Graphene Gate Electrodes Research Articles

Efficient optical phase modulator based on an III–V metal-oxide-semiconductor structure with a doped graphene transparent electrode

We propose a III–V metal-oxide-semiconductor (MOS) optical modulator with a graphene gate electrode along with the analysis of the modulation properties. With p-type doped graphene used as a transparent gate electrode, we can fully utilize the electron-induced refractive index change in an n-type InGaAsP waveguide with the reduction of the hole-induced optical absorption observed in a III–V/Si hybrid MOS optical modulator. Numerical analysis displays that up to the phase modulation efficiency of 0.82 V·cm and 0.22 dB optical loss for π phase shift can be achieved when the gate oxide thickness is 100 nm. With the elimination of the unnecessary parasitic capacitance found in the overlapping of graphene on the slab part of the waveguide, in conjunction with the high electron mobility in InGaAsP, the device also enables a modulation bandwidth of greater than 200 GHz.

Self-aligned capillarity-assisted printing of top-gate thin-film transistors on plastic

Top-gate thin-film transistors (TFTs) are fabricated on plastic using a self-aligned method based on capillarity-assisted lithography and inkjet printing, offering a promising platform for high-throughput manufacturing of flexible electronic devices. Plastic substrates are imprinted with a multi-tier structure containing capillary channels and ink receivers using a precision mold. Liquid inks are sequentially delivered to the microstructured substrate by inkjet printing, and capillary action draws the inks into a multi-tier capillary channel network designed for top-gate TFTs. The combination of imprinting, inkjet printing, and capillary flow yields self-aligned multi-layered devices without requiring precise registration for inkjet printing. The printed top-gate TFTs with Ag/Cu source and drain, poly(3-hexylthiophene) semiconducting channel, ion gel dielectric, and graphene gate electrode have desirable transfer and output characteristics, with a hole mobility of 0.48 cm2 V−1 s−1, threshold voltage of −0.86 V, on/off current ratio of 104.5, and robust tolerance to bending. The top-gate geometry and careful materials selection yields devices with negligible hysteresis and sweep rate dependence, establishing the versatility and utility of this self-aligned strategy for more widespread application in printed and flexible electronics.

Air-stable, transparent flexible ambipolar organic thin film transistors based on CuPc-F16CuPc bi-channel structure

We report the fabrication of air-stable, transparent flexible ambipolar organic thin film transistors based on CuPc and F16CuPc bi-channel films deposited under 10−1 Pa, by using graphene gate electrode, fluoropolymer Cytop dielectric layer, and Au source/drain electrodes on polyethylene terephthalate substrates. The devices show outstanding air-stability and high optical transparency of above 75% with electron and hole mobilities up to 0.031 and 0.029 cm2/V·s, respectively. The excellent ambipolar performance, along with the outstanding air-stability and high optical transparency, make CuPc-F16CuPc bi-channel transistors as a promising candidate for organic integrated circuits.

Characterization of Graphene Gate Electrodes for Metal-Oxide-Semiconductor Devices

ABSTRACT We fabricate and characterize metal-oxide-semiconductor (MOS) devices with graphene as the gate electrode, 5 or 10 nm thick silicon dioxide as the insulator, and silicon as the semiconductor substrate. We find that Fowler-Nordheim tunneling dominates the gate current for the 10 nm oxide device. We also study the temperature dependence of the tunneling current in these devices in the range 77 to 300 K and extract the effective tunneling barrier height as a function of temperature for the 10 nm oxide device. Furthermore, by performing high frequency capacitance-voltage measurements, we observe a local capacitance minimum under accumulation, particularly for the 5 nm oxide device. By fitting the data using numerical simulations based on the modified density of states of graphene in the presence of charged impurities, we show that this local minimum results from the quantum capacitance of graphene. These results provide important insights for the heterogeneous integration of graphene into conventional silicon technology.

