31 research outputs found
Understanding the Bias Dependence of Low Frequency Noise in Sin-gle Layer Graphene FETs
This letter investigates the bias-dependent low frequency noise of single
layer graphene field-effect transistors. Noise measurements have been conducted
with electrolyte-gated graphene transistors covering a wide range of gate and
drain bias conditions for different channel lengths. A new analytical model
that accounts for the propagation of the local noise sources in the channel to
the terminal currents and voltages is proposed in this paper to investigate the
noise bias dependence. Carrier number and mobility fluctuations are considered
as the main causes of low frequency noise and the way these mechanisms
contribute to the bias dependence of the noise is analyzed in this work.
Typically, normalized low frequency noise in graphene devices has been usually
shown to follow an M-shape dependence versus gate voltage with the minimum near
the charge neutrality point (CNP). Our work reveals for the first time the
strong correlation between this gate dependence and the residual charge which
is relevant in the vicinity of this specific bias point. We discuss how charge
inhomogeneity in the graphene channel at higher drain voltages can contribute
to low frequency noise; thus, channel regions nearby the source and drain
terminals are found to dominate the total noise for gate biases close to the
CNP. The excellent agreement between the experimental data and the predictions
of the analytical model at all bias conditions confirms that the two
fundamental 1/f noise mechanisms, carrier number and mobility fluctuations,
must be considered simultaneously to properly understand the low frequency
noise in graphene FETs. The proposed analytical compact model can be easily
implemented and integrated in circuit simulators, which can be of high
importance for graphene based circuits design.Comment: 18 pages, 10 figure
Velocity Saturation effect on Low Frequency Noise in short channel Single Layer Graphene FETs
Graphene devices for analog and RF applications are prone to Low Frequency
Noise (LFN) due to its upconversion to undesired phase noise at higher
frequencies. Such applications demand the use of short channel graphene
transistors that operate at high electric fields in order to ensure a high
speed. Electric field is inversely proportional to device length and
proportional to channel potential so it gets maximized as the drain voltage
increases and the transistor length shrinks. Under these conditions though,
short channel effects like Velocity Saturation (VS) should be taken into
account. Carrier number and mobility fluctuations have been proved to be the
main sources that generate LFN in graphene devices. While their contribution to
the bias dependence of LFN in long channels has been thoroughly investigated,
the way in which VS phenomenon affects LFN in short channel devices under high
drain voltage conditions has not been well understood. At low electric field
operation, VS effect is negligible since carriers velocity is far away from
being saturated. Under these conditions, LFN can be precicely predicted by a
recently established physics-based analytical model. The present paper goes a
step furher and proposes a new model which deals with the contribution of VS
effect on LFN under high electric field conditions. The implemented model is
validated with novel experimental data, published for the first time, from CVD
grown back-gated single layer graphene transistors operating at gigahertz
frequencies. The model accurately captures the reduction of LFN especially near
charge neutrality point because of the effect of VS mechanism. Moreover, an
analytical expression for the effect of contact resistance on LFN is derived.
