Search Results (23)

Frequency multipliers are essential components in communication systems, and graphene’s exceptional electrical properties make it highly promising for flexible electronics. This paper addresses the technical challenges of multi-frequency multipliers based on graphene field-effect transistors (GFETs) and introduces a novel fabrication method using graphene as the channel material and metals with different work functions as the top gate. By employing Ti and Pd with distinct work functions, we develop a dual-gate GFET device that exhibits stable M-shaped resistance characteristics on a flexible polyethylene naphthalate (PEN) substrate. We demonstrate frequency doubler, tripler, and quadrupler on the flexible substrate. The results show that the GFET-based frequency multiplier offers advantages such as low operating voltage (<1 V), high voltage conversion efficiency (up to 8.4% for tripler and 6% for quadrupler), and high spectral purity (up to 88% for tripler and 76% for quadrupler). The intrinsic maximum operating frequency of the frequency quadrupler reaches 54 GHz. The use of a monolayer graphene channel, dual-metal gate control enabling an M-shaped transfer curve, and flexible characteristics all contribute to its superior performance compared to conventional devices. Full article

This paper presents a novel Verilog-A model for the Fermi velocity in Graphene Field-Effect Transistors (GFETs). The Fermi velocity is an important parameter associated with the energy spectrum of the delocalized bonds in graphene which impact the performance of a GFET. Unlike existing GFET models where the Fermi velocity is assumed to have a constant value, the proposed model considers carrier concentrations in the channel and gate dielectrics to create a closed-form solution for the Fermi velocity, a parameter previously demonstrated to vary based on these two factors. The proposed mathematical model is then adapted to Verilog-A for interfacing with computer-aided design (CAD) circuit simulators. To demonstrate the accuracy of the proposed model, the simulation results are compared to measured drain–source currents obtained from various GFET devices (including GFETs measured by authors). The measured results show good agreement with the values predicted using the proposed model (<±1%), demonstrating the superior accuracy of the model compared to other published Verilog-A-based models, especially around the Dirac point. Full article

This study introduces a flexible and low-cost solution for a source measure unit (SMU), which is presented as an alternative to conventional source meter units and a blueprint for sensor FET drivers. An SMU collects current–voltage (I-V) curves with an additional variable voltage or current and is commonly used to characterize semiconductors. We present the hardware design, interfacing, and test results of our SMU. Specifically, we present representative I-V curve measurements for graphene-channel FETs to demonstrate the SMU’s capability to efficiently characterize these devices with minimal noise and sufficient accuracy. This cost-effective solution presents a promising avenue for researchers and developers seeking reliable tools for sensor development and characterization. We demonstrate, with the example of surface illumination, how the sensing behavior of graphene-channel FETs can be characterized without the need for expensive equipment. Additionally, the SMU was validated with known passive and active components, along with probe station integration for semiconductor die-scale connection. The SMU’s focus on collecting I-V curves, coupled with its ability to identify device defects, such as parasitic Schottky junctions or a failed oxide, contributes to its utility in quality testing for semiconductor devices. Its low-cost nature makes it accessible for various research endeavors, enabling efficient data collection and analysis for graphene-based and other nanomaterial-based sensor applications. Full article

Graphene-based materials are actively being investigated as sensing elements for the detection of different analytes. Both graphene grown by chemical vapor deposition (CVD) and graphene oxide (GO) produced by the modified Hummers’ method are actively used in the development of biosensors. The production costs of CVD graphene- and GO-based sensors are similar; however, the question remains regarding the most efficient graphene-based material for the construction of point-of-care diagnostic devices. To this end, in this work, we compare CVD graphene aptasensors with the aptasensors based on reduced GO (rGO) for their capabilities in the detection of NT-proBNP, which serves as the gold standard biomarker for heart failure. Both types of aptasensors were developed using commercial gold interdigitated electrodes (IDEs) with either CVD graphene or GO formed on top as a channel of liquid-gated field-effect transistor (FET), yielding GFET and rGO-FET sensors, respectively. The functional properties of the two types of aptasensors were compared. Both demonstrate good dynamic range from 10 fg/mL to 100 pg/mL. The limit of detection for NT-proBNP in artificial saliva was 100 fg/mL and 1 pg/mL for rGO-FET- and GFET-based aptasensors, respectively. While CVD GFET demonstrates less variations in parameters, higher sensitivity was demonstrated by the rGO-FET due to its higher roughness and larger bandgap. The demonstrated low cost and scalability of technology for both types of graphene-based aptasensors may be applicable for the development of different graphene-based biosensors for rapid, stable, on-site, and highly sensitive detection of diverse biochemical markers. Full article

