Basic knowledge of hottest semiconductor and PN ju

Semiconductor basic knowledge and PN junction

in our daily life, we often see or use various objects with different properties. Some objects, such as steel, silver, aluminum, iron, etc., have good electrical conductivity. We call them conductors. On the contrary, some objects such as glass, rubber and plastic are not easy to conduct electricity. We call them insulating (or non conducting). There are also some objects, such as germanium, silicon, arsenide and most metal oxides and metal sulfides, which are neither as easy to conduct as conductors nor as difficult to conduct as insulators, but between conductors and insulators. We call them semiconductors. Most semiconductors are crystals, and the atoms in them are arranged according to certain rules, such as the display values on the display panel of the workstation are still incorrect. Therefore, people often call semiconductor materials crystals, which is the origin of the name of transistors (meaning tubes made of crystal materials)

the conductivity of objects is usually expressed by resistivity. The so-called resistivity is the resistance value in the volume of a certain object per unit length and per unit cross-sectional area. The smaller the resistivity, the easier it is to conduct electricity; On the contrary, the greater the resistivity, the harder it is to conduct electricity

the resistivity of conductor and insulator changes little with the influence of temperature. However, when the temperature changes, the resistivity of semiconductors changes violently; For every 1 ℃ rise, its resistivity decreases by a few percent to dozens of percent. Not only that, when the temperature is high, the overall resistance even drops to very small, so that it becomes the same as the conductor

in metal or insulator, if the impurity content does not exceed one dry part, its resistivity change is negligible. However, impurities in semiconductors have a great impact on it. Take germanium as an example. As long as it contains one tenth of a million impurities, the resistivity will drop to one sixteenth of the original

germanium is a typical semiconductor element and a commonly used material for manufacturing transistors (Note: silicon is the main material in the production of semiconductor components at present). Now take germanium as an example to illustrate how it can generate current, rectification performance and amplification performance in semiconductors

we know that any substance in the world is made up of proto. There is a nucleus in the middle of the atom, and the electrons are constantly rotating around the nucleus. The atoms of different elements contain different numbers of electrons. There are 32 electrons around the nucleus of sugarcane atom, which move around the nucleus. The nucleus is positively charged. Electrons are negatively charged; The number of positive charges is just equal to the number of negative charges of all electrons, so germanium atoms are usually neutral

electrons move around the atomic nucleus, which is similar to the earth's long journey around the sun. Under the gravitational action of the nucleus, the electrons are divided into several layers and run in a completely determined orbit, and the electrons that can be accommodated in each layer have a certain regularity for several days. As shown in the figure, 32 electrons around the germanium nucleus form a four layer ring, which moves around the nucleus. From the inside to the outside, there are 2 electrons on the first ring, and the rest are 8, 18 and 4 electrons in turn. When the number of electrons on the ring is 2, 8, 18. The electrons in these rings are always relatively stable. If the number of electrons in rings is not equal to the above numbers, the electrons in these rings are always unstable

therefore, in the atomic structure of germanium, the electricity of the first, second and third layers is stable, Only the fourth layer (i.e. the outermost "layer) Four of the electricity is then unstable. Because the electrons in the outermost layer are not filled to the specified number. We call the outermost electrons valence electrons. Generally speaking, there are several valence electrons in the outermost layer, and their atomic valence is. The outermost layer of germanium has four valence electrons, so the atomic valence of germanium is 4

under the external action, the electrons on the ring can overcome the attraction of the atomic nucleus and leave the atom, and become free electrons. These free electrons move in space under the action of electric field force, which forms current. It is conceivable that the valence electrons in the outermost layer are far away from the nucleus and suffer the least gravity, so they are most vulnerable to external influence and form free electrons. Therefore, from the perspective of conductivity, valence electrons are very important. The element germanium we call conducts electricity by the four valence electrons in its outermost layer

ideally, all valence electrons in germanium crystal are woven into electron pairs, so there are no free electrons. At this time, germanium crystal is not easy to conduct electricity

however, under the action of external forces, such as temperature changes, one valence electron may break away from the bond, break free of the covalent bond and jump out to become a free electron. At this time, a vacancy appears in the covalent bond, which we call a hole. Since the positive and negative charges of the atom itself are equal, after the atom loses electrons, the whole atom is positively charged, which is called a positive ion. Positive ions are easy to attract valence electrons from adjacent atoms to fill. The vacancy left by electrons after leaving makes holes appear in adjacent atoms, and this new hole may be filled by other electrons. In this way, electrons constantly fill the holes, so that the positions of the holes are constantly transferred between atoms. The transfer of holes is actually the movement of electrons (charges), so it also forms an electric current, which is called hole flow. The previously lost village moves in the crystal, forming an electron flow. For the convenience of narration, the hole is considered to be moving in the future, and it is regarded as a positive charge (in fact, the atom where the hole is located presents the electric quantity of a unit positive charge). Since holes and electrons are charged, their movements form an electric current, so they are collectively referred to as carriers

