Performance Of Graphene Field-effect Transistors Research Articles

Operando photoelectron spectroscopy analysis of graphene field-effect transistors

In this study, operando photoelectron spectroscopy was used to characterize the performance of graphene field-effect transistors under working conditions. By sweeping the back-gate voltages, the carrier concentration of the graphene channel on the 150 nm Si3N4/Si substrate was tuned. From the C1s core level spectra acquired under the application of different gate voltages, the binding energy shifts caused by electric-field effects were obtained and analyzed. Together with the C1s peak shape information and the photoluminescence spectrum of the Si3N4/Si substrate, the presence of local potential across the x-ray beam spot associated with defects and gate leakage current in amorphous Si3N4 was identified. The presence of defects in Si3N4/Si substrate could not only screen the partial electric field generated by the back gate but also serve as long-range scattering centers to the carriers, thus affecting charge transport in the graphene channel. Our findings will help further investigate the dielectric/graphene interface properties and accelerate the utilization of graphene in real device applications.

Numerical Evaluation of the Effect of Geometric Tolerances on the High-Frequency Performance of Graphene Field-Effect Transistors.

The interest in graphene-based electronics is due to graphene’s great carrier mobility, atomic thickness, resistance to radiation, and tolerance to extreme temperatures. These characteristics enable the development of extremely miniaturized high-performing electronic devices for next-generation radiofrequency (RF) communication systems. The main building block of graphene-based electronics is the graphene-field effect transistor (GFET). An important issue hindering the diffusion of GFET-based circuits on a commercial level is the repeatability of the fabrication process, which affects the uncertainty of both the device geometry and the graphene quality. Concerning the GFET geometrical parameters, it is well known that the channel length is the main factor that determines the high-frequency limitations of a field-effect transistor, and is therefore the parameter that should be better controlled during the fabrication. Nevertheless, other parameters are affected by a fabrication-related tolerance; to understand to which extent an increase of the accuracy of the GFET layout patterning process steps can improve the performance uniformity, their impact on the GFET performance variability should be considered and compared to that of the channel length. In this work, we assess the impact of the fabrication-related tolerances of GFET-base amplifier geometrical parameters on the RF performance, in terms of the amplifier transit frequency and maximum oscillation frequency, by using a design-of-experiments approach.

Ion-Selective Membrane-Coated Graphene-Hexagonal Boron Nitride Heterostructures for Field-Effect Ion Sensing.

An intrinsic ion sensitivity exceeding the Nernst–Boltzmann limit and an sp2-hybridized carbon structure make graphene a promising channel material for realizing ion-sensitive field-effect transistors with a stable solid–liquid interface under biased conditions in buffered salt solutions. Here, we examine the performance of graphene field-effect transistors coated with ion-selective membranes as a tool to selectively detect changes in concentrations of Ca2+, K+, and Na+ in individual salt solutions as well as in buffered Locke’s solution. Both the shift in the Dirac point and transconductance could be measured as a function of ion concentration with repeatability exceeding 99.5% and reproducibility exceeding 98% over 60 days. However, an enhancement of selectivity, by about an order magnitude or more, was observed using transconductance as the indicator when compared to Dirac voltage, which is the only factor reported to date. Fabricating a hexagonal boron nitride multilayer between graphene and oxide further increased the ion sensitivity and selectivity of transconductance. These findings incite investigating ion sensitivity of transconductance in alternative architectures as well as urge the exploration of graphene transistor arrays for biomedical applications.

The Effect of Metal Contact Doping on the Scaled Graphene Field Effect Transistor

Herein, the effect of metal contact doping on the scaled graphene field effect transistor (GFET) is investigated. Different from the traditional semiconductors device, the drain current of GFET is not inversely proportional to the channel length (LCH). The abnormal scaling behavior for drain current in GFETs can be attributed to the modification of channel resistance induced by the penetration of contact metal doping. In addition, the field‐effect mobility (μEF) of long channel GFET trend to saturate with decreasing LCH, which is consistent with the diffusive transport model. As the channel length is further scaled down, the μEF at first increases drastically and then decreases due to the enhanced effect of electrical property of graphene under the metal electrode as well as the contact resistance on the carrier transport of GFET. This study indicates that there will be a trade‐off between scaled channel length and the best performance of GFET. Further efforts should be made to modulate the properties of graphene both in channel and contact region to improve the scaling behavior of GFET.

