We present an analytical device model for a graphene bilayer field-effect transistor (GBL-FET) with a graphene bilayer as a channel, and with back and top gates. The model accounts for the dependences of the electron and hole Fermi energies as well as energy gap in different sections of the channel on the bias back-gate and top-gate voltages. Using this model, we calculate the dc and ac source-drain currents and the transconductance of GBL-FETs with both ballistic and collision dominated electron transport as functions of structural parameters, the bias back-gate and top-gate voltages, and the signal frequency. It is shown that there are two threshold voltages, $V_{th,1}$ and $V_{th,2}$, so that the dc current versus the top-gate voltage relation markedly changes depending on whether the section of the channel beneath the top gate (gated section) is filled with electrons, depleted, or filled with holes. The electron scattering leads to a decrease in the dc and ac currents and transconductances, whereas it weakly affects the threshold frequency. As demonstrated, the transient recharging of the gated section by holes can pronouncedly influence the ac transconductance resulting in its nonmonotonic frequency dependence with a maximum at fairly high frequencies.
Bilayer graphene is a promising material for radio-frequency transistors because its energy gap might result in a better current saturation than the monolayer graphene. Because the great deal of interest in this technology, especially for flexible radio-frequency applications, gaining control of it requires the formulation of appropriate models for the drain current, charge and capacitance. In this work we have developed them for a dual-gated bilayer graphene field-effect transistor. A drift-diffusion mechanism for the carrier transport has been considered coupled with an appropriate field-eff…
Graphene nanoribbon (GNR) field-effect transistors (FETs) with widths down to 12 nm have been fabricated by electron beam lithography using a wafer-scale chemical vapor deposition (CVD) process to form the graphene. The GNR FETs show drain-current modulation of approximately 10 at 300 K, increasing to nearly 106 at 4 K. The strong temperature dependence of the minimum current indicates the opening of a bandgap for CVD-grown GNR-FETs. The extracted bandgap is estimated to be around 0.1 eV by differential conductance methods. This work highlights the development of CVD-grown large-area graphene …
It is promising to apply quantum-mechanically confined graphene systems in field-effect transistors. High stability, superior performance, and large-scale integration are the main challenges facing the practical application of graphene transistors. Our understandings of the adatom-graphene interaction combined with recent progress in the nanofabrication technology indicate that very stable and high-quality graphene nanostripes could be integrated in substrate-supported functionalized (hydrogenated or fluorinated) graphene using electron-beam lithography. We also propose that parallelizing a co…
Fabrication of graphene field effect transistor is described which uses an intrinsic graphene on the surface of as deposited hydrogenated amorphous carbon films. Ambipolar characteristic has been demonstrated typical for graphene devices, which changes to unipolar characteristic if the surface graphene was etched in oxygen plasma. Because amorphous carbon films can be growth easily, with unlimited dimensions and no transfer of graphene is necessary, this can open new perspective for graphene electronics.
We have fabricated transistor structures using fluorinated single-layer graphene flakes and studied their electronic properties at different temperatures. Compared with pristine graphene, fluorinated graphene has very large and strongly temperature dependent resistance in the electro-neutrality region. We show that fluorination creates a mobility gap in graphene's spectrum where electron transport takes place via localised electron states.
We experimentally demonstrate DC functionality of graphene-based hot electron transistors, which we call Graphene Base Transistors (GBT). The fabrication scheme is potentially compatible with silicon technology and can be carried out at the wafer scale with standard silicon technology. The state of the GBTs can be switched by a potential applied to the transistor base, which is made of graphene. Transfer characteristics of the GBTs show ON/OFF current ratios exceeding 50.000.
In this work, double-gated field effect transistors manufactured from monolayer graphene are investigated. Conventional top-down CMOS-compatible processes are applied except for graphene deposition by manual exfoliation. Carrier mobilities in single- and double gated graphene field effect transistors are compared. Even in double-gated graphene FETs, the carrier mobility exceeds the universal mobility of silicon over nearly the entire measured range. At comparable dimensions, reported mobilities for ultra thin body silicon-on-insulator MOSFETs can not compete with graphene FET values.
