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We study the noncoherent transport due to electron–phonon (e-ph) interaction in graphene nanoribbon (GNR) field-effect transistors (GNRFETs) using nonequilibrium Green’s function method in mode space. Phonon dispersion calculations in conjunction with the e-ph interaction computations show that in an armchair GNR, only a small number of phonon modes are coupled to the carriers. Our simulation shows that under a threshold gate voltage, the drain–source current is diminished by the low-energy phonons such as acoustic phonon modes and radial-breathing-like phonon modes, while at great biases of the gate, the current drop is essentially due to high-energy optical phonon modes. The effect of drain voltage on scattering process is discussed, which shows that at higher drain voltages, the ballisticity decreases. We also explore the phonon scattering dependence on the dimensions of GNRFET channel. In larger width ribbon, the impact of phonon scattering decreases. The channel with a length of 30 nm is ballistic up to 90%, and only when its length is increased to approximately 180 nm, transport becomes semiballistic, which infers to an effective mean free path of ~200 nm. [ABSTRACT FROM AUTHOR]

To solve electrical contact problem of horizontal multilayer graphene nanoribbon (GNR), a vertical GNR (VGNR) interconnect scheme is proposed. Performance of the VGNR interconnect is theoretically evaluated and compared with those of conventional interconnect technologies. It is shown that the performance of VGNR is comparable (and even superior) to that of the horizontal GNR (HGNR). Moreover, VGNRs may provide a promising solution to the thermal challenges faced by HGNRs in nanoscale ICs. [ABSTRACT FROM AUTHOR]

The linear ac transport properties are theoretically studied in H-shaped zigzag silicene nanojunction. We numerically calculated the dc conductance and ac emittance by considering the nearest-neighbor hopping, the second-nearest-neighbor spin–orbit interaction (SOI), and external electric field, based on the tight-binding approach and the ac transport theory. It is found that the relatively large SOI induces a topological quantum edge state in the nanojunction, which eliminates the ac emittance response at the Dirac point. Further investigations indicate that the SOI-induced ac transport is topologically protected, despite the changes of geometric size. Moreover, the external electric field would open an energy gap, and destroy the topological quantum state, making the nanojunction a trivial band insulator. [ABSTRACT FROM PUBLISHER]

A thermally evaporated silicon monoxide (SiO) film has been experimented as the gate dielectric in graphene field-effect transistors (GFETs) due to its room-temperature and low-damage deposition without introducing chemical gases or ionized particles as in other film deposition techniques, which may cause damage to graphene. In order to evaluate the dielectric properties, a double-gated GFET was fabricated with a standard commercial thermally grown SiO2 layer as the bottom gate dielectric and thermally evaporated SiO as the top dielectric. The electrical characterizations revealed that the top-gate carrier mobility was 1081.3 cm2/Vs, reasonably comparable to the bottom-gate mobility. Furthermore, the breakdown strength of the SiO film reached 5.7 MV/cm, which was lower than that of the SiO2 dielectric (~10 MV/cm) but in the same order of magnitude. The breakdown mechanism of the SiO film was studied, and the current-voltage characteristics were in agreement with the Frenkel–Poole emission model. Finally, the relative dielectric constant of SiO was found to be 5.3, significantly higher than that of SiO2 (3.9). These results indicate that the thermally evaporated SiO can function as an excellent dielectric for graphene-based devices. [ABSTRACT FROM PUBLISHER]

Thermal performances of light-emitting diode (LED) lamp with three types of thermal interface materials (TIMs) under different convection coefficients and input powers are investigated by using finite-element method. Three types of TIMs are conductive silver, Sn63Pb37 of tin alloy solder, and graphene in this paper. The results demonstrate that the temperature of the LED lamp decreases sharply using the interface material of graphene. With the increasing convection coefficient in a certain range, the junction temperature will be effectively decreased. When under high input powers, graphene has a much better performance than other two TIMs. It implies that graphene may be a possible potential for thermal design, which can effectively prolong the life of LED, especially for high power devices. The simulation offers a test of LED heat dissipation using different TIMs, and the results can provide a reference for the application of graphene in LED thermal design as TIMs and build a motivation for future material research directions. It builds a basis for progressive study of physical property for LED lamp. The obtained results may give insight to the reliability of the LED lamp. [ABSTRACT FROM PUBLISHER]

