The conductivity of semiconductors

The conductive substances of semiconductors can be divided into conductors, semiconductors, and insulators according to their conductivity. As we all know, there are many free electrons in metals. Under the action of an electric field, the free electrons move in a directional manner to form an electric current, so metals have good conductivity and are conductors. In materials such as plastics and mica, although they also contain a large number of electrons, these electrons are tightly bound around the atomic nucleus and cannot move freely. Therefore, even if a strong electric field is applied, these electrons will not undergo directional motion and cannot form an electric current. Therefore, this type of material belongs to insulators.

In addition to conductors and insulators, there is another type of substance They are neither as easy to conduct electricity as conductors nor as difficult to conduct electricity as insulators. Their conductivity is between that of conductors and insulators, and we call these substances semiconductors.

Pure semiconductor materials are non-conductive, but the bound state of semiconductor electrons is relatively loose. As the temperature increases, some electrons will break free and become free electrons, and semiconductors will also have some conductivity. After breaking free from the constraints, electrons will leave a vacancy in their original position, which can be filled by electrons from other atoms, as if they have moved a position in the original vacancy. If an electric field is applied to a semiconductor, the filling motion of this vacancy will be directional, just like the vacancy moving from positive to negative. If free electrons that can move directionally are called charge carriers, then vacancies that can move directionally are also a type of charge carrier, called holes. Free electrons are negatively charged carriers, and because the direction of motion of holes is opposite to that of electrons, holes are positively charged carriers.

Although there are two types of charge carriers in semiconductors But due to its small quantity So the conductivity of pure semiconductors is very weak. If trace amounts of impurity elements are added to pure semiconductors in moderation, the conductivity of the semiconductor can be greatly improved. The impurity elements added to semiconductors can be divided into two types: one type of impurity, when added to a semiconductor, generates many negatively charged electrons in the semiconductor, which is called an N-type semiconductor. The electrons are the majority carriers, and the holes are the minority carriers; Another type of impurity added to a semiconductor produces holes that lack electrons. This type of semiconductor is called a P-type semiconductor, where the holes are the majority carriers and the electrons are the minority carriers.

There is a significant difference in the number of majority carriers and minority carriers between N-type and P-type semiconductors.