The past decade has seen rapid growth in the research area of graphene and its application to novel electronics. With Moore's law beginning to plateau, the need for post-silicon technology in industry is becoming more apparent. Moreover, existing technology is insufficient for implementing terahertz detectors and receivers, which are required for a number of applications including medical imaging and security scanning. Graphene is considered to be a key potential candidate for replacing silicon in existing CMOS technology as well as realizing field effect transistors for terahertz detection, d…
With the motivation of improving the performance and reliability of aggressively scaled nano-patterned graphene field-effect transistors, we present the first systematic experimental study on charge and current distribution in multilayer graphene field-effect transistors. We find a very particular thickness dependence for Ion, Ioff, and the Ion/Ioff ratio, and propose a resistor network model including screening and interlayer coupling to explain the experimental findings. In particular, our model does not invoke modification of the linear energy-band structure of graphene for the multilayer c…
We utilize an organic polymer buffer layer between graphene and conventional gate dielectrics in top-gated graphene transistors. Unlike other insulators, this dielectric stack does not significantly degrade carrier mobility, allowing for high field-effect mobilities to be retained in top-gate operation. This is demonstrated in both two-point and four-point analysis, and in the high-frequency operation of a graphene transistor. Temperature dependence of the carrier mobility suggests that phonons are the dominant scatterers in these devices.
The maximum oscillation frequency (fmax) quantifies the practical upper bound for useful circuit operation. We report here an fmax of 70 GHz in transistors using epitaxial graphene grown on the C-face of SiC. This is a significant improvement over Si-face epitaxial graphene used in the prior high frequency transistor studies, exemplifying the superior electronics potential of C-face epitaxial graphene. Careful transistor design using a high κ dielectric T-gate and self-aligned contacts, further contributed to the record-breaking fmax.
There is a wide range of science and applications accessible via the strain engineering of quantum transport in 2D materials. We propose a realistic experimental platform for uniaxial strain engineering of ballistic charge transport in graphene. We then develop an applied theoretical model, based on this platform, to calculate charge conductivity and demonstrate graphene quantum strain transistors (GQSTs). We define GQSTs as mechanically strained ballistic graphene transistors with on/off conductivity ratios $> 10^{4}$, and which can be operated via modest gate voltages. Such devices would per…
We demonstrate graphene thickness-graded transistors with high electron mobility and low 1/f noise (f is a frequency). The device channel is implemented with few-layer graphene with the thickness varied from a single layer in the middle to few-layers at the source and drain contacts. It was found that such devices have electron mobility comparable to the reference single-layer graphene devices while producing lower noise levels. The metal doping of graphene and difference in the electron density of states between the single-layer and few-layer graphene cause the observed noise reduction. The r…
The speed of silicon-based transistors has reached an impasse in the recent decade, primarily due to scaling techniques and the short-channel effect. Conversely, graphene (a revolutionary new material possessing an atomic thickness) has been shown to exhibit a promising value for electrical conductivity. Graphene would thus appear to alleviate some of the drawbacks associated with silicon-based transistors. It is for this reason why such a material is considered one of the most prominent candidates to replace silicon within nano-scale transistors. The major crux here, is that graphene is intri…
A novel graphene transistor architecture is reported. The transistor has a U-shape geometry and was fabricated using a gallium focused ion beam (FIB). The channel conductance was tuned with a back gate. The Ion/Ioff ratio exceeded 10^5.
