Drain current saturation in graphene field-effect transistors at high fields

Development of competitive high frequency graphene field-effect transistors (GFETs) is hindered, first of all, by a zero-bandgap phenomenon in monolayer graphene, which prevents the drain current saturation and limits significantly the GFET power gain. An approach has been proposed to realise the drain current saturation in GFETs without a bandgap formation, but via velocity saturation of the charge carriers at high fields [1]. In this work, we report on the performance of GFETs fabricated using high quality CVD monolayer graphene and modified technology, which reduce the concentration oftraps generating the charge carriers at high fields [2]. Fig. 1 shows typical output characteristics of GFETs with gate length of 0.5 μm. The drain current clearly reveals the saturation trends at high fields, which we associate with the saturation of the carrier velocity, see inset to Fig. 2 [2]. Fig. 2 shows typical measured (extrinsic) transit frequency (fT) and the maximum frequency of oscillation (fmax), which are characteristics of the current and power gain, respectively. Since fT and fmax are proportional to the carrier velocity, they reveal similar saturation behaviour. We analyse the saturationeffects by applying the Fermi-Dirac carrier statistics. The fT and fmax are up to 34 GHz and 37 GHz, respectively, which are highest among those reported so far for the GFETs with similar gate length and comparable with those reported for Si MOSFETs [3]

Simplified models of three-phase, five-limb transformer for studying GIC effects

The paper describes capabilities of a topological model of three-phase, five-limb transformer to accurately represent its response when subjected to geomagnetically induced currents. The model is validated by close agreement of the predicted values and waveforms of the currents, voltages, and reactive power with those measured in tests performed on two 400 MVA transformers connected back-to-back and to the Fingrid power network. Results demonstrate the importance of incorporating the network representation and dispel some misconceptions about the influence of the hysteretic properties of the core and tank in modeling five-limb transformers

Does carrier velocity saturation help to enhance f(max) in graphene field-effect transistors?

| openaire: EC/H2020/785219/EU//GrapheneCore2 | openaire: EC/H2020/881603/EU//GrapheneCore3It has been argued that current saturation in graphene field-effect transistors (GFETs) is needed to get optimal maximum oscillation frequency (f(max)). This paper investigates whether velocity saturation can help to get better current saturation and if that correlates with enhancedf(max). We have fabricated 500 nm GFETs with high extrinsicf(max)(37 GHz), and later simulated with a drift-diffusion model augmented with the relevant factors that influence carrier velocity, namely: short-channel electrostatics, saturation velocity effect, graphene/dielectric interface traps, and self-heating effects. Crucially, the model provides microscopic details of channel parameters such as carrier concentration, drift and saturation velocities, allowing us to correlate the observed macroscopic behavior with the local magnitudes. When biasing the GFET so all carriers in the channel are of the same sign resulting in highly concentrated unipolar channel, we find that the larger the drain bias is, both closer the carrier velocity to its saturation value and the higher thef(max)are. However, the highestf(max)can be achieved at biases where there exists a depletion of carriers near source or drain. In such a situation, the highestf(max)is not found in the velocity saturation regime, but where carrier velocity is far below its saturated value and the contribution of the diffusion mechanism to the current is comparable to the drift mechanism. The position and magnitude of the highestf(max)depend on the carrier concentration and total velocity, which are interdependent and are also affected by the self-heating. Importantly, this effect was found to severely limit radio-frequency performance, reducing the highestf(max)from similar to 60 to similar to 40 GHz.Peer reviewe

Unveiling the plasma wave in the channel of graphene field-effect transistor

Coupling an electromagnetic wave at GHz to THz frequencies into the channel of a graphene field-effect transistor (GFET) provokes collective charge carrier oscillations of the two-dimensional electron gas (2DEG) known as plasma waves. Here, we report the very first experimental and direct mapping of the electric field distribution in a gated GFET at nanometer length scales using scattering-type scanning near-field microscopy (s-SNOM) at 2 THz. Based on the experimental results we deduce the plasma wave velocity for different gate bias voltages, which is in good agreement with the theoretical prediction.Peer reviewe

Direct nanoscopic observation of plasma waves in the channel of a graphene field-effect transistor

