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Comment On >Assessment Of Field-Induced Quantum Confinement In Heterogate Germanium Electron-Hole Bilayer Tunnel Field-Effect Transistor> Appl. Phys. Lett. 105, 082108 (2014)
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programNSF NASCENT ERCMicroelectronics Research CenterElectrical and Computer Engineerin
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Schrödinger equation Monte Carlo-3D for simulation of nanoscale MOSFETs
Get PDFtextA new quantum transport simulator -- Schrödinger Equation Monte Carlo in Three Dimensions (SEMC-3D) -- has been developed for simulating the carrier transport in nanoscale 3D MOSFET geometries. SEMC-3D self-consistently solves: (1) the 1D quantum transport equations derived from the SEMC method with open boundary conditions and rigorous treatment of various scattering processes including phonon and surface roughness scattering, (2) the 2D Schrödinger equations of the device cross sections with close boundary conditions to obtain the spatially varying subband structure along the conduction channel, and (3) the 3D Poisson equation of the whole device. Therefore, SEMC-3D can provide a physically accurate and electrostatically selfconsistent approach to the quantum transport in the subbands of 3D nanoscale MOSFETs. SEMC-3D has been used to simulate Si nanowire (NW) nMOSFETs to both demonstrate the capabilities of SEMC-3D, itself, and to provide new insight into transport phenomena in nanoscale MOSFETs, particularly with regards to interplay among scattering, quantum confinement and transport, and strain.Electrical and Computer Engineerin
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Schrödinger equation Monte Carlo simulation of nano-scaled semiconductor devices
No full texttextSemiconductor devices have been continuously scaled into the deep submicron
regime. As a result, quantum effects which were neglected in semiclassical
models become more and more important. Meanwhile, scattering still remains
important down to the gate length around 10 nm. Accurate quantum transport
simulators with scattering will be needed to explore the essential device physics. The
work of this dissertation project is aimed at developing an accurate quantum transport
simulation tool for deep submicron device modeling, as well as utilizing this newly
developed simulation tool to study the quantum transport and scattering effects in
ultra-scaled semiconductor devices.
The quantum transport simulator “Schrödinger Equation Monte Carlo”
(SEMC) provides a physically rigorous treatment of quantum transport and phasebreaking
inelastic scattering (in 3D) via real (actual) scattering processes such as
optical and acoustic phonon scattering. SEMC has been used to simulate carrier
transport in nano-scaled devices in order to gauge the potential reliability of
semiclassical models, phase-coherent quantum transport, and other limiting models as
the transition from classical to quantum transport is approached. SEMC has also been
successfully applied to study the carrier capture and transport in tunnel injection
lasers. In this work, a 2D version of SEMC − SEMC-2D − has been developed. The
quantum transport equations are solved self-consistently with Poisson equation.
SEMC-2D has been used to simulate quantum transport in nano-scaled double gate
MOSFETs. Simulation results serve not only to demonstrate the capability of this
new quantum transport simulator, but also to illuminate the importance of physically
accurate simulation of scattering for predictive modeling of transport in nano-scaled
MOSFETs.Electrical and Computer Engineerin
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Modeling of graphene-based FETs for low power digital logic and radio frequency applications
Get PDFtextThere are many semiconductors with nominally superior electronic properties compared to silicon. However, silicon became the material of choice for MOSFETs due to its robust native oxide. With Moore's observation as a guiding principle, the semiconductor industry has come a long way in scaling the silicon MOSFETs to smaller dimensions every generation with engineering ingenuity and technological innovation. As per the 2012 International Technology Roadmap for Semiconductors (ITRS), the MOSFET is expected to be scaled to near 6 nm gate length by 2025. However, materials, design and fabrication capabilities aside, basic physical considerations such as source to drain quantum mechanical tunneling, channel to gate tunneling, and thermionic emission over the channel barrier suggest an end to the roadmap for CMOS is on the horizon. The semiconductor industry is already aggressively looking for the next switch which can replace the silicon FET in the long term. My Ph.D. research is part of the quest for the next switch. The promises of process compatibility with existing CMOS technologies, fast carriers with high mobilities, and symmetric conduction and valence bands have led to graphene