Types of Image Sensors - Essential Principles of Image Sensors - page 65

Overflow control gate
Overflow drain

PD

VCCD

p -Sub

(a)

Excess charges

Overflow barrier

PD

VCCD

Overflow drain

(b)

FIGURE 5.17
Lateral overflow drain: (a) cross-sectional view, (b) potential distribution.

As shown in Figure 5.18, the PD and VCCD are arranged in a p -well formed on the sur-
face side of the n -type substrate. Zero volt or GND potential and positive voltage V-sub
are applied to the p -well and n -substrate, respectively. The potential distribution along
the A-A' line in Figure 5.18a is shown in Figure 5.18b. It is important that the p -region
between the PD and n -substrate is completely depleted and the potential can be controlled
by applied voltage V-sub, so that excess charges in the PD are discharged to the n -substrate
before they overflow to the VCCD channel. Since the potential of the overflow control

Readout part

VCCD

Photodiode

p +

n -

n -

A

p

p -

p -well

V-sub

n -substrate

A′

(a) Depleted

A′ depth direction

A

p -

Readout part

n -substrate

Photodiode

VCCD

Discharge of excess charges

Saturation level

Overflow control barrier

Readout
operation

V-sub

(b)

FIGURE 5.18
Vertical overflow drain structure in an IT-CCD: (a) cross-sectional view of the pixel; (b) potential distribution
along the A-A' line in (a).