Image Processing Reference
In-Depth Information
Conduction
band
Conduction
band
Mobile electron
Acceptorstate
Ionized acceptor
Ionized donor
Donor state
Mobile hole
Valence band
Valence band
(b)
(a)
FIGURE 2.5
Schematic energy diagram: (a)
n
-type silicon, (b)
p
-type silicon. ⦁, electron; ⚬, hole.
A hole is bound by a negatively charged boron ion (B
−
) with a binding energy of 45 meV.
It can be easily freed from the bind by thermal energy and moves in the same way as elec-
trons of phosphorus and the arsenic atom. The boron atom is called an acceptor because it
accepts an electron from the valence band through ionization.
While Figures 2.3 and 2.4 show spatial appearance, Figure 2.5 shows the distributions
of electron and hole in energy space. Figure 2.5a shows diagrammatically that electrons
bound at donor states formed by phosphorus or arsenic leave the bind by thermal excita-
tion to the conduction band to be mobile electrons. Depending on the necessity, the den-
sity of the doped impurity is around 10
14
-10
18
cm
3
, that is, about one impurity atom to
10
5
- 10
9
silicon atoms. Although the doped impurity atom is electrically neutral, the bind-
ing energy of the electron to the ionized donor is of such a level that the electron becomes
free from the bound state by thermal excitation at room temperature. Ionized donor impu-
rities* are positively charged by the release of electrons and the same numbers of mobile
electrons are excited to the conduction band.
As with phosphorus and arsenic, boron is fixed in silicon crystals, as shown in Figure 2.4,
and becomes a negatively charged ionized acceptor by accepting an electron. It generates
the same numbers of holes in the valence band, as shown in Figure 2.5b.
Thus, the semiconductor whose mobile charge is initiated by electrons is called an
n
-type
semiconductor because of its negative charge polarity, and the doped atoms of phosphorus
arsenic are called an
n
-type impurity. When the hole of the positive charge is the initiator,
this is called a
p
-type semiconductor, and boron as the element is called a
p
-type impurity.
The electron and hole are called carriers as they carry the charge that flows to create
an electric current. The directions of carrier flow and electric current are the opposite for
electrons because of their negative charge but the same for holes. While electrons in
n
-type
semiconductors and holes in
p
-type semiconductors are called majority carriers, holes in
n
-type semiconductors and electrons in
p
-type semiconductors are called minority carriers.
2.1.2
pn
-Junction
As Figure 2.6 shows, the structure of a
pn
-junction is literally the connection of a
p
-type semi-
conductor area with an
n
-type semiconductor area. In practical fabrication,
n
-type impurity
atoms whose concentration is more than one digit higher than that of the
p
-type ones are
*
They are fixed spatially in the silicon crystal, as shown in Figure 2.3.