We present the results of the experimental investigation of the low - frequency noise in bilayer graphene transistors. The back - gated devices were fabricated using the electron beam lithography and evaporation. The charge neutrality point for the fabricated transistors was around 10 V. The noise spectra at frequencies above 10 - 100 Hz were of the 1/f - type with the spectral density on the order of 10E-23 - 10E-22 A2/Hz at the frequency of 1 kHz. The deviation from the 1/f spectrum at the frequencies below 10 -100 Hz indicates that the noise is of the carrier - number fluctuation origin due…
Bilayer graphene has the very interesting property of an energy gap tunable with the vertical electric field. We propose an analytical model for a bilayer-graphene field-effect transistor, suitable for exploring the design parameter space and to find a device structure with promising performance in terms of transistor operation. Our model, based on the effective mass approximation and ballistic transport assumptions, takes into account bilayer-graphene tunable gap and self polarization, and includes all band-to-band tunneling current components, which are shown to represent the major limitatio…
We present a novel, graphene-based device concept for high-frequency operation: a hot electron graphene base transistor (GBT). Simulations show that GBTs have high current on/off ratios and high current gain. Simulations and small-signal models indicate that it potentially allows THz operation. Based on energy band considerations we propose a specific materials solution that is compatible with SiGe process lines.
Graphene/silicon heterostructures have attracted tremendous interest as a new platform for diverse electronic and photonic devices such as barristors, solar cells, optical modulators, and chemical sensors. The studies to date largely focus on junctions between graphene and lightly-doped silicon, where a Schottky barrier is believed to dominate the carrier transport process. Here we report a systematic investigation of carrier transport across the heterojunctions formed between graphene and highly-doped silicon. By varying the silicon doping level and the measurement temperature, we show that t…
Two-dimensional (2D) materials, such as molybdenum disulfide (MoS2), have been shown to exhibit excellent electrical and optical properties. The semiconducting nature of MoS2 allows it to overcome the shortcomings of zero-bandgap graphene, while still sharing many of graphene's advantages for electronic and optoelectronic applications. Discrete electronic and optoelectronic components, such as field-effect transistors, sensors and photodetectors made from few-layer MoS2 show promising performance as potential substitute of Si in conventional electronics and of organic and amorphous Si semicond…
We discuss the origin of an additional dip other than the charge neutrality point observed in transfer characteristics of graphene-based field-effect transistors. The double-dip is proved to arise from charge transfer between graphene and metal electrodes, while charge storage at the graphene/SiO2 interface enhances it. Considering different Fermi energy from the neutrality point along the channel and partial charge pinning at the contacts, we propose a model which explains all features in gate voltage loops.
We report the multi-scale modeling and design of a gate-defined single-electron transistor in a MoS$_{2}$ bilayer. By combining density-functional theory and finite-element analysis, we design a surface gate structure to electrostatically define and tune a quantum dot and its associated tunnel barriers in the MoS$_{2}$ bilayer. Our approach suggests new pathways for the creation of novel quantum electronic devices in two-dimensional materials.
We use simulations to examine current saturation in sub-micron graphene transistors on SiO2/Si. We find self-heating is partly responsible for current saturation (lower output conductance), but degrades current densities >1 mA/um by up to 15%. Heating effects are reduced if the supporting insulator is thinned, or in shorter channel devices by partial heat sinking at the contacts. The transient behavior of such devices has thermal time constants of ~30-300 ns, dominated by the thickness of the supporting insulator and that of device capping layers (a behavior also expected in ultrathin body SOI…
We report a bipolar field effect tunneling transistor that exploits to advantage the low density of states in graphene and its one atomic layer thickness. Our proof-of-concept devices are graphene heterostructures with atomically thin boron nitride acting as a tunnel barrier. They exhibit room temperature switching ratios ~50, a value that can be enhanced further by optimizing the device structure. These devices have potential for high frequency operation and large scale integration.
