Biomedical Engineering Reference
In-Depth Information
have been formed with the neighboring Ge atoms. In Fig. 10.17, two short straight
lines are used to represent a pair of electrons shared covalently by neighboring
atoms and the loop represents, very schematically, the orbit of the extra electron
contributed by As
+
, which is in the crystal lattice. There is no state for the extra
electron to occupy in the filled valence band. Since it is only very loosely bound to
the As
+
ion (its orbit can extend over several tens of atomic diameters), this elec-
tron has a high probability of being thermally excited into the conduction band at
room temperature. The conductivity of the doped semiconductor is thus greatly in-
creased over its value as an intrinsic semiconductor. The amount of increase can
be controlled by regulating the amount of arsenic added, which can be as little as
a few parts per million. An impurity such as As that contributes extra electrons is
called a donor and the resulting semiconductor is called n-type (negative).
Since little energy is needed to excite the extra electrons of an n-type semicon-
ductor into the conduction band, the energy levels of the donor impurity atoms
must lie in the forbidden gap just below the bottom of the conduction band. The
energy-level diagram for Ge doped with As is shown in Fig. 10.18. The donor states
are found to lie 0.013 eV below the bottom of the conduction band, as compared
with the total gap energy,
E
G
=
0.67
eV, for Ge. At absolute zero all of the donor
states are occupied and no electrons are in the conduction band. The Fermi en-
ergy lies between the donor levels and the bottom of the conduction band. As
T
is
increased, thermally excited electrons enter the conduction band from the donor
states, greatly increasing the conductivity. Antimony can also be used as a donor
impurity in Ge or Si to make an n-type semiconductor.
Another type of semiconductor is formed when Ge or Si is doped with gallium
or indium, which occur in the adjacent column to their left in the periodic system.
Fig. 10.18
Energy-level diagram for Ge crystal containing As
donor atoms (n-type semiconductor).
