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Strain-Engineered MOSFETs
and geometry dependence. For example, if the drain current noise power
spectral density is caused due to source/drain resistance, the noise is independent of the gate length, while if the noise is caused by channel carrier
fluctuation, the noise changes with gate length. The noise mechanism can
also be revealed from the bias dependence of the low-frequency noise. By
varying the gate voltage in a MOSFET the inversion carrier density will
change, which reflects on the active noise mechanism. From this bias dependency, the dominant source of the 1/f noise, be it mobility fluctuation or
number fluctuation noise, can be identified by analysing the resemblance
with Equation (6.20) or (6.26). Unlike the straightforward way it is described,
the practical measurements pose a lot of problems in identifying the actual
mechanisms involved. For example, both the number fluctuation and the
mobility fluctuation sources can contribute to the excess noise with comparable magnitude. Practically, there are usually large deviations from the
simplistic theoretical description of the mechanisms; the trap density may
vary with energy, correlated mobility fluctuations may have a gate voltage
dependence similar to that of the Hooge noise, and Hooge’s parameter may
vary with inversion carrier density and electric field involving some complex
mechanisms. The trap density and Hooge’s parameter can be used as figures of merit for a given technology or material system. Correlating the noise
level to other device parameters such as oxide charge density, interface state
density, carrier mobility (especially phonon or Coulomb scattering limited
mobility), oxide thickness, etc., can help to establish the noise origin. The
basic understanding of the nature of noise sources allows one to interpret
the noise data obtained from the noise measurements, and thereby enables
one to develop an in-depth knowledge of the semiconductor device physics,
including current conduction mechanisms, defects, interface quality, etc. The
noise spectroscopy study is essential for not only inspecting the defects and
determining the nature and locations of dominant noise source responsible,
but also providing an insight on the remedies of the detrimental effects of
noise in small geometry devices. For example, if oxide traps are found to be
the dominant noise sources behind the 1/f noise, reducing the trap density
by an improved gate oxidation process will reduce the noise. If mobility fluctuation is revealed to be dominant, the quality of the surface or the interface
and the crystal structure will be improving, or incorporation of a strained
channel (to improve carrier mobility) may contribute to the reduction of 1/f
noise. For both mechanisms, a buried channel always provides improved
noise performance. The noise due to source/drain resistance can be mitigated by reducing the source/drain resistances, avoiding current crowding,
and improving the quality of contacts.
G-r noise, RTS noise, and number fluctuation 1/f noise in the MOSFET
drain current originate from oxide traps. G-r and RTS noise are only significant when the trap energies are close to the Fermi level energy, and are
therefore sensitive to bias and temperature. As RTS noise is caused due to a
single trap being active, it is only observed in small area devices or devices
Strain-Engineered MOSFETs
and geometry dependence. For example, if the drain current noise power
spectral density is caused due to source/drain resistance, the noise is independent of the gate length, while if the noise is caused by channel carrier
fluctuation, the noise changes with gate length. The noise mechanism can
also be revealed from the bias dependence of the low-frequency noise. By
varying the gate voltage in a MOSFET the inversion carrier density will
change, which reflects on the active noise mechanism. From this bias dependency, the dominant source of the 1/f noise, be it mobility fluctuation or
number fluctuation noise, can be identified by analysing the resemblance
with Equation (6.20) or (6.26). Unlike the straightforward way it is described,
the practical measurements pose a lot of problems in identifying the actual
mechanisms involved. For example, both the number fluctuation and the
mobility fluctuation sources can contribute to the excess noise with comparable magnitude. Practically, there are usually large deviations from the
simplistic theoretical description of the mechanisms; the trap density may
vary with energy, correlated mobility fluctuations may have a gate voltage
dependence similar to that of the Hooge noise, and Hooge’s parameter may
vary with inversion carrier density and electric field involving some complex
mechanisms. The trap density and Hooge’s parameter can be used as figures of merit for a given technology or material system. Correlating the noise
level to other device parameters such as oxide charge density, interface state
density, carrier mobility (especially phonon or Coulomb scattering limited
mobility), oxide thickness, etc., can help to establish the noise origin. The
basic understanding of the nature of noise sources allows one to interpret
the noise data obtained from the noise measurements, and thereby enables
one to develop an in-depth knowledge of the semiconductor device physics,
including current conduction mechanisms, defects, interface quality, etc. The
noise spectroscopy study is essential for not only inspecting the defects and
determining the nature and locations of dominant noise source responsible,
but also providing an insight on the remedies of the detrimental effects of
noise in small geometry devices. For example, if oxide traps are found to be
the dominant noise sources behind the 1/f noise, reducing the trap density
by an improved gate oxidation process will reduce the noise. If mobility fluctuation is revealed to be dominant, the quality of the surface or the interface
and the crystal structure will be improving, or incorporation of a strained
channel (to improve carrier mobility) may contribute to the reduction of 1/f
noise. For both mechanisms, a buried channel always provides improved
noise performance. The noise due to source/drain resistance can be mitigated by reducing the source/drain resistances, avoiding current crowding,
and improving the quality of contacts.
G-r noise, RTS noise, and number fluctuation 1/f noise in the MOSFET
drain current originate from oxide traps. G-r and RTS noise are only significant when the trap energies are close to the Fermi level energy, and are
therefore sensitive to bias and temperature. As RTS noise is caused due to a
single trap being active, it is only observed in small area devices or devices