Semiconductors
Semiconductors such as silicon conduct electricity better than insulators but far less than metals, because only a small energy gap separates their filled and empty electron bands. Adding tiny amounts of impurity (doping) creates n-type material, rich in electrons, or p-type material, rich in holes. Joining the two makes diodes and transistors — the switches and amplifiers from which every computer chip is built.
From band theory to devices
In a periodic lattice, allowed electron energies form bands. The size of the gap between valence and conduction bands, compared with the thermal energy kT (about 0.026 eV at 300 K, our calculation), decides whether a material is a semiconductor (gap of order 1 eV; silicon ≈ 1.14 eV) or an insulator (several eV, as in diamond).
- Donor binding energies (about 0.02 eV for arsenic in silicon) are comparable to kT, so donor electrons are easily excited into the conduction band at room temperature.
- A p-n junction develops a depletion layer and a potential barrier; forward bias narrows the depletion layer, reverse bias widens it.
- Diode current follows the ideal diode equation; for extreme reverse bias, atoms are ionised and an avalanche of current flows at the breakdown voltage.
- In a junction transistor a small base current controls a much larger collector current — the basis of amplification and switching.
Full explanation — the complete reference version every reading depth is based on
Between conductor and insulator
Band theory explains why materials differ. Electrons in a solid occupy energy bands separated by gaps. In an insulator the gap above the filled valence band is large — several electronvolts for diamond — so almost no electrons can reach the empty conduction band. A semiconductor has the same structure with a much smaller gap — about 1.14 eV for silicon — so at room temperature a useful number of electrons are thermally excited across it and can carry current.
Doping: designing the carriers
- Silicon atoms have four valence electrons.
- n-type: add a donor such as arsenic (five valence electrons). The spare electron is bound by only about 0.02 eV and is easily freed; electrons are the majority carriers.
- p-type: add an acceptor such as aluminium (three valence electrons). This creates holes — missing electrons that behave like positive charges; holes are the majority carriers.
- When an electron moves into a hole, the hole effectively moves the other way, so current can be described as moving holes.
Devices: diodes and transistors
Where p-type and n-type regions meet, a p-n junction forms. Under forward bias current flows easily; under reverse bias very little flows — a diode is a one-way valve for current. A transistor is a valve that can be opened and closed: a small current or voltage controls a larger one. It can amplify a signal, and in computers it works as an on-off switch, which is how chips represent and process bits.
Diode current I versus bias voltage V: large and growing exponentially in forward bias, close to −I₀ in reverse bias.
History in four steps
- 16 December 1947: Bardeen and Brattain achieve transistor action in a germanium point-contact device at Bell Labs.
- 1956: Shockley, Bardeen and Brattain share the Nobel Prize in Physics for the transistor effect.
- Integrated circuits put many components on one chip; Jack Kilby received half of the 2000 Nobel Prize in Physics for his part in their invention.
- 1965: Gordon Moore projects components per chip doubling every year; in 1975 he revises this to every two years ('Moore's law').
Worked example
Moore's revised rate means doubling every 2 years. Over 20 years that is 10 doublings: 2¹⁰ = 1024, roughly a thousandfold increase in components per chip (our calculation). Moore's law is an observed industry trend and projection, not a law of physics.
How we know
Band gaps are measured properties of real materials; the values here come from the band-gap table of an open university physics textbook (OpenStax University Physics, measured at 300 K). The 1947 experiment is documented in Bell Labs notebooks and the 1948 paper by Bardeen and Brattain, as summarised by the Computer History Museum, and the Nobel Prizes are recorded by the Nobel Foundation.
Assumptions and limits
The band picture assumes a perfect, repeating crystal. Real devices have impurities, surfaces and, at today's tiny sizes, quantum effects such as tunnelling, so simple diode and transistor formulas are approximations. Quoted gap values also depend on temperature (1.14 eV is the textbook value for silicon at 300 K).
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Sources and methodology
- A semiconductor has an energy-band structure like an insulator's but with a relatively small energy gap between its filled valence band and the next empty (conduction) band, small enough that a substantial number of electrons are thermally excited into the conduction band at room temperature. (awaiting scientific review)
- Diamond, an insulator, has an energy gap of several electronvolts, much larger than a typical semiconductor's gap of about 1 eV. (awaiting scientific review)
- Silicon has four valence electrons and an energy gap of about 1.14 eV between its valence and conduction bands. (awaiting scientific review)
- The extra electron supplied by an arsenic donor atom in silicon is bound by only about 0.02 eV, so at room temperature it is easily excited into the conduction band and contributes to conduction. (awaiting scientific review)
- Doping silicon with a donor impurity such as arsenic, which has five valence electrons, produces an n-type semiconductor whose majority charge carriers are negative electrons. (awaiting scientific review)
- Doping silicon with an acceptor impurity such as aluminium, which has three valence electrons, produces a p-type semiconductor whose majority charge carriers are positive holes. (awaiting scientific review)
- A p-n junction diode lets current flow freely in one direction (forward bias) but prevents current flow in the opposite direction, so it acts as a one-way valve for current. (awaiting scientific review)
- A transistor can amplify a signal, and in digital circuits such as those in computers it behaves like an on-off switch; integrated circuits contain vast numbers of transistors on a single piece of silicon. (awaiting scientific review)
- John Bardeen and Walter Brattain achieved transistor action in a germanium point-contact device at Bell Labs on 16 December 1947, amplifying an input signal up to 100 times. (awaiting scientific review)
- William Shockley, John Bardeen and Walter Brattain shared the 1956 Nobel Prize in Physics for their researches on semiconductors and their discovery of the transistor effect. (awaiting scientific review)
- The Nobel Prize in Physics 1956 — Summary — Science media
- Jack Kilby was awarded half of the 2000 Nobel Prize in Physics for his part in the invention of the integrated circuit. (awaiting scientific review)
- The Nobel Prize in Physics 2000 — Summary — Science media
- In 1965 Gordon Moore projected that the number of components per integrated circuit would double every year, and in 1975 he revised the projected rate to a doubling every two years. (awaiting scientific review)
Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.
Content status: published 1 October 2026.
- Scientific review: this version has not yet been signed off by a scientific reviewer.
- The Advanced explanation has not yet been reviewed for age suitability.