This contact resistance contribution is experimentally shown to be dominant at
higher gate voltages and is accurately described by the proposed model.Comment: Main Manuscript:10 pages, 6 figure
Adaptive Optics for Astronomy
Adaptive Optics is a prime example of how progress in observational astronomy
can be driven by technological developments. At many observatories it is now
considered to be part of a standard instrumentation suite, enabling
ground-based telescopes to reach the diffraction limit and thus providing
spatial resolution superior to that achievable from space with current or
planned satellites. In this review we consider adaptive optics from the
astrophysical perspective. We show that adaptive optics has led to important
advances in our understanding of a multitude of astrophysical processes, and
describe how the requirements from science applications are now driving the
development of the next generation of novel adaptive optics techniques.Comment: to appear in ARA&A vol 50, 201
From rings to bulges: evidence for rapid secular galaxy evolution at z~2 from integral field spectroscopy in the SINS survey
We present Ha integral field spectroscopy of well resolved, UV/optically
selected z~2 star-forming galaxies as part of the SINS survey with SINFONI on
the ESO VLT. Our laser guide star adaptive optics and good seeing data show the
presence of turbulent rotating star forming rings/disks, plus central
bulge/inner disk components, whose mass fractions relative to total dynamical
mass appears to scale with [NII]/Ha flux ratio and star formation age. We
propose that the buildup of the central disks and bulges of massive galaxies at
z~2 can be driven by the early secular evolution of gas-rich proto-disks. High
redshift disks exhibit large random motions. This turbulence may in part be
stirred up by the release of gravitational energy in the rapid cold accretion
flows along the filaments of the cosmic web. As a result dynamical friction and
viscous processes proceed on a time scale of <1 Gyr, at least an order of
magnitude faster than in z~0 disk galaxies. Early secular evolution thus drives
gas and stars into the central regions and can build up exponential disks and
massive bulges, even without major mergers. Secular evolution along with
increased efficiency of star formation at high surface densities may also help
to account for the short time scales of the stellar buildup observed in massive
galaxies at z~2.Comment: accepted Astrophysical Journal, main July 8 200
Full-bandwidth electrophysiology of seizures and epileptiform activity enabled by flexible graphene microtransistor depth neural probes
Mapping the entire frequency bandwidth of brain electrophysiological signals is of paramount importance for understanding physiological and pathological states. The ability to record simultaneously DC-shifts, infraslow oscillations (<0.1 Hz), typical local field potentials (0.1-80 Hz) and higher frequencies (80-600 Hz) using the same recording site would particularly benefit preclinical epilepsy research and could provide clinical biomarkers for improved seizure onset zone delineation. However, commonly used metal microelectrode technology suffers from instabilities that hamper the high fidelity of DC-coupled recordings, which are needed to access signals of very low frequency. In this study we used flexible graphene depth neural probes (gDNPs), consisting of a linear array of graphene microtransistors, to concurrently record DC-shifts and high-frequency neuronal activity in awake rodents. We show here that gDNPs can reliably record and map with high spatial resolution seizures, pre-ictal DC-shifts and seizure-associated spreading depolarizations together with higher frequencies through the cortical laminae to the hippocampus in a mouse model of chemically induced seizures. Moreover, we demonstrate the functionality of chronically implanted devices over 10 weeks by recording with high fidelity spontaneous spike-wave discharges and associated infraslow oscillations in a rat model of absence epilepsy. Altogether, our work highlights the suitability of this technology for in vivo electrophysiology research, and in particular epilepsy research, by allowing stable and chronic DC-coupled recordings
Evidence for Warped Disks of Young Stars in the Galactic Center
The central parsec around the super-massive black hole in the Galactic Center
hosts more than 100 young and massive stars. Outside the central cusp (R~1")
the majority of these O and Wolf-Rayet (WR) stars reside in a main clockwise
system, plus a second, less prominent disk or streamer system at large angles
with respect to the main system. Here we present the results from new
observations of the Galactic Center with the AO-assisted near-infrared imager
NACO and the integral field spectrograph SINFONI on the ESO/VLT. These include
the detection of 27 new reliably measured WR/O stars in the central 12" and
improved measurements of 63 previously detected stars, with proper motion
uncertainties reduced by a factor of four compared to our earlier work. We
develop a detailed statistical analysis of their orbital properties and
orientations. Half of the WR/O stars are compatible with being members of a
clockwise rotating system. The rotation axis of this system shows a strong
transition as a function of the projected distance from SgrA*. The main
clockwise system either is either a strongly warped single disk with a
thickness of about 10 degrees, or consists of a series of streamers with
significant radial variation in their orbital planes. 11 out of 61 clockwise
moving stars have an angular separation of more than 30 degrees from the
clockwise system. The mean eccentricity of the clockwise system is 0.36+/-0.06.
The distribution of the counter-clockwise WR/O star is not isotropic at the 98%
confidence level. It is compatible with a coherent structure such as stellar
filaments, streams, small clusters or possibly a disk in a dissolving state.
The observed disk warp and the steep surface density distribution favor in situ
star formation in gaseous accretion disks as the origin of the young stars.Comment: ApJ in pres