Gated Graphene Field Effect Transistors Research Articles

Study of graphene p-n junctions formed by the electrostatic modification of the SiO2 substrate.

We study the transport properties of mm-scale CVD graphene p-n junctions, which are formed in a single gated graphene field effect transistor configuration. Here, an electrical-stressing-voltage technique served to modify the electrostatic potential in the SiO2/Si substrate and create the p-n junction. We examine the transport characteristics about the Dirac points that are localized in the perturbed and unperturbed regions in the graphene channel and note the quantitative differences in the Hall effect between the perturbed and unperturbed regions. The results also show that the longitudinal resistance is highly sensitive to the external magnetic field when the Hall bar device operates as a p-n junction.

Bending Stability of Ferroelectric Gated Graphene Field Effect Transistor for Flexible Electronics.

In this work, we explored the potential of the ferroelectric gate of (Pb0.92La0.08)(Zr0.30Ti0.70)O3 (PLZT(8/30/70)) for flexible graphene field effect transistor (GFET) devices. Based on the deep understanding of the VDirac of PLZT(8/30/70) gate GFET, which determines the application of the flexible GFET devices, the polarization mechanisms of PLZT(8/30/70) under bending deformation were analyzed. It was found that both flexoelectric polarization and piezoelectric polarization exist under bending deformation, and their polarization direction is opposite under the same bending deformation. Thus, a relatively stable of VDirac is obtained due to the combination of these two effects. In contrast to the relatively good linear movement of VDirac under bending deformation of relaxor ferroelectric (Pb0.92La0.08)(Zr0.52Ti0.48)O3 (PLZT(8/52/48)) gated GFET, these stable properties of the PLZT(8/30/70) gate GFETs make them have great potential for applications in flexible devices.

Impact of Large Gate Voltages and Ultrathin Polymer Electrolytes on Carrier Density in Electric-Double-Layer-Gated Two-Dimensional Crystal Transistors.

Electric-double-layer (EDL) gating can induce large capacitance densities (∼1-10 μF cm-2) in two-dimensional (2D) semiconductors; however, several properties of the electrolyte limit performance. One property is the electrochemical activity which limits the gate voltage (VG) that can be applied and therefore the maximum extent to which carriers can be modulated. A second property is electrolyte thickness, which sets the response speed of the EDL gate and therefore the time scale over which the channel can be doped. Typical thicknesses are on the order of micrometers, but thinner electrolytes (nanometers) are needed for very-large-scale-integration (VLSI) in terms of both physical thickness and the speed that accompanies scaling. In this study, finite element modeling of an EDL-gated field-effect transistor (FET) is used to self-consistently couple ion transport in the electrolyte to carrier transport in the semiconductor, in which density of states, and therefore quantum capacitance, is included. The model reveals that 50 to 65% of the applied potential drops across the semiconductor, leaving 35 to 50% to drop across the two EDLs. Accounting for the potential drop in the channel suggests that higher carrier densities can be achieved at larger applied VG without concern for inducing electrochemical reactions. This insight is tested experimentally via Hall measurements of graphene FETs for which VG is extended from ±3 to ±6 V. Doubling the gate voltage increases the sheet carrier density by an additional 2.3 × 1013 cm-2 for electrons and 1.4 × 1013 cm-2 for holes without inducing electrochemistry. To address the need for thickness scaling, the thickness of the solid polymer electrolyte, poly(ethylene oxide) (PEO):CsClO4, is decreased from 1 μm to 10 nm and used to EDL gate graphene FETs. Sheet carrier density measurements on graphene Hall bars prove that the carrier densities remain constant throughout the measured thickness range (10 nm-1 μm). The results indicate promise for overcoming the physical and electrical limitations to VLSI while taking advantage of the ultrahigh carrier densities induced by EDL gating.

Preparation and Electrical Testing of Double Top Gate Graphene Field-Effect Transistor

In this paper, we prepare and test a graphene field-effect transistor with two top gates. The Fermi energy level of graphene can be adjusted by applying positive and negative voltages to the two top gates, and N-type and P-type graphene are formed in the channel region, thus inducing a graphene p-n junction. The current model is established using the gradual channel approximation (GCA) method, and the current and p-n junction characteristics of the device were obtained by formula simulations. Based on the principle of p-n junction luminescence, this device with graphene p-n junction is expected to achieve terahertz wave radiation with an appropriate optical resonant cavity.

