Silicon (Si)
Silicon (Si) is a chemical element that constitutes about 25.7% of the Earth's crust, making it the second most abundant element after oxygen. It primarily exists in nature as part of compounds, such as silicates and oxides, rather than in its elemental form. Silicon plays a crucial role in various biological systems and is significant in the field of electronics due to its semiconductor properties. The element, which has an atomic number of 14, is characterized by its hard, gray solid state and unique diamond cubic structure. High-purity silicon is essential for applications in microelectronics, including computer chips and transistors, while polycrystalline and amorphous forms are used in solar panels and thin-film technologies. Silicon also has industrial applications, such as in the production of ferrosilicon, which is vital for the aluminum and steel industries. The production and demand for silicon, particularly in its metal forms, have been rising, with China being the leading producer globally. Silicon's unique properties make it a critical material for technological advancements and renewable energy solutions.
Silicon (Si)
Where Found
Silicon makes up 25.7 percent of the Earth’s crust and is the second most abundant element after oxygen. It is not found in its elemental form, but rather occurs in compounds such as oxides and various silicateminerals. Silicon is a trace element participating in the metabolism of higher animals, and siliceous structures are found in many biological systems in the form of cell walls, scales, and other skeletal features.
Primary Uses
Silicon metal and alloys, including ferrosilicon, are used mainly by producers of aluminum, aluminum alloys, and chemicals. Very pure silicon is an essential component of semiconductors and has given its name to the “silicon age,” a term that came into prominence during the 1990’s.
Technical Definition
Silicon (abbreviated Si) is the fourteenth element of the periodic table, with an atomic number of 28. With carbon, germanium, and tin, it belongs to Group IVA of the periodic table and resembles germanium (Ge) most strongly in its physical, chemical, and electronic properties. Pure silicon is a hard, gray solid with a metallic luster and a cubic crystalline structure similar to that of carbon in diamond form. It has eight isotopes, the most abundant of which are Si28 (92.23 percent), Si29 (4.67 percent), and Si30 (3.10 percent). Its density is 2.329 grams per cubic centimeter, and it has a melting point of 1,410° Celsius and a boiling point of 2,355° Celsius. While the single-crystal form of silicon has been most extensively studied from both basic and practical viewpoints, the polycrystalline and amorphous forms of silicon have also become extremely important: Polycrystalline silicon has been applied in the construction of solar panels and central processing units of computers. Amorphous silicon has been used in thin-film transistors and solar cells.
Description, Distribution, and Forms
Silicon is widely available in oxides and silicates. The oxide forms include sand, quartz, rock crystal, amethyst, agate, flint, and opal. Granite, feldspar, clay, and mica are some of the common forms of silicates. A basic requirement of silicon in all its preeminent electronic applications is extreme purity—to levels much better than parts per billion (ppb).
The single-crystal form of silicon, while essential for computer chips, has cost and size limitations for a host of other potentially high-volume applications. Hence silicon is also produced in polycrystalline and amorphous forms by techniques such as casting and thin-film deposition. Polycrystalline forms (poly-Si) contain crystalline grains separated by grain boundaries, while amorphous silicon lacks the long-range crystalline order completely. However, both have useful semiconducting properties and have been widely developed for a range of uses.
The interesting and extremely useful electronic and optoelectronic properties of silicon stem from its tetrahedral bonding and diamond cubic structure. Replacing a host silicon atom with a Group V element (such as phosphorus) or Group III element (such as boron) adds a free electron or “hole” (an electron vacancy that behaves like a positively charged free particle. Thus the electrical conductivity of silicon can be changed over several powers of ten simply by controlling the trace quantities of phosphorus or boron. The bandgap separating the electron and hole states has a value of 1.12 electron volts for silicon, making it a nearly ideal choice for devices as varied as transistors, diodes, solar cells, and various types of sensors.
Optically, silicon is transparent to infrared wavelengths above 1.1 micrometers while it absorbs the visible spectrum. Silicon is brittle, but its highly directional bonds enable easy “scribing” of the silicon wafer into individual computer chips under properly chosencrystal orientations. The intricate chemical properties of silicon enable deployment of a variety of fabrication techniques, with individual feature sizes falling into the submicron regime. The modest thermal conductivity of silicon places some restraints on thermal dissipation in computer chips.
History
Although many chemists recognized silicon as an element by the early nineteeth century, its tight bonding with oxygen made it difficult to isolate as a separate element. Jöns Jacob Berzelius achieved the isolation of silicon in 1823 using a method similar to one developed by Sir Humphy Davy, who earlier had tried but failed to isolate silicon. The newly isolated element was named for the Latin word for flint, silex, and subsequently was investigated by German chemistFriedrich Wöhler and others.
Obtaining Silicon
Semiconductor-grade silicon requires conversion of raw silicon obtained from reducing silica (SiO2) into gaseous compounds such as chlorosilanes. Multiple fractional distillation of the latter leads to high-purity silicon rods. These rods are subsequently melted and grown into dislocation-free single crystals by either the Czochralski (CZ) crystal pulling process or the float zone (FZ) process. Necessary dopants such as boron (for p-type silicon) and phosphorus (for n-type silicon) are added to the melt. CZ silicon ingots are probably the largest single crystals ever produced—more than 3 meters long, with diameters as large as 300 millimeters. Wafers, about a millimeter thick, sliced from the ingots serve as the starting material for the batch fabrication of microelectronic chips, each containing up to a few million transistors.
Silicon by itself is inert, but a number of source gases and reagents used in manufacturing it are highly toxic, so extreme care must be exercised in waste disposal and protection of assembly workers. Silicon has been implicated in silicotic lung diseases and certain cancers.
Uses of Silicon
The principal applications of high-grade silicon are in microelectronics. The atomic structure of crystalline silicon makes it the most important semiconductor. Silicon in its highly purified form, when “doped” with elements such as boron and phosphorus, becomes the basic element of computer chips, transistors, diodes, and various other electronic switching and control devices. The enormous success of the silicon transistor, the basic electronic amplifying device, was made possible by an extremely pristine interface with silicon dioxide (an insulator readily grown on silicon by heating in oxygen) and by the continual scaling down of transistor feature size, which translates directly to faster computer speed and higher memory capacity.
The field of giant microelectronics, exemplified by portable computer displays and flat-screen television, uses silicon in its polycrystalline or amorphous forms. Another area of great impact for silicon is in terrestrial solar cells, for which extremely large volumes at low cost are necessary. Here computer-grade single crystals are not cost-effective; large-grain polycrystalline silicon holds the key for this crucial renewable energy application.
A late-twentieth century silicon technology extended the same lithographic patterning that helps put millions of transistors on a single computer chip but uses it to make micromachines such as gears, beams, and motors. These microelectromechanical (MEM) structures are entrenched in automotive applications that require pressure and acceleration sensing. Other novel industrial applications (such as microrobots) and biomedical applications (such as microinjection of drugs and remote microsurgery) are coming to fruition through the development of sophisticated microsensors and actuators that can be integrated with silicon electronics on the same chip for signal processing and amplification.
Lower-grade silicon metals, in the form of ferrosilicon and other silicon metals, are used in other industrial applications. Ferrosilicon is used as an alloying element in aluminum, steel, brass, and bronze, as well as in the chemical industry. Demand for this form was on the rise in 2008, and in fact ferrosilicon constitutes about 80 percent of world production of silicon. China leads the pack in all silicon metal production: Top producers of ferrosilicon are, in descending order, China, Russia, Norway, the United States, and South Africa; leaders in other silicon metal production are China, Brazil, France, and Norway.
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