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Semiconductors are materials with electrical properties between conductors and insulators. They are used in integrated circuits for electronics, transistors, radiofrequency devices e.g. mobile phones, and light-emitting diodes (LEDs).
Some devices require semiconductor materials with elements from the “metalloid staircase” (located across groups 13, 14 and 15 of the periodic table); dominating mostly in commercially produced semiconductor devices and components.
Historically, silicon (Si, Group 14) has found the greatest use in semiconductor applications. Dopants, that increase or decrease the conductivity based on the relative increase or decrease of charge carriers (electron or holes), can be used to tailor the properties of Si-semiconductors.
As the second most abundant element in the Earth’s crust after oxygen, with an average crustal abundance of ~29.5%, silicon resources are widespread and voluminous. Elemental silicon is produced via reductive processes from quartz (SiO2) with the purification of ultra-high purity Si required for semiconductor applications.
China is the dominant producer of elemental silicon accounting for ~79% of elemental Si produced in 2016. Silicon still dominates the solar-cell market due to its low prices and abundant supply.
Germanium (Ge, Group 14) found early use in transistors and diodes but has non-ideal properties, e.g. leakage currents, resulting in itbeing supplanted by other elements and materials. In nature, it is typically associated with zinc in minerals such as sphalerite ([Zn,Fe]S), particularly in sedimentary hosted sulfide deposits.
Current production is low at ~120 tonnes in 2018, with coal ash and recycled components from fiber optic cables supplementing Ge from zinc residues. Ge is listed as a strategic element by the United States’ and United Kingdom’s governments due to applications, some established and some emerging, in photovoltaic solar cells, thermal energy conversion, solar-thermal power and energy storage.
Boron (B, Group 13) has limited uses with its historical consumption dominated by the semiconductor industry. It is used to dope into Si semiconductors to produce p-type (i.e. positively charged) semiconductor materials, however, other sectors including glass production, ceramics and agriculture have taken over as dominant end-users for B.
Ore deposits rich in boron are typically found in arid, volcanic settings and evaporate deposits in alkaline lakes, such as in the Mojave Desert in North America, the lithium triangle in the Andes and in Turkey. The minerals tincal, colemanite, ulexite and propertite are all hydrated, alkaline borates. These four minerals account for 90% of global boron production despite the occurrence of over 200 B-containing minerals.
Gallium and Arsenic
Gallium (Ga, Group 13) and arsenic (As, Group 15) are frequently used together in semiconductors requiring better thermal stability than Si semiconductors. The use of GaAs semiconductors in telecommunications devices has seen demand and supply increase in recent years. Gallium is also used alongside nitrogen (N) in GaN semiconductors for LEDs and laser diodes.
Gallium has a crustal abundance of ~0.0019 wt% and is produced as a by-product of bauxite (aluminum ore) and from zinc processing. Canada, China and Japan all produce Ga with aluminum ore being the dominant source with zinc ores and scrap metal making up the remaining supply portion.
Due to its widespread use in technology, its low abundance and a supply chain dominated by three countries, gallium, like germanium, is considered an element with a high supply risk by the UK and US governments.
The development of new technologies for low-carbon energy, such as solar power, has seen the creation of new semiconductor materials for photovoltaic applications such as copper-indium-gallium-selenide (CIGS) cell technology. The amount of material required to convert the sunlight to electricity in CIGS cells is far lower than Si-based solar cells. However, CIGS cells have not yet been able to compete with Si-based solar cells with Si still accounting for 94% of the market.
Copper is sourced from porphyry copper deposits, e.g. Escondida in Chile, magmatic sulfide deposits, e.g. Kambalda in Australia, or from sediment-hosted stratiform deposits in the Central African Copperbelt. Selenium (Se, Group 16) is sourced from copper refineries where it accumulates in anode residues along with tellurium (Te, Group 16). Indium (In, Group 13), like germanium and, to a lesser extent, gallium, is sourced from zinc concentrates as there are no primary deposits of In. Refinement of In from Zn ores is carried out in Belgium, Canada, China, France, Japan and the Republic of Korea.
Technological advances are likely to see other elements used in semiconductor applications in the future, however, the abundance of certain elements will limit their use unless new sources are found.
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