Optimization of graphene transistor sensors based on quantum capacitance and charge carrier mobility analysis

Charge density of molecules (Nm) in hybrid nanostructures that is formed at the interface of graphene and liquid in solution gated graphene field effect transistors (SGFETs) determines the selective response of chemical and biological sensors based on these SGFETs. For optimization of this response it is important to determine how it depends on characteristics of SGFETs such as quantum capacitance (Cq) and charge mobility (μ) which are functionally linked to Nm. The proposed model shows that when the gate voltage (Vgate) is near the minimum point of graphene conductivity (Dirac point) the sensor response is low and increases with gate voltage until Cq is approximately equal to the capacitance of the formed double layer (Cdl) in SGFETs. A decrease in sensor response is predicted upon further increase of Vgate in cases where there is a stronger dependence of μ on Nm than μ propto 1/Nm. A comparison of the predicted results of the model and literature data obtained in SGFET sensors for lysine in an aqueous solution are in agreement with the assumption that the optimal condition of Cq~ Cdl is reached approximately in the Vgate region of (0.5-1.4) V from the Dirac Point. Keywords: graphene, hybrid nanostructures, transistor sensor, charge mobility, interface.

Dependence of charge carrier mobility in hybrid nanostructures at the interface of graphene and molecular ions on their charge density

Hybrid nanostructures with large interface between nanostructural elements play an important role in the modern electronics. Among these nanostructures are hybrid nanostructures formed at the interface of graphene with ensembles of molecular ions in the solution gated Graphene Field Effect Transistors (GFETs) that are promising for chemical and biological sensor fabrication. Therefore investigation of interfacial effects in electrical transport in these systems is interesting. This work is a theoretical study of dependence of the charge carrier mobility (μ) in these nanostructures on density of the interfacial molecular ions (Nii). We show that dependence μ propto 1/(Nii)1/2 obtained in free charge carrier model with short range scattering in case of the weak interaction between the charge carriers and the interfacial ions is in agreement with experimental transistor characteristics obtained at the high gate voltages. Keywords: graphene, hybrid nanostructures, transistors, mobility, interface.

Учет квантовой емкости и подвижности носителей заряда для оптимизации сенсорного отклика в графеновых транзисторах

Charge density of molecules (Nm) in hybrid nanostructures that is formed at the interface of graphene and liquid in solution gated graphene field effect transistors (SGFETs) determines the selective response of chemical and biological sensors based on these SGFETs. For optimization of this response it is important to determine how it depends on characteristics of SGFETs such as quantum capacitance (Cq) and charge mobility (µ) which are functionally linked to Nm . The proposed model shows that when the gate voltage (Vgate) is near the minimum point of graphene conductivity (Dirac point) the sensor response is low and increases with gate voltage until Cq is approximately equal to the capacitance of the formed double layer (Cdl) in SGFETs. A decrease in sensor response is predicted upon further increase of Vgate in cases where there is a stronger dependence of µ on Nm than µ ∝ 1/Nm. A comparison of the predicted results of the model and literature data obtained in SGFET sensors for lysine in an aqueous solution are in agreement with the assumption that the optimal condition of Cq ≈ Cdl is reached approximately in the region of (0.5−1.4) V from the Dirac Point.

Total solution processable and low-cost DNA methylation sensor

DNA methylation sensor is an emerging demand of modern medical for laboratory, though lots of efforts have been contributed in this discipline, the easy-to-operate and efficient sensing terminal is still in deficiency. In this work, a total solution processable printing technology is proposed to fabricate the DNA methylation sensor, which is based on the liquid gate graphene field effect transistor (Lg-GFET). Meanwhile, the graphene ink (G-ink) was prepared by using the modified liquid-exfoliation method, to facilitate printing production. Then, five kinds of tested single strand DNA (ssDNA) which have 0 to 4 5-methylcytosine (5mC) sites were covalently anchored on the graphene channel. X-ray photoelectron spectroscopy (XPS) was conducted to demonstrate the analyte ssDNA was successfully fixed. Thirdly, the 5mC sites were identified by their antibody of 5mC (5mCab) and transduced as the changed current between the source and drain electrodes ( I ds). It is found the proposed sensor has good linear sensitivity (about -1.68% for per 5mC) to the amount of 5mC sites on the tested ssDNAs with Pearson correlation coefficient -0.98 and excellent anti-interference to unmethylated ssDNAs. Besides, the cost is estimated to be about $ 1.5 for each of the proposed DNA methylation sensor. Conclusively, the proposed low-cost component may be an efficient candidate for promoting the instant DNA methylation determination.

