Basic knowledge of semiconductors
According to the electrical conductivity of objects, natural substances can be divided into three categories: conductors, insulators and semiconductors. Generally, substances with a resistivity of less than 10-4Ω. cm are called conductors, and mainly metal materials such as gold, silver, copper, iron, etc. are good conductors. Substances with a resistivity greater than 109Ω. cm are called insulators, such as rubber and plastic. The materials whose conductivity is between the conductor and the insulator are called semiconductors. Common semiconductor materials include silicon (Si), germanium (Ge), gallium arsenide (GaAs) and cadmium sulfide (CdS). In addition to the different conductivity of other materials, semiconductors have many characteristics. For example, when exposed to external influences (light, temperature changes) or doped with impurities, their resistance will change significantly. Using these characteristics of semiconductors, people can manufacture various semiconductor devices.
Semiconductors with high purity and complete crystal structure are called intrinsic semiconductors. The most ideal intrinsic semiconductor is single crystal silicon composed of pure silicon. The schematic diagram of the atomic structure of silicon and germanium is shown in Figure 1(a). Their outermost layer of electrons is four. This layer of electrons is weakly bound by the nucleus, called valence electrons, so silicon and germanium are both four. Valence elements [shown in Figure 1(b)].
In actual silicon crystals, the valence electron of each silicon atom is not only constrained by its own nucleus, but also attracted by the nuclei of neighboring silicon atoms. Therefore, valence electrons move around their own nuclei on the one hand, and on the other hand they appear on the orbits of adjacent nuclei, forming a stable covalent bond structure. Due to the binding of covalent bonds to valence electrons, valence electrons cannot easily get rid of the nucleus binding and become free electrons, so intrinsic semiconductors have poor conductivity. A schematic diagram of the intrinsic semiconductor crystal structure is shown in Figure 2.
If the intrinsic semiconductor can obtain energy from the outside world, a minority of valence electrons can break free from the bondage of covalent bonds. At the same time, a vacancy is left on the covalent bond, which is called a hole. Because of the vacancy, the valence electrons of the nearby covalent bonds can easily come to fill in, so a new vacancy is left. It is possible that electrons elsewhere will fill the latter vacancy. From the effect point of view, this movement of electrons filling holes is equivalent to the movement of positively charged holes. Therefore, holes are positively charged carriers. In intrinsic semiconductors, electrons and holes always appear in pairs, called electron-hole pairs. At this time, the number of electrons and holes are always equal. As the temperature increases, the number of electrons and holes increases, and the conductivity of intrinsic semiconductors increases.
② Impurity semiconductor
The conductivity of intrinsic semiconductors is very small. Intrinsic semiconductors are selectively doped with trace amounts of impurities, and their conductivity will change significantly. According to the different types of doped impurity materials, it can be divided into P-type semiconductors and N-type semiconductors.
(1) P-type semiconductor
Doping a small amount of trivalent element boron (B) into intrinsic semiconductor silicon forms a P-type semiconductor, as shown in Figure 3. When a silicon atom and a boron atom form a covalent bond, one less electron is generated, that is, a hole is generated. Each doped boron atom provides a hole, and the valence electrons in adjacent silicon atoms may come over to fill this hole, making the boron atom an extra electron and become a negatively charged ion. The concentration of holes in a P-type semiconductor is much higher than that of electrons, and it conducts electricity mainly by holes.
(2) N-type semiconductor
Doping a small amount of pentavalent element phosphorus (P) into the intrinsic semiconductor forms an N-type semiconductor, as shown in Figure4. When the phosphorus atom with 5 valence electrons forms a covalent bond with the surrounding silicon atoms, one more valence electron will be generated. This electron can easily become a free electron. Therefore, the concentration of electrons in N-type semiconductors is much higher than the concentration of holes. , It mainly relies on electrons to conduct electricity.
③The formation of PN junction
When a certain method is used to make a part of the semiconductor into a P-type and another part of the semiconductor into an N-type semiconductor, a very thin space charge area will be formed at the junction, as shown in Figure 5. Due to the difference in the carrier concentration on both sides of the interface, the majority carriers (holes) in the P-type region will diffuse to the N-type region; similarly, the majority carriers (electrons) in the N-type region will also diffuse to the P-type region. Zone proliferation. During diffusion, electrons and holes recombine, so at the interface, immovable positively charged ions are left on the side of the N-type region, and immovable negatively charged ions are left on the side of the P-type region. Form a space charge zone. Generated in the space charge region-an internal electric field (self-built electric field) directed from the N-type region to the P-type region.
As the diffusion progresses, the internal electric field continues to increase. The strengthening of the internal electric field in turn hinders the diffusion movement, but makes the minority carrier electrons in the P-type region drift to the N-type region, and the minority carrier holes in the N-type region drift to the P-type region. When the diffusion and drift reach a dynamic balance, that is, when the number of carriers in the diffusion motion is equal to the number of carriers in the drift motion, a space charge region of constant thickness is formed, which is called a PN junction. In this space charge region, energy There are very few moving carriers, so it is also called a depletion layer or barrier layer.
A positive voltage (called forward bias) is applied to the PN junction, that is, the positive electrode of the power supply is connected to the P-type area, and the negative electrode is in the N-type area, as shown in Figure 6 (a). At this time, the direction of the external electric field is opposite to the direction of the internal electric field, the internal electric field is weakened, the space charge region becomes thinner, and the diffusion movement of majority carriers greatly exceeds the drift movement of minority carriers. At the same time, the power supply continuously replenishes holes to the P-type area and electrons to the N-type area. As a result, a larger forward current is formed in the circuit, and the PN junction is in a forward conduction state. PN junction plus reverse
To voltage (called reverse bias) is to connect the positive pole of the power supply to the N-type area and the negative pole to the P-type area, as shown in Figure 6 (b). At this time, the direction of the external electric field is consistent with the direction of the internal electric field, the space charge region becomes thicker, and the diffusion movement of majority carriers is hindered, but the drift movement of minority carriers is strengthened. Due to the small number of minority carriers, only a small current flows, and the PN junction is in a cut-off state where it is almost non-conductive. Therefore, when the PN junction is forward-biased, it is in an on state and a large current flows; when the PN junction is reverse-biased, it is in an off state and there is little reverse current, which is the unidirectional conductivity of the PN junction.