Image Processing Reference
In-Depth Information
n
+
n
+
p
p
-sub.
Depth
(a)
(c)
n
+
n
+
n
p
Watershed
p
-well
n
-sub.
(b)
(d)
Depth
V
sub
FIGURE 2.21
Comparison of
np
and
npn
photodiodes in structure and potential profile: (a)
np
photodiode on
p
-type substrate;
( b)
npn
photodiode in
p
-well on
n
-type substrate; (c) potential profile of
np
photodiode; (d) potential profile of
npn
photodiode.
by the solid line of the
np
photodiode (
p
-type substrate) in Figure 2.22. The absorption
is observed from a little less than 400 nm to the interband absorption edge of silicon,
including the visible region (380 - 780 nm) and near infrared. Especially strong absorp-
tion is seen in the near-infrared region, whose wavelength is longer than that of the vis-
ible region. This means silicon is a superior photosensitive material in the near-infrared
region. However, as for the sensors of color cameras based on the visible range, high
sensitivity in the near-infrared region is rather undesirable because it disrupts color
balance.
1.0
np
photodiode (
p
-type substrate)
0.5
npn
photodiode (
p
-well)
Visible range
400
500
600
700
800
900
1000
1100
1200
Wavelength (nm)
FIGURE 2.22
Measured examples of spectral response of
np
and
npn
photodiodes.
