When a voltage is applied to the two ends of a semiconductor, the valence electron superimposes the directional motion caused by the electric field on the basis of the irregular thermal motion, forming a current, and its motion state also changes, so its motion energy It must be different from the original thermal exercise. In crystals, according to Pauli’s principle of incompatibility, each energy level can hold up to two electrons. Therefore, to change the movement state of electrons in the crystal in order to change the movement energy of the electrons and make it transition to a new energy level, two conditions generally need to be met: ①It has an external function that can provide energy to the electron; ②The electron must The energy level that jumped into is empty.
Because there are a large number of empty energy levels in the conduction band, when there is an electric field, the conduction band electrons can get energy and transition to the empty energy level, that is, the conduction band electrons can change the state of motion. This means that under the action of an electric field, the conduction band electrons can produce a directional movement to form a current, so the conduction band electrons can conduct electricity.
If the valence band is filled with electrons and there is no empty energy level, and under the action of an external electric field, the electrons do not have enough energy to excite the conduction band, then the electron motion state cannot be changed, so directional motion cannot be formed, and there is no current. Therefore, the electrons in the valence band filled with electrons cannot conduct electricity. If some electrons in the valence band jump to the conduction band under external action, then there will be vacancies lacking electrons in the valence band. It is conceivable that under the action of an applied electric field, electrons at adjacent energy levels can jump into these vacancies, and new vacancies appear on the original energy levels of these electrons. After that, other electrons can jump into these new vacancies again, just like the vacancies are moving in the valence band, except that they move in the opposite direction to the electrons. Therefore, for the valence band with electron vacancies, the electronic motion state is no longer immutable. Under the action of an external electric field, some electrons superimpose directional motion on the original thermal motion, thus forming an electric current.
There is a difference between conduction band and valence band, that is: the more electrons in the conduction band, the stronger the conductivity; and the more vacancies in the valence band, that is, the fewer electrons, the stronger the conductivity. The electron vacancies in the valence band are usually imagined as positively charged particles. Obviously, the amount of electricity it carries is equal to that of electrons, and the sign is opposite. Under the action of an electric field, it can move freely in the crystal, and it can conduct electricity like electrons in the conduction band. The electron vacancies in this valence band are usually called holes. Since both electrons and holes can conduct electricity, they are generally collectively referred to as carriers.
Semiconductors that are pure and complete in structure are called intrinsic semiconductors. Its energy band diagram is shown in Figure 1. Figure 1(a) is an intrinsic semiconductor energy band diagram assuming that at absolute zero degrees, it is free from external effects such as light, electricity, and magnetism. At this time, there are no electrons in the conduction band and no holes in the valence band. Therefore, the intrinsic semiconductor at this time, like an insulator, cannot conduct electricity. However, due to the small forbidden band width (Eg) of the semiconductor, under the action of thermal motion or other external factors, electrons in the valence band can be excited to transition to the conduction band. As shown in Figure 1(b). At this time, there are electrons in the conduction band and holes in the valence band, and the intrinsic semiconductor has the ability to conduct electricity. The direct excitation of electrons from the valence band to the conduction band is called intrinsic excitation. For intrinsic semiconductors, their carriers can only be generated by intrinsic excitation. Therefore, the electrons in the conduction band and the holes in the valence band are equal, which is the conductivity characteristic of intrinsic semiconductors.
In fact, crystals always contain defects and impurities, and many of the characteristics of semiconductors are determined by the doped impurities and defects. Impurities and defects have a decisive influence on semiconductors, mainly due to the formation of bound electronic states near the impurities and defects, just like in an isolated atom, electrons are bound near the nucleus. Because the energy of the energy band corresponds to the energy levels of the basic atoms of the crystal (at least when the energy band is not very wide), and the energy level on the impurity atom is different from other atoms in the crystal, so its position It may not be in the range of the crystal energy band. In other words, the energy level of the impurity can be in the forbidden band of the crystal energy level, that is, the energy of the bound state is generally in the forbidden band.
In silicon crystals, silicon has 4 valence electrons, and atoms of group V elements (such as phosphorus, arsenic, antimony, etc.) replace the positions of silicon atoms. Among the 5 valence electrons of group V atoms, 4 valence electrons and silicon Atoms form a covalent bond, and an extra valence electron is not in the valence bond, so it becomes a free electron to participate in conduction. Electrons that can conduct electricity are generally electrons in the conduction band. Therefore, doping a group V impurity into silicon can release an electron to the conduction band of the silicon crystal, and the impurity itself becomes a positive electric center.Impurities with this characteristic are called donor impurities because they can add electrons; in ionic crystals, the positive or negative ions in the gap are absent, and they are actually the positive electric center，so they are also donors and are bound to the donor. The energy state of the electron is called the donor level.
In silicon crystals, when atoms of group III elements with 3 valence electrons (such as boron, aluminum, gallium, steel, etc.) are substituted for silicon atoms to form 4 covalent bonds, one electron is still missing, that is, there is an empty electron energy state that can accept an electron from the valence band of the crystal, which is equivalent to providing a vacancy to the valence band. The group II atom is originally electrically neutral, but when it receives an electron, it becomes a negative negative center of electricity. Impurities with this characteristic are called acceptor impurities because they can accept electrons. The empty energy state of the acceptor is called the acceptor level. In ionic crystals, the absence of positive ions or interstitial negative ions also play the role of negative center and are also acceptors.
The process by which electrons (or holes) on the donor (or acceptor) energy level transition to the conduction band (or valence band) is called ionization, and the energy required for this process is ionization energy. The so-called process of hole excitation from the acceptor energy level to the valence band is actually the process of electron excitation from the valence band to the acceptor energy level. The schematic diagram of the impurity level in the semiconductor is shown in Figure 2.
In Figure 2, E- represents the bottom of the conduction band, and E+ represents the top of the valence band. Generally, the donor energy level is closer to the bottom of the conduction band, that is, the bound state energy level of the impurity is slightly lower than the bottom of the conduction band, so the electrons in the bound state are excited to the conduction band at room temperature, and the electrons in the conduction band are far more than the holes in the valence band. This kind of semiconductor that conducts electricity mainly by electrons is called an N-type semiconductor. Generally, the acceptor energy level is closer to the top of the valence band, that is, when a certain impurity is doped in a semiconductor to make its bound state slightly higher than the top of the valence band, so the electrons in the valence band are excited to the bound stateat room temperature, the holes in the valence band are far more than the electrons in the conduction band. This kind of semiconductor that conducts electricity mainly by holes is called a P-type semiconductor. Since the ionization energy of impurities is much smaller than the band gap, the type and quantity of impurities have a great influence on the conductivity of semiconductors. In N-type semiconductors, since n≥p, electrons are generally called majority carriers, and holes are called minority carriers; in P-type semiconductors, the opposite is true, and holes are called majority carriers, electrons are minority carriers.