We report results of experimental investigation of the low-frequency noise in the top-gate graphene transistors. The back-gate graphene devices were modified via addition of the top gate separated by 20 nm of HfO2 from the single-layer graphene channels. The measurements revealed low flicker noise levels with the normalized noise spectral density close to 1/f (f is the frequency) and Hooge parameter below 2 x 10^-3. The analysis of the noise spectral density dependence on the top and bottom gate biases helped us to elucidate the noise sources in these devices and develop a strategy for the ele…
We improvise a novel approach to carry out first-principles simulations of graphene-based vertical field effect tunneling transistors that consist of a graphene$|${\it h}-BN$|$graphene multilayer structure. Within the density functional theory framework, we exploit the effective screening medium (ESM) method to properly treat boundary conditions for electrostatic potentials and investigate the effect of gate voltage. The distribution of free carriers and the band structure of both top and bottom graphene layers are calculated self-consistently. The dielectric properties of {\it h}-BN thin film…
The performance of graphene-based transistors is often limited by the large electrical resistance across the metal-graphene contact. We report an approach to achieve ultra-low resistance metal contacts to graphene transistors. Through a process of metal-catalyzed etching in hydrogen, multiple nano-sized pits with zigzag edges are created in the graphene that form strong chemical bonds with deposited nickel metallization for source-drain contacts without the need for further annealing. This facile contact treatment prior to electrode metallization results in contact resistance as low as 100 ohm…
The field-effect mobility of graphene devices is discussed. We argue that the graphene ballistic mean free path can only be extracted by taking into account both, the electrical characteristics and the channel length dependent mobility. In doing so we find a ballistic mean free path of 300nm at room-temperature for a carrier concentration of ~1e12/cm2 and that a substantial series resistance of around 300ohmum has to be taken into account. Furthermore, we demonstrate first quantum capacitance measurements on single-layer graphene devices.
The influence of edge vacancies on the working ability of the planar graphene tunnel field-effect transistor (TFET) is studied at various concentrations and distributions (normal, uniform, periodic) of defects. All calculations are performed by using the Green's function method and the tight-binding approximation. It is shown that the transistor performance depends critically on two important factors associated with the defects: the destruction of the edge-localized electronic states and the emergence of subpeaks near the Fermi level. The supportable operation conditions of the TFET are found …
Top-gated, few-layer graphene field-effect transistors (FETs) fabricated on thermally-decomposed semi-insulating 4H-SiC substrates are demonstrated. Physical vapor deposited SiO2 is used as the gate dielectric. A two-dimensional hexagonal arrangement of carbon atoms with the correct lattice vectors, observed by high-resolution scanning tunneling microscopy, confirms the formation of multiple graphene layers on top of the SiC substrates. The observation of n-type and p-type transition further verifies Dirac Fermions unique transport properties in graphene layers. The measured electron and hole …
The performance of field effect transistors based on an single graphene ribbon with a constriction and a single back gate are studied with the help of atomistic models. It is shown how this scheme, unlike that of traditional carbon-nanotube-based transistors, reduces the importance of the specifics of the chemical bonding to the metallic electrodes in favor of the carbon-based part of device. The ultimate performance limits are here studied for various constriction and metal-ribbon contact models. In particular we show that, even for poorly contacting metals, properly taylored constrictions ca…
We observe very small gate-voltage shifts in the transfer characteristic of as-prepared graphene field-effect transistors (GFETs) when the pH of the buffer is changed. This observation is in strong contrast to Si-based ion-sensitive FETs. The low gate-shift of a GFET can be further reduced if the graphene surface is covered with a hydrophobic fluorobenzene layer. If a thin Al-oxide layer is applied instead, the opposite happens. This suggests that clean graphene does not sense the chemical potential of protons. A GFET can therefore be used as a reference electrode in an aqueous electrolyte. Ou…
