Diodes and LEDs
by Chris Woodford. Last updated: September 13, 2018.
Move over bulbs: there are better ways to make light now! There are those compact fluorescent lamps, for example—the ones that save you energy and money. But, even better, there are LEDs (light-emitting diodes) that are just as bright as bulbs, last virtually forever, and use hardly any energy at all. An LED is a special type of diode (a type of electronic component that allows electricity to flow through in only one direction). Diodes have been around for many decades, but LEDs are a more recent development. Let's take a closer look at how they work!
Photo: Unlike incandescent lamp bulbs (used in things like flashlights), which burn out relatively quickly, LEDs are extremely reliable—so much so, that they're typically soldered right onto electronic circuit boards. They virtually never wear out! This is the tiny LED indicator lamp from a computer printer's control panel.
The difference between conductors and insulators
Photo: LEDs are much smaller than lamp bulbs and use a fraction as much energy. They are particularly suitable for use in instrument panels, which have to be lit up for hours at a time. Put many diodes together and you can make as much light as a conventional bulb but use only a fraction as much energy.
If you know a bit about electricity, you'll know that materials fall broadly into two categories. There are some that let electricity flow through them fairly well, known as conductors, and others that barely let electricity flow at all, known as insulators. Metals such as copper and gold are examples of good conductors, while plastics and wood are typical insulators.
What's the difference between a conductor and an insulator? Solids are joined together when their atoms link up. In something like a plastic, the electrons in atoms are fully occupied binding atoms into molecules and holding the molecules together. They're not free to move about and conduct electricity. But in a conductor the atoms are bound together in a different kind of structure. In metals, for example, atoms form a crystalline structure (a bit like equal-sized marbles packed inside a box) and some of their electrons remain free to move throughout the whole material, carrying electricity as they go.
How semiconductors work
Not everything falls so neatly into the two categories of conductor or insulator. Put a big enough voltage across any material and it will become a conductor, whether it's normally an insulator or not. That's how lightning works. When a cloud moves through the air picking up electric charge, it creates a massive voltage between itself and the ground. Eventually, the voltage is so big that the air between the cloud and the ground (which is normally an insulator) suddenly "breaks down" and becomes a conductor—and you get a massive zap of lightning as electricity flows through it.
Certain elements found in the middle of the periodic table (the orderly grouping of chemical elements) are normally insulators, but we can turn them into conductors with a chemical process called doping. We call these materials semiconductors and silicon and germanium are two of the best known examples. Silicon is normally an insulator, but if you add a few atoms of the element antimony, you effectively sprinkle in some extra electrons and give it the power to conduct electricity. Silicon altered in this way is called n-type (negative-type) because extra electrons (shown here as black blobs) can carry negative electric charge through it.
In the same way, if you add atoms of boron, you effectively take away electrons from the silicon and leave behind "holes" where electrons should be. This type of silicon is called p-type (positive type) because the holes (shown here as white blobs) can move around and carry positive electric charge.
Artwork: N-type silicon has extra electrons (black blobs), while p-type silicon has a lack of electrons that we can think of as "extra holes" (white blobs).
How a junction diode works
Interesting things happen when you start putting p-type and n-type silicon together. Suppose you join a piece of n-type silicon (with slightly too many electrons) to a piece of p-type silicon (with slightly too few). What will happen? Some of the extra electrons in the n-type will nip across the join (which is called a junction) into the holes in the p-type so, either side of the junction, we'll get normal silicon forming again with neither too many nor too few electrons in it. Since ordinary silicon doesn't conduct electricity, nor does this junction. Effectively it becomes a barrier between the n-type and p-type silicon and we call it a depletion zone because it contains no free electrons or holes:
Suppose you connect a battery to this little p-type/n-type junction. What will happen? It depends which way the battery is connected. If you put it so that the battery's negative terminal joins the n-type silicon, and the battery's positive terminal joins the p-type silicon, the depletion zone shrinks drastically. Electrons and holes move across the junction in opposite directions and a current flows. This is called forward-bias:
However, if you reverse the current, all that happens is that the depletion zone gets wider. All the holes push up toward one end, all the electrons push up to the other end, and no current flows at all. This is called reverse-bias:
That's how an ordinary diode works and why it allows an electric current will flow through it only one way. Think of a diode as an electrical one-way street. (Transistors, incidentally, take the junction idea a step further by putting three different pieces of semiconducting material side by side instead of two.)