An asymmetric dual-grating gate bilayer graphene-based field effect transistor (ADGG-GFET) with an integrated bowtie antenna was fabricated and its response as a Terahertz (THz) detector was experimentally investigated. The device was cooled down to 4.5 K, and excited at different frequencies (0.15, 0.3 and 0.6 THz) using a THz solid-state source. The integration of the bowtie antenna allowed to obtain a substantial increase in the photocurrent response (up to 8 nA) of the device at the three studied frequencies as compared to similar transistors lacking the integrated antenna (1 nA). The photocurrent increase was observed for all the studied values of the bias voltage applied to both the top and back gates. Besides the action of the antenna that helps the coupling of THz radiation to the transistor channel, the observed enhancement by nearly one order of magnitude of the photoresponse is also related to the modulation of the hole and electron concentration profiles inside the transistor channel by the bias voltages imposed to the top and back gates. The creation of local n and p regions leads to the formation of homojuctions ($np$, $pn$ or $p{p}^{+}$) along the channel that strongly affects the overall photoresponse of the detector. Additionally, the bias of both back and top gates could induce an opening of the gap of the bilayer graphene channel that would also contribute to the photocurrent. Full article

Owing to highly desired requirements in advanced disease diagnosis, therapy, and health monitoring, noncontact mechanosensation active matrix has drawn considerable attention. To satisfy the practical demands of high energy efficiency, in this report, combining the advantage of multiparameter monitoring, high sensitivity, and high resolution of active matrix field-effect transistor (FET) with triboelectric nanogenerators (TENG), we successfully developed the tribotronic mechanosensation active matrix based on tribotronic ion gel graphene scrolls field-effect transistors (GSFET). The tribopotential produced by TENG served as a gate voltage to modulate carrier transport along the semiconductor channel and realized self-powered ability with considerable decreased energy consumption. To achieve high spatial utilization and more pronounced responsivity of the dielectric of this transistor, ion gel was used to act as a triboelectric layer to conduct friction and contact electrification with external materials directly to produce triboelectric charges to power GFET. This tribopotential-driving device has excellent tactile sensing properties with high sensitivity (1.125 mm⁻¹), rapid response time (~16 ms), and a durability operation of thousands of cycles. Furthermore, the device was transparent and flexible with the capability of spatially mapping touch stimuli and monitoring real-time temperature. Due to all these unique characteristics, this novel noncontact mechanosensation GSFET active matrix provided a new method for self-powered E-skin with promising potential for self-powered wearable devices and intelligent robots. Full article

A novel photochemical technological route for one-step functionalization of a graphene surface with an azide-modified DNA aptamer for biomarkers is developed. The methodology is demonstrated for the functionalization of a DNA aptamer for an N-terminal B-type natriuretic peptide (NT-proBNP) heart failure biomarker on the surface of a graphene channel within a system based on a liquid-gated graphene field effect transistor (GFET). The limit of detection (LOD) of the aptamer-functionalized sensor is 0.01 pg/mL with short response time (75 s) for clinically relevant concentrations of the cardiac biomarker, which could be of relevance for point-of-care (POC) applications. The novel methodology could be applicable for the development of different graphene-based biosensors for fast, stable, real-time, and highly sensitive detection of disease markers. Full article

Graphene field-effect transistors (GFETs) exhibit unique switch and sensing features. In this article, GFETs are investigated within the tight-binding formalism, including quantum capacitance correction, where the graphene ribbons with reconstructed armchair edges are mapped into a set of independent dual channels through a unitary transformation. A new transfer matrix method is further developed to analyze the electron transport in each dual channel under a back gate voltage, while the electronic density of states of graphene ribbons with transversal dislocations are calculated using the retarded Green’s function and a novel real-space renormalization method. The Landauer electrical conductance obtained from these transfer matrices was confirmed by the Kubo–Greenwood formula, and the numerical results for the limiting cases were verified on the basis of analytical results. Finally, the size- and gate-voltage-dependent source-drain currents in GFETs are calculated, whose results are compared with the experimental data. Full article

The gate insulator is one of the most crucial factors determining the performance of a graphene field effect transistor (GFET). Good electrostatic control of the conduction channel by gate voltage requires thin gate oxides. Due to the lack of the dangling bond, a seed layer is usually needed for the gate dielectric film grown by the atomic layer deposition (ALD) process. The seed layer leads to the high-quality deposition of dielectric films, but it may lead to a great increase in the thickness of the final dielectric film. To address this problem, this paper proposes an improved process, where the self-oxidized Al₂O₃ seed layer was removed by etching solutions before atomic layer deposition, and the Al₂O₃ residue would provide nucleation sites on the graphene surface. Benefiting from the decreased thickness of the dielectric film, the transconductance of the GFET using this method as a top-gate dielectric film deposition process shows an average 44.7% increase compared with the GFETs using the standard Al evaporation seed layer methods. Full article

