Image Processing Reference
In-Depth Information
Sensor array
Shift register
Output
hν
CCD
CCD transfer
On-chip
output amp.
Low-noise pixel
Low noise
MOSFET switch
& metal wiring
hν
MOS
JFET
off-chip amp.
kTCnoise (high)
Pixel amp.
Multiplied signal charge quantity
×
G
Q
FPN
cancellation
circuit
hν
MOSFET switch
& metal wiring
CMOS
On-chip
output amp.
Low-noise pixel
kTC noise × 1/
G
Q
G
Q
= 10
2
- 10
4
FIGURE 5.82
Comparison from the viewpoint of noise and SNR.
Because of the development of various types of low-noise technologies, in addition to the
low-noise performance bestowed on only CCD by complete charge transfer, CCD was the
market leader for about two decades from the mid-1980s.
MOS sensors made a grand appearance as a leading competitor in the early 1980s by
utilizing LSI processing technology and had a virtual monopoly after their appearance on
the market.
They suffered from high-level kTC noise due to the vertical signal line. While MOS
sensors banished kTC noise by removing the vertical signal line itself in the TSL sensor,
improvement of SNR was restricted by the temporal noise of the off-chip amplifier because
of the current readout. As a result, MOS sensors could not compete with CCDs when they
were launched on the market later.
CMOS sensors did not have the opportunity to appear on the market until the age
of progress in fine-pitch transistor technology, such as 0.35 μm transistor process tech-
nology enough to form three or four transistors in a pixel, in the late 1990s. In the
mid-2000s, widely used cell phone cameras equipped with CMOS sensors were given
a low evaluation. The picture quality of CMOS sensors was quite low compared to that
of CCDs. But CMOS sensors originally aspired to suppress noise impact by charge
quantity multiplication at pixel level before accepting noise at the next stage, and to
cancel FPN caused by variation of pixel amplifier characteristics. Therefore, CMOS
sensors became comparable to CCDs as a result of the realization of low-noise PDs by
importing pixel technologies amassed through CCDs and achievement of suppression
of noise impact to a negligible level by large increases in the charge quantity of the
pixel amplifier. Sensors with a noise electron number less than unity were reported.
This was achieved by additional corrections of FPN due to variations of column circuit
characteristics and procedures of circuit technology such as column amplifiers and
column A/D converters. Thus, CMOS sensors have a SNR advantage over CCDs. It is
