What are Semiconductor Gemstones and Minerals?

A semiconductor material is a nominally small band gap insulator that can be doped with impurities that change the electronic properties of the material in a controllable way. They are often crystalline, inorganic solids which are classified according to their periodic table groups; for example, group IV includes carbon, silicon, germanium and tin, while group VI included sulfur, selenium and tellurium.

They are used in the computer and photovoltaic industry in lasers, transistors and solar cells making the search for new semiconductors an important area for research for materials science.

Pyrite

Pyrite, or Fool’s Gold, is a pale gold cubic crystal system of iron sulfide (FeS2). It’s hard (measuring six out of ten on the Mohr Scale of mineral hardness), brittle and paramagnetic. In addition to being an ornamental material on account of its luster and elegance of crystals, pyrite is an important source of sulfur and is used commercially to produce sulfur dioxide, in the manufacture of sulfuric acid and in the paper industry.

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Pyrite is also a semiconductor, with a band gap of 0.95 eV. In its pure form it is naturally an n-type semiconductor (meaning it is a negative charge carrier), potentially due to the sulfur vacancies in the pyrite crystal structure acting as n-dopants.

It has found use as a cathode in non-rechargeable lithium batteries. The lithium anode and iron sulfide on an aluminum foil substrate is rolled together to form two long electrodes. This configuration gives the battery 20 times more interfacial surface area than standard alkaline batteries, helping to meet the energy demands of today’s devices. Other advantages include a higher operative voltage and flatter discharge curve compared to other battery types, greater power than other primary types and a longer service in moderate to heavy drain applications.

Iron sulfide is abundant, often found with sulfides or oxides as inclusions in quartz, beryl, emerald or sodalite, non-toxic and inexpensive. It has been suggested that it might be a useful in low-cost photovoltaic solar panels; however, it appears that it underperforms. It is unclear why, but it could be because of the inability to understand and control doping in the material; unintentionally-doped iron sulfide single crystals are naturally n-type, while the films are p-type semiconductors.

Franckeite

This mineral (Pb5Sn3Sb2S14) belongs to the sulfosalt family. It is a two-dimensional material of alternating SnS2-like and PbS-like layers stacked on top of each other with semiconducting properties and has promising applications in next-generation electronic devices.

Franckeite has a narrow band gap of < 0.7 eV and is a p-type semiconductor. It has a natural crystal structure, perfect alignment between crystal lattices and an absence of trapped residues between layers. It can be isolated in extremely thin layers, much like the method to obtain graphene, and could be used in photodetectors or solar cells that function in the infrared range. It could be used to produce night vision systems such as video camera or telecommunications sensors.

Sapphire

Sapphire is another hard mineral (measuring nine on the Mohr Scale) that can be used as an infrared optical component, in wristwatch crystals and in very thin electronic wafers for insulating substrates of very special-purpose solid-state electronics.

Sapphire is an excellent electrical insulator and possesses high thermal conductivity. Silicon on sapphire (SOS) circuits see silicon deposited on sapphire and used in CMOS technologies such as high-power radio frequency applications in mobile phones, public safety band radios and satellite communications systems. Wafers of single-crystal sapphire are also used in the semiconductor industry as a substrate for growth of devices based on gallium nitride, such as blue LEDs.

Quartz

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This mineral (SiO4) is the second most abundant material in the continental crust and comes in many forms which are used as gemstones such as amethyst (blue, milky, rose or smoky quartz) and citrine. Naturally occurring quartz of incredibly high purity is utilized for growing silicon wafers in the semiconductor industry, but this is rare and expensive. Pure quartz sand is used to make silicon metal semiconductors for transistors, in the development of microelectronics, integrated circuits and the silicon chip.

These are just a few examples of minerals and gemstones that possess semiconductor properties or find uses in the semiconductor industry. There are many others, including diamond that is believed to be a viable semiconductor, and galena, a natural mineral form of lead sulfide that is a semiconductor with a band gap of 0.4 eV.

References and further reading

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Kerry Taylor-Smith

Written by

Kerry Taylor-Smith

Kerry has been a freelance writer, editor, and proofreader since 2016, specializing in science and health-related subjects. She has a degree in Natural Sciences at the University of Bath and is based in the UK.

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