Types of Image Sensors - Essential Principles of Image Sensors - page 80

Vertical switch Horizontal switch

Photodiode Horizontal signal line

Vertical signal line

Output resistor

Video voltage

V V

C PD ∼ 3 fF << C VSL ∼ 10 pF

20 electrons << 1300 electrons

kTC noise electron number = 400√ C ( C : pF)

FIGURE 5.33
Capacitance and quantities of kTC noise through the charge readout path in an MOS sensor.

be 20 and 1300 electrons, respectively, by Equation 3.9. The latter is a very large noise electron
number. Since the horizontal signal line is kept connected to the video voltage source, it causes
no kTC noise. Compared to the fact that no noise is generated by complete charge transfer in
CCDs, the kTC noise of the vertical signal line is a major disadvantage of MOS sensors.
As we can see, there is a big difference in exposure timing between CCD and MOS sen-
sors. In IT-CCDs, all PDs are accessed at the same time. All pixels start and complete the
exposure period in the same time period in CCDs, as shown in Figure 5.34a. On the other
hand, in MOS sensors, each row is accessed one by one in series, that is, the exposure
period timing of each line shifts serially, as shown in Figure 5.34b. The same is true of
CMOS sensors, as will be discussed in Section 5.3.

5.2.2 Pixel Technology of MOS Sensors

As a characteristic pixel technology of MOS sensors, the npn -PD, 22 which is an antiblooming
device, is shown in Figure 5.35. The PD is arranged in the p -well formed on the n -type substrate.
Although it seems to resemble the VOD structure in IT-CCDs, discussed in Section 5.1.2.1,
the way it suppresses blooming phenomenon is different. In this structure, the p -well is not

Exposure period Exposure period

1st row 1st row

2nd row
3rd row

2nd row
3rd row

Exposure period of final row

Exposure period of final row

(a) Time (b) Time

FIGURE 5.34
Comparison of readout timing and exposure period: (a) CCD, all the same exposure timing; (b) MOS/CMOS
sensor, exposure timing shift of each row.