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To overcome the nonlinear distortion inherent in analog ICs based on Si-MOSFETs, we report here the graphene-silicon hybrid analog amplifier ICs that integrate graphene field-effect transistor (GFET) and Si-MOSFET components. By fabricating homogeneous GFETs on a silicon wafer with prefabricated Si-MOSFETs, we obtained the hybrid ICs that neatly incorporate the high linearity of GFETs. The high-linearity analog amplifier ICs can thus be realized simply using basic MOSFET circuit configurations, without requiring complicated calibration designs. This hybrid integration principle may enable immediate practical applications of graphene electronics. [ABSTRACT FROM AUTHOR]

A complex analysis of spike monitoring in neuro-prosthetic diagnosis demands a high-speed sub-nanoscale transistors with an advanced device technologies. This work reports the high performance of Graphene field-effect transistor (GFET) based front-end amplifier (FEA) design for the neuro-prosthetic application. The 9 nm Graphene FET device is optimized by characterization of transconductance and drain current towards high sensitivity and small factor. The proposed GFET-based FEA with pseudo-resistor technique demonstrates very high-input impedance in Tera-ohms that nullify the input leakage current. Here, gain-bandwidth product and noise optimization of GFET FEA enhances the overall gain with negligible noise. The proposed design operates at low voltage, further reduces the power consumption, and achieves less chip area in sub-nano size so it could be more suitable for implantable devices. The GFET-based FEA architecture achieves an action potential spike of 1.4 µV while the local field potentials spike of 1.8 mV. The proposed architecture is implemented in Advanced Design System using the design kit of the GFET process. Power consumption of 3.14 µW is observed with a supply voltage of 0.9 V. The simulated and experimental results of the proposed design achieve an input impedance of 2 TΩ with excellent noise performance over a wideband of 13.85 MHz. The proposed work demonstrates better neural activity sensing when compared to the state-of-the-artwork, which could be highly beneficial for future neuroscientists. [ABSTRACT FROM AUTHOR]

A simplified region-wise potential-based analytical model is established for boron (B) or nitrogen (N) substitution doped graphene field-effect transistor (GFET). The closed-form direct analytical relation between graphene channel potential and applied bias condition is developed by imposing the effective approximation for Fermi–Dirac integral function in various regions of operation. The boundary for distinct GFET operating regime is separated based on the position of Dirac point and Fermi energy level with respect to applied bias condition. The semiclassical drift and diffusive transport model is utilized to obtain the drain current. In addition to that, the drift component, diffusion coefficient, and its corresponding current component for B and N substitution doped GFETs have been modeled individually and their influence on total current is discussed. The proposed region-potential-based analytical model has shown good agreement with numerically solved self-consistent model. Also, the physical insights in device design, such as threshold voltage and saturation drain potential model with respect to various doping/device parameters, have been examined. [ABSTRACT FROM AUTHOR]

In this work, a new type of graphene ICs only comprising multiple graphene field-effect transistor (GFET) components is first reported. Based on the GFET design that enables homogeneous electrical characteristics in batch-made devices, the practicality and functionality significances of multi-GFET integration are experimentally demonstrated via high-fidelity amplifiers for baseband analog signals. Different from the quadratic relationship of the gate-controlled current output (in saturation region) in the traditional silicon-based MOSFETs, the saturation current output of GFETs relating to the gate-controls possesses high linearity. This advantage overcomes the inherent nonlinear distortion issues in the existing silicon MOSFET amplifiers and potentially provides a promising scenario for the accurate preamplification of weak analog signals. Besides, the novel circuit configuration and signaling principle may expand the developmental paradigm of analog amplifiers and set groundwork for the development and practicalization of advanced carbon-based nanoelectronic ICs. [ABSTRACT FROM AUTHOR]

Sensitive solar-blind ultraviolet (UV) photodetectors are important to various military and civilian applications, such as flame sensors, missile interception, biological analysis, and UV radiation monitoring below the ozone hole. In this paper, a solar-blind UV photodetector based on a buried-gate graphene field-effect transistor (GFET) decorated with titanium dioxide (TiO2) nanoparticles (NPs) was demonstrated. Under the illumination of a 325-nm laser (spot size ~2 μm) with a total power of 0.35 μW, a photoresponsivity as high as 118.3 A/W was obtained, at the conditions of zero gate bias and a source-drain bias voltage of 0.2 V. This photoresponsivity is over 600 times higher than that of a recently reported solar-blind UV photodetector based on graphene/vertical Ga2O3 nanowire array heterojunction (0.185 A/W). Experiments showed that the photoresponsivity of the TiO2 NPs decorated GFET photodetectors can be further enhanced by increasing the source-drain bias voltage or properly tuning the gate bias voltage. Furthermore, the photoresponse time of the TiO2 NPs decorated GFET photodetectors can also be tuned by the source-drain bias and gate bias. This study paves a simple and feasible way to fabricate highly sensitive, cost-efficient, and integrable solar-blind UV photodetectors.

