A semiconductor refrigerating sheet, also called a thermoelectric cooling sheet, is a cooling device composed of a semiconductor.
It has the advantage of no sliding parts and is used in applications where space is limited, reliability is high, and there is no refrigerant contamination. By using the Peltier effect of the semiconductor material, when the direct current is passed through the galvanic couple of two different semiconductor materials in series, the heat can be absorbed and the heat can be released at both ends of the galvanic couple, and the purpose of cooling can be achieved. It is a refrigeration technology that produces negative thermal resistance, which is characterized by no moving parts and high reliability.
This article focuses on the working principle of semiconductor refrigeration.
1.N type semiconductor
Electrons in the far orbit of the nucleus* can often be attracted to the nucleus and move between atoms, called the conductor. If the electrons cannot get out of the orbit to form free electrons, they cannot participate in conduction, called an insulator. The semiconductor's conductivity is between the conductor and the insulator, called a semiconductor. An important characteristic of semiconductors is that after a certain amount of certain impurities penetrate into the semiconductor, not only can the conductivity be greatly increased, but also semiconductors of different properties and different uses can be manufactured according to the kind and amount of impurities incorporated.
The intrinsic semiconductor is doped with a pentavalent impurity element such as phosphorus to form an N-type semiconductor, also called an electron-type semiconductor.
Since only four valence electrons in the pentad impurity atom can form a covalent bond with valence electrons in the surrounding four semiconductor atoms, the excess valence electron can easily form free electrons due to the absence of covalent bonds. In an N-type semiconductor, free electrons are majority carriers, which are mainly provided by impurity atoms; holes are minority carriers, which are formed by thermal excitation.
A pentavalent impurity atom that provides free electrons becomes a positive ion due to a positive charge, and thus a pentavalent impurity atom is also referred to as a donor impurity.
2.P-type semiconductor
P-type semiconductors are electrically conductive by "holes". Under the action of the external electric field, the "hole" flow direction is opposite to the electron flow direction, that is, the "hole" flows from the positive electrode to the negative electrode, which is the principle of the P-type semiconductor.
The intrinsic semiconductor is doped with a trivalent impurity element such as boron, gallium, indium or the like to form a P-type semiconductor, which is also called a hole type semiconductor.
Since a trivalent impurity atom forms a covalent bond with a silicon atom, it lacks a valence electron and leaves a hole in the covalent bond. The holes in the P-type semiconductor are majority carriers, which are mainly formed by doping; electrons are minority carriers and are formed by thermal excitation. The holes easily trap electrons, making the impurity atoms become negative ions. Trivalent impurities are therefore also referred to as acceptor impurities.
Free electrons in N-type semiconductors, "holes" in P-type semiconductors, all of which participate in conduction, collectively referred to as "carriers", are unique to semiconductors and are the result of the incorporation of impurities.
3.PN junction
An N-type semiconductor and a P-type semiconductor are formed by diffusing different impurities on both sides of an intrinsic semiconductor. At this time, the following physical processes are formed on the bonding faces of the N-type semiconductor and the P-type semiconductor:
Concentration difference
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Multi-sub-diffusion motion forms a space charge region from impurity ions
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Space charge region formation forms internal electric field
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Internal electric field promotes minority drift
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Internal electric field prevents multi-sub-diffusion
* After the multi-child diffusion and the drift of the few children reach a dynamic balance. On both sides of the bonding surface of the P-type semiconductor and the N-type semiconductor, a thin layer of ions is left, and the space charge region formed by this ion thin layer is called a PN junction. The direction of the internal electric field of the PN junction is directed from the N region to the P region.
▲ PN junction plus forward voltage when the conduction is as shown
▲ PN junction plus forward voltage when the conduction is as shown
4. Heat dissipation
In principle, a semiconductor cooling sheet is a heat transfer tool. When a current is passed through a pair of thermocouple pairs in which an N-type semiconductor material and a P-type semiconductor material are connected, heat transfer occurs between the two ends, and heat is transferred from one end to the other end, thereby generating a temperature difference to form a hot and cold heat. end.
However, the semiconductor itself has a resistance that generates heat when current passes through the semiconductor, which affects heat transfer. Moreover, the heat between the two plates is also reversed by the air and the semiconductor material itself. When the hot and cold end reaches a certain temperature difference, when the two heat transfer amounts are equal, an equilibrium point is reached, and the reverse heat transfer cancels each other. At this time, the temperature of the hot and cold end will not continue to change. In order to achieve a lower temperature, it is possible to reduce the temperature of the hot end by means of heat dissipation or the like.
The function of the fan and the heat sink is mainly to dissipate heat from the hot end of the cooling fin. Generally, the temperature difference between the cold and hot ends of the semiconductor refrigerating sheet can reach 40 to 65 degrees. If the hot end temperature is lowered by active heat dissipation, the cold end temperature will also drop correspondingly, thereby achieving a lower temperature.
5. Summary of semiconductor refrigeration film features
As a special cold source, the refrigeration chip has the following advantages and features in technical application:
1. No refrigerant is needed, it can work continuously, no pollution source has no rotating parts, and no turning effect will occur;
2. No sliding part is a kind of solid piece, which has no vibration, noise, long life and easy installation;
3. It can not only cool but also heat, the cooling efficiency is generally not high, but the heating efficiency is very high;
4. The semiconductor refrigeration chip is a current-transducing type piece, and the control of the input current can realize high-precision temperature control;
5. The thermal inertia is very small, and the cooling and heating time is very fast;
6. The temperature difference range of the semiconductor refrigeration chip can be realized from a positive temperature of 90 ° C to a negative temperature of 130 ° C.
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