By Ron B. Davis Jr., Georgetown University
Generally speaking, pure metalloids are solids at normal temperatures and have a lustrous appearance. To look at a sample of boron, silicon, germanium, arsenic, or antimony, one might think one is looking at a metallic element. But their chemical and physical properties often deviate from what one might consider normal metallic behavior.
Discovery of Boron
Boron was discovered at nearly the same time by French chemists Joseph-Louis Gay-Lussac and Louis-Jaques Thenard as well as by English chemistry Humphry Davy.
Boron, like most of the metalloids, seemed to combine with oxygen in ways that made it difficult to isolate. Elemental boron is very difficult to extract and recombines with oxygen quickly, making it highly flammable. Pure boron has proved useful—as an igniter for rocket fuels!
Boron was first recognized as an essential nutrient for plants in the 1920s. Fertilizers are often added with small amounts of boron-oxygen compounds, in part because boron apparently helps build the cell walls of plants—especially tall plants, such as trees.
Moreover, Boron has a role to play in our everyday lives as well.
Use of Boron in Everyday Life
In the late 1800s, Otto Schott, the son of a window glass maker and a student of chemistry, was working in the town of Jena, Germany, where he had established a glass technology laboratory after receiving his doctorate.
Prior to this time, nearly all glass was what chemists refer to as ‘soda-lime’ glass: a silicate mineral combining silicon, oxygen, sodium and potassium to produce the hard, transparent materials used in windows, drinking glasses etc.
But soda-lime glass has a shortcoming. It expands and contracts significantly when heated to extremes, ultimately weakening the glass and making it prone to cracking or even shattering under stress. Naturally, this property made glass a less-than-perfect choice for the manufacture of lab equipment and baking products.
Schott discovered that by including boron in the production of glass, he could create a product that was clear and colorless like soda-lime glass, but had much greater resistance to thermal expansion and contraction. In this new material, boron substitutes for a portion of the silicon atoms in the glass, leading to a material that is remarkably resistant to thermal stresses.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Silicon and Germanium
As we move from boron’s group 15 to group 16, we find two metalloids—silicon and germanium. Germanium, once given the provisional name of ‘eka-silicon’ by Dimitri Mendeleev, was discovered in Germany in 1886.
Like boron, both germanium and silicon are brittle solids that conduct electricity poorly. But unlike boron, these two elements are stable enough in air that even as pure elements they have found an array of uses, especially in the semiconductor industry.
Although we all think immediately of the element silicon when we think of computer technology, the truth is that the seminal invention in microcomputing—the transistor—was first developed using the metalloid germanium instead.
Metalloid in Transistor
William Shockley, working at Bell Laboratories in Murray Hill, New Jersey in the 1940s, led the team that is widely credited with the invention of the transistor—a device that can conduct large amounts of electricity or practically none at all, by applying a much weaker signaling voltage.
Until the 1940s, transistor technology required the use of vacuum tubes. This wasn’t so bad when designing a radio, for example, which only requires a single transistor to work. But the ENIAC, the first computer ever built, consisted of 18,000 vacuum tubes and took up an entire basement on the campus of the University of Pennsylvania when completed in 1946.
To accomplish such a design, Shockley needed a material that had only a moderate ability to conduct electricity so that this ability could be amplified or reduced to near-zero with relative ease. Clearly, metallic substances would be too conductive and nonmetals not conductive enough. So he built his first transistor out of a metalloid.
Changing Conductivity of Silicon
Let’s consider how we can take an element like silicon, a metalloid that has a relatively poor conductivity, and change that conductivity using other elements from the periodic table.
In its pure state, silicon is actually not terribly conductive; it’s a pretty good resistor, in fact. But we can change that with the addition of just a very small amount of other elements in a process called doping.
If we grab a few atoms of boron, an element that has one less valence electron, and replace a few atoms of silicon with them, we create a situation where there are some gaps; there are some empty spaces that are missing electrons. So, it becomes an electron deficient silicon. It becomes a positive or p-type semiconductor because the gaps allow electrons that are usually confined within the relatively covalent silicon-silicon bonds to move.
We can also create semiconductors using elements that are more electron-rich than silicon.
Arsenic is a group 15 element, and so it has five valence electrons. And just as before, when we replace a few of the silicon atoms with arsenic, we have a little bit of arsenic doped into our silicon. In this case, when we apply our voltage, these additional electrons are free to move through the valence band because they’re not confined in covalent bonds between the atoms.
And so, that voltage now generates a current again. So, by doping silicon with either elements from group 13 or 15 adjacent to silicon, we can create conductors based on different types of chemistries.
By carefully doping and stacking N- and P-type semiconductors, engineers can create transistors—devices that can deliver a large current from end-to-end when a small voltage is applied to the middle of the device. Essentially, this creates a switch that can be on or off. Also known as a gated circuit, this produces the ones and zeros that make up the binary language of computers.
Common Questions about Uses of Metalloids in Everyday Life
Pure boron is used as an igniter for rocket fuels. Elemental boron is very difficult to extract and recombines with oxygen quickly, making it highly flammable.
Soda-lime glass expands and contracts significantly when heated to extremes, ultimately weakening the glass and making it prone to cracking or even shattering under stress.
William Shockley led the team that is widely credited with the invention of the transistor.