What is a semiconductor?
The conductivity of metals such as copper, silver, aluminum, iron, etc. very strong, so it is called an electrical conductor. And plastics, glass, rubber, ceramics, etc. hardly conductive, so it is called an insulator. There is also a type of matter where the ability to conduct electricity lies between the conductor and the insulator, the semiconductor. Semiconductors’ ability to conduct electricity can vary with the variation of physical factors: at extremely low temperatures, pure semiconductors cannot conduct electricity, like insulators. However, at relatively high temperatures, either when light is illuminated or after impurities have mixed in, the semiconductor’s electrical conductivity greatly increases, possibly close to metal’s conductivity. People immediately take advantage of semiconductors’ property to make key components of semiconductors and integrated circuits (IC), which apply in places in electronic engineering. Silicon and germanium are the two most widely used semiconductor elements today.
Why is the conductivity capacity of conductors, semiconductors, and insulators so different? That is due to the difference in their material structure. We know that matter is made up of atoms. Electrons in an atom move around the nucleus of an atom. Whether it is a conductor, a semiconductor or an insulator, there are lots of electrons inside. In metals, the atomic nucleus’ attraction for the electron is very weak. There are quite a few electrons that can move freely. So, electrons in the metal are called free electrons. Once an electric field is applied, the free electrons in the radiant metal subject to the electric field’s direction all move in one direction. So the electric current is formed. But in insulators, negatively charged electrons are subject to the attraction of the positively charged atomic nucleus, which cannot be separated arbitrarily, like falling into a “trap.” If the atomic nucleus’s affected electrons are very strong, then like a very deep “trap,” they cannot “escape” to become free electrons. That is, they cannot form an electric current.
The semiconductor state falls between the two. At low temperatures, electrons are bound by an atomic nucleus that cannot conduct electricity. But that binding is a little weaker than the effect in the insulator. As the temperature rises, the electron’s motion becomes stronger. An integral part of electrons can escape from the binding, turning into free electrons that participate in conducting electricity. Using illumination can also power electrons. The higher the temperature, the more electrons escape the binding, the stronger the ability to conduct electricity, thereby altering the semiconductor’s conductivity.
Impurity mixing is the most important way to increase the conductivity of semiconductors. Mixing only one part per million impurities could improve the semiconductor conductivity by more than a million times. The silicon atom has quaternary; if mixed with a little phosphorus (P) or arsenic (As), etc., all have a valence of five; they replace the position of a silicon atom to yield an electron. That electron immediately participates in conducting electricity. Such a semiconductor mixed with impurities is called an n-type semiconductor. If the impurities are boron or indium having only trivalent value, one electron will be missing, leaving a positively charged hole. It is the positively charged hole that will conduct electricity. Such a type of impurity-mixed semiconductor is called a p-type semiconductor. Semiconductors of type n and type p contact each other to form a p-n junction. Taking advantage of the p-n junction can make semiconductor components such as resistors, bipolar lamps, triode lamps, etc.
Use those semiconductor components to progress to making electrical circuits. It can be seen that semiconductor materials play a very important role in electronic engineering.