Characteristics Of Graphene Field-effect Transistors Research Articles

Charge distribution in turbostratic few-layer graphene studied by carbon isotope labeling

Elucidating layer-resolved charge distribution in van der Waals stacked crystals is crucial for electronic applications based on 2D materials. Here, we use CVD-grown few-layer graphene (FLG) labeled by carbon isotopes as a prototype, in which Raman features of two isotope components identify distinct layers. Electrical transfer characteristics of graphene field-effect transistors (GFETs) are employed to quantitatively calibrate the correspondence of Fermi level and G-phonon frequency that shifts with the charge-carrier concentration (n) variation. The isotope-assisted spectroscopy reveals previously unprobed characteristics that the n value of both top and bottom layers in FLG is close rather than following an exponential decay law. This negligible gradient likely benefits from the π−π interaction that favors the interlayer diffusion of charges. In addition, each extra layer reduces the degree of charge exchange at the graphene/dopant interface. These results have important implications for FLG nanoelectronics and provide a robust framework by which one can further investigate the critical properties of other 2D systems.

Impact of contact resistance on the performances of graphene field-effect transistor through analytical study

Currently, owing to its remarkable electro-mechanical, thermal, and optical properties, graphene has attracted tremendous attention in the research community as one of the most prominent materials in modern electronic technology. In recent years, the graphene field-effect transistor (G-FET) has exhibited outstanding radio frequency performance and unprecedented sensitivity. Generally, the contact or parasitic resistance significantly influences the different characteristics of a large area G-FET. In this work, we have determined the effect of contact resistance from different characteristics of a G-FET. We have found that contact or parasitic resistance has a meaningful impact on the device’s different characteristics, i.e., transfer characteristics, transconductance, cut-off frequency, etc. The analytical results have indicated that the transconductance and cut-off frequency of a G-FET decrease significantly with a higher value of contact resistance. Thereafter, reducing contact resistance according to experimental conditions will predict revolutionary changes in fabrication technology for graphene-based devices.

Adsorption Site-Dependent Mobility Behavior in Graphene Exposed to Gas Oxygen

Transport characteristics of graphene field-effect transistors were measured in situ in oxygen/nitrogen atmospheres and at various temperatures. Mobilities of holes were extracted from transport ch...

Improvement of the Frequency Characteristics of Graphene Field-Effect Transistors on SiC Substrate

Analog applications attract increasing interest for graphene field-effect transistors (GFETs). GFET with a cutoff frequency of up to 427 GHz has been reported; however, the device suffered from the large parasitic parameters and poor drain current saturation, which made their maximum oscillation frequency lower than the cutoff frequency. In this letter, quasi-free-standing bilayer graphene transistors with a gate length of 60 nm and ultra-thin gate dielectric are fabricated by an improved, self-aligned, process. Good gate coupling is achieved, and parasitic parameters are suppressed to a significant extent. The as-measured extrinsic cutoff frequency reaches 70 GHz and the maximum oscillation frequency reaches 120 GHz, which are the highest extrinsic frequencies reported for graphene transistors to date. Our results show the application potential of graphene RF devices in future high-speed electronic systems.

A Fitting Model for Asymmetric <inline-formula> <tex-math notation="LaTeX">$I$ </tex-math> </inline-formula>–<inline-formula> <tex-math notation="LaTeX">$V$ </tex-math> </inline-formula> Characteristics of Graphene FETs for Extraction of Intrinsic Mobilities

A fitting model is developed for accounting the asymmetric ambipolarities in the I-V characteristics of graphene field-effect transistors (G-FETs) with doped channels, originating from the thermionic emission and interband tunneling at the junctions between the gated and access regions. Using the model, the gate-voltage-dependent intrinsic mobility as well as other intrinsic and extrinsic device parameters can be extracted. We apply it to a top-gated G-FET with a graphene channel grown on a SiC substrate and with SiN gate dielectric that we reported previously, and we demonstrate that it can excellently fit its asymmetric I-V characteristic.

Influence of electron-beam lithography exposure current level on the transport characteristics of graphene field effect transistors

Many factors have been identified to influence the electrical transport characteristics of graphene field-effect transistors. In this report, we examine the influence of the exposure current level used during electron beam lithography (EBL) for active region patterning. In the presence of a self-assembled hydrophobic residual layer generated by oxygen plasma etching covering the top surface of the graphene channel, we show that the use of low EBL current level results in higher mobility, lower residual carrier density, and charge neutrality point closer to 0 V, with reduced device-to-device variations. We show that this correlation originates from the resist heating dependent release of radicals from the resist material, near its interface with graphene, and its subsequent trapping by the hydrophobic polymer layer. Using a general model for resist heating, we calculate the difference in resist heating for different EBL current levels. We further corroborate our argument through control experiments, where radicals are either intentionally added or removed by other processes. We also utilize this finding to obtain mobilities in excess of 18 000 cm2/V s on silicon dioxide substrates. We believe these results are applicable to other 2D materials such as transition metal dichalcogenides and nanoscale devices in general.

Dramatic vapor-phase modulation of the characteristics of graphene field-effect transistors.

