154
Strain-Engineered MOSFETs
6.3 1/f Noise in MOSFETs
The origin of the 1/f noise in MOS transistors has been much debated
concerning whether carrier number fluctuation noise due to traps in the
gate oxide or bulk mobility fluctuations dominates the 1/f noise. In 1957,
McWorther presented a 1/f noise model based on quantum mechanical
tunneling transitions of electrons between traps in the gate oxide and the
channel [22]. The tunneling time varies exponentially with distance from
the trap, and the 1/f noise is obtained for a trap density that is uniform
in both energy and distance from the channel interface. The McWorther
model is widely accepted for simplicity and excellent agreement with
experiments, especially for n-MOSFETs. However, the mobility fluctuation
noise model explains the 1/f noise in p-MOSFETs better [9]. It was later
explained by the unified flicker noise theory that a trapped carrier also
affects the surface mobility through Coulombic interaction. This correlated
mobility fluctuation model gave a correction to the number fluctuation
noise model, which resolves the deviations of the theory for p-MOSFETs.
However, the correction factor was criticised for being unfeasibly high
since screening was not accounted for. Also, the carrier mobility at the
surface is reduced compared to the bulk mobility due to additional surface scattering (by acoustic phonons and surface roughness), which has an
impact on the mobility fluctuations. Moreover, the Hooge mobility noise is
sensitive to the crystalline quality, which is deteriorated close to the interface. The most feasible explanation for the higher 1/f noise, with the carriers being in close proximity of the gate oxide surface, is increased mobility
fluctuation noise.
6.3.1 Number Fluctuations
The physical mechanism behind the number fluctuation noise is interaction between slow traps in the gate oxide and the carriers in the channel, which is schematically illustrated in Figure 6.4. The interaction and
exchange of carriers between the channel and the oxide traps results in a
fluctuation in the surface potential, thereby causing variation in inversion
charge density and effectively noise in the drain current. Although the
fluctuation in inversion charge density causes no current flow, the drain
current is needed to sense the fluctuation externally. The fluctuating oxide
charge density δQ ox is equivalent to a variation in the flat-band voltage
(V fb ).
V
Q C
/
fb
ox
ox
δ
= −δ
(6.17)
Strain-Engineered MOSFETs
6.3 1/f Noise in MOSFETs
The origin of the 1/f noise in MOS transistors has been much debated
concerning whether carrier number fluctuation noise due to traps in the
gate oxide or bulk mobility fluctuations dominates the 1/f noise. In 1957,
McWorther presented a 1/f noise model based on quantum mechanical
tunneling transitions of electrons between traps in the gate oxide and the
channel [22]. The tunneling time varies exponentially with distance from
the trap, and the 1/f noise is obtained for a trap density that is uniform
in both energy and distance from the channel interface. The McWorther
model is widely accepted for simplicity and excellent agreement with
experiments, especially for n-MOSFETs. However, the mobility fluctuation
noise model explains the 1/f noise in p-MOSFETs better [9]. It was later
explained by the unified flicker noise theory that a trapped carrier also
affects the surface mobility through Coulombic interaction. This correlated
mobility fluctuation model gave a correction to the number fluctuation
noise model, which resolves the deviations of the theory for p-MOSFETs.
However, the correction factor was criticised for being unfeasibly high
since screening was not accounted for. Also, the carrier mobility at the
surface is reduced compared to the bulk mobility due to additional surface scattering (by acoustic phonons and surface roughness), which has an
impact on the mobility fluctuations. Moreover, the Hooge mobility noise is
sensitive to the crystalline quality, which is deteriorated close to the interface. The most feasible explanation for the higher 1/f noise, with the carriers being in close proximity of the gate oxide surface, is increased mobility
fluctuation noise.
6.3.1 Number Fluctuations
The physical mechanism behind the number fluctuation noise is interaction between slow traps in the gate oxide and the carriers in the channel, which is schematically illustrated in Figure 6.4. The interaction and
exchange of carriers between the channel and the oxide traps results in a
fluctuation in the surface potential, thereby causing variation in inversion
charge density and effectively noise in the drain current. Although the
fluctuation in inversion charge density causes no current flow, the drain
current is needed to sense the fluctuation externally. The fluctuating oxide
charge density δQ ox is equivalent to a variation in the flat-band voltage
(V fb ).
V
Q C
/
fb
ox
ox
δ
= −δ
(6.17)