A Disposable Printed Liquid Gate Graphene Field Effect Transistor for a Salivary Cortisol Test.

Circadian rhythm of salivary cortisol is of clinical significance, tracking salivary cortisol in domicile is welcomed by both doctor and patient, due to its merits of noninvasion, ease of sampling, and free-of-stress response. Here, we present a portable salivary cortisol test setup based on a liquid gate graphene field effect transistor (Lg-GFET) for the first time. In this work, the Lg-GFET was prepared by the printing technology and exploited as a sensitive material. In the procedures of device preparation, the modified liquid exfoliation method and direct-ink-write technology were utilized for synthesizing the graphene ink and printing Lg-GFETs; then, the as-prepared Lg-GFETs were decorated and functionalized by tetrakis(4-carboxyphenyl) porphyrin and the cortisol aptamer, successively. Their sensitivity, selectivity, and robustness are seriously examined. The test results indicate that the sensors have good linear sensitivities over a seven-log analyte concentration range (0.01 to 104 nM) and the anti-interference ability to distinguish from the substancess with similar chemical structures. Moreover, the conceptual application for tracking circadian rhythm was carried out successfully. Conclusively, the proposed flexible Lg-GFET-based salivary cortisol detection platform can satisfy the requirements of the salivary cortisol's assay for instant detection. Additionally, it also provides an alternative solution for developing similar household medical appliances.

Self-Consistent Modeling of B or N Substitution Doped Bottom Gated Graphene FET With Nonzero Bandgap

A phenomenological all region drain current model for boron (B) or nitrogen (N) substitution-doped bottom gated graphene field effect transistor (GFET) is developed. In this work, a self-consistent approach is utilized to obtain appropriate potential-charge relation. The effects of substitution doping such as shift in Dirac point with significant nonzero bandgap, change in carrier sheet density and mobility are explicitly captured in this model. In addition to that the semiclassical diffusive mobility is modeled extensively as a function of two predominant parameters such as interaction parameter ( r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ) and impurity concentration ( n <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> ). The proposed drain current model and diffusive mobility model are predicting an excellent agreement with experimental data from fabricated B-doped bottom gated GFET. B/N substitution-doped bottom gated GFETs completely suppress the bipolar behavior and exhibit significant reduction in OFF-current. And, the ON/ OFF-ratio has been enhanced significantly from 1 to 8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> as compared from undoped to 25% B-doped GFET, which makes B/N-doped GFETs well-suitable in digital applications. Also, B/N-doped GFETs enhance the saturation behavior which is highly desirable in analog/RF applications. Hence, B/N substitution doping in graphene fulfill the requirement of GFET in both analog/RF and digital applications.

Зависимость подвижности носителей заряда в гибридных наноструктурах на интерфейсе графена с молекулярными ионами от их зарядовой плотности

A.V. Butko, V.Y. Butko, Y.A.Kumzerov Ioffe Institute, 194021, St. Petersburg, Russia Hybrid nanostructures with large interface between nanostructural elements play an important role in the modern electronics. Among these nanostructures are hybrid nanostructures formed at the interface of graphene with ensembles of molecular ions in the solution gated Graphene Field Effect Transistors (GFETs) that are promising for chemical and biological sensor fabrication. Therefore investigation of interfacial effects in electrical transport in these systems is interesting. This work is a theoretical study of dependence of the charge carrier mobility (µ) in these nanostructures on density of the interfacial molecular ions (Nii). We show that dependence µ~1/(Nii)^1/2 obtained in free charge carrier model with short range scattering in case of the weak interaction between the charge carriers and the interfacial ions is in agreement with experimental transistor characteristics obtained at the high gate voltages.

Detection of lysine molecular ions in solution gated field effect transistors based on unmodified graphene

The electrical transport in graphene interfaced with different ions in solution gated graphene field effect transistors (GFETs) is the subject of active studies due to its importance in sensor fabrication. Most of the developed GFET biological sensors use graphene that has been modified. The difficulty in the modification procedure and the reduction in quality of graphene that it causes are important drawbacks for applications. Therefore, we focus on GFETs based on unmodified graphene gated by aqueous solutions containing lysine amino acids. We observed that an increase in the ionic concentration of lysine in these solutions leads to a suppression of unipolar electron conductance of graphene in GFETs. This dependence is opposite to the dependence typically observed in gating solutions containing smaller atomic ions. We attribute the observed suppression to electric field screening of the graphene surface from water molecules by lysine ions which are larger and have lower charge density compared to atomic ions. This novel phenomenon leads to an overall decrease of surface charge density in molecular layers formed at the graphene interface and can be applied in GFET sensors with unmodified graphene that detect the presence and concentration of large molecules in the gating solutions.

