Basic Theory Of Semiconductor Operation

Semiconductors are important to understand because they play such a prominent part in automotive electronics. You will deal with them nearly every time you diagnose a Toyota electronic system.

Some materials conduct electrical current better than others. This is due to the number of electrons in the outermost ring, or shell, of electrons of the atoms that make up the materials. The outer shell is called the valence shell" or "ring." If the valence ring has five to eight electrons, it takes a large amount of force to cause one of the electrons to break free from the atom, making that material a poor conductor. Such materials are often used as insulators to block current. materials that are made up of atoms with one to three electrons in their valence ring are good conductors because a small force will cause the electrons to break free. Semiconductors fall somewhere in the middle. Since they have four electrons in their valence rings, they are not good insulators or conductors.



Semiconductor Basics Theory

Semiconductors are usually made from germanium or silicon which, in their natural states, are pure crystals. Neither have enough free electrons to support significant current flow, but by adding atoms from other materials—a process called doping— the crystals will conduct electricity in a way that is useful in electronic circuits. The semiconductor material, after it has been doped, becomes either N-type material or P-type material.


Silicon is the most commonly used semiconductor material. The outer shell of a silicon atom contains four electrons, but it needs eight to be stable. Therefore, the atoms link together to share electrons. In this state, silicon will not conduct current.

When silicon is doped with a material such as phosphorous, which has five electrons, the resultant material contains free electrons—known as carriers—and therefore conducts electricity. This creates N-type material, named for its negative charge caused by the excess of electrons.

Silicon can also be doped with a material that has fewer than four electrons in its outer shells, as is the case with boron and its three electrons. The resultant structure has "holes" left by the missing electrons. As discussed earlier, an electron can move into these holes and, in effect, the hole moves in the opposite direction. The abundance of holes creates P-type material, named for its positive charge due the lack of electrons or excess of holes. By joining this N-type and P-type material, diodes and transistors can be formed.

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