Here we report on dramatic and favorable changes to the operating characteristics in monolayer graphene field-effect transistors (FETs) exposed to vapor-phase, polar organic molecules in ambient. These changes include significant reduction of the Dirac voltage, accompanied by both an increase in electron and hole mobility, μ, and a decrease in residual carrier density, N0, to < 3 × 10(11) cm(-2). In contrast to graphene FET modulation with various liquid- and solid-phase dielectric media present in the literature, we attribute these changes to screening by polar vapor-phase molecules of fields induced by charged impurities and defects, n(imp), in or near the active layer. The magnitude of the changes produced in the graphene FET parameters scales remarkably well with the dipole moment of the delivered molecules. These effects are reversible, a unique advantage of working in the vapor phase. The changes observed upon polar molecule delivery are analogous to those produced by depositing and annealing fluoropolymer coatings on graphene that have been reported previously, and we attribute these changes to similar charge screening or neutralization phenomena.

New analytical drain current model for the sub-linear region of output characteristics of graphene field-effect transistors in the low carrier density limit.

A new analytical drain current model for a quantitative description of output characteristics of graphene field-effect transistors (GFETs) in the sub-linear region is derived from a previously-developed diffusion-drift theory for GFETs. To simplify calculation, the diffusion-to-drift current ratio is assumed to be constant along the graphene channel, and a reasonable representative value of the ratio is used instead. In addition, the analytical modeling is conducted in the low-carrier-density limit where carrier velocity at the source end is lower than saturation velocity caused by optical phonon emission. This limit facilitates correct explanation of the peculiar behavior of output characteristics that has been attributed to an ambipolar property of graphene. For realistic simulation, extrinsic series resistances are considered, and carrier mobility degraded by vertical electric field is calculated from a modified classical formula. The output characteristics of GFETs in the sub-linear region can properly be reproduced by the new model, and good agreement between simulation results and several sets of experimental data taken from previous literatures is obtained in this region.

Improvement of graphene field-effect transistors by hexamethyldisilazane surface treatment

We report the improvement of the electrical characteristics of graphene field-effect transistors (FETs) by hexamethyldisilazane (HMDS) treatment. Both electron and hole field-effect mobilities are increased by 1.5 × –2×, accompanied by effective residual carrier concentration reduction. Dirac point also moves closer to zero Volt. Time evolution of mobility data shows that mobility improvement saturates after a few hours of HMDS treatment. Temperature-dependent transport measurements show small mobility variation between 77 K and room temperature (295 K) before HMDS application. But mobility at 77 K is almost 2 times higher than mobility at 295 K after HMDS application, indicating reduced carrier scattering. Performance improvement is also observed for FETs made on hydrophobic substrate—an HMDS-graphene-HMDS sandwich structure. Raman spectroscopic analysis shows that G peak width is increased, G peak position is down shifted, and intensity ratio between 2D and G peaks is increased after HMDS application. We attribute the improvements in electronic transport mainly to enhanced screening and mitigation of adsorbed impurities from graphene surface upon HMDS treatment.

N‐Doped Graphene Field‐Effect Transistors with Enhanced Electron Mobility and Air‐Stability

Although graphene can be easily p-doped by various adsorbates, developing stable n-doped graphene that is very useful for practical device applications is a difficult challenge. We investigated the doping effect of solution-processed (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) on chemical-vapor-deposited (CVD) graphene. Strong n-type doping is confirmed by Raman spectroscopy and the electrical transport characteristics of graphene field-effect transistors. The strong n-type doping effect shifts the Dirac point to around -140 V. Appropriate annealing at a low temperature of 80 ºC enables an enhanced electron mobility of 1150 cm(2) V(-1) s(-1). The work function and its uniformity on a large scale (1.2 mm × 1.2 mm) of the doped surface are evaluated using ultraviolet photoelectron spectroscopy and Kelvin probe mapping. Stable electrical properties are observed in a device aged in air for more than one month.

The Restorative Effect of Fluoropolymer Coating on Electrical Characteristics of Graphene Field-Effect Transistors

We report on the improvement of the electronic characteristics of mono-layered graphene field-effect transistors by an interacting capping layer of a suitable fluoropolymer. Favorable shifts in the Dirac voltage toward zero with shift magnitudes in excess of 60 V are observed. Furthermore, the field-effect mobility is increased and the on-off current ratio is improved by up to a factor of 2. The residual carrier concentration is reduced to ~ 4.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . We hypothesize that this improvement is due to the strongly polar nature of the C-F chemical bond in the fluoropolymer. Significantly, these results have been achieved in graphene grown by wafer-scale chemical vapor deposition process and lift-off transfer.