Gate tunneling current and quantum capacitance in metal-oxide-semiconductor devices with graphene gate electrodes

Metal-oxide-semiconductor (MOS) devices with graphene as the metal gate electrode, silicon dioxide with thicknesses ranging from 5 to 20 nm as the dielectric, and p-type silicon as the semiconductor are fabricated and characterized. It is found that Fowler-Nordheim (F-N) tunneling dominates the gate tunneling current in these devices for oxide thicknesses of 10 nm and larger, whereas for devices with 5 nm oxide, direct tunneling starts to play a role in determining the total gate current. Furthermore, the temperature dependences of the F-N tunneling current for the 10 nm devices are characterized in the temperature range 77–300 K. The F-N coefficients and the effective tunneling barrier height are extracted as a function of temperature. It is found that the effective barrier height decreases with increasing temperature, which is in agreement with the results previously reported for conventional MOS devices with polysilicon or metal gate electrodes. In addition, high frequency capacitance-voltage measurements of these MOS devices are performed, which depict a local capacitance minimum under accumulation for thin oxides. By analyzing the data using numerical calculations based on the modified density of states of graphene in the presence of charged impurities, it is shown that this local minimum is due to the contribution of the quantum capacitance of graphene. Finally, the workfunction of the graphene gate electrode is extracted by determining the flat-band voltage as a function of oxide thickness. These results show that graphene is a promising candidate as the gate electrode in metal-oxide-semiconductor devices.

Highly sensitive glucose sensors based on enzyme-modified whole-graphene solution-gated transistors.

Noninvasive glucose detections are convenient techniques for the diagnosis of diabetes mellitus, which require high performance glucose sensors. However, conventional electrochemical glucose sensors are not sensitive enough for these applications. Here, highly sensitive glucose sensors are successfully realized based on whole-graphene solution-gated transistors with the graphene gate electrodes modified with an enzyme glucose oxidase. The sensitivity of the devices is dramatically improved by co-modifying the graphene gates with Pt nanoparticles due to the enhanced electrocatalytic activity of the electrodes. The sensing mechanism is attributed to the reaction of H2O2 generated by the oxidation of glucose near the gate. The optimized glucose sensors show the detection limits down to 0.5 μM and good selectivity, which are sensitive enough for non-invasive glucose detections in body fluids. The devices show the transconductances two orders of magnitude higher than that of a conventional silicon field effect transistor, which is the main reason for their high sensitivity. Moreover, the devices can be conveniently fabricated with low cost. Therefore, the whole-graphene solution-gated transistors are a high-performance sensing platform for not only glucose detections but also many other types of biosensors that may find practical applications in the near future.

Vertically Architectured Stack of Multiple Graphene Field‐Effect Transistors for Flexible Electronics

Vertically architectured stack of multiple graphene field-effect transistors (GFETs) on a flexible substrate show great mechanical flexibility and robustness. The four GFETs are integrated in the vertical direction, and dually gated GFETs with graphene channel, PMMA dielectrics, and graphene gate electrodes are realized.

ITO-Free Organic Light-Emitting Transistors with Graphene Gate Electrode

In this work, we report on the fabrication and characterization of organic light-emitting transistors (OLETs) within an indium–tin-oxide (ITO)-free platform, using graphene-based transparent conductive electrodes in place of ITO as gate electrode. A direct comparison between twin bottom-gate/top-contacts OLETs, where a standard ITO layer is replaced with a film made of a few graphene layers, shows that comparable electrical characteristics can be obtained along with a clear improvement in the electroluminescence generation characteristics. Our experimental findings pave the way to the exploitation of graphene-based transparent conductive electrodes within this class of emerging devices on flexible substrates, further promoting the novel era of flexible organic electronics.

High-Performance Biosensors Based on Solution-Gated Thin Film Transistors

Solution-gated transistors have shown promising applications in biosensors due to the high sensitivity, low working voltage and the simple design of the devices. Solution-gated transistors normal have no gate dielectric and the gate voltages are applied directly on the solid/electrolyte interfaces or electric double layers near the channel and the gate, which lead to very low working voltages (about 1 V) of the transistors. On the other hand, the devices can be easily prepared by solution process or other convenient methods because of the much simpler device structure compared with that of a conventional field effect transistor with several layers. Many biosensors can be developed based on the detection of potential changes across solid/electrolyte interfaces induced by electrochemical reactions or interactions. The devices normally can show high sensitivity due to the inherent amplification function of the transistors. Here, I will introduce several types of biosensors studied by our group recently, including DNA, glucose, dopamine, uric acid, cell, and bacteria sensors, based on solution-gated organic electrochemical transistors or graphene transistors. The biosensors show high sensitivity and selectivity when the devices are modified with functional nano-materials (e.g. graphene, Pt nanoparticles) and biomaterials (e.g. enzyme, antibody, DNA) on the gate electrodes or the channel. Furthermore, the devices are miniaturized successfully for the applications as sensing arrays. It is expected that the solution-gated transistors will find more important applications in the future. Some representative works are presented as follows.Figure 1 shows an organic electrochemical transistor (OECT) based on poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS) integrated in a flexible microfluidic system. We find that the device performance is not influenced by the bending status of the device. A novel label-free DNA sensor is developed using the OECT with single-stranded DNA probes immobilized on the gate electrode. The device can successfully detect a complementary DNA target down to 1 nM. The detection limit is extended to 10 pM by pulse-enhanced hybridization process of DNA in the microfluidic channel. Therefore OECTs are excellent candidates for flexible and low-cost biosensors.Figure 2 shows a solution-gated graphene transistor with graphene as both channel and gate electrodes. The device is used as a dopamine sensor with the detection limit down to 1nM, which is three orders of magnitude better than that of conventional electrochemical measurements. The sensing mechanism is attributed to the change of effective gate voltage applied on the transistors induced by the electro-oxidation of dopamine at the graphene gate electrodes. The interference from glucose, uric acid and ascorbic acid on the dopamine sensor is characterized. The selectivity of the dopamine sensor is dramatically improved by modifying the gate electrode with a thin Nafion film by solution process. This work paves the way for developing many other biosensors based on the solution-gated graphene transistors by specifically functionalizing the gate electrodes. Because the devices are mainly made of graphene, they are potentially low cost and ideal for high-density integration as multifunctional sensor arrays.

