances; and (2) the cut-off frequency is near the minimum of the two contacts, thereby reducing the spillage of the states into the tunneling regime. For an ultimately scaled single atom/spring junction, the matching criteria transforms to the arithmetic mean for mass and the harmonic mean for spring constant. The matching can be further improved using a composite graded junction with an exponential varying broadening that functions like a broadband antireflection coating. There is however a trade off as the increased length of the interface brings in additional intrinsic sources of scattering.

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arXiv2015

Impedance Matching of Atomic Thermal Interfaces Using Primitive Block Decomposition

We explore the physics of thermal impedance matching at the interface between two dissimilar materials by controlling the properties of a single atomic mass or bond. The maximum thermal current is transmitted between the materials when we are able to decompose the entire heterostructure solely in terms of primitive building blocks of the individual materials. Using this approach, we show that the minimum interfacial thermal resistance arises when the interfacial atomic mass is the arithmetic mean, while the interfacial spring constant is the harmonic mean of its neighbors. The contact induced broadening matrix for the local vibronic spectrum, obtained from the self-energy matrices, generalizes the concept of acoustic impedance to the nonlinear phonon dispersion or the short-wavelength (atomic) limit.

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arXiv2015

Chiral tunneling of topological states: towards the efficient generation of spin current using spin-momentum locking

We show that the interplay between chiral tunneling and spin-momentum locking of helical surface states leads to spin amplification and filtering in a 3D Topological Insulator (TI). Chiral tunneling across a TI pn junction allows normally incident electrons to transmit, while the rest are reflected with their spins flipped due to spin-momentum locking. The net result is that the spin current is enhanced while the dissipative charge current is simultaneously suppressed, leading to an extremely large, gate tunable spin to charge current ratio (~20) at the reflected end. At the transmitted end, the ratio stays close to one and the electrons are completely spin polarized.

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arXiv2014

Reducing error rates in straintronic multiferroic dipole-coupled nanomagnetic logic by pulse shaping

Dipole-coupled nanomagnetic logic (NML), where nanomagnets with bistable magnetization states act as binary switches and information is transferred between them via dipole coupling and Bennett clocking, is a potential replacement for conventional transistor logic since magnets dissipate less energy than transistors when they switch in response to the clock. However, dipole-coupled NML is much more error-prone than transistor logic because thermal noise can easily disrupt magnetization dynamics. Here, we study a particularly energy-efficient version of dipole-coupled NML known as straintronic multiferroic logic (SML) where magnets are clocked/switched with electrically generated mechanical strain. By appropriately shaping the voltage pulse that generates strain, the error rate in SML can be reduced to tolerable limits. In this paper, we describe the error probabilities associated with various stress pulse shapes and discuss the trade-off between error rate and switching speed in SML.

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arXiv2013

Manipulating chiral transmission by gate geometry: switching in graphene with transmission gaps

We explore the chiral transmission of electrons across graphene heterojunctions for electronic switching using gate geometry alone. A sequence of gates is used to collimate and orthogonalize the chiral transmission lobes across multiple junctions, resulting in negligible overall current. The resistance of the device is enhanced by several orders of magnitude by biasing the gates into the bipolar $npn$ doping regime, as the ON state in the near homogeneous $nn^-n$ regime remains highly conductive. The mobility is preserved because the switching involves a transmission gap instead of a structural band-gap that would reduce the number of available channels of conduction. Under certain conditions this transmission gap is highly gate tunable, allowing a subthermal turn-on that beats the Landauer bound on switching energy limiting present day digital electronics.

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arXiv2013

Computing with Non-equilibrium 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 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.

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arXiv2013

Atomistic deconstruction of current flow in graphene based hetero-junctions

We describe the numerical modeling of current flow in graphene heterojunctions, within the Keldysh Landauer Non-equilibrium Green's function (NEGF) formalism. By implementing a $k$-space approach along the transverse modes, coupled with partial matrix inversion using the Recursive Green's function Algorithm (RGFA), we can simulate on an atomistic scale current flow across devices approaching experimental dimensions. We use the numerical platform to deconstruct current flow in graphene, compare with experimental results on conductance, conductivity and quantum Hall, and deconstruct the physics of electron `optics' and pseudospintronics in graphene $p-n$ junctions. We also demonstrate how to impose exact open boundary conditions along the edges to minimize spurious edge reflections.

