Biomedical Engineering Reference
In-Depth Information
Fig. 10.16
Occupation of energy states in an intrinsic
semiconductor at room temperature. A relatively small number
of electrons (-) are thermally excited into the conduction band,
leaving an equal number of holes (+) in the valence band. The
Fermi energy
E
F
lies at the middle of the forbidden gap.
Fig. 10.17
Addition of a small quantity of pentavalent As to Ge
crystal lattice provides very loosely bound “extra” electrons that
have a high probability of being thermally excited into the
conduction band at room temperatures. Arsenic is called a
donor impurity and the resulting semiconductor, n-type.
position in the germanium lattice, as indicated schematically in Fig. 10.17. (The
As atom has a radius of 1.39 Å compared with 1.37 Å for Ge.) Since As has five
valence electrons, there is one electron left over after all of the eight covalent bonds