In this paper, we investigate a graphene nanoribbon FET (GNRFET) with locally embedded Stone–Wales (SW) defects near its drain contact. The simulation procedure is preformed via the self-consistent solution of the full 3-D Schrödinger and Poisson equations. Localized states induced by the SW defects are able to reduce ambipolar conduction and provide higher ON–OFF ratio and attenuated kink effect. Causing outgoing carriers decelerated in the channel, the use of such defected region in the GNRFET yields less vulnerability to hot-carrier degradation than its conventional counterpart. Our proposed structure also enhances the device transconductance, making it more appealing in high-frequency applications. [ABSTRACT FROM AUTHOR]

This paper presents the frequency and output power characteristics of oscillators in the graphene tunneling FETs (GTFET) technology. In order to acquire these characteristics, the practical operating conditions of GTFET oscillators are carefully analyzed, and then, the device parameters are simplified to an equivalent circuit and the I/V large signal model is approximated by a compact form. Expressions for the maximum oscillation frequency, maximum output power, and the relationship between frequency and power are given combing the equivalent circuit and the simplified model. In addition, the key device parameters that can be used to improve the performance of a GTFET oscillator are also discussed. [ABSTRACT FROM PUBLISHER]

We present a circuit-compatible compact model of the intrinsic capacitances of GFETs. Together with a compact drain current model, a large-signal model is developed combining both models as a tool for simulating the electrical behavior of graphene-based integrated circuits, dealing with the dc, transient behavior, and frequency response of the circuit. The drain current model is based on a drift-diffusion mechanism for the carrier transport coupled with an appropriate field-effect approach. The intrinsic capacitance model consists of a 16-capacitance matrix including self-capacitances and transcapacitances of a four-terminal GFET. To guarantee charge conservation, a Ward–Dutton linear charge partition scheme has been used. The large-signal model has been implemented in Verilog-A, being compatible with conventional circuit simulators and serving as a starting point toward the complete GFET device model that could incorporate additional nonidealities. [ABSTRACT FROM PUBLISHER]

An optimization of the current-to-voltage transfer characteristic of a graphene FET (GFET) compact model, based on drift-diffusion carrier transport, is presented. The improved accuracy at Dirac point extends the model usability for GFETs when scaling parameters, such as voltage supply, gate length, oxide thickness, and mobility, for circuit design exploration. The model’s accuracy is demonstrated through fitting to GFETs processed in-house. The model has been written in a standard behavioral language, and extensively run in an analog circuit simulator for designing basic circuits, such as inverters and cascode cells, demonstrating its robustness. [ABSTRACT FROM AUTHOR]

While different RF functionalities, such as an amplifier or a mixer, have been designed using the graphene FET (GFET) devices, the balun circuit has not been explored. In this paper, two innovative active balun architectures are presented taking advantage of the GFET’s unique symmetrical and ambipolar behavior, respectively. The symmetry-based active balun circuit is realized using an advanced SiC-based GFET RF technology. After circuit design and optimization using a large signal GFET compact model, the circuit has been fabricated and characterized. Measurement results confirm its excellent functionality. Circuit simulation shows that the second architecture exploring the GFETs ambipolar behavior gives equivalent results compared with the first architecture. Both topologies avoid asymmetric impedance matching and result in the accurate amplitude and phase balance of the balun. [ABSTRACT FROM AUTHOR]

Based on realistic circuit- and system-level simulations, graphene interconnects are analyzed in terms of multiple material properties, such as the mean free path (MFP), the contact resistance, and the edge roughness. The benchmarking results indicate that the advantage of using graphene interconnects occurs only under certain circumstances. The device-level parameters, including the supply and threshold voltages, and the circuit-level parameters, including the wire length and width, have large impacts on both the delay and energy-delay product (EDP). At the circuit level, one representative circuit, a 32-bit adder, is investigated, where up to 40% and 70% improvements in delay and EDP are observed. At the system-level analysis, an ARM Cortex-M0 processor is synthesized, and placement and routing are performed. After replacing copper interconnects with multilayer graphene interconnects, up to 15% and 22% performance improvements in clock frequency and EDP have been observed. It is also demonstrated that the benefits of using graphene for the ARM core processor are strongly dependent on the quality of the graphene, such as the MFP and the edge roughness. [ABSTRACT FROM AUTHOR]

