holes, creates a POSITIVE or P-TYPE semiconductor, and the impurity that was added to it is known as a
P-type impurity. Semiconductors which are doped in this manner either with N- or P-type impurities
are referred to as EXTRINSIC semiconductors.
The N-type impurity loses its extra valence electron easily when added to a semiconductor material,
and in so doing, increases the conductivity of the material by contributing a free electron. This type of
impurity has 5 valence electrons and is called a PENTAVALENT impurity. Arsenic, antimony, bismuth,
and phosphorous are pentavalent impurities. Because these materials give or donate one electron to the
doped material, they are also called DONOR impurities.
When a pentavalent (donor) impurity, like arsenic, is added to germanium, it will form covalent
bonds with the germanium atoms. Figure 1-10 illustrates this by showing an arsenic atom (AS) in a
germanium (GE) lattice structure. Notice the arsenic atom in the center of the lattice. It has 5 valence
electrons in its outer shell but uses only 4 of them to form covalent bonds with the germanium atoms,
leaving 1 electron relatively free in the crystal structure. Pure germanium may be converted into an
N-type semiconductor by "doping" it with any donor impurity having 5 valence electrons in its outer
shell. Since this type of semiconductor (N-type) has a surplus of electrons, the electrons are considered
MAJORITY carriers, while the holes, being few in number, are the MINORITY carriers.
Figure 1-10.Germanium crystal doped with arsenic.
The second type of impurity, when added to a semiconductor material, tends to compensate for its
deficiency of 1 valence electron by acquiring an electron from its neighbor. Impurities of this type have
only 3 valence electrons and are called TRIVALENT impurities. Aluminum, indium, gallium, and boron
are trivalent impurities. Because these materials accept 1 electron from the doped material, they are also
called ACCEPTOR impurities.
A trivalent (acceptor) impurity element can also be used to dope germanium. In this case, the
impurity is 1 electron short of the required amount of electrons needed to establish covalent bonds with 4
neighboring atoms. Thus, in a single covalent bond, there will be only 1 electron instead of 2. This
arrangement leaves a hole in that covalent bond. Figure 1-11 illustrates this theory by showing what
happens when germanium is doped with an indium (In) atom. Notice, the indium atom in the figure is 1