A small-signal equivalent circuit for graphene field-effect transistors (GFETs) is proposed considering the explicit contribution of effects at the metal–graphene interfaces by means of contact resistances. A methodology to separate the contact resistances from intrinsic parameters, obtained by a deembedding process, and extrinsic parameters of the circuit is considered. The experimental high-frequency performance of three devices from two different GFET technologies is properly described by the proposed small-signal circuit. Some model parameters scale with the device footprint. The correct detachment of contact resistances from the internal transistor enables to assess their impact on the intrinsic cutoff frequency of the studied devices. [ABSTRACT FROM AUTHOR]

The trap-induced hysteresis on the performance of a graphene field-effect transistor is experimentally diminished here by applying consecutive gate-to-source voltage pulses of opposing polarity. This measurement scheme is a practical and suitable approach to obtain reproducible device characteristics. Trap-affected and trap-reduced experimental data enable a discussion regarding the impact of traps on static and dynamic device performance. An analytical drain current model calibrated with the experimental data enables the study of the trap effects on the channel potential within the device. High-frequency (HF) figures of merit and the intrinsic gain of the device obtained from both experimental and synthetic data with and without hysteresis show the importance of considering the generally overlooked impact of traps for analog and HF applications. [ABSTRACT FROM AUTHOR]

Sensitive solar-blind ultraviolet (UV) photodetectors are important to various military and civilian applications, such as flame sensors, missile interception, biological analysis, and UV radiation monitoring below the ozone hole. In this paper, a solar-blind UV photodetector based on a buried-gate graphene field-effect transistor (GFET) decorated with titanium dioxide (TiO2) nanoparticles (NPs) was demonstrated. Under the illumination of a 325-nm laser (spot size ~2 μm) with a total power of 0.35 μW, a photoresponsivity as high as 118.3 A/W was obtained, at the conditions of zero gate bias and a source-drain bias voltage of 0.2 V. This photoresponsivity is over 600 times higher than that of a recently reported solar-blind UV photodetector based on graphene/vertical Ga2O3 nanowire array heterojunction (0.185 A/W). Experiments showed that the photoresponsivity of the TiO2 NPs decorated GFET photodetectors can be further enhanced by increasing the source-drain bias voltage or properly tuning the gate bias voltage. Furthermore, the photoresponse time of the TiO2 NPs decorated GFET photodetectors can also be tuned by the source-drain bias and gate bias. This study paves a simple and feasible way to fabricate highly sensitive, cost-efficient, and integrable solar-blind UV photodetectors. [ABSTRACT FROM AUTHOR]

A unified quasi-ballistic transport model is developed for single- and double-gate graphene fieldeffect transistors (GFETs) using the McKelvey flux theory approach. The proposed model is compact, scalable, and compatible for the simulation of I-V characteristics of GFET for all regions of device operation. The drain current equation ( IDS) incorporates the formulation of quasithermal velocity, quasi-ballistic mobility of charge carrier (describe the carrier transportof 2-Dmaterial likegraphene), and source/drain backscattering coefficient. This model is also capable to describe the mobility of graphene material in degenerate and nondegenerate states. The GFET with different channel lengths, widths, and oxide thicknesses is simulated using this model for single- and double-gate devices.The proposedmodel synchronizedwith experimental results and explains the peculiar transport characteristics of GFET with normalized root-mean-square error less than 9%. [ABSTRACT FROM AUTHOR]

This work develops the first frequency-dependent small-signal model for graphene electrolyte-gated field-effect transistors (EGFETs). Graphene EGFETs are microfabricated to measure intrinsic voltage gain, frequency response, and to develop a frequency-dependent small-signal model. The transfer function of the graphene EGFET small-signal model is found to contain a unique pole due to a resistive element, which stems from electrolyte gating. Intrinsic voltage gain, cutoff frequency, and transition frequency for the microfabricated graphene EGFETs are approximately 3.1 V/V, 1.9 kHz, and 6.9 kHz, respectively. This work marks a critical step in the development of high-speed chemical and biological sensors using graphene EGFETs. [ABSTRACT FROM AUTHOR]

