Lesson 05 · Semiconductors

Semiconductors and doping

Copper conducts, glass does not, and silicon sits in between. See why: each silicon atom shares its four outer electrons with four neighbours, so a pure crystal has almost no free charge. Then swap in a few phosphorus atoms (five outer electrons) or boron atoms (three) and watch free electrons and holes carry current. A pinch of about one dopant atom per five million silicon atoms gives about a million times more free carriers.

10 min10 steps5 questionsBeginner

Fig 5 · Interactive

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5 quick questions

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01Why does copper conduct electricity so well?

In copper about one electron per atom is free to roam. A battery's push makes those free electrons drift, and that drift is the current. In an insulator such as glass, the electrons are held tight.

02How many valence (outer) electrons does a silicon atom have?

Silicon has 14 electrons in all, but only the 4 in its outer shell take part in bonding. Each one pairs with an electron of a neighbouring atom, so every atom has 4 bonds.

03You swap a few silicon atoms for phosphorus, which has 5 valence electrons. What do you get?

Four of phosphorus's electrons fill the four bonds. The fifth has no bond to join, is only weakly held, and breaks free at room temperature. Free negative charges make the silicon n-type.

04In p-type silicon, a hole drifts toward the - contact. What is really moving?

A hole is a bond that is one electron short. A neighbouring electron hops into it, leaving a new hole where it was. The electrons step toward +, so the hole steps toward -, just like a positive charge would.

05A typical doping level is about 1 phosphorus atom per 5 million silicon atoms. Roughly how many more free electrons does that give than pure silicon?

Pure silicon has about 10^10 free electrons per cubic centimetre. 1 dopant per 5 million atoms is 10^16 per cubic centimetre, each giving one free electron: 10^16 / 10^10 = a million times more.

Key terms

Glossary

Conductor: A material whose electrons move freely, so current flows easily. Copper has about one free electron per atom.
Insulator: A material whose electrons are all held tight, so almost no current flows. Glass is an example.
Semiconductor: A material in between a conductor and an insulator, such as silicon, whose conductivity can be tuned by doping.
Valence electrons: The electrons in an atom's outer shell, which take part in bonding. Silicon has 4, phosphorus 5, boron 3.
Covalent bond: A bond in which two atoms share a pair of electrons, one from each atom.
Doping: Adding a tiny, controlled amount of another element to a semiconductor to change how well it conducts.
n-type: Silicon doped with atoms that have 5 valence electrons, such as phosphorus. Its current is carried by free electrons (negative).
Hole: A bond that is one electron short. Electrons hopping into it make the hole move like a positive charge; p-type silicon carries current this way.

Fine print

Notes and sources

The board is a teaching model: the battery, the switch and the bulb show the idea. Doped silicon conducts well enough to light a small bulb; pure silicon does not.
The flat square grid is the usual simplified sketch. Real silicon is a 3D diamond lattice; every atom still bonds to four neighbours.
The drawing has 2 dopant atoms in 15 sites so you can see them. A typical real level, 1 × 10^16 per cm³, is 1 dopant per 5 million silicon atoms (0.2 ppm).
Silicon numbers at 300 K (Ioffe Institute): 5 × 10^22 atoms per cm³, 1 × 10^10 free electrons per cm³ in pure silicon, resistivity 3.2 × 10^5 Ω·cm, 0.045 eV to free the extra electron of phosphorus or fill the hole of boron.
Wire dashes show conventional current (from + to -). Electrons drift the other way; holes drift the same way as conventional current.

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