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Every phone, computer, car and solar panel depends on a small set of elements that conduct electricity somewhat — not as well as metals, not as poorly as insulators, and, crucially, in a way that engineers can control with extraordinary precision. They’re called semiconductors, and the most important of them, silicon, is made from ordinary sand.
Conductors, insulators and in-between
How well a material conducts depends on whether its electrons can move freely.
- In metals, the outer electrons form a mobile “sea”, so metals conduct very well. See metallic bonding.
- In insulators like diamond or glass, every electron is locked into a bond; there are essentially no free charge carriers.
- In semiconductors, electrons are mostly locked in bonds too — but it takes only a small amount of energy to free some of them.
Band gaps in plain language
In a solid, electron energy levels merge into bands. The valence band holds electrons that are tied up in bonds. The conduction band holds electrons free to move. The energy difference between them is the band gap.
| Material | Band gap | Behaviour |
|---|---|---|
| Metals | none (bands overlap) | Conductor |
| Germanium | ~0.67 eV | Semiconductor |
| Silicon | ~1.12 eV | Semiconductor |
| Gallium arsenide | ~1.42 eV | Semiconductor |
| Gallium nitride | ~3.4 eV | Wide-gap semiconductor |
| Diamond | ~5.5 eV | Insulator |
With a small band gap, heat or light can kick some electrons across it. Each electron that jumps leaves behind a hole — a missing electron — which behaves like a positive charge that can also move. Unlike metals, semiconductors conduct better as they get hotter, because more electrons are excited across the gap.
Where the semiconductors are on the table
The classic semiconductor elements are silicon and germanium in group 14 — the carbon group — each forming four bonds in a diamond-like crystal. Many other semiconductors are compounds that mimic that structure by pairing elements from either side of group 14:
- III–V compounds (groups 13 and 15): gallium arsenide, gallium nitride, indium phosphide.
- II–VI compounds: cadmium telluride, zinc selenide.
The semiconductor elements sit along the metalloid staircase — see metals, nonmetals and metalloids.
Doping: the key trick
Pure silicon conducts only weakly. Its power comes from doping — deliberately adding tiny, precise amounts of other elements, often around one atom in a million or less.
n-type (negative): add a group 15 element such as phosphorus or arsenic. Each has five outer electrons; four bond with the surrounding silicon, and the fifth is spare, free to carry current. The charge carriers are extra electrons.
p-type (positive): add a group 13 element such as boron. Each has only three outer electrons, leaving one bond short — a hole. Neighbouring electrons can hop into the hole, so the hole effectively moves through the crystal. The charge carriers are holes.
From junctions to chips
Put p-type and n-type silicon side by side and you get a p–n junction, the building block of electronics:
- A diode lets current flow in one direction only.
- A transistor uses junctions so a small voltage can switch or amplify a larger current. Modern processors contain tens of billions of transistors, each only nanometres across.
- An LED is a p–n junction in which electrons and holes recombine and release their energy as light. The band gap sets the colour — which is why blue LEDs needed wide-gap gallium nitride. Isamu Akasaki, Hiroshi Amano and Shuji Nakamura won the 2014 Nobel Prize in Physics for efficient blue LEDs, which made white LED lighting possible.
- A solar cell is the reverse: light creates electron–hole pairs at a junction, and the junction pushes them apart to drive a current.
Why silicon?
Germanium was used in the first transistor in 1947, but silicon took over because:
- it’s the second most abundant element in the Earth’s crust;
- its larger band gap makes devices work reliably at higher temperatures;
- it forms a superb natural insulator, silicon dioxide, on its surface — essential for building transistors.
Making electronic-grade silicon requires purity of around 99.9999999% (“nine nines”) or better, one of the purest materials humans produce.
Quick answers
Is silicon a metal? No. It’s a metalloid and a semiconductor.
What does doping do? It adds impurity atoms that supply extra electrons (n-type) or holes (p-type), increasing and controlling conductivity.
Why do semiconductors conduct better when hot? Heat excites more electrons across the band gap, creating more charge carriers. Metals do the opposite.
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