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]

Biosensors with high selectivity, high sensitivity, and real-time detection capabilities are of significant interest for diagnostic applications as well as human health and performance monitoring. Graphene field-effect transistor (GFET) based biosensors are suitable for integration into wearable sensor technology and can potentially demonstrate the sensitivity and selectivity necessary for real-time detection and monitoring of biomarkers. Previously reported DC-mode GFET biosensors showed a high sensitivity for sensing biomarkers in solutions with a low salt concentration. However, due to Debye length screening, the sensitivity of the DC-mode GFET biosensors decreases significantly during operation in a physiological fluid such as sweat or interstitial fluid. To overcome the Debye screening length limitation, we report here alternating current (AC) mode heterodyne-based GFET biosensors for sensing neuropeptide-Y (NPY), a key stress biomarker, in artificial sweat at physiologically relevant ionic concentrations. Our AC-mode GFET biosensors show a record ultralow detection limit of 2 × 10 -18 M with an extensive dynamic range of 10 orders of magnitude in sensor response to target NPY concentration. The sensors were characterized for various carrier frequencies (ranging from 30 kHz to 2 MHz) of the applied AC voltages and various salt concentrations (10, 50, and 100 mM). Contrary to DC-mode sensing, the AC-mode sensor response increases with an increase in salt concentration in the electrolyte. The sensor response can be further enhanced by tuning the carrier frequency of the applied AC voltage. The optimum response frequency of our sensor is approximately 400-600 kHz for salt concentrations of 50 and 100 mM, respectively. The salt-concentration- and frequency-dependent sensor response can be explained by an electrolyte-gated capacitance model.

This work proposes a novel geometry field effect transistor with graphene as a channel--graphene field-effect transistor (GFET), having a hybrid contact that consists of an ohmic source/drain and its extended part towards the gate, which is capacitively coupled to the channel. The ohmic contacts are used for direct current (DC) biasing, whereas their capacitive extension reduces access region length and provides the radio frequency (RF) signal a low impedance path. Minimization of the access region length, along with the paralleling of ohmic contact's resistance and resistive part of capacitively coupled contact's impedance, lower the overall source/drain resistance, which results in an increase in current gain cut-off frequency, fT. The DC and high-frequency characteristics of the two chosen conventional baseline GFETs, and their modified versions with proposed hybrid contacts, have been extensively studied, compared, and analyzed using numerical and analytical techniques. [ABSTRACT FROM AUTHOR]

We present the characterization of ballistic graphene field-effect transistors (GFETs) with an effective oxide thickness of 3.5 nm. Graphene channels are fully encapsulated within hexagonal boron nitride, and self-aligned contacts are formed to the edge of the single-layer graphene. Devices of channel lengths ( LG) down to 67 nm are fabricated, and a virtual-source transport model is used to model the resulting current–voltage characteristics. The mobility and source-injection velocity as a function of LG yields a mean-free-path, ballistic velocity, and effective mobility of 850 nm, 9.3\times 10^7 cm/s, and 13 700 cm2/Vs, respectively, which are among the highest velocities and mobilities reported for GFETs. Despite these best-in-class attributes, these devices achieve transconductance ( gm) and output conductance ( $g_{{\mathrm{ds}}}) of only 600 and 300 \mu \textS/\mu \textm , respectively, due to the fundamental limitations of graphene’s quantum capacitance and zero-bandgap. g_m values, which are less than those of comparable ballistic silicon devices, benefit from the high ballistic velocity in graphene but are degraded by an effective gate capacitance reduced by the quantum capacitance. The $g_{{\mathrm{ds}}}$ values, which limit the effective power gain achievable in these devices, are significantly worse than comparable silicon devices due to the properties of the zero-bandgap graphene channel. [ABSTRACT FROM PUBLISHER]

Through self-consistent quantum transport simulations, we evaluate the RF performance of monolayer graphene field-effect transistors in the bias region of negative output differential resistance. We show that, compared with the region of quasi-saturation, a voltage gain larger than 10 can be obtained, at the cost of a decrease in the maximum oscillation frequency of about a factor of 1.5–3 and the need for a careful circuit stabilization. [ABSTRACT FROM PUBLISHER]