Phase Shift Induced by Gate-Controlled Quantum Capacitance in Graphene FET

The gate-controlled quantum capacitance and the channel resistance play an important role in the performance of graphene field-effect transistors (GFETs). This paper experimentally verifies that the output phase of the graphene field-effect transistor changes under the influence of quantum capacitance and the channel resistance, which are controlled by the gate voltage. This phenomenon is theoretically analyzed, and a model is established to simulate the phase shift. The obtained simulation results of the model are in good agreement with the experimental results. This work reveals the influence of gate voltage variation on the phase characteristics of GFETs and provides a research basis for the application of GFETs in phase shifter and the establishment of the small-signal model.

Study of Graphene field effect transistor (GFET) for chemical sensing application.

This paper presents a model that provides the transfer and output characteristic of a grapheme FET transistor. This model also provides close form inquiring expressions for the drain current, trance conductance of the device. In this paper we have investigate the transfer characteristic curve for chemical sensing application. In this present scenario this work proposes to develop a superfast mycotoxin sensor based on Graphene field effect transistor (GFET) using standard microelectronics technology. GFET would be able work in liquid and also in the biological substances. In this work, we will use the simulation program to explore the performance of GFET. The GFET sensor will be integrated to IoT and mobile phone platforms. This study will show that the fully integrated mycotoxin sensor with high sensitivity, fast response, and high dynamic range of toxins concentrations is feasible by proper GFET functionalization.GFET was modelled and simulated Using MATLAB software Introduction.

Contact Resistance Induced Variability in Graphene Field Effect Transistors

The success of the graphene field-effect transistor (GFET) is primarily based on solving the problems associated with the growth and transfer of high-quality graphene, the deposition of dielectrics and contact resistance. The contact resistance between graphene and metal electrodes is crucial for the achievement of high-performance graphene devices. This is because process variability is inherent in semiconductor device manufacturing. Two units, even manufactured in the same batch, never show identical characteristics. Therefore, it is imperative that the effect of variability be studied with a view to obtain equivalent performance from similar devices. In this study, we undertake the variability of source and drain contact resistances and their effects on the performance of GFET. For this we have used a simulation method developed by us. The results show that the DC characteristics of GFET are highly dependent on the channel resistance. Also the ambipolar characteristics are strongly affected by the variation of source and drain resistances. We have captured their impact on the output as well as transfer characteristics of a dual gate GFET.

Enhanced Gas Sensing Properties of Graphene Transistor by Reduced Doping with Hydrophobic Polymer Brush as a Surface Modification Layer.

Surface modification layer of a silicon substrate has been used to enhance the performance of graphene field-effect transistors (FETs). In this report, ultrathin and chemically robust polymer brush was used as a surface modification to enhance the gas sensing properties of graphene FETs. The insertion of the polymer brush decreased substrate-induced doping of graphene. This leads to a huge increase in field-effect mobility as well as a minimum shift of the Dirac point voltage. The use of the polymer brush enables fast detection of target gas molecules because graphene sensing modality can be maximized at the undoped state of graphene. The increase of source-drain current, as well as the abrupt decrease of electron mobility upon NO2 exposure, was utilized for the instantaneous detection, and a limit of detection of 4.8 ppb was achieved with graphene FETs on PS brush. We also showed excellent cross-sensitivity of graphene gas sensors to NH3, CO2, and relative humidity condition; the source-drain current decreases upon NH3 exposure, while response to CO2 or relative humidity condition is extremely low. Our results prove that reducing the substrate-induced doping of graphene with a polymer brush is a direct method for boosting the gas sensing properties of graphene FETs.

