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A diode allows electrical current to flow in one direction but not the other. i.e. a diode is a one-way gate. In essence, a diode is analogous to a 'guard dog' in an electronic circuit preventing current flowing in the wrong direction and potentially damaging sensitive devices.

A diode is a simple semiconductor device made up of two layers of 'doped' semiconductor materials i.e. n type and p type.

Doping is the addition of a small percentage of impurity into the regular crystal lattice of the element silicon, which can produce dramatic changes in its electrical properties. Typical impurities used include arsenic and boron. The impurity used decides whether the semiconductor produced becomes n type or p type.

Diodes have two terminals, the positive anode and the negative cathode. Current will only flow when the anode is at a more positive voltage than the cathode.

There are a wide range of diodes available, three of the more common types are listed below:-

Light Emitting Diodes (LED)

These are diodes that emit light when connected to a closed electrical circuit. The diode itself is encased in a hard epoxy bulb. LEDs are frequently used as "status indicators" in electronic appliances to indicate whether the circuit is closed or not. e.g. PC monitor indicator light lets you know the monitor is on.

Why do LEDs emit Light?

When a particular voltage is applied to the LED across its leads, electrons flow in only one direction across the junction between the p and n type materials.

In the p layer there are more positive than negative charges. In the n layer there are more negatively charged electrons than positive electric charges. When a voltage is applied and the current starts to flow, electrons in the n layer are excited and move across the junction into the p Layer. Once in the p layer the electrons are attracted to the positive charges and 'combine' with them.

Each time an electron recombines with a positive charge, potential energy is converted into electromagnetic energy. Electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the doped semi-conductor material.

Different colours of LEDs can be achieved by using different types of semi-conductor materials in the diode.

Schottky Diodes

A metal to semiconductor diode is called a Schottky diode. Schottky diodes (also known as Schottky barrier diodes) consist of a metal layer which is in contact with a semiconductor element such as silicon.

A Schottky diode has a very low forward-voltage drop. When current flows through a diode, it has some internal resistance to that current flow, which causes a small voltage drop across the diode terminals. A Schottky diode can have a voltage drop up to 5 times smaller than that of a standard diode, this lower voltage drop translating into higher system efficiency. As a result, Schottky diodes are widely used for radio frequency (RF) applications. The Schottky diode is also used in power applications operating as a rectifier.

Zener Diodes

are used to maintain a fixed voltage. A Zener Diode will allow current to flow in the forward direction, but will also allow it to flow in the reverse direction when the voltage is above a certain value - this is known as the breakdown voltage or Zener voltage.

Zener diodes can be distinguished from ordinary diodes by their code and breakdown voltage which are printed on them. Zener diode codes typically begin BZX... or BZY...

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Guide Written by Paul Wilson

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