Diodes 101: What do they do and how do they work?

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Diodes are critical safeguards that protect sensitive components in the event of unexpected power surges, but how do they work? How do you use diodes? And whose diodes can you trust?

What is a diode?

At the most basic level, a diode is a component that creates a one-way street for electric currents flowing through a circuit. By funneling and blocking these currents, diodes keep power surges and other unplanned effects from damaging sensitive components. They also play essential roles in signal processing, especially in radio-frequency applications.

The first diodes were invented in the early 20th Century by wireless telegraph companies. From there, diodes rose to become essential components of early radio technology — believe it or not, today’s digital devices wouldn’t be possible without these small (but mighty) components.

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What does a diode do?

Diodes are invaluable assets in modern technology because they only allow currents to pass in one direction. When the current flows forward from the diode’s anode to its cathode, the diode acts like a bare wire and permits the current to go through. When that same current flows in the opposite direction, the diode becomes a powerful resistor and blocks passage.

Therefore, protecting electronics is one of the most common uses for diodes.

Electromagnetic components, like motors and relays, release built-up charges when turned off, sending currents back through a circuit where they can damage sensitive parts. A diode placed across motors and relays will channel that current away from the rest of the circuit.

Processing radio signals (the original reason diodes were invented) are still a common use of diodes, even today. Branching beyond old-school AM radios, diodes are also integrated into a variety of modern devices, ranging from television sets to microwave ovens and even to communication satellites.

How does a diode work?

Today’s diodes are made from semiconductor materials such as silicon, germanium, or gallium arsenide. Semiconductor manufacturers introduce specific impurities into these materials to give a diode its particular properties.

This “doping” process creates three regions in the diode. First, there’s a positively-charged p-region at the anode where doping creates a large number of electron holes. Then, there’s a negatively-charged n-region at the cathode, where doping creates a large number of free electrons. The two meet at the p-n junction where n-region electrons jump over to fill the p-region’s electron holes — this creates a third “depletion region” which acts as an insulator and resists current flow.

Forward-bias

The diode is “forward-biased” when the current flows from the positive anode to the negative cathode. In the ideal case, the current eliminates the depleted region and passes through the diode as if it wasn’t there.

In reality, the diode needs an initial voltage, called the forward voltage (Vf), to get working. In effect, a real diode acts like an ideal diode in serial with a resistor to create a small, but noticeable voltage drop. In the most common signal diodes, Vf is about 0.7 V.

Something to watch out for though is temperature — a diode’s Vf will shrink as the temperature rises. In some cases, you can turn that temperature sensitivity into a feature. For example, an article on Hackaday explains how to use this characteristic to make a single-diode temperature sensor.

Reverse-bias

When current flows through the diode in the opposite direction, from cathode to anode, it’s called “reverse-biased.” The aforementioned depleted region grows even larger, turning the diode into a powerful resistor and blocking the current flow.

Every electronic component has a limit however, and diodes aren’t an exception. A diode’s threshold is determined by what’s known as a breakdown voltage (Vbr). Past this Vbr the depleted region stops working, freeing the reverse current to flow through the circuit and wreak havoc — often resulting in a blue smoke-related burnout.

Diode symbol

The symbol for a diode resembles a sideways triangle with a vertical bar at the tip. The triangle points in the direction of the forward-bias current and the bar represents the diode’s blocking action when current is reverse-biased.

Since diodes are polarized, you need to know which way to put them into your circuits. Because of this, the diode’s packaging will have a band at the cathode end just like the vertical bar in the diode symbol.

What kinds of diodes are there?

Diodes in electronic circuit design have two main differentiations: power diodes and signal diodes. Within those broad categories, you’ll find all sorts of sub-specialized diodes created to handle specific tasks. Certain diodes can also generate light — aptly titled Light-Emitting Diodes, or LEDs, as they’re more popularly known by — but that’s a big enough topic for its own article.

Power diodes

Surge suppression, as we talked about earlier, is one method of using power diodes. Besides blocking inductive surges from components like motors, you can also use diodes to prevent electrostatic discharge (ESD). This is the spark you sometimes see when you plug your charger into your phone — that little flash sends an instantaneous spike of 15,000 volts straight into your device, and it’s an ESD protection diode that keeps your cherished phone from frying.

Another everyday use of power diodes is found within your phone’s charger. Power diodes help convert the alternating current (AC) coming out of the wall into the direct current (DC) that receiving electronics use.

Signal diodes

Signal diodes deal with much smaller currents and voltages. These type of diodes are used in circuits that process analog signals, and in some cases, these applications help the rest of the circuit do its job better.

A fun experiment to showcase how signal diodes work is with a DIY AM radio, which consists of some wire, a metal stake, a diode, and an earpiece. Using the ungrounded wire as an antenna converts the incoming radio wave into an alternating current. Just like the incoming radio wave, that current has a constant frequency of fast-paced swings between voltage peaks and troughs. The size of those peaks and troughs (AKA the amplitude) is the signal that matters, unfortunately, on its own, those waves tend to cancel each other out in the earphone. Incorporating a diode however, block the troughs and allow the peaks to pass through for the earphone to convert into an audio signal.

Another way diodes process analog signals is to protect delicate electronics from signals that get too strong. The diodes in a clipper circuit, for example, will cut off signals once the voltage goes beyond a certain point. Depending on the circuit’s design, the clipper circuit will cap positive voltages, negative voltages, or both.

Instructables contributor gmoon showcases how a diode clipper circuit can give a guitar amp a more tube-like sound.

Special diodes

Manufacturers can select materials to give diodes specific properties to make circuits perform better.

A Schottky diode, for example, has a very low forward voltage. Where a typical signal diode has a Vf of 0.7 V, a Schottky diode’s Vf could get as low as 0.15 V. Hobby electronic supplier SparkFun recommends Schottky diodes “when every last bit of voltage must be spared.”

Zener diodes are a kind of anti-diode. Manufacturers design Zener diodes to allow reverse currents to flow through without destroying the diode. The diode will block a reverse-biased current as long as the voltage stays below a certain point. After that, the Zener diode lets currents through at a constant Zener voltage (Vz).

Who makes diodes?

According to market research firm Technavio, sales of discrete diodes alone generate more than $3 billion a year. The laser diode market is worth another $55 billion. That says something about how valuable these tiny components are to modern electronics.

Unlike some industries however, the diode market is fragmented and therefore not dominated by a handful of big players. Some know reliable suppliers of quality diodes include:

These companies sell to manufacturers around the world, but their products are available in smaller quantities from resellers like Mouser Electronics or DigiKey Electronics. Lesser-known diode manufacturers like Chanzon and T&F Electronics have good reputations among Amazon customers.

Who is your recommended source for high-quality diodes? What features do you look for in securing a reputable diode supplier? Share with us in the comments below!