Effects of Self-Heating on ${f}_{\text{T}}$ and ${f}_{\text{max}}$ Performance of Graphene Field-Effect Transistors

It has been shown that there can be a significant temperature increase in graphene field-effect transistors (GFETs) operating under high drain bias, which is required for power gain. However, the possible effects of self-heating on the high-frequency performance of GFETs have been weakly addressed so far. In this article, we report on an experimental and theoretical study of the effects of self-heating on dc and high-frequency performance of GFETs by introducing a method that allows accurate evaluation of the effective channel temperature of GFETs with a submicrometer gate length. In the method, theoretical expressions for the transit frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) and the maximum frequency of oscillation (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ) based on the small-signal equivalent circuit parameters are used in combination with the models of the field- and temperature-dependent charge carrier concentration, velocity, and saturation velocity of GFETs. The thermal resistances found by our method are in good agreement with those obtained by the solution of the Laplace equation and by the method of thermo-sensitive electrical parameters. Our experiments and modeling indicate that the self-heating can significantly degrade the f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> of GFETs at power densities above 1 mW/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , from approximately 25 to 20 GHz. This article provides valuable insights for further development of GFETs, taking into account the self-heating effects on the high-frequency performance.

The Dependence of the High-Frequency Performance of Graphene Field-Effect Transistors on Channel Transport Properties

This paper addresses the high-frequency performance limitations of graphene field-effect transistors (GFETs) caused by material imperfections. To understand these limitations, we performed a comprehensive study of the relationship between the quality of graphene and surrounding materials and the high-frequency performance of GFETs fabricated on a silicon chip. We measured the transit frequency (fT) and the maximum frequency of oscillation (fmax) for a set of GFETs across the chip, and as a measure of the material quality, we chose low-field carrier mobility. The low-field mobility varied across the chip from 600 cm2/Vs to 2000 cm2/Vs, while the fT and fmax frequencies varied from 20 GHz to 37 GHz. The relationship between these frequencies and the low-field mobility was observed experimentally and explained using a methodology based on a small-signal equivalent circuit model with parameters extracted from the drain resistance model and the charge-carrier velocity saturation model. Sensitivity analysis clarified the effects of equivalent-circuit parameters on the fT and fmax frequencies. To improve the GFET high-frequency performance, the transconductance was the most critical parameter, which could be improved by increasing the charge-carrier saturation velocity by selecting adjacent dielectric materials with optical phonon energies higher than that of SiO2.

Graphene electrical properties modulated by swift heavy ion irradiation

Changes in electrical properties of graphene devices induced by the energetic ion irradiation are very important for their application in harsh radiation environment. This paper presents the modulating behavior of electrical properties of graphene-based devices induced by swift heavy ions (SHIs). Graphene field effect transistors (GFETs) were irradiated by 1.79 GeV Ta ions and it was found that at lower fluence (109-1010 ions/cm2), the SHIs irradiation can effectively optimize the performance of GFETs, while at higher fluence (∼1011 ions/cm2), the electrical properties of the devices were significantly deteriorated after the irradiation process. The effective length and width of the graphene strip and irradiation fluence are the main factors that determine the improvement in performance of GFETs. Raman spectroscopy was employed to figure out the correlation between the initial defect density in graphene and changes in electrical performance of GFETs. It was shown that the competition among various factors such as the doping, local annealing and defect creation dominates the GFET performance. This work explores the best conditions for improving the electrical properties of GFETs and provides an important reference data for the utilization of graphene based irradiated devices in aerospace electronics.

