Semiconductors
A semiconductor is a material whose electrical conductivity lies between that of a good conductor (like copper) and a good insulator (like glass), and — crucially for technology — whose conductivity can be precisely and predictably controlled by introducing small, deliberate amounts of impurity atoms, a process called doping, making semiconductors the foundational material of essentially all modern electronics.
Curious
A pure, unmodified semiconductor like silicon is actually a fairly poor conductor at room temperature — its small but nonzero band gap (see the Codex's band theory entry) means only a small fraction of electrons have enough thermal energy to jump across the gap and participate in conduction. What makes semiconductors technologically transformative isn't this modest intrinsic conductivity, but how dramatically and precisely that conductivity can be altered by adding tiny, carefully controlled amounts of specific impurity atoms — a process called doping.
Doping a semiconductor with atoms that have one extra outer electron (like phosphorus, added to silicon) creates n-type material, with extra mobile negative charge carriers (electrons). Doping instead with atoms having one fewer outer electron (like boron) creates p-type material, with mobile positive charge carriers called "holes" — the absence of an electron behaving, in many important respects, like a positively charged particle in its own right.
Exploring
| Type | Dopant | Majority carrier |
|---|---|---|
| n-type | Group 15 element (e.g. phosphorus, arsenic) in silicon | Electrons (negative) |
| p-type | Group 13 element (e.g. boron, gallium) in silicon | Holes (positive) |
Deep Dive
The invention of the point-contact transistor at Bell Labs in December 1947, by John Bardeen and Walter Brattain (with William Shockley developing the more practically important junction transistor shortly afterward), marked the beginning of the semiconductor electronics era — replacing bulky, fragile, power-hungry vacuum tubes with a small, robust, low-power solid-state device capable of amplifying or switching electrical signals. The trio shared the 1956 Nobel Prize in Physics for this work, which is widely regarded as one of the most economically and technologically consequential inventions of the 20th century.
The p-n junction: Bringing a p-type and n-type semiconductor region into contact creates a p-n junction, at which mobile charge carriers from each side diffuse briefly across the boundary and recombine, creating a thin "depletion region" depleted of mobile carriers and establishing a built-in electric field. This junction behaves as a diode, conducting current readily in one direction (forward bias) while strongly resisting current flow in the other (reverse bias) — the fundamental building block underlying not just simple diodes, but transistors, solar cells, LEDs, and virtually all semiconductor electronic devices, through various combinations and refinements of this basic junction structure.
Silicon remains the dominant semiconductor material for most electronics due to a favourable combination of properties (an appropriately sized band gap, natural abundance, and the ability to grow a stable, high-quality native oxide layer essential for transistor fabrication), though other semiconductors — gallium arsenide, gallium nitride, silicon carbide — are used for specialised applications requiring properties silicon doesn't provide, such as high-frequency operation or LED light emission.
Concept Relationship Map
Requires: band theory
Enables: p-n junction
Commonly confused with: treating a "hole" as a literal particle rather than a description of collective electron motion
Sources
- Bardeen, J., Brattain, W.H. (1948). Physical Review, 74(2), 230–231.
- Kittel, C. (2004). Introduction to Solid State Physics, 8th ed. Wiley, Ch. 8.