Natural substances can be divided into conductors, insulators and semiconductors according to their conductive capacity. Semiconductor materials refer to a class of functional materials with electrical conductivity between conductive materials and insulating materials at room temperature. Conduction is achieved by the two carriers of electrons and holes, and the resistivity is generally between 10-5 and 107 ohms at room temperature. Generally, the resistivity increases with the increase of temperature. If it is mixed with active impurities or irradiated with light and radiation, its resistivity can be changed by several orders of magnitude. A silicon carbide detector was made in 1906.
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After the invention of the transistor in 1947, semiconductor materials as an independent material field has been greatly developed, and has become an indispensable material in the electronics industry and high-tech fields. Properties and parameters The electrical conductivity of semiconductor materials is extremely sensitive to certain trace impurities. Semiconductor materials with high purity are called intrinsic semiconductors, which have high resistivity at room temperature and are poor conductors of electricity. When appropriate impurities are added into the high purity semiconductor material, the resistivity of the material is greatly reduced because the impurity atoms provide conductive carriers. Such doped semiconductors are often called impurity semiconductors. Impurity semiconductors conducting by conduction band electrons are called N-type semiconductors, and valence band holes conducting are called P-type semiconductors.
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Contact between different types of semiconductors (form PN junction) or semiconductor and metal contact, due to the difference in electron (or hole) concentration caused by diffusion, the formation of a barrier at the contact, so this type of contact has unidirectional conductivity. Using the unidirectional conductivity of PN junction, semiconductor devices with different functions can be made, such as diodes, transistors, thyristors and so on.
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In addition, the conductivity of semiconductor materials is very sensitive to changes in external conditions (such as heat, light, electricity, magnetic and other factors), according to which various sensitive components can be manufactured for information conversion. The characteristic parameters of semiconductor materials are bandgap width, resistivity, carrier mobility, non-equilibrium carrier lifetime and dislocation density. The bandgap width is determined by the electronic state and atomic configuration of the semiconductor, reflecting the energy required for valence electrons in the atoms composing the material to excite from the bound state to the free state. The resistivity and carrier mobility reflect the conductivity of the material. The non-equilibrium carrier lifetime reflects the relaxation characteristics of the internal carrier transition from the non-equilibrium state to the equilibrium state under external action (such as light or electric field). Dislocation is one of the most common defects in crystals. Dislocation density is used to measure the degree of lattice integrity of semiconductor single crystal materials, but this parameter is not available for amorphous semiconductor materials. The characteristic parameters of semiconductor materials can not only reflect the difference between semiconductor materials and other non-semiconductor materials, but more importantly, it can reflect the quantitative difference between various semiconductor materials and even the same material in different circumstances.
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Types of semiconductor materials
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Commonly used semiconductor materials are divided into elemental semiconductors and compound semiconductors. Elemental semiconductors are semiconductor materials made from a single element. There are mainly silicon, germanium, selenium, etc., silicon and germanium are the most widely used. Compound semiconductors are divided into binary systems, ternary systems, multicomponent systems and organic compound semiconductors. Binary compound semiconductors have Ⅲ-Ⅴ group (such as gallium arsenide, gallium phosphide, indium phosphide, etc.), Ⅱ-Ⅵ group (such as cadmium sulfide, cadmium selenide, zinc telluride, zinc sulfide, etc.), Ⅳ-Ⅵ group (such as lead sulfide, lead selenide, etc.), Ⅳ-Ⅳ group (such as silicon carbide) compounds. Ternary and multicomponent compound semiconductors are mainly ternary and multicomponent solid solutions, such as gallium aluminum arsenic solid solution, gallium germanium arsenic phosphorus solid solution, etc. Organic compound semiconductors are naphthalene, anthracene, polyacrylonitrile, etc., which are still in the research stage.
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In addition, there are amorphous and liquid semiconductor materials, and the biggest difference between these semiconductors and crystalline semiconductors is that they do not have a strictly periodic crystal structure. The preparation of different semiconductor devices has different morphological requirements for semiconductor materials, including single crystal slicing, grinding, polishing, thin film and so on. Different forms of semiconductor materials require different processing techniques. The commonly used semiconductor material preparation processes are purification, single crystal preparation and thin film epitaxial growth.
