Image Processing Reference
In-Depth Information
S1
S2
Horizontal shift register
Horizontal switch
MOSFET
Horizontal signal line
Vertical signal line
Photodiode
Vertical switch
MOSFET
Pitch of pixel interpolation array
Pitch of pixel square array
FIGURE 5.36
Device configuration of pixel interpolation array MOS sensor.
pitch displacement in the horizontal direction every two lines, while pixels are in square
array in the first-generation sensor, similar to that shown in Figure 5.31a. In the sen-
sors, two adjacent rows are read out simultaneously, the same as in the first generation.
Therefore, there are twice as many sampling points in the horizontal direction as in the
square array sensor. This means enhancement of horizontal resolution. This is effective
not only in monochrome images, but also in SCCCs because of the employed color filter
array
24
in which a green component is contained at every pixel site.
5.2.3.2 Transversal Signal Line Imager
The third generation of MOS sensors are transversal signal line (TSL) imagers.
21,25
As
shown in Figure 5.33, the major noise in MOS sensors is kTC noise of the vertical signal
line. As the device configuration in Figure 5.37 shows, the vertical signal line is not formed
in the TSL. Signal charges are read out from the PD to the horizontal signal line directly
through vertical and horizontal MOSFET switches formed in each pixel. In the figure,
V
R
,
R
P
,
S
V
,
C
V
,
C
H
, and
R
f
mean reset voltage, reset pulse, signal row select MOS switch, vertical
line capacitance, horizontal signal line capacitance, and output resistor, respectively. Using
this method, the source of large kTC noise is removed. The remaining kTC noise of PD is
about 20 electrons. Since readout drains that collect smear charges are connected to the
horizontal signal line, the smear charge integration period is one pixel readout time, that
is, smear is reduced to one thousandth. Although TSL imagers need some complications,
such as both vertical and horizontal switch MOSFETs in each pixel and both addressing
wiring to each pixel, it has achieved drastic performance advances by extreme reduction of
sensor noise itself and smear, which is virtually absent. But larger random noise, of around
300 electrons, which is caused in the off-chip preamplifier originating from the current
readout mode, remains. As a result, MOS sensors disappeared in the early 1990s, because
they could not compete against CCDs, which entered the market in the mid-1980s with
lower noise of about 10 electrons at that time. But MOS sensors did not completely vanish;
they came back in the late 1990s as CMOS sensors.