The gate-voltage (VG) dependence of the contact resistance (RC) in graphene field-effect transistors is characterized by the transmission line model. The RC-VG characteristics of Ag, Cu, and Au contacts display a dip around the charge neutrality point, and become even negative with Ag contacts. The dip structure is well reproduced by a model calculation that considers a metal-contact-induced potential variation near the metal contact edges. The apparently negative RC originates with the carrier doping from the metal contacts to the graphene channel and appears when the doping effect is more su…
The discovery of electric field induced bandgap opening in bilayer graphene opens new door for making semiconducting graphene without aggressive size scaling or using expensive substrates. However, bilayer graphene samples have been limited to um size scale thus far, and synthesis of wafer scale bilayer graphene posts tremendous challenge. Here we report homogeneous bilayer graphene films over at least 2 inch x 2 inch area, synthesized by chemical vapor deposition on copper foil and subsequently transferred to arbitrary substrates. The bilayer nature of graphene film is verified by Raman spect…
Using first-principles calculations, we study the structural and electronic properties of the bilayer SnS/graphene, bilayer SnS/bilayer graphene (AA-stacked), bilayer SnS/bilayer graphene (AB-stacked) and monolayer SnS/graphene/monolayer SnS van der Waals (vdW) heterostructures. Electronic properties of all components of the vdW heterostructures are well preserved, which reflects the weakness of the vdW interaction. In the cases of bilayer SnS/graphene and bilayer SnS/bilayer graphene (AA-stacked), an Ohmic contact is formed which can be turned first into p-type and then into n-type Schottky c…
In this work, we investigate the intrinsic limits of subthreshold slope in a dual gated bilayer graphene transistor using a coupled self-consistent Poisson-bandstructure solver. We benchmark the solver by matching the bias dependent bandgap results obtained from the solver against published experimental data. We show that the intrinsic bias dependence of the electronic structure and the self-consistent electrostatics limit the subthreshold slope obtained in such a transistor well above the Boltzmann limit of 60mV/decade at room temperature, but much below the results experimentally shown till …
We have studied the phonon specific heat in single-layer, bilayer and twisted bilayer graphene. The calculations were performed using the Born-von Karman model of lattice dynamics for intralayer atomic interactions and spherically symmetric interatomic potential for interlayer interactions. We found that at temperature T<15 K, specific heat varies with temperature as T^n, where n = 1 for graphene, n = 1.6 for bilayer graphene and n = 1.3 for the twisted bilayer graphene. The phonon specific heat reveals an intriguing dependence on the twist angle in bilayer graphene, which is particularly pron…
We demonstrate a graphene-MoS2 architecture integrating multiple field-effect transistors and we independently probe and correlate the conducting properties of van der Waals coupled graphene-MoS2 contacts with the ones of the MoS2 channels. Devices are fabricated starting from high-quality single-crystal monolayers grown by chemical vapor deposition and characterized by scanning Raman and photoluminescence spectroscopies. Transconductance curves of MoS2 are compared with the current-voltage characteristics of graphene contact stripes, revealing a significant suppression of transport on the n-s…
For the first time, n-type few-layer MoS2 field-effect transistors with graphene/Ti as the hetero-contacts have been fabricated, showing more than 160 mA/mm drain current at 1 μm gate length with an on-off current ratio of 107. The enhanced electrical characteristic is confirmed in a nearly 2.1 times improvement in on-resistance and a 3.3 times improvement in contact resistance with hetero-contacts compared to the MoS2 FETs without graphene contact layer. Temperature dependent study on MoS2/graphene hetero-contacts has been also performed, still unveiling its Schottky contact nature. Transfer …
We have found that the Berry phase of bilayer graphene becomes from 2$π$ estimated by Shubnikov-de Haas oscillations when the A-B stacked pristine bilayer graphene experiences the Li-intercalation and sequential Li-desorption process in ultrahigh vacuum. Furthermore, the mobility of such processed bilayer graphene increases around four times larger, ~ 8,000 cm2/V$\cdot$s, than that of the pristine bilayer graphene. This is mainly due to increment of the scattering time and decrement of the cyclotron mass, which can be interpreted as a result of the change of the stacking structure of bilayer g…
We investigate band-gap tuning of bilayer graphene between hexagonal boron nitride sheets, by external electric fields. Using density functional theory, we show that the gap is continuously tunable from 0 to 0.2 eV, and is robust to stacking disorder. Moreover, boron nitride sheets do not alter the fundamental response from that of free-standing bilayer graphene, apart from additional screening. The calculations suggest that the graphene-boron nitride heterostructures could provide a viable route to graphene-based electronic devices.
