Image Processing Reference
In-Depth Information
Readout part
P
Photodiode
VCCD
φ
read
φ
PD
Potential barrier
(a)
Drain gate source
I
∼
i
0
{exp(
q
φ / kT)-1}
kT
=
26 meV (300 K)
V
s
φ
φ
→
Time
Very slow electron movement
and no stop at φ = 0
V
g
V
d
Image lag
(b)
(c)
FIGURE 5.22
Schematic view of charge transfer in a nondepleted PD: (a) potential distribution; (b) weak inversion mode in
MOSFET operation model; (c) time dependence of current by thermoelectron emission.
takes a very long time to reach equilibrium condition, although
V
s
does not stop decreas-
ing at
V
s
=
V
g
. As a behavior of image sensors, all the signal charges cannot be read out in
a field and are read out throughout many fields. This is image lag in motion pictures. Since
complete charge transfer is achieved in depleted PDs, image lag does not occur. kTC noise
does not occur either, as mentioned in Section 3.2.
While the example of incomplete charge transfer described here is a case of the charge
transfer from the PD to VCCD channel, the mechanism is the same for charge transfer in
the CCD channel.
Another operation mode that needs a completely depleted PD is electronic shutter mode,
mentioned in Section 4.3. In electronic shutter operation in VOD structure image sensors,
all the signal charges integrated in the PDs are flushed to the substrate at the same time.
After a prescribed integration period, signal charges in the PDs are read out to the VCCD
channel.
As shown in Figure 5.23a, the discharge operation is carried out by applying a high
positive voltage pulse to the
n
-type substrate to push down the
p
-region potential barrier
so that charges in the PD are flushed to the substrate. As shown in Figure 5.23b, discharge
and readout operations have different paths. If there is excess or insufficient charge quan-
tity at discharge or readout operation, fixed-pattern noise (FPN) is caused. Therefore, it
is necessary that the PD situations just after both discharge and readout operations are
perfectly equal.
14
Since the potential of the PD is determined to be fixed depleted potential
after discharge or readout operation in the case of a fully depleted PD, this kind of prob-
lem does not occur.
15
To achieve this effect, it is necessary that both readout and discharge
potentials are higher than the depleted potential of the PD.
The discharge and readout operations in MOS image sensors, which will be discussed
in Section 5.2, are not done in transfer mode but in reset mode, by a direct connection,
so there is no such problem. The three-transistor pixel configuration in the CMOS image
sensor, which will be mentioned in Section 5.3, has the same reset operation as that of the