In this paper, we report on the development of a versatile compact model for graphene FETs (GFETs). Aging studies have been performed on the GFETs via bias stress measurements and aging laws were implemented in the compact model, including failure mechanisms in the GFETs. The failure mechanisms are identified to be originated from the generation of traps and interface states causing a shift in the transfer characteristics and mobility degradation, respectively. For the development of the aging compact model, the trap density is implemented in the prestress compact model to modulate the channel potential. Moreover, the interface state generation is implemented to reflect on the modification of the source/drain access region charges. The implemented aging model is compared with reported bias-stress measurement results as well as the aging measurements carried out on chemical vapor deposition GFETs which show a very good agreement. [ABSTRACT FROM AUTHOR]

A reduced-order method is presented to efficiently calculate Green’s functions connecting contacts or leads to all the points in a nanostructure in the coherent transport limit. The proposed approach samples a small subset of spatial grid points on the lead and a small subset of energy grid points to build a reduced-order model. The efficacy of the algorithm is demonstrated by applying it to calculate both the electron density and transmission in a resonant tunneling structure, a MOSFET, and a bilayer graphene device. The match in features of both the electron density and transmission versus energy with conventional methods to model devices is excellent while a large reduction in computational time is demonstrated. [ABSTRACT FROM AUTHOR]

We present a closed-form continuous model for the electrical conductivity of a single layer graphene (SLG) sheet in the presence of short-range impurities, long-range screened impurities, and acoustic phonons. The validity of the model extends from very low doping levels (chemical potential close to the Dirac cone vertex) to very high doping levels. We demonstrate complete functional relations of the chemical potential, polarization function, and conductivity with respect to both doping level and temperature ( \(T\) ), which were otherwise developed for SLG sheet only in the very low and very high doping levels. The advantage of the continuous conductivity model reported in this paper lies in its simple form which depends only on three adjustable parameters: the short-range impurity density, the long-range screened impurity density, and temperature \(T\) . The proposed theoretical model was successfully used to correlate various experiments in the midtemperature and moderate density regimes. [ABSTRACT FROM AUTHOR]

In this paper, we investigate with theory and experiments the performance improvements achievable using bilayer graphene as channel material in field effect transistors for analog applications. Bilayer graphene provides larger output resistance than monolayer graphene, which translates in both higher voltage gain and higher maximum frequency oscillation. To experimentally prove bilayer graphene potential as a channel material, simple circuits have been fabricated and tested, i.e., an amplifier and a frequency doubler. We show that they largely outperform similar circuits built with monolayer-graphene devices. [ABSTRACT FROM PUBLISHER]

In this paper, for the first time device characteristics of field-effect tunneling transistors based on vertical graphene-hBN heterostructure (VTGFET) and vertical graphene nanoribbon (GNR)-hBN heterostructure (VTGNRFET) are theoretically investigated and compared. An atomistic simulation based on the nonequilibrium Green's function (NEGF) formalism is employed. The results indicate that due to the presence of an energy gap in GNRs, the ION/IOFF ratio of VTGNRFET can be much larger than that of VTGFET, which renders VTGNRFETs as promising candidates for future electronic applications. Furthermore, it can be inferred from the results that due to smaller density of states and as a result smaller quantum capacitance of GNRs in comparison with that of graphene, better switching and frequency response can be achieved for VTGNRFETs. [ABSTRACT FROM PUBLISHER]

In this paper, the effect of structural nonidealities on the electronic transport properties of ultranarrow zigzag graphene nanoribbons (ZGNRs) is systemically investigated for the first time, employing the nonorthogonal third nearest neighbor mean-field Hubbard model along with the nonequilibrium Green's function formalism. We have evaluated the influence of line-edge roughness, single atom vacancies, and substrate-induced potential fluctuations on the transport gap, ON-and OFF-state conductances, and the ON/OFF conductance ratio of 12-nm-length ultranarrow ZGNRs. The results reveal that while even moderate amounts of edge roughness lead to a nonuniform suppression of the transmission probability and increase the transport gap, the presence of single atom vacancies tends to decrease the induced transport gap. Furthermore, it is shown that the transport properties of ZGNRs are more robust against potential fluctuations compared with their armchair counterparts. [ABSTRACT FROM PUBLISHER]