being considered as a possible alternative to silicon. This work looks at three devices based on graphene using first principles atomistic transport simulations and compact models capturing essential physics: the large-area graphene RF FET, the Bilayer pseudoSpin FET, and the double electron layer resonant tunneling transistor. The characteristics and performance of each device is explored with a combination of SPICE simulations and atomistic quasi static transport simulations. The BiSFET device was found to be a promising alternative to CMOS due to extremely low power dissipation. Finally, I have presented formalism for efficient simulation of time dependent transport in graphene for beyond quasi static performance analysis of the graphene based devices explored in this work.Electrical and Computer Engineerin
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Schrödinger equation Monte Carlo simulation of nanoscale devices
Get PDFSome semiconductor devices such as lasers have long had critical dimensions on the nanoscale where quantum effects are critical. Others such as MOSFETs are now being scaled to within this regime. Quantum effects neglected in semiclassical models become increasing important at the nanoscale. Meanwhile, scattering remains important even in MOSFETs of 10 nm and below. Therefore, accurate quantum transport simulators with scattering are needed to explore the essential device physics at the nanoscale. The work of this dissertation is aimed at developing accurate quantum transport simulation tools for deep submicron device modeling, as well as utilizing these simulation tools to study the quantum transport and scattering effects in the nano-scale semiconductor devices. The basic quantum transport method "Schrödinger Equation Monte Carlo" (SEMC) provides a physically rigorous treatment of quantum transport and phasebreaking inelastic scattering (in 3D) via real (actual) scattering processes such as optical and acoustic phonon scattering. The SEMC method has been used previously to simulate carrier transport in nano-scaled devices in order to gauge the potential reliability of semiclassical models, phase-coherent quantum transport, and other limiting models as the transition from classical to quantum transport is approached. In this work, SEMC-1D and SEMC-2D versions with long range polar optical scattering processes have been developed and used to simulate quantum transport in tunnel injection lasers and nanoscaled III-V MOSFETs. Simulation results serve not only to demonstrate the capabilities of the developed quantum transport simulators, but also to illuminate the importance of physically accurate simulation of scattering for the predictive modeling of transport in nano-scaled devices.Electrical and Computer Engineerin
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Voltage and temperature dependent gate capacitance and current model for high-K gate dielectric stack
Get PDFtextElectrical and Computer Engineerin
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Advanced semi-classical Monte Carlo modeling of Si, Ge, InGaAs, and MoS₂ n-channel FETs for novel CMOS
Get PDFScaling-down of silicon (Si) based complementary-metal-oxide-semiconductor (CMOS) technologies are approaching material limits. For high-performance applications, high thermal velocity channel materials, such as indium-gallium-arsenide (InGaAs) and germanium (Ge), are viable alternatives to Si to extend the limits of CMOS downscaling. The unique mechanical and electrical properties of two-dimensional atomic crystals, such as single-layer molybdenum disulfide (MoS₂), combined with soft, flexible, and curvilinear substrates, enable new device functionalities and concepts in the field of low-power flexible electronics not achievable with Si channels. While the intrinsic electron mobility of MoS₂ is rather low, strain engineering may provide a pathway for improving electron transport. Silicon, InGaAs, Ge, and MoS₂ n-channel MOSFETs were explored via first-principles computational tools including density functional theory and particle-based ensemble semi-classical Monte Carlo methods to better understand and enable the rational design of end-of-the-roadmap CMOS and potential beyond-CMOS technologies. The impact of contact geometry and transmissivity and gate length scaling on quasi-ballistic nanoscale Si, Ge, and InGaAs n-channel FinFETs was studied. FinFETs with end, saddle/slot, and raised source and drain contacts and the same saddle/slot contact geometry with different gate lengths, according to the projections of industry roadmaps, were simulated. Simulated Si FinFETs exhibited relatively limited degradation in performance due to non-ideal contact transmissivities, more limited sensitivity to contact geometry with non-ideal contact transmissivities, some contact-related advantage for Si 〈110〉 channel devices, and limited sensitivity to gate length scaling. Simulated InGaAs FinFETs were highly sensitive to modeled contact geometry, specific contact resistivity, the band structure model, and gate length scaling. Simulated Ge FinFETs showed substantial degradation due to non-ideal contact transmissivities, sensitivity to gate length scaling, and a large orientation-related advantage for Ge 〈110〉 channel devices. The impact of tensile strain on the intrinsic performance limits of monolayer MoS₂ n-channel MOSFETs was studied. 200 and 15 nm gate length MoS₂ MOSFETs with end contacts subject to different types and amounts of strain were simulated. Simulated MoS₂ MOSFETs displayed improved performance with strain due to lower effective mass and larger inter-valley separation, which is largely reduced due to non-ideal contact transmissivities.Materials Science and Engineerin