Nitrogen-doped NDR behavior of double gate graphene field effect transistor

Abstract This paper presents the negative differential resistance (NDR) behavior of double gate graphene field effect transistor. The channel length of the device is considered as 4 nm and source/drain regions are doped with nitrogen atoms. The carrier transport is assumed to be ballistic for the simulation. The semi-empirical Slater-Koster method is used to model the material and Nonequilibrium Green's Function (NEGF) is used for the electron transport study of the device. The NDR behavior of the device is discussed by using the transmission spectrum and device density of state (DDOS). The peak to valley current ratio (PVCR) of 105 is achieved with the peak current of -0.6 μA at room temperature. The other parameters like the NDR voltage window and the peak value of drain current are also discussed quantitatively. The simulation study shows the importance of graphene for negative differential resistance for future scaled electronic device applications.

Empirical Drain Current Model of Graphene Field-Effect Transistor for Application as a Circuit Simulation Tool

In this paper, an empirical model of dual gate graphene field-effect transistor (GFET) has been developed. The explicit form of drain current expression has been established analytically based on a drift-diffusion approach. The channel sheet charge density helps us to determine the quantum capacitance more accurately. The channel potential has been derived from the equivalent circuit. The improvement of carrier mobility has been included. The closed form expression is obtained with a few fitting parameters to explore the model characteristics. The model is also valid for single gate technology. The model has been verified against some pioneering experimental results reported in literatures. Model simulation agrees well with existing experimental data. We have also compared the results of the model obtained from NANOHUB online simulator for further verification. The characteristics of small signal parameters of this GFET model have been observed and cut-off frequency has been determined. The developed analytical GFET model is also found to be computationally efficient. It is observed that with ± 10% variation in top gate oxide thickness, maximum drain current and peak transconductance vary by (−11.53%/ + 14%) and (−25%/+31.25%), respectively. The double gate bilayer GFET model is implemented in Verilog-A and verified for a voltage amplifier in SPICE environment.

A critical assessment on the electron transport through dehydrogenated intrinsically conducting channels in graphane-graphene hybrids

We investigate the electron transport properties of different extended line/ribbon defects in hydrogenated graphene (graphane) by means of density functional theory combined with the non-equilibrium Green’s function formalism. The calculated zero-bias transmission profiles show that dehydrogenated tracks in graphane are promising candidates for fabricating printed nanocircuits. Our calculations also indicate that the gate-induced transmission probability of a completely dehydrogenated zigzag carbon line (DHZCL) in a freestanding graphane sheet opens a conduction channel in its insulating structure. The voltage-dependent transmission tends to increase for two weakly interacting DHZCL defects, although the transmission spectra critically depend on the proximity of these dehydrogenated lines. The current response to a bias voltage is also analyzed for the proposed defective devices and the calculated I-V characteristics show that one or two weakly interacting DHZCL defects in graphane behave as fair conduction channels from a certain threshold voltage. However, two immediately neighbor line defects (i.e., acene-type defects) exhibit a conducting behavior increasing from 0.0 to 0.2 V and a feature of a negative differential resistance effect under bias voltage in the 0.25–0.5 V range. These results suggest that extended 1D/2D defects in graphane-graphene hybrid platforms could be properly exploited for electronic applications in solid-state chemically gated graphene field-effect transistor.

A novel hot embossing Graphene transfer process for flexible electronics

In this work a new Single Layer Graphene (SLG) transfer technique exploiting a hot embossing process was carried out. Flexible electrolyte gated Graphene Field Effect Transistors (G-FET) were fabricated and tested electrically. A polymeric transparent foil suitable for optics and flexible electronics, Cyclic Olefin Copolymer (COC) was used as flexible substrate. Raman characterization confirmed that the new Hot Embossing Graphene Transfer (HEGT) is suitable for the deposition of SLG and the fabrication of G-FETs. A Comparison with SLG common transfer method was carried out and proven for G-FETs fabrication. The HEGT devices showed typical characteristics and maintained the same performances when the substrate was bent. This demonstrated that the HEGT allows for efficient transfer of high quality SLG on large area thus providing the opportunity for the exploitation on a large scale production process for flexible substrates.

