Image Processing Reference
In-Depth Information
Bottom of
conduction band
(a)
V
J
> 0 V
Top of
valence band
Depletion layer
(b)
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
---
---
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---
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-
-
-
-
-
-
-
-
V
J
+
+
-
-
-
-
-
-
-
-
+
+
n
-type
p
-type
(c)
Conductionband
V
J
< - |φ
B
|
Valence band
n
-type
p
-type
-
: Electron
: Hole
+
: Ionized donor
: Ionized acceptor
+
-
FIGURE 2.8
Potential distribution model of biased
pn
-junction: (a) potential and charge distribution of reverse-biased
pn
-
junction; (b) spatial distribution of charge of reverse-biased
pn
-junction; (c) carrier flow through forward-biased
pn
-junction.
2.1.3 MOS Structure
As a typical example of a metal-oxide semiconductor (MOS) structure, a cross-sectional
view of a gate electrode-silicon dioxide film (SiO
2
)-
p
-type silicon structure is shown in
Figure 2.9. A conductive gate electrode is formed on silicon via silicon dioxide film.
Let us consider the behavior from the viewpoints of spatial and energetic distributions
using Figure 2.10. Figure 2.10a and b show spatial and energetic distribution of charges,
respectively, at the state that the gate voltage
V
G
is 0 V. The same voltage is applied to
both the
p
-type Si and the gate electrode. It is supposed that there is no potential differ-
ence between the silicon and the gate electrode in this condition. This is called a flat-band
condition. In real devices, the gate voltage at a flat-band condition generally deviates from
0 V because of the difference of concentration of impurities between semiconductor and
gate electrode materials, and also because of the existence of ions in the silicon oxide film.
However, these are ignored to simplify the arguments in this topic.