We address a physics-based solution of joule heating phenomenon in a single-layer graphene (SLG) sheet under the presence of Thomson effect. We demonstrate that the temperature in an isotopically pure (containing only C^12) SLG sheet attains its saturation level quicker than when doped with its isotopes (C^13). From the solution of the joule heating equation, we find that the thermal time constant of the SLG sheet is in the order of tenths of a nanosecond for SLG dimensions of a few micrometers. These results have been formulated using the electron interactions with the inplane and flexural phonons to demonstrate a field-dependent Landauer transmission coefficient. We further develop an analytical model of the SLG specific heat using the quadratic (out of plane) phonon band structure over the room temperature. Additionally, we show that a cooling effect in the SLG sheet can be substantially enhanced with the addition of C^13. The methodologies as discussed in this paper can be put forward to analyze the graphene heat spreader theory. [ABSTRACT FROM AUTHOR]

A simulation study aimed at investigating the main features in dc and small-signal operating conditions of the hot-electron graphene base transistor (GBT) for analog terahertz operation is presented. Intrinsic silicon is used as reference material. The numerical model is based on a self-consistent Schrödinger–Poisson solution, using a 1-D transport approximation and accounting for multiple-valley and nonparabolicity band effects. Some limitations in the extension of the saturation region and in the output conductance related to the finite quantum capacitance of graphene and to space charge effects are discussed. A small-signal model is developed that catches the essential physics behind the voltage gain and the cutoff frequency, which shows that the graphene quantum capacitance severely limits the former but not the latter. According to simulations carried out within the ballistic transport approximation, a 20-nm-long GBT can achieve at the same time a voltage gain larger than 10 and a cutoff frequency largely above 1 THz within a reasonably wide bias range. [ABSTRACT FROM AUTHOR]

We study theoretically the different transport behaviors and the electrical characteristics of a top-gated graphene field-effect transistor where boron nitride is used as the substrate and gate insulator material, which makes the ballistic transport realistic. Our simulation model is based on the Green's function approach to solving a tight-binding Hamiltonian for graphene, self-consistently coupled with Poisson's equation. The analysis emphasizes the effects of the chiral character of carriers in graphene in the different transport regimes including the Klein and band-to-band tunneling processes. In particular, the Klein tunneling is shown to have an important role on the onset of the current saturation which is analyzed in detail as a function of the device parameters. Additionally, we predict the possible emergence of negative differential conductance and investigate its dependence on the BN-induced bandgap, the temperature, and the gate insulator thickness. Short-channel effects are evaluated from the analysis of transfer characteristics as a function of gate length and gate insulator thickness. They manifest through the shift of the Dirac point and the appearance of current oscillations at short gate length. [ABSTRACT FROM AUTHOR]

With the recent upsurge in experimental efforts toward fabrication of short-channel graphene field-effect transistors (GFETs) for analog and high-frequency RF applications—where the advantages of distinctive intrinsic properties of gapless graphene are expected to be leveraged—a critical understanding of the factors affecting both output and transfer characteristics is necessary for device optimization. Analyzing the device characteristics through ballistic electronic transport simulations within the nonequilibrium Green's function formalism, we show that a doping in the drain underlap region can significantly improve the quasi-saturation behavior in the GFET output characteristics and, hence, the output resistance and intrinsic gain. From this understanding, we provide a unified and coherent explanation for seemingly disparate phenomena—quasi-saturation and the recently reported three-terminal negative differential resistance in GFETs. We also investigate the scaling behavior of cutoff frequency and comment on some of the observed scaling trends in recent experiments. [ABSTRACT FROM AUTHOR]

In this paper, an analytical model for calculating the channel potential and current–voltage characteristics in a symmetric tunneling field-effect transistor (SymFET) is presented. The current in a SymFET flows by tunneling from an n-type graphene layer to a p-type graphene layer. A large current peak occurs when the Dirac points are aligned at a particular drain-to-source bias VDS. Our model shows that the current of the SymFET is very weakly dependent on temperature. The resonant current peak is controlled by chemical doping and applied gate bias. The on/off ratio increases with graphene coherence length and doping. The symmetric resonant peak is a good candidate for high-speed analog applications and can enable digital logic similar to the BiSFET. Our analytical model also offers the benefit of permitting simple analysis of features such as the full-width at half-maximum (FWHM) of the resonant peak and higher order harmonics of the nonlinear current. The SymFET takes advantage of the perfect symmetry of the band structure of 2-D graphene, a feature that is not present in conventional semiconductors. [ABSTRACT FROM AUTHOR]