Diodes: Electronic Products Word of the Week

Diodes today are solid-state devices, but in the beginning of the electronics age, they were crystals, as it was the German physicist Ferdinand Braun who first discovered in 1874 that crystals had rectifying properties, critical to diodes.

All diodes consist of an anode (negative pole) and a cathode (positive pole). Diodes always allowed current to flow in only one direction, a process known as rectification (a unidirectional behavior used to convert alternating current to direct current, including extraction of modulation from radio signals in radio receivers). The first semiconductor diodes, called cat's whisker diodes, were developed around 1906 and made of mineral crystals like galena. Soon, though, electron tubes came to contain diodes. Transistors, an outgrowth of solid-state diodes, replaced triodes and other electron tubes and made possible the miniaturization of electronics – and made Moore’s Law necessary.
Now, while it was my original intention to have this series of electronics words of the week to be alphabetical, the subject of diodes has proven to be so broad that the alphabetization could almost take place inside my explanation of diodes. You see, there are several kinds of diodes — everything from avalanche diodes to Zener diodes, and much, much more. And while Electronic Products hasn’t exactly fully defined the diode (until now), the publication does give ample help in finding samples and manufacturers.
Here are a few examples of the different diodes out there:
An avalanche diode is designed for use in the avalanche region, when the reverse electric field across the p–n junction causes a wave of ionization, reminiscent of an avalanche.
Cat's whisker or crystal diodes are among the oldest of diodes, consisting of a thin or sharpened metal wire pressed against a semiconducting crystal, typically galena or a piece of coal. The wire forms the anode and the crystal forms the cathode. Cat's whisker diodes were also called crystal diodes and found application in crystal radio receivers. A hobbyist can concoct a simple radio using a crystal, headphones, and a ground to a radiator to pick up the local station.
A light-emitting diode (LED) is a semiconductor device that emits incoherent narrow-spectrum light when electrically forward biased, resulting in a form of electroluminescence. An LED is used as a light source in many illumination applications, often promising cooler operation than alternative light sources, but its heat must be managed.
An ORing diode blocks current from the bus to the output capacitors .
A p-n diode is a type of semiconductor diode based upon the p–n junction. The diode conducts current in only one direction, and it is made by joining a p-type semiconducting layer to an n-type semiconducting layer.
A photodiode is intended to sense light (be a photodetector), so photdiodes are packaged in materials that allow light to pass, and are usually PIN (the kind of diode most sensitive to light). A photodiode can be used in solar cells, in photometry, or in optical communications.
A PIN diode has a central undoped, or intrinsic, layer, forming a p-type/intrinsic/n-type structure. A PIN diode is used as a radio frequency switch and attenuator. It is also used as a large-volume, ionizing-radiation detector and as a photodetector. PIN diodes are also used in power electronics, as their central layer can withstand high voltages.
A Schottky diode is a semiconductor diode with a low forward voltage drop.
A tunnel diode has a region of operation showing negative resistance caused by quantum tunneling. They are often used in spacecraft because they are very fast, may be used at low temperatures, high magnetic fields, and in high radiation environments.
A varicap (or varactor diode) allows tuning circuits, such as those in televisions, to lock quickly on a station.
A Zener diode can act as a control element in a circuit. The internal design features include a temperature compensation circuit to minimize variation in the voltage clamp over its operating temperature range.

A diode family portrait (Source: Wikipedia)

Among the oldest and most basic electronic components, diodes still play a vital role in today’s up-and-coming electronic devices.