Diode Image

A two-terminal semiconductor (rectifying) device that exhibits a nonlinear current-voltage characteristic. The function of a diode is to allow current in one direction and to block current in the opposite direction. The terminals of a diode are called the anode and cathode. There are two kinds of semiconductor diodes: a P-N junction diode, which forms an electrical barrier at the interface between N- and P-type semiconductor layers, and a Schottky diode, whose barrier is formed between metal and semiconductor regions.

But this discussion really ought to start with a bit about semiconductors as materials.

Semiconductors are crystals that, in their pure state, are resistive (that is, their electrical properties lie between those of conductors and insulators) -- but when the proper impurities are added (this process is called doping) in trace amounts (often measured in parts per billion), display interesting and useful properties.

A bit of history
The oldest ancestor of semiconductor devices was the crystal detector, used in early wireless radios. This device (patented by a German scientist, Ferdinand Braun, in 1899) was made of a single metal wire (fondly called a "cat's whisker") touching against a semiconductor crystal. The result was a "rectifying diode" (so called because it has two terminals), which lets current through easily one way, but hinders flow the other way. By 1930, though, vacuum-tube diodes had all but replaced the smaller but much quirkier crystal detector. The crystal and "cat's whisker" were left to languish as a kids' toy in the form of "crystal radios."

The development of radar during World War II did much to revive the fortunes of crystal detectors (and, as a result, that of semiconductors) -- although temperamental, crystals were better than vacuum-tube diodes at rectifying the high frequencies used by radar. So, during the war, much effort was put into improving the semiconductors, mostly silicon and germanium, used in crystal detectors. At about the same time, Russell Ohl at Bell Laboratories discovered that these materials could be "doped" with small amounts of foreign "impurity" atoms to create interesting new properties.

Depending on the selection of impurities (often called dopants) added, semiconductor material of two electricallly-different types can be created -- one that is electron-rich (called N-type, where N stands for Negative), or one that is electron-poor (called P-type, where P stands for Positive). Most of the "magic" of semiconductor devices occurs at the boundary between P-type and N-type semiconductor material -- such a boundary is called a P-N junction. Ohl and his colleagues found that such a P-N junction made an effective diode.

Like many components, diodes have a positive side or leg (a.k.a, their anode), and a negative side (cathode). When the voltage on the anode is higher than on the cathode then current flows through the diode (the resistance is very low). When the voltage is lower on the anode than on the cathode then the current does not flow (the resistance is very high).

An easy way to remember this is to look at the symbol for a diode -- the "arrow" in the diode symbol points the direction in which it allows current (hole flow) to flow.

The cathode of a diode is generally marked with a line next to it (on the diode body). You can see a similar line in the schematic symbols, above.

Diodes are also some times marked with an identifying color code (similar, but not identical, to that used for resistors); a good explanation is given here.

Note that when current is flowing through a diode, the voltage on the positive leg is higher than on the negative leg (this is often referred to as the diode's "forward voltage drop"). The magnitude of the voltage drop is a function of (among other things) the semiconductor material that the diode is made from.

Silicon diodes are the most common and cheapest, and have a forward voltage drop of about 0.65 volts. Germanium diodes have a forward voltage drop of about 0.1 volt. Germanium diodes, though, are typically much more expensive than silicon diodes; luckily, they're salvageable from lots of circuit boards.

Zener diodes TBR
The Zener diode is designed to have a specific reverse breakdown voltage (i.e., conduction voltage when reverse-biased). Because of this, Zener diodes can be used by themselves as voltage-sensitive switches, or in series with a current-limiting resistor to provide voltage regulation.

Photodiodes TBR
All P-N junctions are light sensitive; photodiodes are just P-N junctions that are designed to optimize this effect. Photodiodes can be used two ways -- in a photovoltaic (here it becomes a current source when illuminated -- see solar cell), or photoconductive role.

  • To use a photodiode in its photoconductive mode, the photodiode is reverse-biased; the photodiode will then allow a current to flow when it is illuminated.

    ThermoCentrovision has an interesting site on the technology behind photodiodes here.

    Light-Emitting Diodes (LEDs) TBR
    All diodes emit some light when forward-biased. LEDs are made from a special semiconductor (like gallium arsenide phosphide) which optimizes this light output. Unlike light bulbs, LEDs rarely burn out unless their current limit is passed.

    When current is flowing through an LED the voltage on the positive leg is about 1.4 volts higher than the voltage on the negative side (this varies with LED type -- infrared LEDs have a lower forward voltage requirement, others may need up to 1.8 V). Remember that there is very little resistance to limit the current, so a resistor must be used in series with the LED to avoid destroying it (note, though, that some panel-mount LEDs come from the factory with a current-limiting resistor soldered to them).

    Also note that LEDs can be used as photodiodes (tho' their sensitivity is relatively low, so they're only useable this way in very bright conditions).

    Flashing LEDs (FLEDs)
    A flashing LED is just an LED with a built-in microcircuit to cause it to flash periodically. Since the FLED draws current when it flashes, we can use FLEDs to drive a number of timing-dependent circuits (via the fact that it periodically becomes conductive). In particular, see the discussion of the FLED solarengine design. Like other LEDs, FLEDs are light-sensitive, and so flash faster in brighter light. Note that some FLEDs need 3 V minimum to work in, but FLEDs don't in general require current-limiting resistors (at least, I've never seen one that does).

For more information, see the AMS tutorial page on diodes here.

For diode selection and comparison information, see the diode section of the BEAM Reference Library's BEAM Pieces collection.


Legalities  Image 

Page author: Eric Seale  
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