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Diode What are diodes and what are they used for?

| Author/ Editor: Luke James / Erika Granath

A diode may be the simplest of all semiconductor components, however, it performs many critical functions, including the control of the flow of an electrical current. Here’s a brief overview of the humble diode and what it is commonly used for.

A diode is a device that allows current to flow in one direction but not the other. This is achieved through a built-in electric field.
A diode is a device that allows current to flow in one direction but not the other. This is achieved through a built-in electric field.
(Bild: Public Domain)

A diode is a device that allows current to flow in one direction but not the other. This is achieved through a built-in electric field.

Although the earliest diodes consisted of red-hot wires running through the middle of a metal cylinder which itself was located inside of a glass vacuum tube, modern diodes are semiconductor diodes. As the name suggests, these are made from semiconductor materials, primarily doped silicon.

Conducting an electric current in one direction

Despite being nothing more than a simple two-pin semiconductor devices, diodes are vital to modern electronics. Some of their most common applications include turning AC to DC, isolating signals from a supply, and mixing signals.

A diode has two ‘sides’ and each side is doped differently. One side is the “p-side”, this has a positive charge. The other side is the “n-side”, this has a negative charge. Both of these sides are layered together to form what is known as the “n-p junction” where they meet.

I–V (current vs. voltage) characteristics of a p–n junction diode.
I–V (current vs. voltage) characteristics of a p–n junction diode.
(Bild: CC BY-SA 4.0)

When a negative charge is applied to the n-side and a positive to the p-side, electrons ‘jump’ over this junction and current flows in one direction only. This is the diode’s core property; conventional current flows from the positive side to the negative side in that direction only. At the same time, electrons flow in a single direction only from the negative side to the positive side. This is because electrons are negatively charged and are attracted to the positive end of a battery.

What are diodes used for?

Diodes are extremely useful components and are widely used in modern technology.

Light-emitting diodes (LEDs)

Perhaps the most widely known modern application for diodes is in LEDs. These use a special kind of doping so that when an electron crosses the n-p junction, a photon is emitted, which creates light. This is because LEDs glow in the presence of a positive voltage. The type of doping can be varied so that any frequency (colour) of light can be emitted, from infrared to ultraviolet.

Power conversion

Although LEDs may be the most widely known application to the average person, the most common application is by far the use of diodes for the rectification of AC power to DC power. Using diodes, different types of rectifier circuits can be created, the most basic of which are half wave, full wave centre tapped, and full bridge rectifiers. These are extremely important in electronics power supplies --- for example, a laptop’s charger --- where an AC current, which comes from the mains power supply, must be converted to a DC current which can then be stored.

Over-voltage protection

Sensitive electronic devices need to be protected from surges in voltage, and the diode is perfect for this. When used as voltage protection devices, diodes are nonconducting, however, they immediately short any high-voltage spike by sending it to the ground where it cannot harm sensitive integrated circuits. For this use, specialized diodes known as “transient voltage suppressors” are designed. These can handle large power spikes over short time periods which would normally damage sensitive components.

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