The PN Junction
The modern semiconductor diode is built by creating two regions with different electrical properties inside semiconductor material such as silicon. One region is called P-type; the other is N-type. The boundary between them is the PN junction.
Pure semiconductor
Silicon has four valence electrons and forms a crystal in which each atom shares electrons with neighboring atoms. At ordinary temperatures, pure silicon conducts electricity much less readily than a metal. Its conductivity can be changed dramatically by adding carefully controlled amounts of other atoms.
Doping
Adding a small quantity of selected impurity atoms is called doping.
- N-type: donor atoms provide electrons that can move through the crystal more easily.
- P-type: acceptor atoms create mobile vacancies called holes.
A hole is not a positively charged particle sitting inside the crystal in the same sense as a proton. It is a useful description of a missing electron in the bond structure; that vacancy can move through the crystal as neighboring electrons change positions.
What happens when P and N meet
Near the junction, some electrons from the N side diffuse into the P side and recombine with holes. Some holes similarly disappear by recombination near the boundary. The mobile carriers close to the junction are therefore depleted.
The uncovered dopant ions are fixed in the crystal. They create an internal electric field that opposes further diffusion. The region containing these fixed charges but few mobile carriers is called the depletion region.
Forward bias
Connecting the P side more positively and the N side more negatively reduces the junction barrier. Carriers can cross the junction much more readily and forward current increases rapidly.
Reverse bias
Reversing the applied voltage widens the depletion region. Only a small reverse leakage current normally flows until the electric field becomes strong enough for breakdown.
Why silicon became dominant
Early semiconductor rectifiers used materials such as selenium, copper oxide, galena, and germanium. Silicon eventually became dominant because of its useful electrical properties, excellent native oxide, thermal performance, abundance, and the manufacturing technology developed around purified silicon wafers.
PN junction and light
The same junction physics connects electronics and light. In an LED, carrier recombination releases energy as photons. In a photodiode, absorbed photons create carriers that an electric field can separate and collect as current. In a solar cell, the PN junction converts optical energy into useful electrical power.
One junction as a foundation
The PN junction introduces the ideas needed for later semiconductor devices: carrier concentration, depletion regions, electric fields, minority carriers, recombination, diffusion, and bias. Bipolar transistors use two closely spaced junctions so that carriers injected by one junction can influence current at the other.