In a semiconductor, electrons flow, but not as well as they do in a conductor. Some semiconductors carry electrons almost as well as good electrical conductors like copper or aluminum; others are almost as bad as insulating materials.

Semiconductors are not the same as resistors. In a semiconductor, the material is treated so that it has very special properties. Semiconductors include certain substances such as silicon, selenium, or gallium, that have been “doped” by the addition of impurities such as indium or antimony. Have you heard of such things as gallium arsenide, metal oxides, or silicon rectifiers? Electrical conduction in these materials is always a result of the motion of electrons. But this can be a quite peculiar movement, and sometimes engineers speak of the movement of holes rather than electrons. A hole is a shortage of an electron—you might think of it as a positive ion—and it moves along in a direction opposite to the flow of electrons
(following Figure).


In a semiconducting material, holes travel in a direction opposite to the direction in which the electrons travel.

When most of the charge carriers are electrons, the semiconductor is called N-type, because electrons are negatively charged. When most of the charge carriers are holes, the semiconductor material is known as P-type because holes have a positive electric charge. But P-type material does pass some electrons, and N-type material carries some holes. In a semiconductor, the more abundant type of charge carrier is called the majority carrier. The less abundant kind is known as the minority carrier. Semiconductors are used in diodes, transistors, and integrated circuits. These substances are what make it possible for you to have a computer or a television receiver in a package small enough to hold in your hand.