Biomedical Engineering Reference
In-Depth Information
Fig. 10.19
Energy-level diagram for Ge crystal containing Ga
acceptor atoms (p-type semiconductor).
In this case, the valence shell of the interposed impurity atom has one less electron
than the number needed to form the regular covalent crystal. Thus, the doped crys-
tal contains positively charged “holes,” which can accept electrons. The dopant is
then called an acceptor impurity and the resulting semiconductor, p-type (positive).
Holes in the valence band move like positive charges as electrons from neighbor-
ing atoms fill them. Because of the ease with which valence-band electrons can
move and leave holes with the impurity present, the effect of the acceptor impurity
is to introduce electron energy levels in the forbidden gap slightly above the top
of the valence band. Figure 10.19 shows the energy-level diagram for a p-type Ge
semiconductor with Ga acceptor atoms added. The action of the p-type semicon-
ductor is analogous to that of the n-type. At absolute zero all of the electrons are in
the valence band, the Fermi energy lying just above. When
T
>0
, thermally excited
electrons occupy acceptor-level states, giving enhanced conductivity to the doped
crystal.
Semiconductor Junctions
The usefulness of semiconductors as electronic circuit elements and for radiation
measurements stems from the special properties created at a diode junction where
n- and p-type semiconductors are brought into good thermodynamic contact. Fig-
ure 10.20 shows an electron energy-level diagram for an n-p junction. The two
semiconductor types in contact form a single system with its own characteristic
Fermi energy
E
F
. Because
E
F
lies just below the conduction band in the isolated
n region and just above the valence band in the isolated p region, the bands must
become deformed over the junction region, as shown in the figure. When the n-
and p-type semiconductors are initially brought into contact, electrons flow from
the donor impurity levels on the n side over to the lower-energy acceptor sites on