Transformation of the Electrical Characteristics of Graphene Field-Effect Transistors with Fluoropolymer

We report on the improvement of the electronic characteristics of monolayer graphene field-effect transistors (FETs) by an interacting capping layer of a suitable fluoropolymer. Capping of monolayer graphene FETs with CYTOP improved the on-off current ratio from 5 to 10 as well as increased the field-effect mobility by as much as a factor of 2 compared to plain graphene FETs. Favorable shifts in the Dirac voltage toward zero with shift magnitudes in excess of 60 V are observed. The residual carrier concentration is reduced to ~2.8 × 10(11) cm(-2). Removal of the fluoropolymer from graphene FETs results in a return to the initial electronic properties before depositing CYTOP. This suggests that weak, reversible electronic perturbation of graphene by the fluoropolymer favorably tune the electrical characteristics of graphene, and we hypothesize that the origin of this improvement is in the strongly polar nature of the C-F chemical bonds that self-organize upon heat treatment. We demonstrate a general method to favorably restore or transform the electrical characteristics of graphene FETs, which will open up new applications.

Explicit Drain-Current Model of Graphene Field-Effect Transistors Targeting Analog and Radio-Frequency Applications

We present a compact physics-based model of the current-voltage characteristics of graphene field-effect transistors, of especial interest for analog and radio-frequency applications where bandgap engineering of graphene could be not needed. The physical framework is a field-effect model and drift-diffusion carrier transport. Explicit closed-form expressions have been derived for the drain current covering continuosly all operation regions. The model has been benchmarked with measured prototype devices, demonstrating accuracy and predictive behavior. Finally, we show an example of projection of the intrinsic gain as a figure of merit commonly used in RF /analog applications.

Modeling of the Output and Transfer Characteristics of Graphene Field-Effect Transistors

We obtain the output and transfer characteristics of graphene field-effect transistors by using the charge-control model for the current, based on the solution of the Boltzmann equation in the field-dependent relaxation time approximation. Closed expressions for the conductance, transconductance, and saturation voltage are derived. We found good agreement with the experimental data of Meric et al. [Nat. Nanotechnol. vol. 3, p. 684, 2008] without assuming carrier density-dependent velocity saturation.

Empirical Modeling of Metal-Contact Effects on Graphene Field-Effect Transistors

An empirical model is developed to characterize metal-contact effects on the transfer characteristics of graphene field-effect transistors. The present model is based on a diffusive transport, and considers charge carrier doping from metallic electrodes to the adjacent graphene channel. An electron–hole conductivity asymmetry, one of the main features in the transfer characteristics, is well reproduced by the model. Model calculations with varied parameters show that the conductivity asymmetry becomes more distinct with higher charge-carrier mobility, higher doping level at the metal contacts, larger extent of the doped region near the contacts, or shorter channel lengths.

Compact Virtual-Source Current–Voltage Model for Top- and Back-Gated Graphene Field-Effect Transistors

This paper presents a compact model for the current-voltage characteristics of graphene field-effect transistors (GFETs), which is based on an extension of the “virtual-source” model previously proposed for Si MOSFETs and is valid for both saturation and nonsaturation regions of device operation. This GFET virtual-source model provides a simple and intuitive understanding of carrier transport in GFETs, allowing extraction of the virtual-source injection velocity <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VS</sub> , which is a physical parameter with great technological significance for short-channel graphene transistors. The derived <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> characteristics account for the combined effects of the drain-source voltage <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VDS</i> , the top-gate voltage <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VTGS</i> , and the back-gate voltage <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VBGS</i> . With only a small set of fitting parameters, the model shows excellent agreement with experimental data. It is also shown that the extracted virtual-source carrier injection velocity for graphene devices is much higher than in Si MOSFETs and state-of-the-art III-V heterostructure FETs with similar gate length, demonstrating the great potential of GFETs for high-frequency applications. Comparison with experimental data for chemical-vapor-deposited GFETs from our group and epitaxial GFETs in the literature confirms the validity and flexibility of the model for a wide range of existing GFET devices.

Electrical and noise characteristics of graphene field-effect transistors: ambient effects, noisesources and physical mechanisms

We fabricated a large number of single and bilayer graphene transistors and carriedout a systematic experimental study of their low-frequency noise characteristics.Special attention was given to determining the dominant noise sources in thesedevices and the effect of aging on the current–voltage and noise characteristics. Theanalysis of the noise spectral density dependence on the area of graphene channelshowed that the dominant contributions to the low-frequency electronic noise comefrom the graphene layer itself rather than from the contacts. Aging of graphenetransistors due to exposure to ambient conditions for over a month resulted insubstantially increased noise, attributed to the decreasing mobility of graphene andincreasing contact resistance. The noise spectral density in both single and bilayergraphene transistors either increased with deviation from the charge neutralitypoint or depended weakly on the gate bias. This observation confirms that thelow-frequency noise characteristics of graphene transistors are qualitatively different fromthose of conventional silicon metal–oxide–semiconductor field-effect transistors.

Charge-density depinning at metal contacts of graphene field-effect transistors

An anomalous distortion is often observed in the transfer characteristics of graphene field-effect transistors. We fabricate graphene transistors with ferromagnetic metal electrodes, which reproducibly display distorted transfer characteristics, and show that the distortion is caused by metal-graphene contacts with no charge-density pinning effect. The pinning effect, where the gate voltage cannot tune the charge density of graphene at the metal electrodes, has been experimentally observed; however, a pinning-free interface is achieved with easily-oxidizable metals. The distortion should be a serious problem for flexible electronic devices with graphene.