Electrical Components Basics: The Standard Diode

The simplest (and of the most common) semiconductor electrical component is the rectifier diode.

In the simplest terms possible, a diode blocks electric current in one direction and one direction only. Place a diode across a battery one way and you will essentially short the battery, but reverse the diode and nothing will happen (the battery will not discharge), as if the diode was never connected.

What does a Diode do?

The circuit symbol for a diode looks like this:

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Unlike components like resistors or inductors, the orientation of a diode in a circuit matters. Flipping a diode and putting it back in a circuit will dramatically change the way the circuit works, and may prevent it from working entirely.

A typical rectifier diode. The silver band corresponds to the flat stripe on the diode schematic symbol above.
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That's because diodes have polarity, and only work in one direction. If a diode is oriented such that a positive voltage is applied to the black end (the anode) and the other end (the cathode) is brought to a lower potential, electrical current will flow across the diode. But if the voltage at the cathode is higher than the anode voltage, no current will flow and the diode acts like a break in the wire (up to a certain point - at high enough voltages the diode will break down and conduct even in reverse).

Diode Construction

The precursor to the diode is the rectifier vacuum tube. This device uses the fact that electrons are repelled by like charges to only allow current to flow one way across the tube. However, vacuum tubes are big, use a lot of power, and require a fragile vacuum cavity.

The diode does the same thing, but without needing a second voltage supply to run the heated filament. Diodes are all solid state, meaning no moving parts and no vacuum tubes. The standard pn junction diode contains two semiconductor materials that meet at a junction. One material is doped (given impurities) in such a way as to give it an excess of negative charges (n-type) and the other is doped in a way to give it an excess of positive charges (p-type). Put them together and you have a p-n junction, which is the core of important things like solar cells, LEDs, and regular diodes.

A tiny M7 surface mount diode on a USB battery charger. Notice that the cathode is marked as usual. This image is my own.

Current can only flow one way across this p-n junction, giving the diode it's primary characteristic.

What can diodes be used for?

The standard rectifier diode is incredibly useful.

An obvious application of diodes is to convert AC (alternating current, like what comes out of the wall) to DC (direct current, like what comes out batteries and USB ports). Alternating current typically involves a sine wave, where voltage alternates between positive and negative. Placing a diode in series with an alternating current output will cut off all of the negative voltages. The result is that only pulsed positive voltage comes out of the diode, letting you charge things like capacitors without having the negative parts of the AC signal discharge them shortly afterward.

4-diode full-wave rectifier found onboard a cheap LED bulb. This specific device converts AC 120V input from the wall into direct current for the LEDs. This image is my own.

Using four diodes, you can make something called a full-wave rectifier that will convert all of the AC pulses to positive voltage. Now, instead of every other voltage peak not even making it through the diode, all peaks come out, all with positive voltage. Here's an example of what a fully rectified output looks like:

Output from a full wave rectifier.
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Diodes can also be used to boost voltages. If you have an alternating current signal that you would like to raise the voltage of, you can use a single diode and a single capacitor to double the voltage (outputting pulsed direct current) using something called a voltage doubler.

Add more diodes and more capacitors and you can continue to multiply the voltage, creating what is called a CW multiplier. This technique is used in a lot of high voltage power supplies, including one that I personally own that goes up to 35 kV. It's also used in microwave ovens to drive the magnetron vacuum tube inside.

Example of a CW multiplier
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There are lots of other ways to use diodes. Some I haven't mentioned yet involve stopping batteries from discharging into the solar panels that charge them when the sun goes down and removing painful voltage spikes across coils when the power is turned off.

But there are also many types of diodes. I will probably cover some of these individually in the future, but here's a quick rundown:

• Light Emitting Diodes (LEDs) produce light when current flow across them. They can also be used to detect light.
• Solar Panels (yes, these are diodes) produce electrical power when put in sunlight.
• Zener Diodes, a diode that can reliably work when it breaks down (this occurs when you put a high reverse voltage across a diode).
• Schottky Diodes, diodes that turn on much lower voltages than regular silicon diodes.
• Gunn "Diodes", diode-like devices that can be used to make microwave transmitters.
• Tunnel Diodes that use quantum tunneling to switch on and off very quickly.
• Photodiodes, a half-way point between tiny LEDs and huge solar panels that can be used to detect light (or radiation!)
• Laser Diodes that form the core of cheap laser pointers. More powerful ones can melt things (including your eyes so don't build anything with these in it without proper protection).

There are also lots of different semiconductors to make diodes out of, all of which change the properties of the subsequent diode.

Diodes form the backbone of modern electronics along with transistors. They aren't quite as common as capacitors, but almost every electrical device will have at least a few diodes in it. If you're ever taking something apart and get curious, diodes can be easily identified as they typically look like black rectangles or cylinders with one end painted silver. Some other diodes will look like clear glass capsules that contain an orange and white device inside.

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Unfortunately it would be impossible to cover everything that diodes can do in one post (if I could even remember/research all of these uses!) as they are so versatile and widely used. I hope that I've been able to give you a little bit of a better understanding of how these devices work. Rest assured that whatever you are using to view this post online contains a huge amount of diodes.

Let me know if you have any questions and I'll do my best to answer. If this post was too technical (or not technical enough) please let me know so that I can better tune the way I write these explanations.

Thanks for reading!

For Additional Reading:
HyperPhysics PN Junction
SparkFun Diodes