Image Processing Reference
In-Depth Information
material for the electrode. There are a few examples of transparent electrodes such as
indium tin oxide (ITO), but polysilicon, which is heavily used in the processes of silicon
LSI circuit production, is the most commonly used. The light-absorbing characteristic of
polysilicon is basically the same as that of silicon. Section 2.2 discusses how the absorp-
tion coefficient at longer wavelengths is lower. However, it tends to be higher for shorter
wavelengths, so that the amount of blue light that reaches silicon is reduced and the
sensitivity tends to be low.
2.1.6 Photodiode
The structure of the photodiode is the same as that of the
pn
-junction, as Figure 2.17 shows,
and it does not have a gate electrode, which attenuates the intensity, especially of blue
light. In operation, positive voltage is applied to the
n
-type area to create a reverse condi-
tion, and the reverse-biased
pn
-junction is isolated electrically and left in a floating state,
as mentioned in Section 2.1.2. The
n
-type region is not completely depleted in an ordinary
photodiode. The energy of light that silicon absorbs excites an electron from the valence
band to the conduction band and a hole is left in the valence band. Electrons flow into the
highly potential
n
-type region and are stored, and holes flow toward the substrate and are
discharged. The behavior in silicon is the same as in the photogate and photodiode; while
only the method of biasing is different.
2.1.7 Buried Photodiode/Pinned Photodiode
A buried or pinned photodiode is used in most of sensors for high image quality applica-
tions. The difference from an ordinary photodiode is that it has a
pnp
structure created
by forming a high concentration of
p
+
layer at the surface, the
n
-type region is completely
depleted, and the potential is pinned in the depleted region, as shown in Figure 2.18.
The potential profile in silicon is the same as with a buried MOS structure, as dis-
cussed in Section 2.1.4. It will be explained in Section 5.1.2.3 that the existence of a
high-concentration hole layer facing the interface greatly suppresses dark output (dark
current) noise generation by the electrons, which are thermally excited to the conduction
band through the interface state on the silicon oxide interface. The absorption of light
and the integration of signal charges in silicon are the same as in an ordinary photo-
diode. The names buried and pinned photodiode originate from the structure and the
function, respectively.
Conduction band
Light
Signal charge
Silicon surface
Interface
state
SiO
2
Light
n
Valence band
p
Electron-hole
pair generation
Depletion
layer
n
-type
p
-type
(a)
(b)
FIGURE 2.17
Photodiode: (a) cross-sectional view; (b) operation in energy space.
