What is the unidirectional conduction of semiconductor diodes


1.1 The mechanism of semiconductor and metal conductor conduction

The term “semiconductor” is relative to “conductor”. Objects with high conductivity are called conductors; objects that cannot conduct electricity are called insulators. Objects with conductivity much smaller than conductors and much larger than insulators are called semiconductors.

The mechanism of conduction between semiconductors and metal conductors is fundamentally different. Compared with metals (conductors), the conductivity of semiconductors is at least two or three orders of magnitude smaller than that of metals. This is just the difference in the amount of conductivity between metals and semiconductors, and more importantly, they have essential differences. . The concentration of free electrons in a metal is constant. Even if the temperature is reduced to absolute zero, the concentration is still that large. Temperature and foreign impurities only slightly affect its mobility. Therefore, the electrical conductivity of the metal has a relatively small relationship with temperature and impurities. In contrast to semiconductors, there are no free electrons at absolute zero. The increase in temperature and the addition of impurities (especially the addition of certain impurities) can significantly increase the concentration of free electrons in the semiconductor, that is, the conductivity of the semiconductor and the The relationship between temperature and impurity content is very close.

1.2 Physical characteristics of semiconductor diodes

1.2.1 Unidirectional conductivity of semiconductor diodes Semiconductor diodes (diodes for short) have two electrodes: one is the anode; the other is the cathode.

When the diode circuit is connected to the external voltage in the forward direction, the bulb passes a larger current called the forward current; when the diode circuit is connected to the external voltage in the reverse direction, the current through the bulb is very weak, and the bulb does not light at this time, which is called reverse Leakage current,As shown in Figure 2-3.Therefore, it can be considered that the diode only allows current to flow in one direction. This characteristic that only allows current to flow in one direction is called the unidirectional conductivity characteristic of the diode. Silicon crystal, the material used for solar photovoltaic power generation, has the same unidirectional conductivity characteristics as a diode after being doped with certain impurities.

Unidirectional conduction phenomenon of diode

 Figure 1-1 Unidirectional conduction phenomenon of diode

Why can silicon crystals have unidirectional conductivity like semiconductor diodes? Silicon (Si) has an atomic number of 14, and its crystal structure is the same as that of diamond crystals. Its electron configuration is 1s22s22p63s23p2. The inner layer of 10 electrons (1s22s22p6) is The nucleus is tightly bound, and the outer 4 electrons (3s23p2) are also called valence electrons (the silicon atom has 4 valence electrons). In silicon crystals, each silicon atom forms a covalent bond with its four valence electrons and four adjacent atoms, which is less bound by the nucleus, as shown in Figure 1-2(a) . Due to the role of covalent bonds, electrons cannot move (although the outermost electrons of silicon atoms are less bound by the nucleus). But when it is exposed to enough light or heat, that is, when it has enough energy, it will break away from the bondage of the atomic nucleus and become a free electron.At the same time, it will lack an electron in the original position and leave a hole, as shown in Figure 1-2(b). In a pure semiconductor, with a free electron, there must be a hole, and the number of the two is equal. Free electrons and holes are constantly produced and “resurrected” constantly.

The result of covalent bonding of silicon atoms (a) and the crystal structure of silicon and the generation of electron-hole pairs (b)

 Figure 1-2 The result of covalent bonding of silicon atoms (a) and the crystal structure of silicon and the generation of electron-hole pairs (b)

When there is an external electric field, free electrons move in the opposite direction to the electric field. At the same time, the electrons adjacent to the hole fill the hole due to the thermal movement of the original atom, but leave a new void in the original place. hole. In this way, the hole is also moving, and its direction of movement is exactly opposite to that of the electron. Therefore, the hole can be regarded as a positively charged particle. Semiconductors can conduct electricity because they have these two types of carriers (current carriers and hole carriers) inside. Since there are very few electrons and holes in a pure semiconductor, its conductivity is far inferior to that of a conductor. If a small amount of impurities are added to pure semiconductor silicon, such as the pentavalent element phosphorus (P) with 5 electrons in the outermost layer of the nucleus, there will inevitably be an extra electron that cannot form an electron pair (i.e., a covalent bond), such as As shown in Figure 1-3(a). In this way, many free electrons that are excluded from covalent bonds will appear in the crystal. The number of these newly emerged free electrons far exceeds the number of electrons and holes that were originally not doped with impurities. Therefore, in all the carriers (the free electron is the load of the current at this time, called the electron “carrier”), the vast majority are electrons, and the number of holes is very small. In this kind of semiconductor, the dominant carrier is the electron, so the electron is called the majority carrier, and the hole is only the minority carrier. This type of semiconductor mainly relies on electrons to conduct electricity, so it is called an electronic semiconductor, also called an N-type semiconductor; on the contrary, if the pure semiconductor silicon is doped with the trivalent element boron (B) with 3 electrons in the outermost layer of the nucleus, As shown in Figure 1-3(b), there are some covalent bonds in the crystal that lack electrons to form holes. The number of these holes far exceeds the original number of electrons and holes when it is undoped. Therefore, in all carriers, the vast majority are holes, while the number of electrons is very small. In this type of semiconductor, the dominant position is the hole, so it is called a hole semiconductor, also called a P-type semiconductor.

Schematic diagram of silicon single crystal doping

(a) Silicon doped with phosphorus to form electrons (N-type) (b) Silicon doped with boron to form holes P-type)

Figure 1-3 Schematic diagram of silicon single crystal doping

It can be seen that due to the different impurities doped in the pure crystal, the majority carriers and minority carriers in the two types of semiconductors are also different. Most carriers can appear in N-type semiconductors and P-type semiconductors because of doping, and their conductivity is much stronger than that of pure semiconductors. Taking germanium as an example, as long as it is doped with one-tenth millionth of an impurity, its conductivity is increased by 16 times. Crystal diodes are composed of these two types of semiconductors. This shows that the basic principle of why silicon is used as a material for solar cells is the same.

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