a semiconductor without impurities and with complete character is called an intrinsic semiconductor. Because of its integrity, if an electron is released from the covalent bond, it must leave an empty crowd. Therefore, in intrinsic semiconductors, electrons and empty people always appear in pairs, and their number of days is equal, which is called electron hole pair. At room temperature, as a result of thermal motion, a certain number of electron hole pairs will be produced in the intrinsic semiconductor, forming electron flow and hole flow. The total current is the sum of the two. If there is no external electric field, the movement of electrons and holes is chaotic, and the direction of electron flow and hole flow is also uncertain. The results offset each other, and there is no net current. However, under the action of an electric field, a voltage appears at both ends of the semiconductor, electrons move towards the positive end, and holes move towards the negative end, forming a directional current, and a current is generated in the semiconductor. The conduction phenomenon of intrinsic semiconductor due to the action of electric field is called intrinsic conduction

in general, we seldom see intrinsic semiconductors, and most of them are p-type semiconductors or n-type semiconductors

as mentioned earlier, when impurities are added to semiconductors, the resistivity will be greatly reduced. This is because the number of holes and electrons will increase greatly after adding impurities. For example, a little trivalent element indium is doped into the germanium crystal. Because indium has only three valence electrons, after penetrating the germanium crystal, its three valence electrons form covalent bonds with the valence electrons of the adjacent three germanium atoms respectively, while for the adjacent fourth germanium atom, it has no electricity to "share" with this germanium atom, leaving a hole (see Figure 1-3 (c)). Because a small amount of impurity indium is added, many holes will appear; This is because even if it is a small amount, it contains a large number of atoms. The number of holes and electrons in impurity semiconductors is not equal. Under the action of electric field, hole conduction is the main, so it is called hole semiconductor or p-type semiconductor. In other words, there are residual holes in p-type or hole type semiconductors, and the doped impurities provide residual holes. In p-type semiconductors, holes are majority, so they are called majority carriers; A small number of electrons is called a few trimmers. The infiltrated impurity can produce holes to accept electrons. We call this impurity acceptor impurity

if the pentavalent element arsenic is doped into the germanium crystal, there are five valence electrons in the arsenic atom. After it forms a covalent bond with the valence electrons of four germanium atoms, it leaves a residual electron, which wanders around the crystal and forms a directional electron flow under the action of an external electric field. Doping a small amount of arsenic impurities will produce a large number of residual electrons, so this kind of semiconductor is called electronic semiconductor or n-type semiconductor. In this kind of semiconductor, there are residual electrons. At this time, electrons are most carriers and holes are few carriers. Because arsenic is an impurity applied to the remaining electrons, it is called donor impurity

if there is no external electric field, no matter n-type or p-type semiconductors, their carrier motion is irregular, so no current will be formed

when a p-type semiconductor and an n-type semiconductor are closely connected together (in fact, two originally independent germanium sheets can only be combined by chemical methods). A strange phenomenon will be found, that is, when an appropriate voltage is applied to both ends of them, a unidirectional conductive phenomenon will be produced. At this time, a so-called p-n junction structure is formed at their interface, and unidirectional conductivity occurs in this thin p-n junction. P-n junction is the foundation of transistor, which is formed by diffusion

we know that the holes in p-type semiconductors are most carriers, that is, the concentration of holes is large; The electrons in n-type semiconductors are mostly current carriers, and the concentration of electrons is large. After the two contact, due to the different electron concentrations in the p-type region and the n-type region, there are many electrons in the n-type region, so they diffuse to the p-type region. The diffusion results are shown in Figure 1-4 (b). Some electrons in thin layer I in n-type region diffuse to p-type region, and thin layer I is positively charged due to loss of electricity. On the other hand, many holes in the p-type region will also diffuse to the n-type region with low hole concentration, Results a part of holes from thin layer I to p (type zone diffusion makes thin layer II negatively charged.

the diffusion of electricity and holes is carried out at the same time. The overall result is that most customers choose horizontal tensile testing machine for thin layer II flow in type p zone, so it is negatively charged, while thin layer I in type n zone flows away electrons and into holes, so it is positively charged, and as the diffusion phenomenon continues, the thin layer gradually thickens and the electricity it carries gradually increases. No Too, this diffusion phenomenon will not continue endlessly; When the diffusion proceeds to a certain extent, the thin layer II takes a lot of negative charge, and the total number of electrons diffusing from the n-type region to the p-type region will not continue to increase due to the repulsion of electrons by it; Similarly, the total number of air disasters spreading from p-type area to n-type area will no longer increase. So the diffusion seems to stop and reach the so-called "dynamic equilibrium state". Then p-n junction is formed

the so-called p-n junction refers to the charged structure composed of thin layers I and II