P-N Junction
A p-n junction is the interface between a p-type region (doped to have excess holes) and an n-type region (doped to have excess electrons) within one semiconductor crystal; carrier diffusion across the interface leaves a depletion region of fixed ionised dopants and a built-in potential that blocks further diffusion. Applied voltage raises or lowers this barrier, giving strongly asymmetric (rectifying) current, and the junction is the basis of diodes, LEDs, solar cells and bipolar transistors.
Curious
Pure semiconductors conduct poorly, but adding tiny amounts of impurities (doping) changes that. Phosphorus in silicon adds spare electrons (n-type); boron leaves missing electrons, called holes, which act as positive charges (p-type). When the two types meet, electrons drift into the p side and holes into the n side, cancelling each other near the boundary. That leaves a thin zone with no free carriers, the depletion region, and an electric field across it (see Electric Field).
Push current one way (forward bias) and the barrier shrinks so current flows; push the other way and the barrier grows so almost nothing flows. That is a diode. The same junction, under light, makes a solar cell, and in direct-gap materials, passing current through it makes light, as in an LED.
Exploring
| Device | Junction role |
|---|---|
| Rectifier diode | One-way current flow |
| Light-emitting diode | Electron-hole recombination emits photons (direct gap) |
| Solar cell / photodiode | Absorbed light creates carriers separated by the built-in field |
| Bipolar transistor | Two back-to-back junctions; small base current controls large collector current |
Deep Dive
Shockley diode equation. In the ideal model, minority carriers injected across the depletion region diffuse away, giving I = Is[exp(qV/nkBT) − 1], where Is depends on ni², diffusion lengths and doping, and the ideality factor n is between 1 and 2 (recombination in the depletion region pushes it toward 2). Because Is ∝ ni² ∝ exp(−Eg/kBT), the silicon forward turn-on voltage is about 0.6–0.7 V and falls by roughly 2 mV per kelvin.
Breakdown. Under large reverse bias, junctions break down by Zener tunnelling (heavily doped, thin junctions, below about 5 V; see Quantum Tunnelling) or by avalanche multiplication (wider junctions). Zener diodes exploit this as voltage references.
History. Russell Ohl at Bell Labs identified the p-n junction in silicon in 1940 while studying photovoltaic effects. Shockley's 1949 theory and the junction transistor followed the 1947 point-contact transistor; Shockley, John Bardeen and Walter Brattain shared the 1956 Nobel Prize in Physics (see John Bardeen). The 2014 Nobel Prize recognised Isamu Akasaki, Hiroshi Amano and Shuji Nakamura for efficient blue LEDs based on gallium nitride junctions.
Concept Relationship Map
Requires: semiconductors, density of states, effective mass
Enables: diodes, LEDs, solar cells, transistors
Commonly confused with: a mechanical contact between two semiconductor pieces
Sources
- Shockley, W. (1949). Bell System Technical Journal, 28, 435–489.
- Sze, S.M., Ng, K.K. (2007). Physics of Semiconductor Devices (3rd ed.). Wiley.
- The Nobel Foundation (1956, 2014). The Nobel Prize in Physics. nobelprize.org.