How Diodes Work

Diode Basics

A diode is one of the simplest and most useful types of semiconductors. It serves as a sort of electric one-way valve, allowing current to flow in one direction but stopping it from going in the opposite direction. One specialized kind of diode, called a light-emitting diode or LED, is becoming increasingly important because of its ability to generate light very cheaply and efficiently.

P-N Junctions

A diode has two types of doped silicon--p-type and n-type. Silicon is a semi-conductor, a material which has a weak ability to conduct electricity which can be changed by adding trace amounts of different materials in it. P-type silicon is doped with a material which gives it a positive charge by taking away electrons, leaving electric "holes". N-type silicon is doped to give it a negative charge with extra, free-floating electrons. The p-type and n-type silicon are layered together, forming a p-n junction.

How it Works

The "p" and "n" designation gives you a clue as to how diodes work--they only operate when the "p" side has a positive voltage applied to it and the "n" side has a negative voltage. The extra electrons from the negative current create enough pressure on the electrons on the n side to make them jump into the p side, which attracts them because it is missing electrons. If the current is reversed, however, the flow stops. When the n-side is exposed to a positive current, electrons are drawn away from the junction. Similarly, a negative current on the p side reduces the positive charge. The boundary becomes strongly resistant, and only a little current can leak across.

LEDs

LEDs work like other diodes with one important difference. In all diodes, electrons need a certain amount of force (voltage) to cross the p-n junction. In an LED, when the electrons store up this force and jump across the junction to land in one of the holes on the p side, some of this force is discharged as a photon--a small packet of light. Depending on the type of material used, the photon will have a different color. LEDs can be made in nearly any color including some outside the visible spectrum. LEDs range from ultraviolet to infrared.