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Name: Nathan P.
Status: student
Age: 17
Location: N/A
Country: N/A
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
Physics Instructor
Illinois Central College

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