Channel length scaling and electrical characterization of graphene field effect transistor (GFET)

&lt;p&gt;The exclusive monoatomic framework of graphene makes it as an alluring material to be implemented in electronic devices. Thus, using graphene as charge carrying conducting channel material in Field Effect Transistors (FET) expedites the opportunities for production of ultrasensitive biosensors for future device applications. However, performance of GFET is influenced by various parameters, particularly by the length of conducting channel. Therefore, in this study we have investigated channel length scaling in performance of graphene field effect transistor (GFET) via simulation technique using Lumerical DEVICE software. The performance was analyzed based on electrical characterization of GFET with long and short conducting channels. It proves that conducting channel lengths have vast effect on ambipolar curve where short channel induces asymmetry in transfer characteristics curve where the n-branch is suppressed. Whereas for output characteristics, the performance of GFET heavily degraded as the channel length is reduced in short channels of GFET. Therefore, channel length scaling is a vital parameter in determining the performance of GFET in various fields, particularly in biosensing applications for ultrasensitive detection.&lt;/p&gt;

Graphene Field-Effect Transistors With High Extrinsic &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${f}_{T}$&lt;/tex-math&gt; &lt;/inline-formula&gt; and &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${f}_{\mathrm{max}}$&lt;/tex-math&gt; &lt;/inline-formula&gt;

In this letter, we report on the performance of graphene field-effect transistors (GFETs) in which the extrinsic transit frequency ( ${f}_{T}$ ) and maximum frequency of oscillation ( ${f}_{\text {max}}$ ) showed improved scaling behavior with respect to the gate length ( ${L}_{g}$ ). This improvement was achieved by the use of high-quality graphene in combination with successful optimization of the GFET technology, where extreme low source/drain contact resistances were obtained together with reduced parasitic pad capacitances. GFETs with gate lengths ranging from $0.5~\mu \text{m}$ to $\text {2}~\mu \text{m}$ have been characterized, and extrinsic ${f}_{T}$ and ${f}_{\text {max}}$ frequencies of up to 34 and 37 GHz, respectively, were obtained for GFETs with the shortest gate lengths. Simulations based on a small-signal equivalent circuit model are in good agreement with the measured data. Extrapolation predicts extrinsic ${f}_{T}$ and ${f}_{\text {max}}$ values of approximately 100 GHz at ${L}_{g}=\text {50}$ nm. Further optimization of the GFET technology enables ${f}_{\text {max}}$ values above 100 GHz, which is suitable for many millimeter wave applications.

Effect of channel-width and chirality on graphene field-effect transistor based real-time biomolecule sensing

Graphene field-effect transistors (GFET) hold promise in biomolecule sensing due to the outstanding properties of graphene materials. Charges in biomolecules are transduced into a change in the GFET current, which allows real-time monitoring of the biomolecule concentrations. Here we theoretically evaluate the performance of GFET based real-time biomolecule sensing, aiming to better understand the width-scaling limit in GFET based biosensors. In particular, we study the effect of the channel-width and the chirality on FET sensitivity by taking the percentage change of the FET current per unit charge density as the sensing signal. Firstly, GFETs made of graphene nanoribbons (GNR) and graphene sheets (GS) show comparable sensing signals to each other when gated at 1011 – 1012 cm-2 carrier densities. Sensing signals in GNRs are enhanced when gated near the sub-band thresholds, and increase their values in wider GNRs due to the change in device conductance and quantum capacitance. Secondly, the GNR chirality is found to fine tune the sensing signals. Armchair GNRs with smaller energy bandgaps appear to have an enhanced sensing signal close to 1011 cm-2 carrier densities. These results may help understand the scaling limit in GFET based biosensors along the width direction, and shed light on forming all-electrical bio-arrays.

High-pressure oxygen annealing of Al2O3 passivation layer for performance enhancement of graphene field-effect transistors

High-pressure annealing in oxygen ambient at low temperatures (∼300 °C) was effective in improving the performance of graphene field-effect transistors. The field-effect mobility was improved by 45% and 83% for holes and electrons, respectively. The improvement in the quality of Al2O3 and the reduction in oxygen-related charge generation at the Al2O3-graphene interface, are suggested as the reasons for this improvement. This process can be useful for the commercial implementation of graphene-based electronic devices.