Plasma waves play an important role in many solid-state phenomena and devices. They also become significant in electronic device structures as the operation frequencies of these devices increase. A prominent example is field-effect transistors (FETs), that witness increased attention for application as rectifying detectors and mixers of electromagnetic waves at gigahertz and terahertz frequencies, where they exhibit very good sensitivity even high above the cut-off frequency defined by the carrier transit time. Transport theory predicts that the coupling of radiation at THz frequencies into the channel of an antenna-coupled FET leads to the development of a gated plasma wave, collectively involving the charge carriers of both the two-dimensional electron gas and the gate electrode. In this paper, we present the first direct visualization of these waves. Employing graphene FETs containing a buried gate electrode, we utilize near-field THz nanoscopy at room temperature to directly probe the envelope function of the electric field amplitude on the exposed graphene sheet and the neighboring antenna regions. Mapping of the field distribution documents that wave injection is unidirectional from the source side since the oscillating electrical potentials on the gate and drain are equalized by capacitive shunting. The plasma waves, excited at 2 THz, are overdamped, and their decay time lies in the range of 25–70 fs. Despite this short decay time, the decay length is rather long, i.e., 0.3-0.5 μm, because of the rather large propagation speed of the plasma waves, which is found to lie in the range of 3.5–7  7 10^6 m/s, in good agreement with theory. The propagation speed depends only weakly on the gate voltage swing and is consistent with the theoretically predicted\ua01414 power law

Direct nanoscopic observation of plasma waves in the channel of a graphene field-effect transistor

Plasma waves play an important role in many solid-state phenomena and devices. They also become significant in electronic device structures as the operation frequencies of these devices increase. A prominent example is field-effect transistors (FETs), that witness increased attention for application as rectifying detectors and mixers of electromagnetic waves at gigahertz and terahertz frequencies, where they exhibit very good sensitivity even high above the cut-off frequency defined by the carrier transit time. Transport theory predicts that the coupling of radiation at THz frequencies into the channel of an antenna-coupled FET leads to the development of a gated plasma wave, collectively involving the charge carriers of both the two-dimensional electron gas and the gate electrode. In this paper, we present the first direct visualization of these waves. Employing graphene FETs containing a buried gate electrode, we utilize near-field THz nanoscopy at room temperature to directly probe the envelope function of the electric field amplitude on the exposed graphene sheet and the neighboring antenna regions. Mapping of the field distribution documents that wave injection is unidirectional from the source side since the oscillating electrical potentials on the gate and drain are equalized by capacitive shunting. The plasma waves, excited at 2 THz, are overdamped, and their decay time lies in the range of 25-70 fs. Despite this short decay time, the decay length is rather long, i.e., 0.3-0.5 μm, because of the rather large propagation speed of the plasma waves, which is found to lie in the range of 3.5-7 × 106 m/s, in good agreement with theory. The propagation speed depends only weakly on the gate voltage swing and is consistent with the theoretically predicted 1/4 power law

Revisiting an old antimicrobial drug: amphotericin B induces interleukin-1-converting enzyme as the main factor for inducible nitric-oxide synthase expression in activated endothelia. Molecular Pharmacology 62

ABSTRACT We have investigated the impact of the widely used antifungal agent Amphotericin B (AmB) on cytokine activated aortic endothelial cells (AEC) and their inflammatory response as monitored by cytokine and inducible nitric-oxide synthase (iNOS) expression as well as high-output nitric oxide synthesis. Because both blood-borne infections and systemically administered drugs will first encounter vessel lining endothelial cells, this cell type represents an important participant in innate immune reactions against xenobiotics. Culturing cytokine-activated AEC in the presence of 1.25 g/ml AmB, a concentration equivalent to serum levels during patient treatment, we find increases in iNOS promoter activity up to 120%, in iNOS mRNA or protein expressions by factors of up to 3.5 Ϯ 1.1, and in iNOS activity of up to 180% compared with cells with cytokines only. In parallel, a strong increase in endothelial interleukin (IL)-1␤-converting enzyme (ICE) and IL-1␤ expression and activity was observed. Specific inhibition of ICE activity or IL-1␤ functionality significantly reduces expression and activity of the iNOS to control values. Because ICE activity is essential for the endogenous synthesis of active IL-1␤, ICE overexpression represents the key signal in the AmB-induced and IL-1␤-mediated effects on iNOS activity. In summary, in endothelial cells, AmB strongly augments cytokine-induced iNOS expression and activity by increasing the expression and activity of the ICE. This adjuvant activity for augmented endogenous cytokine processing adds to the efficacy of the antimycotic activity of AmB. Furthermore, our data underline the relevance of the endothelial iNOS as a potent effector of the innate immune system