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Quantum corrected full-band semiclassical Monte Carlo simulation research of charge transport in Si, stressed-Si, and SiGe MOSFETs
Get PDFThis Ph.D. research is centered around a full-band Monte Carlo device simulator
(“Monte Carlo at the University of Texas”, MCUT) with quantum corrections
(based on one-dimensional Schrödinger equation solver). The code itself was
based on a solid infrastructure of a Monte Carlo simulator, “MoCa” from the
University of Illinois at Urbana-Champaign. To that there were added new
methods and features during my Ph.D. program, including strained band
structures, alternative (to conventional 100 ) surface orientations, full-band
scattering mechanisms, and valley-dependent quantum correction. These
features enable “MCUT” to be used to model various strained and/or alloyed
silicon MOSFETs, as well as the MOSFETs composed of alternative materials
such as Ge, in sub-100 nm regime. Monte Carlo simulation, itself, handles short
channel effects and hot carriers in ultra small device well; full-band structure
replaces the inaccurate and unknown (for new/strained materials) analytical
formulae; and the quantum corrections approximate quantum-confinement effects
on device performance. The goal is to understand and predict the device
behavior of the so called “non-classical” CMOS ― beyond bulk Si based
CMOS ― in the sub-100 nm regime.Electrical and Computer Engineerin
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Semiclassical Monte Carlo simulation of nano-scaled semiconductor devices
Get PDFtextAs the channel lengths of MOSFETs are being scaled down, the focus is on
replacing silicon by high mobility channel materials, such as Ge and III-V
semiconductors. This is because mobility and saturation velocity determine the on current
of short channel MOSFETs. However, a priori, it is not possible to determine the material
that will maximize the ratio of ON current to OFF current. Hence it is interesting to
perform simulations to compare the performance of various semiconductor devices with
their silicon counterparts.
In this work, a semiclassical Monte Carlo simulator, Monte Carlo University of
Texas (MCUT), has been used and modified to handle Ge and III-V MOSFETs. It is
capable of handling full bandstructure and incorporates various scattering models,
including, inelastic acoustic phonon scattering with longitudinal and transverse modes,
optical phonon scattering, impact ionization, ionized impurity scattering, surface
roughness scattering, remote Coulomb, remote surface roughness scattering and polar
optical phonon scattering.
Quantum correction in the inversion layer is taken into account in the form of a
modified potential that reproduces the correct concentration of carriers. Germanium Nand PMOSFETs and GaAs and InP NMOSFETs seem to perform worse than their silicon
counterparts when the saturation currents are compared at the same gate overdrive. The
results on GaAs and InP NMOSFETs are considered preliminary at this stage.Electrical and Computer Engineerin
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Multistate spin-transfer-torque random access memory
Get PDFSpin-transfer-torque random access memory (STT-RAM) is an emerging non-volatile memory technology that stores information as the relative alignment of two ferromagnets in a magnetic tunnel junction stack. Due to high scalability, speed and endurance STT-RAM is being considered as a promising candidate for future universal memory. To improve storage density various multi-state configurations have been proposed for STT-RAM. Previously, using micromagnetic simulations, it was shown that shape anisotropy of a cross-shaped ferromagnet can be used to achieve multi-state operation in a STT-RAM bit. In this work, we attempt to demonstrate the multi-state operation of such cross-shaped ferromagnet experimentally. We have explored different approach to fabricate cross-shaped magnetic tunnel junctions. Using magnetic force microscopy we demonstrate equilibrium magnetization states of a patterned cross-shaped ferromagnet. Challenges and future perspectives have been discussed.Electrical and Computer Engineerin
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