High‐Performance Dopamine Sensors Based on Whole‐Graphene Solution‐Gated Transistors

Solution‐gated graphene transistors with graphene as both channel and gate electrodes are fabricated for the first time and used as dopamine sensors with the detection limit down to 1 nM, which is three orders of magnitude better than that of conventional electrochemical measurements. The sensing mechanism is attributed to the change of effective gate voltage applied on the transistors induced by the electro‐oxidation of dopamine at the graphene gate electrodes. The interference from glucose, uric acid, and ascorbic acid on the dopamine sensor is characterized. The selectivity of the dopamine sensor is dramatically improved by modifying the gate electrode with a thin Nafion film by solution process. This work paves the way for developing many other biosensors based on the solution‐gated graphene transistors by specifically functionalizing the gate electrodes. Because the devices are mainly made of graphene, they are potentially low cost and ideal for high‐density integration as multifunctional sensor arrays.

Flexible and Transparent MoS2 Field-Effect Transistors on Hexagonal Boron Nitride-Graphene Heterostructures

Atomically thin forms of layered materials, such as conducting graphene, insulating hexagonal boron nitride (hBN), and semiconducting molybdenum disulfide (MoS2), have generated great interests recently due to the possibility of combining diverse atomic layers by mechanical "stacking" to create novel materials and devices. In this work, we demonstrate field-effect transistors (FETs) with MoS2 channels, hBN dielectric, and graphene gate electrodes. These devices show field-effect mobilities of up to 45 cm(2)/Vs and operating gate voltage below 10 V, with greatly reduced hysteresis. Taking advantage of the mechanical strength and flexibility of these materials, we demonstrate integration onto a polymer substrate to create flexible and transparent FETs that show unchanged performance up to 1.5% strain. These heterostructure devices consisting of ultrathin two-dimensional (2D) materials open up a new route toward high-performance flexible and transparent electronics.

Controlling Ionic Currents in Batteries Using Graphene Gate Electrode

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Towards bi-carrier ion-transistors: DC and optically induced effects in electrically controlled electrochemical cells

By applying a DC electrical bias to a central graphene gate electrode, which separated the anode and the cathode in an electrochemical cell, we controlled the external cell's current, Icell, electrically. By irradiating the cell with modulated light intensity, we obtained an AC open-circuit response. The optically induced open-circuit AC voltage could be tuned by the DC electrical bias. Such findings suggest novel methods to control and monitor ion currents in electrochemical cells.

Graphene Gate Electrode for MOS Structure-Based Electronic Devices

We demonstrate that the use of a monolayer graphene as a gate electrode on top of a high-κ gate dielectric eliminates mechanical-stress-induced-gate dielectric degradation, resulting in a quantum leap of gate dielectric reliability. The high work function of hole-doped graphene also helps reduce the quantum mechanical tunneling current from the gate electrode. This concept is applied to nonvolatile Flash memory devices, whose performance is critically affected by the quality of the gate dielectric. Charge-trap flash (CTF) memory with a graphene gate electrode shows superior data retention and program/erase performance that current CTF devices cannot achieve. The findings of this study can lead to new applications of graphene, not only for Flash memory devices but also for other high-performance and mass-producible electronic devices based on MOS structure which is the mainstream of the electronic device industry.