The top-gated graphene field effect transistor (GFET) with electric-field induced doping polarity conversion has been demonstrated. The polarity of channel conductance in GFET can be transition from p-type to n-type through altering the gate electric field scanning range. Further analysis indicates that this complementary doping is attributed to the charge exchange between graphene and interface trap sites. The oxygen vacancies in Al₂O₃filmare are considered to be the origin of the trap sites. The trapping–detrapping process, which may be tuned by the electric field across the metal/oxide/graphene gate stack, could lead to the changing of the intrinsic electric property of graphene. This study promises to produce the complementary p- and n-type GFET for logic applications. Full article

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. Full article

In this work, we have designed and simulated a graphene field effect transistor (GFET) with the purpose of developing a sensitive biosensor for methanethiol, a biomarker for bacterial infections. The surface of a graphene layer is functionalized by manipulation of its surface structure and is used as the channel of the GFET. Two methods, doping the crystal structure of graphene and decorating the surface by transition metals (TMs), are utilized to change the electrical properties of the graphene layers to make them suitable as a channel of the GFET. The techniques also change the surface chemistry of the graphene, enhancing its adsorption characteristics and making binding between graphene and biomarker possible. All the physical parameters are calculated for various variants of graphene in the absence and presence of the biomarker using counterpoise energy-corrected density functional theory (DFT). The device was modelled using COMSOL Multiphysics. Our studies show that the sensitivity of the device is affected by structural parameters of the device, the electrical properties of the graphene, and with adsorption of the biomarker. It was found that the devices made of graphene layers decorated with TM show higher sensitivities toward detecting the biomarker compared with those made by doped graphene layers. Full article

This paper presents a simple and comprehensive model of a dual-gate graphene field effect transistor (FET). The quantum capacitance and surface potential dependence on the top-gate-to-source voltage were studied for monolayer and bilayer graphene channel by using equivalent circuit modeling. Additionally, the closed-form analytical equations for the drain current and drain-to-source voltage dependence on the drain current were investigated. The distribution of drain current with voltages in three regions (triode, unipolar saturation, and ambipolar) was plotted. The modeling results exhibited better output characteristics, transfer function, and transconductance behavior for GFET compared to FETs. The transconductance estimation as a function of gate voltage for different drain-to-source voltages depicted a proportional relationship; however, with the increase of gate voltage this value tended to decline. In the case of transit frequency response, a decrease in channel length resulted in an increase in transit frequency. The threshold voltage dependence on back-gate-source voltage for different dielectrics demonstrated an inverse relationship between the two. The analytical expressions and their implementation through graphical representation for a bilayer graphene channel will be extended to a multilayer channel in the future to improve the device performance. Full article

We report on the fabrication and characterisation of Graphene field-effect transistor (GFET) Biosensors for detecting clusterin, a prominent protein biomarker of Alzheimer’s disease (AD). There are approximately 54 million people currently living with dementia worldwide and this is expected to rise to 130 million by 2050. Although there are over 400 different types of dementia, AD is the most common type, affecting between 50–75% of those diagnosed with dementia. Diagnosis of AD can take up to 2 years currently using MRI, PET, CT scans and memory tests. There is, therefore, an urgent need to develop low-cost, accurate, non-invasive and point-of-care (PoC) sensors for early diagnosis of AD. The GFET sensors we are developing to address this challenge were fabricated on Si/SiO₂ substrate through processes of photolithographic patterning and metal lift-off techniques with evaporated chromium and sputtered gold contacts. Raman Spectroscopy was performed on the devices to determine the quality of the graphene. The GFETs were annealed to improve their performance before the channels were functionalized by immobilising the graphene surface with a linker molecule and anti-clusterin antibody. The detection was achieved through the binding reaction between the antibody and varying concentrations of clusterin antigen from 1 pg/mL to 1 ng/mL. The GFETs were characterized using 4-probe direct current (DC) electrical measurements which demonstrated a limit of detection of the biosensors to be below 1 pg/mL. Full article

A reference electrode is necessary for the working of ion-sensitive field-effect transistor (ISFET)-type sensors in electrolyte solutions. The Ag/AgCl electrode is normally used as a reference electrode. However, the Ag/AgCl reference electrode limits the advantages of the ISFET sensor. In this work, we fabricated a two-channel graphene solution gate field-effect transistor (G-SGFET) to detect pH without an Ag/AgCl reference electrode in the electrolyte solution. One channel is the sensing channel for detecting the pH and the other channel is the reference channel that serves as the reference electrode. The sensing channel was oxygenated, and the reference channel was fluorinated partially. Both the channels were directly exposed to the electrolyte solution without sensing membranes or passivation layers. The transfer characteristics of the two-channel G-SGFET showed ambipolar field-effect transistor (FET) behavior (p-channel and n-channel), which is a typical characteristic curve for the graphene ISFET, and the value of V_Dirac was shifted by 18.2 mV/pH in the positive direction over the range of pH values from 4 to 10. The leakage current of the reference channel was 16.48 nA. We detected the real-time pH value for the two-channel G-SGFET, which operated stably for 60 min in the buffer solution. Full article