Results of quantum mechanical simulations of the influence of edge disorder on transport in graphene nanoribbon metal oxide semiconductor field-effect transistors (MOSFETs) are reported. The addition of edge disorder significantly reduces ON-state currents and increases OFF-state currents, and introduces wide variability across devices. These effects decrease as ribbon widths increase and as edges become smoother. However the bandgap decreases with increasing width, thereby increasing the band-to-band tunneling mediated subthreshold leakage current even with perfect nanoribbons. These results …
We study the mid-infrared plasmonic response in Bernal-stacked bilayer graphene. Unlike its monolayer counterpart, bilayer graphene accommodates optically active phonon modes and a resonant interband transition at infrared frequencies. They strongly modifies the plasmonic properties of bilayer graphene, leading to Fano-type resonances, giant plasmonic enhancement of infrared phonon absorption, narrow window of optical transparency, and a new plasmonic mode at higher energy than the classical plasmon.
We prepare twist-controlled resonant tunneling transistors consisting of monolayer (Gr) and Bernal bilayer (BGr) graphene electrodes separated by a thin layer of hexagonal boron nitride (hBN). The resonant conditions are achieved by closely aligning the crystallographic orientation of the graphene electrodes, which leads to momentum conservation for tunneling electrons at certain bias voltages. Under such conditions, negative differential conductance (NDC) can be achieved. Application of in-plane magnetic field leads to electrons acquiring additional momentum during the tunneling process, whic…
This paper reviews the current status of graphene transistors as potential supplement to silicon CMOS technology. A short overview of graphene manufacturing and metrology methods is followed by an introduction of macroscopic graphene field effect transistors (FETs). The absence of an energy band gap is shown to result in severe shortcomings for logic applications. Possibilities to engineer a band gap in graphene FETs including quantum confinement in graphene Nanoribbons (GNRs) and electrically or substrate induced asymmetry in double and multi layer graphene are discussed. Graphene FETs are sh…
In this paper, we propose a novel S/D engineering for dual-gated Bilayer Graphene (BLG) Field Effect Transistor (FET) using doped semiconductors (with a bandgap) as source and drain to obtain unipolar complementary transistors. To simulate the device, a self-consistent Non-Equilibrium Green's Function (NEGF) solver has been developed and validated against published experimental data. Using the simulator, we predict an on-off ratio in excess of $10^4$ and a subthreshold slope of ~110mV/decade with excellent scalability and current saturation, for a 20nm gate length unipolar BLG FET. However, th…
We report the batch fabrication of graphene field-effect-transistors (GFETs) with nanoperforated graphene as channel. The transistors were cut and encapsulated. The encapsulated GFETs display saturation regions that can be tuned by modifying the top gate voltage, and have on/off ratios of up to 10^8 at room temperature. In addition, the nanoperforated GFETs display orders of magnitude higher photoresponses than any room-temperature graphene detector configurations that do not involve heterostructures with bandgap materials.
We invented a novel method to fabricate graphene transistors on oxidized silicon wafers without the need to transfer graphene layers. By means of catalytic chemical vapor deposition (CCVD) the in-situ grown bilayer graphene transistors (BiLGFETs) are realized directly on oxidized silicon substrate, whereby the number of stacked graphene layers is determined by the selected CCVD process parameters, e.g. temperature and gas mixture. BiLGFETs exhibit ultra-high on/off-current ratios of 107 at room temperature, exceeding previously reported values by several orders of magnitude. This will allow a …
In this manuscript, we present a field effect transistor with a channel consisting of a two-dimensional electron gas located at the interface between an ultrathin metallic film of Ni and a p-type Si(111) substrate. We have demonstrated that the two-dimensional electron gas channel is modulated by the gate voltage. The dependence of the drain current on the drain voltage has no saturation region, similar to a field effect transistor based on graphene. However, the transport in this transistor is not ambipolar, as in graphene, but unipolar.
We propose a concept for a graphene tunnel field-effect transistor. The main idea is based on the use of two graphene electrodes with zigzag termination divided by a narrow gap under the influence of the common gate. Our analysis shows that such device will have a pronounced switching effect at low gate voltage and high on/off current ratio at room temperature.
Potential steps naturally develop in graphene near metallic contacts. We investigate the influence of these steps on the transport in graphene Field Effect Transistors. We give simple expressions to estimate the voltage-dependent contribution of the contacts to the total resistance and noise in the diffusive and ballistic regimes.
