Image Processing Reference
In-Depth Information
(g)
(i)
V
G
> 1 V
V
G
> 1 V
p
p
Depletion layer
Surface inversion layer
Deep depletion layer
(j)
(h)
Conduction band
Conduction band
Surface inversion layer
Deep depletion layer
Depletion layer
p
p
- 0 V
- 0 V
V
G
> 1 V
V
G
> 1 V
Valence band
Valence band
: Electron
: Hole
: Acceptor
: Ionized acceptor
FIGURE 2.10
(Continued)
electrons are supplied into the conduction band by some means, electrons gather at the
hollow of potential at the surface, as Figure 2.10i and j show. The width of the depletion
layer shrinks so that the total area density of negatively ionized acceptors and electrons
is maintained. Since electrons have the opposite polarity to
p
-type semiconductors, this
electron layer is called the surface inversion layer.
As will be described, when the means to supply electrons is light in the above situation,
it plays a sensing role and is named a photogate sensor.
In contrast, the hole layer collected on the silicon surface by the application of a negative
voltage to the gate electrode in an
n
-type silicon MOS state is also called an inversion layer.
The MOS field-effect transistor (MOSFET) is a device to control carrier density in the sur-
face inversion layer by applying voltage to the gate electrode. Figure 2.11 shows that, adjacent
to both sides of the channel under the gate electrode in the MOS structure, the highly con-
centrated
n
-type regions are formed as a source for electrons and also as a drain to accept
electrons. The voltage applied to a gate electrode controls the density of current. As the
electrons carry electric current in a MOSFET of
p
-type substrate, it is called an
n
-channel or
n
-type MOSFET. On the other hand, a MOSFET composed of
n
-type substrate, source, drain
of
p
-type region with negative voltage, and holes whose density at the channel is controled
by negative voltage applied to a gate electrode is called a
p
-channel or
p
-t y p e MO SF ET.
2.1.4 Buried MOS Structure
Following the ordinal MOS structure, the buried MOS structure, which is used in buried-
channel charged-couple devices (CCDs) and buried MOSFETs, is described.
