Image Processing Reference
In-Depth Information
Silicon surface
High-density
hole layer
Conduction band
p
+
SiO
2
Interface state
Interface
state
n
Valence
band
p
Depletion layer
n
-type
p
-type
p
+
(a)
(b)
FIGURE 5.25
Buried/pinned PD: (a) cross-sectional view; (b) energy diagram.
currents of a normal photodiode (
n
+
p
-PD) and pinned photodiode (
p
+
np
-PD), photographs
taken at 73°C with a 2 s exposure period are shown in Figure 5.26. Image quality degrada-
tion caused by FPN originating from variation of dark current is strongly seen in normal
PDs. By using buried PDs, dark current can be reduced to less than one-tenth of that of a
normal PD.
As discussed above, one role of the
p
+
layer is to suppress dark current. But it has another
role, to stabilize the potential of the PD and suppress the potential increase of the depleted
PD by pulling down from the surface side as well as the bottom side. If a PD without a high
surface concentration
p
-type layer is fully depleted, its potential is affected by the charges
in and on the SiO
2
layer on the PD and the electric line of force radiating from neighbor-
ing gates, although a pinned PD is realized. Since electric potentials of gates change with
time, the potential of a PD is unstable. It causes the increase of readout pulse voltage for
complete transfer readout. As the surface
p
+
layer is connected with GND level by way of
the
p
-well, this kind of instability is avoided.
Buried/pinned PDs are widely used for high-performance sensors also in CMOS image
sensors because of low dark current, low-noise, and image lag-free performance.
From the operational principle of pinned PDs by complete depletion, it is clear that the
maximum signal electron number of a PD is the same as the
n
-type impurity atom number
at the highest. However, in practice it is a lower number because of the limitations placed
73°C
73°C
(a)
(b)
FIGURE 5.26
Comparison of dark current images at 2 s exposure at 73°C: (a) normal PD (
n
+
p
-PD); (b) buried/pinned PD
(
p
+
np
-PD).
