Image Processing Reference
In-Depth Information
After the transistor turns to off-state, the potential of node A does not change, as node
A is floating electrically. Thus, the potential of node A fluctuates at every reset operation.
The noise charge quantity, q n , is expressed as follows: 1
(
)
2
q
=
kTC
1
exp
2
t CR
n
(
)
(3.6)
=
kTC
tCR
As shown, this expression is the origin of the name kTC noise.
From Equation 3.6, the noise charge quantity, q n , and the noise voltage, v n , are shown as
follows:
q n =
kTC
(3.7)
vqC
n
=
=
kT/C
(3.8)
n
Thus, the noise amplitude as an electron number and voltage are directly and inversely
proportional to √ C , respectively.
In the field of image sensors, the amplitude of noise is often expressed by an electron
number. Hence, the noise electron number, n n , at room temperature is expressed as follows:
(
)
n
n =
400
C CpF
:
(3.9)
Since the amplitude of kTC noise can be easily estimated, this formula is useful.
Although the noise source of kTC noise exists in the channel of a transistor, the amplitude
of noise is determined by circuit construction. Therefore, it is classified as circuit noise.
3.3 Transistor Noise
3.3.1 1/f Noise
Figure 3.6 shows a measured example of the noise power spectrum of a MOSFET with a fre-
quency of 10 MHz. Since the noise power appears inversely proportional to the frequency
at less than approximately 100 kHz by plotting on logarithmic coordinates, the noise in
this area is called 1/f noise. To date, the mechanism of this noise is unclear, although some
models indicate that the charge trap and release at interface states in a Si-SiO 2 interface are
related. There is an empirical rule that the larger the gate area size that MOSFETs achieve,
the lower the 1/f noise power is.
As 1/f noise power is higher in the lower frequency range, the higher-level noise in
the lower frequency region overlaps with the image signal, depending on the position of
the noise-generating transistor. Since human eyes can follow noise fluctuating at low fre-
quency, this noise is apt to be noticeable as transversal noise visually.
3.3.2 Thermal Noise
The noise spectrum in the higher frequency shown in Figure 3.6 is almost flat and inde-
pendent from the frequency. The noise in this area is called thermal noise. Noise with a flat
 
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