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arXiv2012

Manifestation of chiral tunneling at a tilted graphene pn junction

Electrons in graphene follow unconventional trajectories at PN junctions, driven by their pseudospintronic degree of freedom. Significant is the prominent angular dependence of transmission, capturing the chiral nature of the electrons and culminating in unit transmission at normal incidence (Klein tunneling). We theoretically show that such chiral tunneling can be directly observed from the junction resistance of a tilted interface probed with separate split gates. The junction resistance is shown to increase with tilt in agreement with recent experimental evidence. The tilt dependence arises because of the misalignment between modal density and the anisotropic transmission lobe oriented perpendicular to the tilt. A critical determinant is the presence of edge scattering events that can completely reverse the angle-dependence. The absence of such reversals in the experiments indicates that these edge effects are not overwhelmingly deleterious, making the premise of transport governed by electron `optics' in graphene an exciting possibility.

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arXiv2012

A quasi-analytical model for energy-delay-reliability tradeoff studies during write operations in perpendicular STT-RAM cell

One of the biggest challenges the current STT-RAM industry faces is maintaining a high thermal stability while trying to switch within a given voltage pulse and energy cost. In this paper, we present a physics based analytical model that uses a modified Simmons' tunneling expression to capture the spin dependent tunneling in a magnetic tunnel junction(MTJ). Coupled with an analytical derivation of the critical switching current based on the Landau-Lifshitz-Gilbert equation, and the write error rate derived from a solution to the Fokker-Planck equation, this model provides us a quick estimate of the energydelay- reliability tradeoffs in perpendicular STTRAMs due to thermal fluctuations. In other words, the model provides a simple way to calculate the energy consumed during a write operation that ensures a certain error rate and delay time, while being numerically far less intensive than a full-fledged stochastic calculation. We calculate the worst case energy consumption during anti-parallel (AP) to parallel (P) and P to AP switchings and quantify how increasing the anisotropy field HK and lowering the saturation magnetization MS, can significantly reduce the energy consumption. A case study on how manufacturing variations of the MTJ cell can affect the energy consumption and delay is also reported.

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arXiv2011

Energy dissipation and switching delay in spin-transfer torque switching of nanomagnets with low-saturation magnetization in the presence of thermal fluctuations

A common ploy to reduce the switching current and energy dissipation in spin-transfer-torque driven magnetization switching of shape-anisotropic single-domain nanomagnets is to employ magnets with low saturation magnetization $M_s$ and high shape-anisotropy. The high shape-anisotropy compensates for low $M_s$ to keep the static switching error rate constant. However, this ploy increases the switching delay, its variance in the presence of thermal noise, and the dynamic switching error rate. Using the stochastic Landau-Lifshitz-Gilbert equation with a random torque emulating thermal noise, we show that pumping some excess spin-polarized current into the nanomagnet during switching will keep the mean switching delay and its variance constant as we reduce $M_s$, while still reducing the energy dissipation significantly.

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arXiv2011

Graphene Nanoribbons: from chemistry to circuits

The Y-chart is a powerful tool for understanding the relationship between various views (behavioral, structural, 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 (GNR) 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 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.

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arXiv2010

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 two-dimensional single sheet of graphene. In our simulated devices, the source/drain and interconnect regions are composed of wide graphene nanoribbon (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 three-dimensional Poisson's equation to capture the nontrivial contact electrostatics, along with a quantum kinetic formulation of transport based on non-equilibrium Green's functions (NEGF). 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.

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arXiv2010

From low-bias mobility to high-bias current saturation: fundamental trade-offs in graphene based transistors

We describe the fundamental trade-offs in engineering the mobility, current saturation and ON- OFF ratios in graphene transistors. Surprisingly, the trade-offs arise solely from an asymptotic constraint on the high energy bandstructure and independent of scattering processes. This places graphite derivatives (bulk monolayer graphene, uniaxially strained graphene nanoribbons, carbon nanotubes and bilayer graphene) on the same 3-parameter mobility-bandgap-scattering length (μ - Egap -λ) plot, proximal to other semiconductors. In addition to this low-bias trade-off, the high bias current bears signatures of the underlying saturation mechanism, arising through phonon scattering or Γ-point suppressed density of states opening bandgap.

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