There are all kinds of substances in nature, and they can be gases, liquids or solids. According to the arrangement of its atoms, solids can be divided into two types: crystalline and amorphous. According to the conductivity, they can be divided into three types: conductors, insulators, and semiconductors between the two.
The resistivity of different materials is very different. Usually, substances with resistivity in the range of 10-6~10-3 Ωcm are called conductors (such as silver, copper, aluminum, iron and other metals); substances with resistivity above 1012Ωcm It is called an insulator (such as plastic, ceramic, rubber, quartz glass, etc.); a substance with a resistivity between a conductor and an insulator is called a semiconductor. The representative materials are arranged in a straight line according to the size of the resistivity, as shown in Figure 1, the intermediate state of the material is a semiconductor.
Although there is no absolutely clear boundary on the distinction between conductors, insulators and semiconductors in terms of resistivity, there are huge differences in nature. Because semiconductors have many special properties, they occupy an extremely important position in the electronics industry and the optoelectronic industry.
The conductivity of semiconductors can change significantly due to the influence of very small amounts of impurities. For example, the conductivity of pure silicon at room temperature is 5×10-6/Ωcm, when doped with impurities of one millionth of silicon atoms Although its purity is still as high as 99.9999%, the conductivity has risen to 2/Ωcm, which is almost a million times increase. In addition, depending on the types of impurities doped, semiconductors of opposite conductivity types can be obtained. If boron is doped into silicon, a P-type semiconductor can be obtained; when antimony is added, an N-type semiconductor can be obtained.
The conductivity and properties of semiconductors will undergo very important changes due to external effects such as heat, light, electricity, and magnetism. For example, the impedance of a cadmium sulfide layer deposited on an insulating substrate can be as high as tens or even hundreds of megaohms when exposed to light, but once exposed to light, the resistance will drop to tens of kiloohms or even less.
Common semiconductor materials include elemental semiconductors such as silicon, germanium, selenium, and compound semiconductors such as gallium arsenide (GaAs), aluminum gallium arsenide (GaAlAs), indium antimonide (Insb), cadmium sulfide (CdS), and lead sulfide (PbS). There are also oxide semiconductors such as cuprous oxide such as nickel arsenide-gallium phosphide solid solution semiconductors, as well as organic semiconductors, glass semiconductors, rare earth semiconductors, and so on. Using the special properties of semiconductors, heat-sensitive devices, optoelectronic devices (solar cells), field-effect devices, body-effect devices, Hall devices, infrared receiving devices, charge-coupled devices, camera tubes and various light-emitting diodes (LED) have been made , Transistors, integrated circuits and other semiconductor devices.
In order to explain the different conductive properties of solid materials, energy band theory is introduced from the concept of electronic energy level. It is the theoretical basis of semiconductor physics. The application of energy band theory can explain various physical phenomena occurring in semiconductors and the working principles of various semiconductor devices.
Energy Band Theory
Atom is composed of a positively charged nucleus and some negatively charged electrons. These electrons are constantly moving around the nucleus in their respective orbits. According to quantum theory, electron motion has the following three important characteristics:
(1) The electron moves around the nucleus and has a completely definite energy. This stable state of motion is called a quantum state. The energy that determines each quantum state is called the energy level. In a silicon atom, the orbit of electrons moving around the nucleus and the corresponding energy level diagram are shown in Figure 2.
The 14 electrons in the silicon atom have 14 different quantum states, which are distributed in three-layer orbits at different distances from the nucleus. In the innermost quantum state, the electron is closest to the nucleus, is bound by the nucleus the strongest, and has the lowest energy. The more the outer quantum state, the weaker the electron is bound by the nucleus, and the higher the energy. Electrons can absorb energy and jump from a low energy level to a high energy level. Electrons can also release energy under certain conditions and fall back to a low energy level, but they cannot exist in a quantum state between energy levels.
(2) Because microscopic particles have the duality of particle and wave, strictly speaking, there is no completely definite orbit for electrons in atoms. But for the convenience of description, the term “orbital” is still used, where “orbital” represents the part of the area where electrons are most likely to appear.
(3) In an atom or a system composed of atoms, two electrons cannot be in the same quantum state, that is, at most two electrons with opposite spin directions can be accommodated in each energy level. Principle of compatibility. In addition, electrons fill up the lower energy level first, and then fill up sequentially until all electrons are filled.
Read more: What is the mechanism of conduction in semiconductors?