Image Processing Reference
In-Depth Information
V
s
V
G
V
d
Source
Gate
SiO
2
Drain
n
+
n
+
p
FIGURE 2.11
Cross-sectional view of MOSFET.
In contrast with the ordinal MOS structure, in the buried MOS structure the
n
-type
layer is formed in the channel area under the gate electrode, having antipolarity and lower
impurity concentration compared to the
p
-type substrate, as shown in Figure 2.12.
A
pn
-junction is formed with low impurity concentration of
n
-type layer at the silicon
surface.
It is considered how this
n
-type area plays a role in Figure 2.13, where (a) indicates charge
distribution in real space, while (b) indicates charge distribution in energy space. Ground-
level voltage is applied to both the
p
-type substrate and gate electrode. The system is in
thermal equilibrium, and a built-in potential with a depletion layer exists between both
sides of the
pn
-junction. In other areas, ionized impurity, which is spatially fixed, and an
antipolarity carrier are distributed and are electrically neutral, as shown in the figure.
Here, if electrons in the
n
-type layer are pulled out, for example in such a way as to set
up a higher-concentration
n
-type drain adjacent to the
n
-type layer and apply a higher
V
G
SiO
2
n
-
p
FIGURE 2.12
Cross-sectional view of buried MOS structure.
