18 research outputs found
On the possibility of obtaining MOSFET-like performance and sub-60 mV/decade swing in 1D broken-gap tunnel transistors
Tunneling field-effect transistors (TFETs) have gained a great deal of recent
interest due to their potential to reduce power dissipation in integrated
circuits. One major challenge for TFETs so far has been achieving high drive
currents, which is a prerequisite for high-performance operation. In this paper
we explore the performance potential of a 1D TFET with a broken-gap
heterojunction source injector using dissipative quantum transport simulations
based on the nonequilibrium Green's function formalism, and the carbon nanotube
bandstructure as the model 1D material system. We provide detailed insights
into broken-gap TFET (BG-TFET) operation, and show that it can indeed produce
less than 60mV/decade subthreshold swing at room temperature even in the
presence of electron-phonon scattering. The 1D geometry is recognized to be
uniquely favorable due to its superior electrostatic control, reduced carrier
thermalization rate, and beneficial quantum confinement effects that reduce the
off-state leakage below the thermionic limit. Because of higher source
injection compared to staggered-gap and homojunction geometries, BG-TFET
delivers superior performance that is comparable to MOSFET's. BG-TFET even
exceeds the MOSFET performance at lower supply voltages (VDD), showing promise
for low-power/high-performance applications.Comment: 34 pages, 11 figure
Ballisticity of nanotube field-effect transistors: Role of phonon energy and gate bias
We investigate the role of electron-phonon scattering and gate bias in degrading the drive current of nanotube field-effect transistors FETs. Optical phonon scattering significantly decreases the drive current only when gate voltage is higher than a well-defined threshold. For comparable electron-phonon coupling, a lower phonon energy leads to a larger degradation of drive current. Thus in semiconductor nanowire FETs, the drive current will be more sensitive than in carbon nanotube FETs because of the smaller phonon energies in semiconductors. Acoustic phonons and other elastic scattering mechanisms are most detrimental to nanotube FETs irrespective of biasing conditions
Influence of Phonon Scattering on the Performance of p-i-n Band-to-Band-Tunneling Transistors
Power dissipation has become a major obstacle in performance scaling of
modern integrated circuits, and has spurred the search for devices operating at
lower voltage swing. In this letter, we study p-i-n band-to-band tunneling
field effect transistors (TFET) taking semiconducting carbon nanotubes as the
channel material. The on-current of these devices is mainly limited by the
tunneling barrier properties, and phonon scattering has only a moderate effect.
We show, however, that the off-current is limited by phonon absorption assisted
tunneling, and thus is strongly temperature-dependent. Subthreshold swings
below the 60mV/decade conventional limit can be readily achieved even at room
temperature. Interestingly, although subthreshold swing degrades due to the
effects of phonon scattering, it remains low under practical biasing
conditions.Comment: 14 pages, 3 figure
Scalability of Atomic-Thin-Body (ATB) Transistors Based on Graphene Nanoribbons
A general solution for the electrostatic potential in an atomic-thin-body
(ATB) field-effect transistor geometry is presented. The effective
electrostatic scaling length, {\lambda}eff, is extracted from the analytical
model, which cannot be approximated by the lowest order eigenmode as
traditionally done in SOI-MOSFETs. An empirical equation for the scaling length
that depends on the geometry parameters is proposed. It is shown that even for
a thick SiO2 back oxide {\lambda}eff can be improved efficiently by thinner top
oxide thickness, and to some extent, with high-k dielectrics. The model is then
applied to self-consistent simulation of graphene nanoribbon (GNR)
Schottky-barrier field-effect transistors (SB-FETs) at the ballistic limit. In
the case of GNR SB-FETs, for large {\lambda}eff, the scaling is limited by the
conventional electrostatic short channel effects (SCEs). On the other hand, for
small {\lambda}eff, the scaling is limited by direct source-to-drain tunneling.