Abstract: In this paper, we developed a low operation voltage and a single reaction step of a graphene field-effect transistor (GFET) biosensor device to detect alanine aminotransferase (ALT). The device is fabricated using large-area graphene thin films by means of chemical vapor deposition. The GFET device operated at 0.1V drain voltage and −0.1 to 0.3V sweep gate voltage exhibited a high pH sensitivity of 23.12mV/pH, a low hysteresis voltage of 1.2mV, and a small drift rate of 4.74mV/h. In this research, the hybrid configuration of l-alanine and α-ketoglutarate immobilized on the graphene membrane was proved to be able to detect ALT with good linearity (R 2 =0.99) in the ALT concentration range of 10–100U/L. [Copyright &y& Elsevier]

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]

We present a study on the metal–graphene contact properties. Utilizing a dual-gate field-effect transistor device, an energetic separation between the Fermi level and the Dirac point in the contact areas can be adjusted deliberately by applying an appropriate front-gate voltage that acts only on the channel. This front-gate voltage is compensated by an opposite large-area back-gate voltage, thereby mimicking the metal induced doping effect. A back-gate voltage sweep enables identifying two distinct resistance peaks—a result of the combined impact of the graphene cones in the contact and in the channel region. Comparing our experimental data with simulations allows extracting the coupling strength between metal and graphene and also estimating the magnitude of the metal-induced doping concentration in the case of palladium contacts. In contrast to conventional metal–semiconductor contacts, our simulations predict a decreased on-current for increased coupling strength in graphene field-effect transistors. [ABSTRACT FROM PUBLISHER]

The graphene field-effect transistor (GFET) with a thin insulating oxide layer can regulate electrical performance under small voltage. After reactive ion etching of 300 nm SiO 2 , the substrates with different shapes and thicknesses are used to fabricate GFET. The electrical performance test proves that GFETs with suspended structures have high carrier mobility. Compared with the suspended structures that should uphold voltage within 15 V, a device with SiO 2 thickness of 10 nm can achieve superior electrical regulation only within 4 V. The thin insulating oxide layer increases the risk of breakdown. Under the high electric field, the thin oxide layer will be broken down to form a conductive channel, and the interface between graphene and substrate will generate metal-semiconductor contact, forming Schottky-like diode. The electrical properties have changed at this stage. There is an electric current flowing through internal conduction, and the transfer characteristics change from bipolar to unipolar. This research provides a new idea to realize small voltage regulation and a basis for judging graphene transistor failure. • Suspended graphene field-effect transistor is fabricated by transferring monolayer graphene to the patterned substrate. • Achieve superior electrical regulation under smaller gate voltage by reducing the thickness of oxide layer. • The carrier mobility of the graphene field-effect transistor with the suspended structure is remarkably improved. • The contact interface forms metal-semiconductor contact and device forms a Schottky-like diode after electrical breakdown. • These results provide a theoretical basis for judging contamination of channel material or device breakdown. [ABSTRACT FROM AUTHOR]

This letter presents a detailed study of transport in graphene field-effect transistors (GFETs) with various channel lengths, from 5 \mu\m down to 90 nm, using transferred graphene grown by chemical vapor deposition. An electron–hole asymmetry observed in short-channel devices suggests a strong impact from graphene/metal contacts. In addition, for the first time, we observe a shift of the gate voltage at the Dirac point in graphene devices as a consequence of gate length scaling. The unusual shift of the Dirac point voltage has been identified as one of the signatures of short-channel effects in GFETs. [ABSTRACT FROM AUTHOR]

This paper presents an approach to the real-time, label-free, specific, and sensitive monitoring of insulin using a graphene aptameric nanosensor. The nanosensor is configured as a field-effect transistor, whose graphene-based conducting channel is functionalized with a guanine-rich IGA3 aptamer. The negatively charged aptamer folds into a compact and stable antiparallel or parallel G-quadruplex conformation upon binding with insulin, resulting in a change in the carrier density, and hence the electrical conductance, of the graphene. The change in the electrical conductance is then measured to enable the real-time monitoring of insulin levels. Testing has shown that the nanosensor offers an estimated limit of detection down to 35 pM and is functional in Krebs-Ringer bicarbonate buffer, a standard pancreatic islet perfusion medium. These results demonstrate the potential utility of this approach in label-free monitoring of insulin and in timely prediction of accurate insulin dosage in clinical diagnostics.