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All semiconductor materials require the raw material to be purified, requiring a purity of more than 6 \"9\
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Because each method has certain limitations, a process that combines several purification methods is often used to obtain qualified materials. The vast majority of semiconductor devices are made on a single wafer or epitaxial wafer on a single wafer substrate. Bulk semiconductor single crystals are made by melt growth method. Czochralase method is the most widely used, 80% of silicon single crystals, most of the germanium single crystals and indium antimonide single crystals are produced by this method, of which the largest diameter of silicon single crystals has reached 300 mm. The direct pulling method by which a magnetic field is introduced into the melt is called the magnetic-controlled pulling method, and high uniformity silicon single crystals have been produced by this method. The single crystals of gallium arsenide, gallium phosphide, indium phosphide and so on were prepared by the liquid-sealed czotylar method by adding liquid covering agent to the surface of the crucible melt. High purity silicon single crystal is grown by the suspension zone melting method where the melt is not in contact with the container.
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The horizontal zone-melting method is used to produce germanium single crystal. The horizontal directional crystallization method is mainly used to prepare gallium arsenide single crystal, while the vertical directional crystallization method is used to prepare cadmium telluride and gallium arsenide. The bulk single crystal produced by various methods then goes through all or part of the processes of crystal orientation, rolling, reference surface, slicing, grinding, chamfering, polishing, corrosion, cleaning, testing, packaging, etc., to provide the corresponding wafer. The growth of a single crystal film on a single crystal substrate is called epitaxy. Epitaxy methods include gas phase, liquid phase, solid phase, molecular beam epitaxy, etc.
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Chemical vapor phase epitaxy is mainly used in industrial production, followed by liquid phase epitaxy. Metal-organic compound gas phase epitaxy and molecular beam epitaxy are used to prepare quantum Wells and superlattices. Amorphous, microcrystalline, polycrystalline films are mostly made on glass, ceramic, metal substrates by different types of chemical vapor deposition, magnetron sputtering and other methods.
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The difference between semiconductors and insulators mainly comes from the difference in band width between the two. The energy band of the insulator is wider than that of the semiconductor, which means that the carrier in the valence band of the insulator must obtain a higher energy than in the semiconductor in order to skip the band and enter the conduction band. A semiconductor at room temperature conducts electricity like an insulator, with only a few carriers having enough energy to enter the conduction band. Thus, an intrinsicsemiconductor and an insulator with the same electric field have similar electrical properties, but the smaller band width of the semiconductor than that of the insulator means that the conductivity of the semiconductor is more easily controlled and changed.
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The electrical properties of a pure semiconductor can be permanently altered by the insertion of impurities, a process commonly referred to as \"doping.\" Depending on the impurity used in the doping, the doped semiconductor atom may have an extra electron or a hole around it, making the conductive properties of the semiconductor material different from the original. If the concentration of impurities doped into the semiconductor is high enough, the semiconductor may also behave like a metal conductor. There is a built-inelectricfield at the junction of a semiconductor doped with impurities of different polarity, and the built-inelectricfield is closely related to the operation principle of many semiconductor components.
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In addition to permanently changing electrical properties by doping processes, semiconductors can also change dynamically due to changes in the electric field applied to them. Semiconductor materials are also suitable for use as circuit components, such as transistors, because of this property. Transistors are active (active) semiconductor devices (activesemiconductordevices), When active components and passive (passive) semiconductor devices (passivesemiconductordevices) such as resistors, resistor or capacitor (capacitor) together, can be used to design all kinds of IC products, such as the microprocessor.
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When electrons fall from the conduction band to the valence band, the reduced energy may be released as light. This process is the basis for the manufacture of light-emitting diodes (leds) and semiconductor lasers, both of which are critical for commercial applications. Conversely, a semiconductor can also absorb photons and, through the photoelectric effect, excite electrons in the valence band to produce a signal. This is the source of photodetectors, which are the most important components in the field of fiber-opticcommunications or solar cells.
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Semiconductors may be composed of a single element, such as silicon. Can also is a compound of two or more elements (compound), common compound semiconductor are gallium arsenide (GaAs galliumarsenide) or aluminum indium gallium phosphide (aluminiumgalliumindiumphosphide, AlGaInP), etc. Alloy (alloy) is also one of the sources of the semiconductor material, such as silicon germanium (silicongermanium, SiGe) or gallium arsenide aluminum (aluminiumgalliumarsenide, AlGaAs), etc.