7
Introduction
1.4 Electronic Properties of Strained Semiconductors
The band structure provides the information about the states of energy and
the electronic dispersion relation under a specific condition. It is known that
if the band structure of the material is modified, mechanical and electrical
properties of the material will be also changed, such as effective mass and
corresponding mobility. Band structure analysis provides details about strain
effects on the electron/hole transport property. For instance, strain-induced
lattice constant change will induce band warping in both the conduction
band and the valence band. However, the effective mass change is much
more important for holes in the valence band due to a strong correlation
between six subbands. In Chapter 4, we shall briefly discuss the stress–strain
relationships and their effects on the band structure, and a representative
method of strain components in terms of elastic compliance constants is
given. The basic physical definitions, such as the strain and stress tensors,
are introduced. Different methods of calculating the effect of strain on the
band structure are presented. The deformation potentials of the conduction
and valence bands are calculated, and the band edge shifts and splitting are
discussed in detail as strain effects. Since carrier mobility is a key parameter
for the simulation of the electrical characteristics of semiconductor devices,
several analytical models capable of capturing the dependence of mobility
on temperature, doping, and electric field will be introduced. Various types
of mobility models commonly used in simulation will be described in detail.
1.5 Strain-Engineered MOSFETs
As the MOSFET channel length enters the nanometer regime, short-channel
effects (SCEs), such as threshold voltage roll-off and drain-induced barrier lowering (DIBL), become high, which hinders the scaling of planar
bulk or silicon-on-insulator (SOI) MOSFETs. To overcome these problems,
new device architectures as well as new gate stacks have been proposed.
Multigate (also known as FinFET) devices are considered a promising architecture for replacement of conventional planar MOSFETs, offering a solution for overcoming the short-channel effects and providing better threshold
voltage control at short gate lengths. In Chapter 5, different schemes of multigate devices are reviewed. The tri-gate devices will be the focus of this
chapter because they are a good compromise between processing complexity and electrical performance. Although the gate-all-around (GAA) and the
Π-gate structures show better electrical properties, they require more complex and costly processing for implementation. According to the ITRS, the
strongest driver for high-k gate dielectrics comes from the need to extend
Introduction
1.4 Electronic Properties of Strained Semiconductors
The band structure provides the information about the states of energy and
the electronic dispersion relation under a specific condition. It is known that
if the band structure of the material is modified, mechanical and electrical
properties of the material will be also changed, such as effective mass and
corresponding mobility. Band structure analysis provides details about strain
effects on the electron/hole transport property. For instance, strain-induced
lattice constant change will induce band warping in both the conduction
band and the valence band. However, the effective mass change is much
more important for holes in the valence band due to a strong correlation
between six subbands. In Chapter 4, we shall briefly discuss the stress–strain
relationships and their effects on the band structure, and a representative
method of strain components in terms of elastic compliance constants is
given. The basic physical definitions, such as the strain and stress tensors,
are introduced. Different methods of calculating the effect of strain on the
band structure are presented. The deformation potentials of the conduction
and valence bands are calculated, and the band edge shifts and splitting are
discussed in detail as strain effects. Since carrier mobility is a key parameter
for the simulation of the electrical characteristics of semiconductor devices,
several analytical models capable of capturing the dependence of mobility
on temperature, doping, and electric field will be introduced. Various types
of mobility models commonly used in simulation will be described in detail.
1.5 Strain-Engineered MOSFETs
As the MOSFET channel length enters the nanometer regime, short-channel
effects (SCEs), such as threshold voltage roll-off and drain-induced barrier lowering (DIBL), become high, which hinders the scaling of planar
bulk or silicon-on-insulator (SOI) MOSFETs. To overcome these problems,
new device architectures as well as new gate stacks have been proposed.
Multigate (also known as FinFET) devices are considered a promising architecture for replacement of conventional planar MOSFETs, offering a solution for overcoming the short-channel effects and providing better threshold
voltage control at short gate lengths. In Chapter 5, different schemes of multigate devices are reviewed. The tri-gate devices will be the focus of this
chapter because they are a good compromise between processing complexity and electrical performance. Although the gate-all-around (GAA) and the
Π-gate structures show better electrical properties, they require more complex and costly processing for implementation. According to the ITRS, the
strongest driver for high-k gate dielectrics comes from the need to extend