We fabricate back-gated field effect transistors using Niobium electrodes on mechanically exfoliated monolayer graphene and perform electrical characterization in the pressure range from atmospheric down to 10-4 mbar. We study the effect of room temperature vacuum degassing and report asymmetric transfer characteristics with a resistance plateau in the n-branch. We show that weakly chemisorbed Nb acts as p-dopant on graphene and explain the transistor characteristics by Nb/graphene interaction with unpinned Fermi level at the interface.
We discuss the high-bias electrical characteristics of back-gated field-effect transistors with CVD-synthesized bilayer MoS2 channel and Ti Schottky contacts. We find that oxidized Ti contacts on MoS2 form rectifying junctions with ~0.3 to 0.5 eV Schottky barrier height. To explain the rectifying output characteristics of the transistors, we propose a model based on two slightly asymmetric back-to-back Schottky barriers, where the highest current arises from image force barrier lowering at the electrically forced junction, while the reverse current is due to Schottky-barrier limited injection …
We show simultaneous p and n type carrier injection in bilayer graphene channel by varying the longitudinal bias across the channel and the top gate voltage. The top gate is applied electrochemically using solid polymer electrolyte and the gate capacitance is measured to be 1.5 $μF/cm^2$, a value about 125 times higher than the conventional SiO$_2$ back gate capacitance. Unlike the single layer graphene, the drain-source current does not saturate on varying the drain-source bias voltage. The energy gap opened between the valence and conduction bands using top and back gate geometry is estimate…
We invented a method to fabricate graphene transistors on oxidized silicon wafers without the need to transfer graphene layers. To stimulate the growth of graphene layers on oxidized silicon a catalyst system of nanometer thin aluminum/nickel double layer is used. This catalyst system is structured via liftoff before the wafer enters the catalytic chemical vapor deposition (CCVD) chamber. In the subsequent methane based growth process monolayer graphene field-effect transistors (MoLGFETs) and bilayer graphene transistors (BiLGFETs) are realized directly on oxidized silicon substrate, whereby t…
Proximity orbital and spin-orbit effects of bilayer graphene on monolayer WSe$_2$ are investigated from first-principles. We find that the built-in electric field induces an orbital band gap of about 10 meV in bilayer graphene. Remarkably, the proximity spin-orbit splitting for holes is two orders of magnitude---the spin-orbit splitting of the valence band at K is about 2 meV---more than for electrons. Effectively, holes experience spin-valley locking due to the strong proximity of the lower graphene layer to WSe$_2$. However, applying an external transverse electric field of some 1 V/nm, coun…
We report room-temperature Coulomb blockade in a single layer graphene three-terminal single-electron transistor (SET) fabricated using feedback-controlled electroburning. The small separation between the side gate electrode and the graphene quantum dot results in a gate coupling up to 3 times larger compared to the value found for the back gate electrode. This allows for an effective tuning between the conductive and Coulomb blocked state using a small side gate voltage of about 1V. The technique can potentially be used in the future to fabricate all-graphene based room temperature single-ele…
We propose use of disorder to produce a field effect transistor (FET) in biased bilayer and trilayer graphene. Modulation of the bias voltage can produce large variations in the conductance when the disorder's effects are confined to only one of the graphene layers. This effect is based on the bias voltage's ability to select which of the graphene layers carries current, and is not tied to the presence of a gap in the density of states. In particular, we demonstrate this effect in models of gapless ABA-stacked trilayer graphene, gapped ABC-stacked trilayer graphene, and gapped bilayer graphene…
Tens of graphene transistors with nanoperforated channels and different channel lengths were fabricated at the wafer scale. The nanoholes have a central diameter of 20 nm and a period of 100 nm, the lengths of the channel being of 1, 2, 4 or 8 micrometers. We have found that the mobility in these 2 micrometer-wide transistors varies from about 10400 cm2/Vs for a channel length of 1 micrometer to about 550 cm2/Vs for a channel length of 8 micrometer. Irrespective of the mobility value, in all transistors the on-off ratio is in the range 10^3-10^4 at drain and gate voltages less than 2 V. The ch…
