332
Compact Models for Integrated Circuit Design
Similar to bulk MOSFETs (Section 5.3.1), these mechanisms together are
modeled through a submodel called low field mobility degradation and used to
get the effective mobility [71].
At high lateral field due to high applied V ds , the dominant scattering mechanism is optical phonon scattering since the electrons are able to gain enough
energy to emit optical phonons. This high lateral field scattering causes the carrier velocity saturation. The velocity saturation is calculated using a submodel
called current saturation and it degrades the drain-to-source current directly [71].
9.3.2.4 Series Resistances
In thin body source-drain transistors, series resistance is large. In order to
reduce the parasitic resistances in FinFETs and UTB transistors, raised sourcedrain regions are used in device architecture [Figure 9.4]. Thus, the parasitic
source-drain resistance submodel includes a bias-dependent extension resistance R ext , a spreading resistance R sp , and a distributed contact resistance R con .
The components of contact resistance include resistance ΔR s of the raised
source-drain bulk regions and silicon/silicide inter-face resistance ΔR c . And,
R con is modeled as a lumped resistance using a distributed network.
The spreading resistance R sp is due to current crowding as the current
flows from the raised drain region into the drain extension; this results in
an increase in the resistance by R sp . The spreading resistance is, modeled in
terms of the device and source-drain areas and a shape parameter [81].
The extension resistance R ext contributes the most to the series resistance.
The fringe field from the gate can cause surface accumulation at the interfaces of the extension region and the gate oxide/offset spacer; this modulates
the resistivity of the region and makes R ext bias-dependent. R ext is modeled
as a resistance network with two bias-independent resistances R ext1 and R ext2 ,
and a bias-dependent resistance R acc . Since the exact extension doping profile
is often unknown, analytical expressions with fitting parameters are used to
obtain the values of these components of R ext [81].
9.4 Independent Multiple-Gate FET Model
The model developed for common-gate FinFETs cannot be used for transistors with different gate dielectric thickness and independently biased gate
terminals. In this section, we will derive a surface potential–based compact
model targeted for UTB-SOI MOSFETs. The model could be used for computer analysis of emerging devices including graphene nanoribbon transistors [22,23,52]. Many of the real-device effects presented for a CMG model
can be used with appropriate changes for independent gate operation. Thus,
only a description of the core model is presented in the following section.
Compact Models for Integrated Circuit Design
Similar to bulk MOSFETs (Section 5.3.1), these mechanisms together are
modeled through a submodel called low field mobility degradation and used to
get the effective mobility [71].
At high lateral field due to high applied V ds , the dominant scattering mechanism is optical phonon scattering since the electrons are able to gain enough
energy to emit optical phonons. This high lateral field scattering causes the carrier velocity saturation. The velocity saturation is calculated using a submodel
called current saturation and it degrades the drain-to-source current directly [71].
9.3.2.4 Series Resistances
In thin body source-drain transistors, series resistance is large. In order to
reduce the parasitic resistances in FinFETs and UTB transistors, raised sourcedrain regions are used in device architecture [Figure 9.4]. Thus, the parasitic
source-drain resistance submodel includes a bias-dependent extension resistance R ext , a spreading resistance R sp , and a distributed contact resistance R con .
The components of contact resistance include resistance ΔR s of the raised
source-drain bulk regions and silicon/silicide inter-face resistance ΔR c . And,
R con is modeled as a lumped resistance using a distributed network.
The spreading resistance R sp is due to current crowding as the current
flows from the raised drain region into the drain extension; this results in
an increase in the resistance by R sp . The spreading resistance is, modeled in
terms of the device and source-drain areas and a shape parameter [81].
The extension resistance R ext contributes the most to the series resistance.
The fringe field from the gate can cause surface accumulation at the interfaces of the extension region and the gate oxide/offset spacer; this modulates
the resistivity of the region and makes R ext bias-dependent. R ext is modeled
as a resistance network with two bias-independent resistances R ext1 and R ext2 ,
and a bias-dependent resistance R acc . Since the exact extension doping profile
is often unknown, analytical expressions with fitting parameters are used to
obtain the values of these components of R ext [81].
9.4 Independent Multiple-Gate FET Model
The model developed for common-gate FinFETs cannot be used for transistors with different gate dielectric thickness and independently biased gate
terminals. In this section, we will derive a surface potential–based compact
model targeted for UTB-SOI MOSFETs. The model could be used for computer analysis of emerging devices including graphene nanoribbon transistors [22,23,52]. Many of the real-device effects presented for a CMG model
can be used with appropriate changes for independent gate operation. Thus,
only a description of the core model is presented in the following section.