A subthreshold swing below 100mV/dec is still possible with a sub-10nm gate
length in GNR SB-FETs.Comment: 4 figures, accepted by ED
Influence of phonon scattering on the performance of p-i-np-i-n band-to-band tunneling transistors
Power dissipation has become a major obstacle in performance scaling of modern integrated circuits and has spurred the search for devices operating at lower voltage swing. In this letter, we study p-i-n band-to-band tunneling field effect transistors taking semiconducting carbon nanotubes as the channel material. The on current of these devices is mainly limited by the tunneling barrier properties, and phonon-scattering has only a moderate effect.We show, however, that the off current is limited by phonon absorption assisted tunneling, and thus is strongly temperature dependent. Subthreshold swings below the 60 mV/decade conventional limit can be readily achieved even at room temperature. Interestingly, although subthreshold swing degrades due to the effects of phonon scattering, it remains low under practical biasing conditions
Performance comparison between p-i-n tunneling transistors and conventional MOSFETs
Field-effect transistors based on band-to-band tunneling (BTBT) have gained a
lot of recent interest due to their potential for reducing power dissipation in
integrated circuits. In this paper we present a detailed performance comparison
between conventional n-i-n MOSFET transistors, and BTBT transistors based on
the p-i-n geometry (p-i-n TFET), using semiconducting carbon nanotubes as the
model channel material. Quantum transport simulations are performed using the
nonequilibrium Green's function formalism including realistic phonon
scattering. We find that the TFET can indeed produce subthreshold swings below
the conventional MOSFET limit of 60mV/decade at room temperature leading to
smaller off-currents and standby power dissipation. Phonon assisted tunneling,
however, limits the off-state performance benefits that could have been
achieved otherwise. Under on-state conditions the drive current and the
intrinsic device delay of the TFET are mainly governed by the tunneling barrier
properties. On the other hand, the switching energy for the TFET is observed to
be fundamentally smaller than that for the MOSFET, reducing the dynamic power
dissipation. Aforementioned reasons make the p-i-n geometry well suited for low
power applications.Comment: 37 pages, 12 figure
Computational study of exciton generation in suspended carbon nanotube transistors
Optical emission from carbon nanotube transistors (CNTFETs) has recently
attracted significant attention due to its potential applications. In this
paper, we use a self-consistent numerical solution of the Boltzmann transport
equation in the presence of both phonon and exciton scattering to present a
detailed study of the operation of a partially suspended CNTFET light emitter,
which has been discussed in a recent experiment. We determine the energy
distribution of hot carriers in the CNTFET, and, as reported in the experiment,
observe localized generation of excitons near the trench-substrate junction and
an exponential increase in emission intensity with a linear increase in current
versus gate voltage. We further provide detailed insight into device operation,
and propose optimization schemes for efficient exciton generation; a deeper
trench increases the generation efficiency, and use of high-k substrate oxides
could lead to even larger enhancements.Comment: 17 pages, 5 figure
Simulation of phonon-assisted band-to-band tunneling in carbon nanotube field-effect transistors
Electronic transport in a carbon nanotube (CNT) metal-oxide-semiconductor
field effect transistor (MOSFET) is simulated using the non-equilibrium Green's
functions method with the account of electron-phonon scattering. For MOSFETs,
ambipolar conduction is explained via phonon-assisted band-to-band
(Landau-Zener) tunneling. In comparison to the ballistic case, we show that the
phonon scattering shifts the onset of ambipolar conduction to more positive
gate voltage (thereby increasing the off current). It is found that the
subthreshold swing in ambipolar conduction can be made as steep as 40mV/decade
despite the effect of phonon scattering.Comment: 13 pages, 4 figure
Influence of Metal-Graphene Contact on the Operation and Scalability of Graphene Field-Effect-Transistors
We explore the effects of metal contacts on the operation and scalability of
2D Graphene Field-Effect-Transistors (GFETs) using detailed numerical device
simulations based on the non-equilibrium Green's function formalism
self-consistently solved with the Poisson equation at the ballistic limit. Our
treatment of metal-graphene (M-G) contacts captures: (1) the doping effect due
to the shift of the Fermi level in graphene contacts, (2) the density-of-states
(DOS) broadening effect inside graphene contacts due to Metal-Induced-States
(MIS). Our results confirm the asymmetric transfer characteristics in GFETs due
to the doping effect by metal contacts. Furthermore, at higher M-G coupling
strengths the contact DOS broadening effect increases the on-current, while the
impact on the minimum current (Imin) in the off-state depends on the source to
drain bias voltage and the work-function difference between graphene and the
contact metal. Interestingly, with scaling of the channel length, the MIS
inside the channel has a weak influence on Imin even at large M-G coupling
strengths, while direct source-to-drain (S -> D) tunneling has a stronger
influence. Therefore, channel length scalability of GFETs with sufficient gate
control will be mainly limited by direct S -> D tunneling, and not by the MIS.Comment: 7 figures, accepted by TE