A diode is the simplest semiconductor component you can make. Take some P material. Fuse it with some N material. In the result, current will flow from the P to N, but not backwards.

Most diodes always a voltage drop of about .7V across the component. If you run the voltage high enough in the reverse direction, you will eventually hit the breakdown voltage, above which the diode will act as a resistanceless voltage drop. It will also probably blow out at that voltage, unless you take the proper precautions. This is what zener diodes are designed to do.

The normal diode is an electronic device that lets electronic current flow in one direction but not the other. The first diodes were electron tubes. These were relatively big and not very efficient. Later a smaller kind of diode was developed, it consisted of a metal and a metaloxide. the metal/metaloxside is probably the easyest diode to make, all you need is to put a pice of metal against an oxidized metal and you have a simple diode. Eventualy the tubes and metal/metaloxide diodes was replaced in most devices by semiconductors.

The most used semiconductor in diodes are silicon, but germanium and gallium is also used. The basic diode consists of a positively doped and a negatively doped material on a semiconducting substrate. The place the two meet is called a PN junction.

The silicon diode is the most used today. It has a forward voltage drop from approximatly 0.7 volt for a small signal diode, to 1V or more for a high-power rectifier diode.

The fast recovery diode is a diode that switches faster between a conducting and nonconducting state. their switching time may be as low as 1ns.

The Zener diode functions as a normal diode in the forward direction. Also when connected backwards it will seem as a normal diode, until a spesific voltage is reached, then it starts to conduct. The zener diode is must allways be used in series with a resistor or it'll be fried. When in series with a resistor the voltage over the diode is stable at the zener voltage, and therefore the zener is used in voltage stabilizer circiuts.

The Avalanche diode is used for similar purposes as the zener diode but is available for voltages from 100v to 300v

The germanium diode has a voltage drop between 0.2v and 0.5v. It was dominant before the 1960s, when the silicon diod became available , but are still in use in some radio and video detector circiuts.

Schottky diode: This diode can in most cases replace the germanium diode. It has a voltage drop of about 0.4v, and fast switching. This means it's a good choice for high frequency applications.

The capacitancediode(varactor/varicaps): All diodes have a backwards capacitance that falls when the voltage rises, but the capacitance diod is optimized for this. These diodes are used in radios instead of the variable capacitor. Without this component we might not have the pocket radio with digital tuning.

The Light emitting diode(LED) is a diode that, as the name suggests, sends out light. They are available in just about any colour from infra red to blue(blue LEDs are still expensive so they are not much used). the forward voltage drop of a LED is between 1.4v and 3v depending on type.

The LASER diode. This is a more refined type of LED that emitts LASER light. By the addition of an appropriate lens (collimator) you will get a thin high intencity beam. The LASER diode is available in power from 1 mW to 10 mW.
Some variations of the basic diode are:

the Zener diode which doesn't conduct unless the voltage is over a threshold, written Vz. The Zener effect, for which this diode is named, was discovered by Clarence Melvin Zener, in 1915.

The Light Emitting Diode, or LED. All diode radiate energy from the P-N junction, LEDs are made to make that light visible. LEDs are the little lights on your computer, or monitor.

                                      _ |\
                                     |\   \
                    _                  \
    |  /|            |  /|            |  /|
    | / |            | / |            | / |
____|/  |____    ____|/  |____    ____|/  |____
    |\  |            |\  |            |\  |
    | \ |            | \ |            | \ |
    |  \|            |_ \|            |  \|

   Diode          Zener Diode     Light-Emitting Diode

Electrons flow against the direction of the arrow. The side with the stripe is called the cathode; the side without the anode. At voltages below the breakdown voltage, electrons can flow into the cathode and out the anode, but not the other way round.

Most diodes are cylinders, with one wire coming out of each end. The side of the diode with a stripe corresponds to the side of the symbol with a stripe. LEDs do not have stripes, but indicate the cathode using a shorter wire. The larger side of the symbol corresponds to the longer wire of the component.

The diode symbol may optionally be enclosed in a circle. This is merely a matter of preference. A diode with a circle is the same as a diode without.

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