Diode: Most basic semiconductor device
The basis of all semiconductor components is the pn junction. Made from the union of two semiconductor materials with different electrical characteristics, most often a silicon substrate with impurities that give it an overall charge.
These two types of materials are called N for negative type with excess electrons, and P for positive type with lack of electrons (excess holes).
When these two material initially come in contact with each other, a portion of the extra electrons in the N type material rush to meet the holes in the P type material, creating a zone where there are neither extra electrons or holes. This region is called a Depletion zone, since the extra charges are depleted by combining with each other.
This depletion zone works as an insulator, separating the N and P layers of the material. Since on one side you have more electrons and on the other you have more holes separated by an insulating layer, the PN junction resembles a little battery, by creating a potential difference across its terminals.
The diode is just a basic two terminal device made of a pn junction. Each terminal is given a specific name, now that it is part of a component, it's better to differentiate it from the P and N type materials. The terminal connected to the P material is called anode (A in schematics) and the terminal connected to the N material is called Cathode (K in schematics).
The pn junction has some properties when an external voltage source is applied to it. When you connect a positive voltage to the cathode with respect to the anode, the electrons are pushed towards the depletion zone.
When the external voltage if higher than the internal junction voltage, also called forward voltage drop the electrons get enough energy to cross the depletion zone and meet with the holes on the other side.
In case of a negative voltage to anode with respect to cathode, the electrons in the cathode are drawn towards the holes in the positive of the voltage source. Same happens with the holes in the anode, which are filled and drawn towards the electrons in the negative terminal.
This has the overall effect of drawing the internal charges away from the depletion zone, effectively widening it, so the electrons don't have enough energy to cross it.
In any bias mode, when a diode conducts, the voltage at which it start to do so remains constant even with increases in the external applied voltage.
Diode varieties and common configurations
Diodes have a wide range of uses depending on their structure and exploited characteristic.
The simple rectifier diode employs the basic properties of the PN junction, specially the fact that it only conducts when forward biased, to create useful circuits. Its most important uses are, as its name implies, in rectifying alternating current into direct current.
The most basic of these rectifier circuits is the half wave rectifier, which basically consists of a diode in series with the source. This diode blocks the negative half of the input wave from reaching the load, creating a pulsating but one way flow of current. An improvement to this and any rectifier circuit is the use of an output capacitor that will charge close to the highest peak of the signal and keep the output dc from varying as much.
Using only one half of the incoming wave is not very efficient, since the energy from the other half is not used. An improved rectifier circuit is the full wave rectifier.
In the simplest form of the full wave rectifier, a special transformer with a central tap is used. In this configuration, the full wave rectifier functions as two half wave rectifiers working each on a different half of the input wave.
Using a center tap transformer is not very practical for small applications, since tapped transformers tend to be bigger than two terminal ones. Since we no longer have a common return path for the current, we need to find a way to make sure that whatever polarity the input has, the output will "see" current flowing in the same direction.
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