On the impact of high-κ gate stacks on mobility: A Monte Carlo study including coupled SO phonon-plasmon scattering

HfO₂ based high-κ dielectrics are among the most likely candidates to replace SiO₂ and the currently favoured oxinitride in the next generation of MOSFETs. High-κ materials allow the use of a thicker gate dielectric, maintaining the gate capacitance at reduced gate leakage. However, they lead to mobility degradation due to among other factors the coupling of carriers to surface soft optical phonons. Comparing the vertical field dependence of the mobility for HfO₂ and SiO₂, the severe degradation in mobility in the presence of high-κ becomes evident. The introduction of a SiO₂ interfacial layer between the channel and the HfO₂ mitigates the interaction with the SO phonons, but increases the equivalent oxide thickness (EOT) of the gate dielectric. The material of choice for the first commercial introduction of high-κ gate stacks is Hafnium Silicate (Si _x Hf_1-x O₂). This alloy stands up better to the processing challenges and as a result suffers less from dielectric fluctuations.     

Semiclassical transport in silicon nanowire FETs including surface roughness

In this paper we investigate the effect of surface roughness scattering on transport in silicon nanowire FETs using a deterministic Boltzmann equation solver previously developed by the authors. We first solve the coupled Schrödinger-Poisson equations to extract the subband profiles along the channel, and then address the transport problem. Some features of the low-field mobility as a function of the wire diameter and gate bias are discussed and the effect of surface roughness on the I–V characteristics is presented.     

Physical modeling of hole mobility in silicon inversion layers under uniaxial stress

A physical model for hole mobility under either biaxial or uniaxial stress has been developed. The six-band k ? p theory is used to obtain the bandstructure through stress-dependent Hamiltonian. The hole mobility in the silicon inversion layer is studied in details using Monte Carlo method. A numerically robust method has been applied to achieve self-consistent solution of Poisson’s and Schrödinger equations.     

Model plasma dispersion functions for SO phonon scattering in Monte Carlo simulations of high-κ dielectric MOSFETS

We have developed an accurate Padé approximant for the plasma dispersion function that is valid for degenerate semiconductors that occur in ultra-small MOSFETs. The new approximant is based on a two pole model that enables a simple evaluation of the Lindhard dielectric function for the full dynamic response of electrons of any degeneracy. The importance of this result is that it enables a fast numerical algorithm for determining the energies and scattering strengths of coupled plasmon-phonon modes in silicon MOSFET devices with high-κ gate stacks. Moreover, the formalism allows the systematic inclusion of Landau damping and other processes such as collisional damping that damp out some of the modes at particular ranges of wave vector. The new model is a non-trivially scaled model of a previous approximant derived for Boltzmann statistics. The new model reduces to the classical result in the appropriate limit. Results are presented that compare the exact numerically computed complex plasma dispersion function with the new Padé approximant model. Comparison is also made between exact numerical calculations and the Padé approximant model for static screening. A brief outline is made of the potential application to high-κ gate stack devices where the formalism should provide a significantly large reduction in complexity that will enable efficient Monte Carlo simulation of SO phonon and plasmon scattering.     

Cross-sectional dependence of electron mobility and lattice thermal conductivity in silicon nanowires

The thermoelectric efficiency of a material depends on the ratio of its electrical and thermal conductivity. In this work, the cross-sectional dependence of electron mobility and lattice thermal conductivity in silicon nanowires has been investigated by solving the electron and phonon Boltzmann transport equations. The effects of confinement on acoustic phonon scattering (both electron–phonon and phonon–phonon) are accounted for in this study. With decreasing wire cross-section, the electron mobility shows a non-monotonic variation, whereas the lattice thermal conductivity exhibits a linear decrease. The former is a result of the decrease in intervalley and intersubband scattering due to a redistribution of electrons among the twofold-degenerate Δ₂ and fourfold-degenerate Δ₄ valley subbands when the cross-section is below 5×5 nm², while the latter is because of the monotonic increase of three phonon umklapp and boundary scattering with decreasing wire cross-section. Among the wires considered, those with a cross-section between 3×3 nm² and 4×4 nm² have the maximal ratio of the electron mobility to lattice thermal conductivity, and are expected to provide the maximal thermoelectric figure of merit.     

Effect of elastic processes and ballistic recovery in silicon nanowire transistors

Scaling of silicon devices is fast approaching the limit where a single gate may fail to retain effective control over the channel region. Of the alternative device structures under focus, silicon nanowire transistors (SNWT) show great promise in terms of scalability, performance, and ease of fabrication. Here we present the results of self-consistent, fully 3D quantum mechanical simulations of SNWTs to show the role of surface roughness (SR) and ionized dopant scattering on the transport of carriers. We find that the addition of SR, in conjunction with impurity scattering, causes additional quantum interference which increases the variation of the operational parameters of the SNWT. However, we also find that quantum interference and elastic processes can be overcome to obtain nearly ballistic behavior in devices with preferential dopant configurations.     

Self-consistent calculation for valence subband structure and hole mobility in <Emphasis Type="Italic">p</Emphasis>-channel inversion layers

Using six- and eight-band k⋅p models—with parameters calibrated against the bulk band structure obtained using non-local empirical pseudopotentials—we have employed a new hybrid self-consistent method to calculate the valence subband structure in p-channel inversion layers of InAs, InSb, GaAs, In_0.53Ga_0.47As, and GaSb. This method involves two separate stages: first, density-of-states (DOS) of the three lowest-energy subbands (heavy, light, and split-off holes) is calculated using the triangular-well approximation. Then, the self-consistent calculation is performed using the DOS previously obtained, but shifting each subband by the amount obtained from the self-consistent eigenvalues obtained during the self-consistent iteration. Finally, we present results regarding the hole mobility in Ge p-channel inversion layers. The results are compared to those obtained employing the subband structure computed with the triangular-well approximation and also with experimental data.     

Value of electric interconnection links in remote island power systems: The Spanish Canary and Balearic archipelago cases

This paper tackles the technical and economical value of island interconnection links in remote island power systems. For this purpose, a novel deterministic hourly unit commitment on a weekly basis is formulated including the possibility of interconnection links between islands. The unit commitment reflects the common practice of the majority of real island power system operators when operating their systems: the economic dispatch is constrained in order to cover the loss of any on-line generating unit and the loss of any interconnection link between islands. Several islands of the Spanish Balearic and Canary archipelagos are used as illustrative real cases to assess the impact of existing and projected links between islands. The paper shows on one hand how reserve constraints drive the economical operation of real island power system. On the other hand, how the use of interconnection links not only enable the flow of cheaper generation power between islands, but also significantly contribute to the fulfillment of reserve constraints which translates into a cheaper and more sustainable island operation.     

The role of electric vehicles in a decarbonized economy: Supporting a reliable,affordable and efficient electric system

Reductions in the carbon intensity of electricity generation, coupled with technological improvements in the end uses that it can power, have created opportunities for the electrification of large segments of the economy. As the largest source of greenhouse gas (GHG) emissions in our economy — 29% of total 2017 U.S. energy-related emissions — electrifying transportation is a key opportunity. Although the technology needed to decarbonize ground transportation exists today, an affordable and reliable transition will require a focus on policy and regulatory changes. Accommodating and correctly managing this growth in electric transportation will be critical to the development of a low-carbon future. If not properly planned for through direct and indirect charging controls, this new load could produce major impacts on the power system and its operations. However, managed correctly, EVs can serve as a useful tool and asset for grid managers and can be accommodated even under aggressive EV adoption models.