Types of Image Sensors - Essential Principles of Image Sensors - page 67

Light

PD

Light shield film

Potential

n -

VCCD

n

p

Smear charge

p -well

V-sub

Dividing
ridge of
potential

n -sub

FIGURE 5.20
Smear suppression mechanism in p -well structure image sensors. Generated charges in regions deeper than the
dividing ridge of potential are discharged through the n -type substrate.

start, final state of readout operation with little charges left, and completed state of readout
operation are shown in Figure 5.21a-d, respectively. On readout operation, the read pulse
shown in Figure 5.11 is applied to the transfer gates, which play roles as not only transfer
gates of the VCCD but also as readout transfer gates from the PD to the VCCD channel. Then,
a readout channel potential rises from ϕ off to ϕ read , as shown in Figure 5.21b. ϕ PD , ϕ off , and
ϕ read are completely depleted PD potential, readout channel potential at OFF, and READOUT
situation, respectively. The potential of PD is increased until all the charges are transferred
to the VCCD channel and the PD is completely depleted, as shown in Figure 5.21d.
Here we trace the process of charge transfer from start to finish. In Figure 5.21b, the
readout channel has just been opened and many charges are still in the PD. Therefore,
almost all charges are transferred by self-induced drift, that is, mutual coulomb repulsion
between charges. In the final state of transfer operation, as in Figure 5.21c, since charge
quantity is very small, coulomb repulsion is not a leading mechanism of transfer any-
more. If there is graduation of potential, shown by the broken line in Figure 5.21c, that is,
an electric field toward the VCCD channel exists, remaining charges are transferred to the
VCCD channel quickly and smoothly. But if there is no electric field, shown by the straight
line in Figure 5.21c, each charge moves in a random direction based on thermal motion.
Since there is no direction to move, only charges that happen to move to the VCCD channel
direction and fall into it are transferred to the VCCD channel. It takes a long time for all
the charges to be transferred by falling in by chance and thus completing transfer to the
situation in Figure 5.18d. This causes a profound difference in the length of time necessary
to complete transfer.
For complete charge transfer, it is clear that the potential of ϕ read should be higher than
that of ϕ PD . A situation where there is a potential barrier or a dip in the charge transfer
path, as shown in Figure 5.22a, must be avoided. As shown in Figure 5.22b, the PD, readout
channel, and VCCD channel can be considered as sources, with voltage V s , gate of V g , and
drain of V d of a MOSFET. Let us consider the transfer operation as a behavior of a MOSFET.
In the initial situation of transfer, because V g ‒ V s >> kT, channel current I s is expressed as
follows:

( ) 2

I VV
s

∝− (5.1)

g s