Why don't the semiconductor refrigeration fins realize refrigeration with refrigerant?

　　【】Semiconductor cooling fins, also called thermoelectric cooling fins, are cooling devices composed of semiconductors.

　　 Its advantage is that there are no sliding parts, and it is used in some occasions where space is limited, high reliability is required, and no refrigerant pollution. Utilizing the Peltier effect of semiconductor materials, when direct current passes through a galvanic couple formed by two different semiconductor materials in series, the two ends of the galvanic couple can absorb heat and release heat respectively, which can achieve the purpose of cooling. It is a refrigeration technology that produces negative thermal resistance. It is characterized by no moving parts and high reliability.

　　 This article focuses on the working principle of the semiconductor refrigeration film.

　　1, N-type semiconductor

　　 Electrons in orbits far away from the nucleus z* can often move away from the nucleus and move between atoms, called conductors. If electrons cannot leave the orbit to form free electrons, they cannot participate in conduction, which is called an insulator. The conductivity of a semiconductor lies between a conductor and an insulator and is called a semiconductor. The important characteristic of semiconductors is that after a certain amount of certain impurity penetrates into the semiconductor, not only can the conductivity be greatly increased, but also semiconductors with different properties and different uses can be manufactured according to the type and quantity of doped impurities.

　　Intrinsic semiconductors are doped with pentavalent impurity elements, such as phosphorus, to form N-type semiconductors, also known as electronic semiconductors.

　　Because only four valence electrons in the pentavalent impurity atom can form a covalent bond with the valence electrons in the surrounding four semiconductor atoms, the extra valence electron is easy to form because there is no covalent bond bound Free electrons. In N-type semiconductors, free electrons are majority carriers, which are mainly provided by impurity atoms; holes are minority carriers, which are formed by thermal excitation.

　　 The pentavalent impurity atom that provides free electrons becomes a positive ion because it is positively charged, so the pentavalent impurity atom is also called a donor impurity.

　　2, P-type semiconductor

　　P-type semiconductor conducts electricity by holes. Under the action of an external electric field, the flow direction of holes is opposite to that of electrons, that is, holes flow from the positive electrode to the negative electrode, which is the principle of P-type semiconductor.

　　Intrinsic semiconductors are doped with trivalent impurity elements, such as boron, gallium, indium, etc., to form P-type semiconductors, also known as hole-type semiconductors.

　　Because the trivalent impurity atom lacks a valence electron when forming a covalent bond with the silicon atom, a hole is left in the covalent bond. Holes in P-type semiconductors are majority carriers and are mainly formed by doping; electrons are minority carriers and are formed by thermal excitation. Holes easily trap electrons, making impurity atoms become negative ions. Trivalent impurities are therefore also called acceptor impurities.

　　 The free electrons in N-type semiconductors and the holes in P-type semiconductors are all involved in conduction, collectively called carriers, which are unique to semiconductors and are the result of doping impurities.

　　3, PN junction

　　 By diffusing different impurities on both sides of an intrinsic semiconductor, an N-type semiconductor and a P-type semiconductor are formed respectively. At this time, the following physical process will be formed on the bonding surface of the N-type semiconductor and the P-type semiconductor: The electric field ↓　　 internal electric field promotes the minority carrier drift ↓　　 internal electric field prevents the multi-carrier diffusion

　　z*, the multi-carrier diffusion and the minority carrier drift reach a dynamic balance. On both sides of the junction surface of the P-type semiconductor and the N-type semiconductor, a thin ion layer is left. The space charge area formed by this thin ion layer is called a PN junction. The direction of the internal electric field of the PN junction points from the N area to the P area.

　　4. Heat dissipation

　　 In principle, the semiconductor refrigeration chip is a heat transfer tool. When a current flows through a thermocouple pair formed by a piece of N-type semiconductor material and a piece of P-type semiconductor material, heat transfer will occur between the two ends, and the heat will be transferred from one end to the other end, thereby generating a temperature difference to form cold and heat end.

　　 But the semiconductor itself has resistance when current passes through the semiconductor, it will generate heat, which will affect the heat transfer. Moreover, the heat between the two plates will be reversely transferred through the air and the semiconductor material itself. When the hot and cold ends reach a certain temperature difference and the two kinds of heat transfer are equal, an equilibrium point will be reached, and the forward and reverse heat transfer will cancel each other out. At this time, the temperature of the hot and cold ends will not continue to change. In order to achieve a lower temperature, it can be achieved by reducing the temperature of the hot end by means of heat dissipation.

The role of 　　 fans and heat sinks is mainly to dissipate heat from the hot end of the cooling fins. Generally, the temperature difference between the hot and cold ends of the semiconductor refrigeration fins can reach between 40 and 65 degrees. If the temperature of the hot end is reduced by active heat dissipation, the temperature of the cold end will also decrease accordingly, thereby reaching a lower temperature.

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