When an impurity like arsenic is added to
germanium it will change the germanium crystal lattice in such a way as to
leave one electron relatively free in the crystal structure. Because this type
of material conducts by electron movement, it is called a negative carrier
(N-type) semiconductor.
Pure germanium may be converted into an N-type
semiconductor by doping it with a donor impurity consisting of any element
containing five electrons in its outer shell. The amount of the impurity added
is very small.
An impurity element can also be added to
pure germanium to dope the material so as to leave one electron lacking in the
crystal lattice, thereby creating a hole in the lattice. Because this
semiconductor material conducts by the movement of holes which are positive
charges, it is called a positive carrier (P-type) semiconductor.
When an electron fills a hole, the hole
appears to move to the spot previously occupied by the electron. As stated
previously, both holes and electrons are involved in conduction.
In N-type material the electrons are the
majority carriers and holes are the minority carriers. In P-type material the
holes are the majority carriers and the electrons are the minority carriers.
Conduction in this type of semiconductor is
similar to conduction in a copper conductor. That is, an application of voltage
across the material will cause the loosely bound electron to be released from
the impurity atom and move toward the positive potential point.
In current flow through a P-type material,
conduction in this material is by positive carrier (holes) from the positive to
the negative terminal. Electrons from the negative terminal cancel holes in the
vicinity of the terminal, while, at the positive terminal, electrons are being
removed from the crystal lattice, thus creating new holes.
The new holes then move toward the negative
terminal (the electrons shifting to the positive terminal) and are canceled by
more electrons emitted into the material from the negative terminal. This
process continues as a steady stream of holes (hole current) move toward the
negative terminal.