High-frequency noise characterization of graphene field effect transistors on SiC substrates

Considering its high carrier mobility and high saturation velocity, a low-noise amplifier is thought of as being the most attractive analogue application of graphene field-effect transistors. The noise performance of graphene field-effect transistors at frequencies in the K-band remains unknown. In this work, the noise parameters of a graphene transistor are measured from 10 to 26 GHz and noise models are built with the data. The extrinsic minimum noise figure for a graphene transistor reached 1.5 dB, and the intrinsic minimum noise figure was as low as 0.8 dB at a frequency of 10 GHz, which were comparable with the results from tests on Si CMOS and started to approach those for GaAs and InP transistors. Considering the short development time, the current results are a significant step forward for graphene transistors and show their application potential in high-frequency electronics.

Current crowding mediated large contact noise in graphene field-effect transistors.

The impact of the intrinsic time-dependent fluctuations in the electrical resistance at the graphene–metal interface or the contact noise, on the performance of graphene field-effect transistors, can be as adverse as the contact resistance itself, but remains largely unexplored. Here we have investigated the contact noise in graphene field-effect transistors of varying device geometry and contact configuration, with carrier mobility ranging from 5,000 to 80,000 cm2 V−1 s−1. Our phenomenological model for contact noise because of current crowding in purely two-dimensional conductors confirms that the contacts dominate the measured resistance noise in all graphene field-effect transistors in the two-probe or invasive four-probe configurations, and surprisingly, also in nearly noninvasive four-probe (Hall bar) configuration in the high-mobility devices. The microscopic origin of contact noise is directly linked to the fluctuating electrostatic environment of the metal–channel interface, which could be generic to two-dimensional material-based electronic devices.

Organic-Inorganic Heterointerfaces for Ultrasensitive Detection of Ultraviolet Light.

The performance of graphene field-effect transistors is limited by the drastically reduced carrier mobility of graphene on silicon dioxide (SiO2) substrates. Here we demonstrate an ultrasensitive ultraviolet (UV) phototransistor featuring an organic self-assembled monolayer (SAM) sandwiched between an inorganic ZnO quantum dots decorated graphene channel and a conventional SiO2/Si substrate. Remarkably, the room-temperature mobility of the chemical-vapor-deposition grown graphene channel on the SAM is an order-of-magnitude higher than on SiO2, thereby drastically reducing electron transit-time in the channel. The resulting recirculation of electrons (in the graphene channel) within the lifetime of the photogenerated holes (in the ZnO) increases the photoresponsivity and gain of the transistor to ∼10(8) A/W and ∼3 × 10(9), respectively with a UV to visible rejection ratio of ∼10(3). Our UV photodetector device manufacturing is also compatible with current semiconductor processing, and suitable for large volume production.

Impact of contact and access resistances in graphene field-effect transistors on quartz substrates for radio frequency applications

High-frequency performance of graphene field-effect transistors (GFETs) has been limited largely by parasitic resistances, including contact resistance (RC) and access resistance (RA). Measurement of short-channel (500 nm) GFETs with short (200 nm) spin-on-doped source/drain access regions reveals negligible change in transit frequency (fT) after doping, as compared to ∼23% fT improvement for similarly sized undoped GFETs measured at low temperature, underscoring the impact of RC on high-frequency performance. DC measurements of undoped/doped short and long-channel GFETs highlight the increasing impact of RA for larger GFETs. Additionally, parasitic capacitances were minimized by device fabrication using graphene transferred onto low-capacitance quartz substrates.

Impact of graphene polycrystallinity on the performance of graphene field-effect transistors

We have used a multi-scale physics-based model to predict how the grain size and different grain boundary morphologies of polycrystalline graphene will impact the performance metrics of graphene field-effect transistors. We show that polycrystallinity has a negative impact on the transconductance, which translates to a severe degradation of the maximum and cutoff frequencies. On the other hand, polycrystallinity has a positive impact on current saturation, and a negligible effect on the intrinsic gain. These results reveal the complex role played by graphene grain boundaries and can be used to guide the further development and optimization of graphene-based electronic devices.