What is Doping?
Name: Nathan P.
Date: Thursday, June 06, 2002
What is electron doping?
I have heard the term "electron doping" used in two different ways. One
definition involves making something negatively charged. One involves
semiconductors as used in diodes and transistors.
Electron doping can be adding electrons to a material to give it a negative
charge. This can apply to something such as a capacitor. It is often done
to produce an electric field that extends out for a significant distance.
Electron doping can also be a procedure that makes semiconductors such as
silicon and germanium ready for diodes and transistors. Semiconductors in
their undoped form are actually electrical insulators that don't insulate
very well. They form a crystal pattern where every electron has a definite
place. Most semiconductor materials have four valence electrons, four
electrons in the outer shell. By putting one or two percent of atoms with
five valence electrons such as arsenic in with a four valence electron
semiconductor such as silicon, something interesting happens. There are not
enough arsenic atoms to affect the overall crystal structure. Four of the
five electrons are used in the same pattern as for silicon. The fifth atom
doesn't fit well in the structure. It still prefers to hang near the
arsenic atom, but it is not held tightly. It is very easy to knock it loose
and send it on its way through the material. A doped semiconductor is much
more like a conductor than an undoped semiconductor. You can also dope a
semiconductor with a three-electron atom such as aluminum. The aluminum
fits into the crystal structure, but now the structure is missing an
electron. This is called a hole. Making a neighboring electron move into
the hole is sort of like making the hole move. Putting an electron-doped
semiconductor (n-type) with a hole-doped semiconductor (p-type) creates a
diode. Other combinations create devices such as transistors.
Dr. Ken Mellendorf
Illinois Central College
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