What is a metalloid?
Metalloids are elements which are, in a way, half metal and half nonmetal. They have properties of both metals and nonmetals. All metalloids are semiconductors, meaning that they are not highly effective conductors. One may think this would make them fairly useless when, on the contrary, they’re the reason for our digital age! They don’t conduct much charge, but they have high mobilities, meaning that current can travel through them very quickly.
Current is the basis of electronics and computing. To the computer, current essentially translates to a series of 1’s and 0’s: the faster the charge can pass through the material, the faster the information will travel. This is great for computing, but currents generate heat, hence the need for cooling systems for many electronics (take computer fans, for example).
Silicon is one of the most important elements in the electronics world and is the most commonly used semiconductor. Silicon never appears isolated in nature, but with other elements in compounds. Silica, or silicon dioxide (SiO2), is the main constituent in sand. Commercial silicon comes from sand, but purer forms for the electronics industry come from trichlorosilane (HCl3Si).
Types of semiconductors
Intrinsic semiconductors are chemically very pure, but conduct electricity poorly. These are not doped, meaning that no impurities have been added. They have an equal amount of holes and electrons.
Holes are seen as positive charge carriers which flow in the opposite direction as electrons. They “carry” the same amount of charge as an electron, but with opposite polarity. I use quotation marks on “carry” because holes aren’t actually particles and therefore don’t literally carry anything. Treating them as particles simplifies visualization. So when referring to them as particles, holes essentially carry charge.
In reality, holes are mobile pockets of positive charge left behind when an electron leaves its spot. More technically, holes appear when electrons leave the outer (valence) shell of an atom and become mobile, entering the conduction band. It takes a certain amount of energy for an electron to do this, which may be added through heat, light, material deformation, etc.
Extrinsic (n-type and p-type)
Extrinsic semiconductors are doped, meaning that impurities (other elements) are added to enhance its conductivity.
N-type semiconductors have more electrons moving than holes (“n” for negative). P-type semiconductors have more holes moving than electrons (“p” for positive). Note that semiconductors are electrically neutral: the two extrinsic types are differentiated by how mobile their electrons or holes are.
- Doped with pentavalent ions (5 valence electrons)
- Doping agents include arsenic, antimony, and phosphorous
- These dopants add valence electrons
- Ready to donate electrons because valence electrons become free more easily
- Free electrons have high mobility
- Conductivity based on mobility of free electrons
- Doped with trivalent ions (3 valence electrons)
- Doping agents include indium, boron, aluminum
- These dopants add holes in the semiconductor
- Ready to accept electrons
- Accepting electrons causes holes to form
- Holes have high mobility
- Conductivity based on hole mobility