Quantum and electrochemical interplays in hydrogenated graphene

The design of electrochemically gated graphene field-effect transistors for detecting charged species in real time, greatly depends on our ability to understand and maintain a low level of electrochemical current. Here, we exploit the interplay between the electrical in-plane transport and the electrochemical activity of graphene. We found that the addition of one H-sp3 defect per hundred thousand carbon atoms reduces the electron transfer rate of the graphene basal plane by more than five times while preserving its excellent carrier mobility. Remarkably, the quantum capacitance provides insight into the changes of the electronic structure of graphene upon hydrogenation, which predicts well the suppression of the electrochemical activity based on the non-adiabatic theory of electron transfer. Thus, our work unravels the interplay between the quantum transport and electrochemical kinetics of graphene and suggests hydrogenated graphene as a potent material for sensing applications with performances going beyond previously reported graphene transistor-based sensors.

A novel technique for detection of biomolecules and its aqueous concentration using a double gate graphene field effect transistor

• A novel technique for detection of biomolecules has been proposed using adsorption on the gate material, Au (100). • Calculation of aqueous concentration of biomolecules present in the analyte solution introduced to the nanobiosensor. • The nanobiosensor exhibits excellent sensitivity in the sub threshold region. • Analytical model of the proposed nanobiosensor has been developed.

Increasing the Room-Temperature Electric Double Layer Retention Time in Two-Dimensional Crystal FETs.

Poly(vinyl alcohol) (PVA) and LiClO4, a solid polymer electrolyte with a glass transition temperature (Tg) of 80 °C, is used to electrostatically gate graphene field-effect transistors. The ions in PVA:LiClO4 are drifted into place by field-effect at T > Tg, providing n- or p-type doping, and when the device is cooled to room temperature, the polymer mobility and, hence ion mobility are arrested and the electric double layer (EDL) is "locked" into place in the absence of a gate bias. Unlike other electrolytes used to gate two-dimensional devices for which the Tg, and therefore the "locking" temperature, is well below room temperature, the electrolyte demonstrated in this work provides a route to achieve room-temperature EDL stability. Specifically, a 6 orders of magnitude increase in the room temperature EDL retention time is demonstrated over the commonly used electrolyte, poly(ethylene oxide) (PEO) and LiClO4. Hall measurements confirm that large sheet carrier densities can be achieved with PVA:LiClO4 at top gate programming voltages of ±2 V (-6.3 ± 0.03 × 1013 cm-2 for electrons and 1.6 ± 0.3 × 1014 cm-2 for holes). Transient drain current measurements show that at least 75% of the EDL is retained after more than 4 h at room temperature. Unlike PEO-based electrolytes, PVA:LiClO4 is compatible with the chemicals used in standard photolithographic processes enabling the direct deposition of patterned, metal contacts on the surface of the electrolyte. A thermal instability in the electrolyte is detected by both I-V measurements and differential scanning calorimetry, and FTIR measurements suggest that thermally catalyzed cross-linking may be driving phase separation between the polymer and the salt. Nevertheless, this work highlights how the relationship between polymer and ion mobility can be exploited to tune the state retention time and the charge carrier density of a 2D crystal transistor.

Characterization of Electrochemically Gated Graphene Field-Effect Transistor for Bioelectronic Applications

This work provides an experimental characterization and equivalent circuit modelling of electrochemically gated field effect transistors (EGFETs) using a graphene layer. The aim is to demonstrate the detection limit of these sensing devices for bioelectronics applications. Towards that, we present a detail study of the; (i) sources of intrinsic noise, (ii) the effect of ionic density in charge transfer resistance and (iii) the existence of parasitic conducting paths in the surrounding electrolyte. To investigate the liquid-based applications of graphene based EGFETs, quantifying the basic physical properties of graphene/solution interface is crucial. Towards that, we present a detail study of zero bias charge transfer resistance by using different type of ionic solutions and cell culture mediums. Measurements of the electrical noise were also performed to determine the EGFET detection limit. The performance of the EGFET to record the electrophysiological signals was evaluated using adult zebra fish heart and electrogenic cell cultures. The electrocardiogram of the zebrafish heart was monitored with signal-to-noise-ratio higher than 7. This contribution also discusses the application of graphene based EGFETS to measure electrophysiological signals in cell cultures. For instance, ultra-week extracellular signals resulting from cell metabolic activity or low frequency calcium waves. Keywords. Graphene, electrochemically gated field effect transistor, charge transport, intrinsic noise, extracellular signals. Figure 1