61 research outputs found
Small-signal model for 2D-material based field-effect transistors targeting radio-frequency applications: the importance of considering non-reciprocal capacitances
A small-signal equivalent circuit of 2D-material based field-effect
transistors is presented. Charge conservation and non-reciprocal capacitances
have been assumed so the model can be used to make reliable predictions at both
device and circuit levels. In this context, explicit and exact analytical
expressions of the main radio-frequency figures of merit of these devices are
given. Moreover, a direct parameter extraction methodology is provided based on
S-parameter measurements. In addition to the intrinsic capacitances,
transconductance and output conductance, our approach allows extracting the
series combination of drain/source metal contact and access resistances.
Accounting for these extrinsic resistances is of upmost importance when dealing
with low dimensional field-effect transistors.Comment: 8 pages, 10 figures, 4 table
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
Laterally Inhomogeneous Au Intercalation in Epitaxial Graphene on SiC(0 0 0 1): A Multimethod Electron Microscopy Study
Epitaxial graphene is of particular interest because of its tunable electronic structure. One important approach to tune the electronic properties of graphene relays on intercalating atomic species between graphene and the topmost silicon carbide layer. Here, we investigated the morphology and electronic structure of gold-intercalated epitaxial graphene using a multitechnique approach combining spectroscopic photoemission low-energy electron microscopy (SPELEEM) for chemical and structural characterization at mesoscopic length scale and with transmission electron microscopy (STEM) at the atomic level. Deposition of gold on ex situ prepared graphene on SiC(0 0 0 1) results in the partial intercalation of Au adatoms under graphene, with the formation of a buffer layer of variable thickness. Gold has also shown to aggregate in nanometer-sized clusters lying on top of the same graphene film. X-ray photo-emission electron microscopy measurements indicate that Au induces only small changes in the doping of the graphene layer, which does not develop a quasi free-standing behavior
Disorder-perturbed Landau levels in high electron mobility epitaxial graphene
We show that the Landau levels in epitaxial graphene in presence of localized
defects are significantly modified compared to those of an ideal system. We
report on magneto-spectroscopy experiments performed on high quality samples.
Besides typical interband magneto-optical transitions, we clearly observe
additional transitions that involve perturbed states associated to short-range
impurities such as vacancies. Their intensity is found to decrease with an
annealing process and a partial self-healing over time is observed.
Calculations of the perturbed Landau levels by using a delta-like potential
show electronic states both between and at the same energies of the Laudau
levels of ideal graphene. The calculated absorption spectra involving all
perturbed and unperturbed states are in very good agreement with the
experiments
Low-frequency noise parameter extraction method for single layer graphene FETs
In this paper, a detailed parameter extraction methodology is proposed for
low-frequency noise (LFN) in single layer (SL) graphene transistors (GFETs)
based on a recently established compact LFN model. Drain current and LFN of two
short channel back-gated GFETs (L=300, 100 nm) were measured at lower and
higher drain voltages, for a wide range of gate voltages covering the region
away from charge neutrality point (CNP) up to CNP at p-type operation region.
Current-voltage (IV) and LFN data were also available from a long channel SL
top solution-gated (SG) GFET (L=5 um), for both p- and n-type regions near and
away CNP. At each of these regimes, the appropriate IV and LFN parameters can
be accurately extracted. Regarding LFN, mobility fluctuation effect is dominant
at CNP and from there the Hooge parameter aH can be extracted while the carrier
number fluctuation contribution which is responsible for the well-known M-shape
bias dependence of output noise divided by squared drain current, also observed
in our data, makes possible the extraction of the NT parameter related to the
number of traps. In the less possible case of a Lambda-shape trend, NT and aH
can be extracted simultaneously from the region near CNP. Away from CNP,
contact resistance can have a significant contribution to LFN and from there
the relevant parameter SDR^2 is defined. The LFN parameters described above can
be estimated from the low drain voltage region of operation where the effect of
Velocity Saturation (VS) mechanism is negligible. VS effect results in the
reduction of LFN at higher drain voltages and from there the IV parameter
hOmega which represents the phonon energy and is related to VS effect can be
derived both from drain current and LFN data
Persistent photoconductivity in 2-dimensional electron gases at different oxide interfaces
We report on the transport characterization in dark and under light
irradiation of three different interfaces: LaAlO3/SrTiO3, LaGaO3/SrTiO3, and
the novel NdGaO3/SrTiO3 heterostructure. All of them share a perovskite
structure, an insulating nature of the single building blocks, a polar/non-
polar character and a critical thickness of four unit cells for the onset of
conductivity. The interface structure and charge confinement in NdGaO3/SrTiO3
are probed by atomic-scale- resolved electron energy loss spectroscopy showing
that, similarly to LaAlO3/SrTiO3, extra electronic charge confined in a sheet
of about 1.5 nm in thickness is present at the NdGaO3/SrTiO3 interface.
Electric transport measurements performed in dark and under radiation show
remarkable similarities and provide evidence that the persistent perturbation
induced by light is an intrinsic peculiar property of the three investigated
oxide-based polar/non-polar interfaces. Our work sets a framework for
understanding the previous contrasting results found in literature about
photoconductivity in LaAlO3/SrTiO3 and highlights the connection between the
origin of persistent photoconductivity and the origin of conductivity itself.
An improved understanding of the photo- induced metastable electron-hole pairs
might allow to shed a direct light on the complex physics of this system and on
the recently proposed perspectives of oxide interfaces for solar energy
conversion.Comment: 11 pages, 7 figure
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