A diode is:
TL;DR version: a simple non-linear electronic device made by an engineered attachment of at least 2 different material types.
a thermionic diode for posterity. There are few applications left that use these kind of devices today, mainly in high power radio electronics, but they will soon be replaced by SiC & GaN transistors, where appropriate.
the difference between this and a transistor (or other devices) is that a diode cannot be controlled.
it is based around a semiconductor junction
(intrinsic, raw) semiconductors are materials that have the unique property that when adding some related atoms into it (impurities or dopants), they will behave more like a conductor. Appropriately named, the conductance(1/resistance) of most semconductors is between that of a conductor and an insulator
unlike a conductor it exhibits a non-linear voltage-current characteristic a diode is thus formed by bringing 2 semiconductor materials together, which form a junction. The first ever diode, a cat's whisker detector, was made this way, though modern devices are a bit more complicated than that.
1960s style diode packaging (1N917). The actual diode is the ugly blob inside the glass
The most common diode types, pn, pin, Zenner and Schottky allow the current to flow only in one way. In reality, the electrical current isn't the nice, directed flow of electrons that you were told about in schoo:
In a diode, both directions of a current exist. It's just that one of them is so weak, that it practically doesn't count.
The tilt in the above diagrams represents the nearing of 2 materials, with a fuzzy region inbetween. When you apply a voltage, this tilt further increases or decreases. Think of electrons as rolling balls: they roll down the hill naturally under gravity (electric field), but you need to push them to go upward. Sometimes the balls collide, which can generate light (LEDs). As a matter of fact, any such semiconductor junction will generate photons, but not a lot of them.
Look how many different types of diodes there are there:
A zenner is usually heavily doped, so you want to artificially increase the tilt of the above bands. In case you were wondering, the bands are directly related to the electron shells of an atom.
A Schottky is a metal-semiconductor diode, that behaves slightly differently from a pn.
Semiconductors: What is meant by the term tunnel diode
A photo diode is usually connected in reverse, to capture light and generate a small current. Solar cells are a grouping of photodiodes that are optimised to deliver the largest current possible. Other photo diodes may be optimised to be very sensitive instead (light sensor)
A varactor is another special kind of device used in radios, and it has the attached capacitor symbol to it, because it changes its capacitance significantly as voltage (bias) is applied to it. Most diodes do this in virtue of what is called a piezoelectric effect, but this one is specifically engineered to enhance this effect because it behaves like an oscillator.
finally, a LASER(light by amplified spontaneous emission of radiation) diode further exploits the spontaneous collisions between electrons and tries to increase them (stimulated emission) and amplifiy them. A lot of semiconductor lasers tend to be good conductors in reverse...
There are other kind of material junctions that are not usually recognised as diodes for instance: thermocouples, peltier elements. Their non-linear effects are not strong enough to be used in electrical applications, but are exploited for their thermal properties.
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