Phophorus, which lies to the right of silicon in the periodic
table, adds conduction electrons to silicon. One naturally
wonders (particularly if one carefully considered the Discussion
Questions) what might be the effect of aluminum, which lies to
the left of silicon in the periodic table. As it turns out, aluminum
tends to gather in clumps rather than scatter throughout the silicon, so
it is not the best choice for doping silicon. Instead we use gallium,
which also falls to the left of silicon in the periodic table. Gallium
has three valence
electrons; when a gallium atom takes the place of a silicon atom,
the valence band ends up with only seven electrons, not the desired eight
that will fill the valence shell of a silicon atom. The eighth energy
state is not included in the valence
band since the gallium atom has one fewer proton to go with the
one fewer electron, but the extra state is not in the conduction
band either. Again, the impurity provides an energy state
in the band gap. This time, however, the energy state is empty and
is closer to the valence band than to the conduction band. (Electrons
in a completly full atomic energy level are more tightly bound than those
that start filling the next energy level, even when filling the level provides
a net negative charge to that atom.) An electron from the filled
valence band can easily move up into this surplus energy state, leaving
behind a hole that can then move through the valence band. This process
is shown below, again in both a band diagram (Figure (a)) and a lattice
diagram (Figure (b)).
The "missing" valence electron of the gallium atom can be represented as a missing bond in the lattice diagram. This bond is easily filled by an electron from an adjacent atom. When this occurs, however, a charge separation arises between the now negatively-charged phosphorus core and the positively charged hole.
The charge carriers contributed by the gallium impurity
again far outnumber any "intrinsic" charge carriers from the pure silicon.
The majority charge carriers in this case are positively-charged
holes, so we call gallium-doped silicon a p-type
semiconductor. Since gallium provides an extra energy state
that "accepts" an electron, it is also called an acceptor.
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Copyright © 2003 Doris Jeanne Wagner and Rensselaer Polytechnic Institute. All Rights Reserved.