Control Freak – How the Diode Works in Its Many Uses, Including the LED!

Welcome back, Component Captains! Today it’s time to level up your knowledge and move beyond simple passive components into the realm of semiconductor components. These parts come to life when wired into a circuit, and can manipulate electricity in many ways!  There are two semiconductor components that you’ll be working with, the diode and transistor. Today, we’ll be talking about the diode, the notorious control freak that only allows electricity to flow in one direction! If you’ve seen a LED in action, then you’re already well ahead of the game, let’s get started.

Control the Flow

The diode is well known for its ability to control the flow of electrical current in a circuit. Unlike passive components that sit idly by resisting or storing, diodes actively have their hands deep in the ebb and flow of current as it courses throughout our devices. There are two ways to describe how current will or won’t flow through a diode, and they include:

• Forward-Biased. When you insert a battery correctly into a circuit, then current will be allowed to flow through a diode; this is called a forward-biased state.
• Reverse-Biased. When you manage to slip a battery into a circuit backward, then your diode will block any current from flowing, and this is called a reverse-biased state.

An easy way to visualize the difference between forward-biased and reverse-biased states of a diode in a simple circuit

While these two terms might seem overly complicated, think of a diode as a switch. It’s either closed (on) and letting current flow through it, or it’s open (off), and no current can flow through.

Diode Polarity & Symbols

Diodes are polarized components, meaning that they have a very specific orientation that they need to be connected in a circuit to work correctly. On a physical diode, you’ll notice two terminals extending from a tin can shape in the middle. One side is the positive terminal, called the anode. The other terminal is the negative end, called the cathode. Going back to our flow of electricity, current can only move in a diode from the anode to the cathode, never the other way around.

You can spot the cathode side on a physical diode by looking for the silver strip near one of the terminals. (Image source)

You can easily spot a diode on a schematic, just look for the large arrow with a line running through it as shown below. Some diodes will have both their anode and cathode marked as positive and negative, but a simple way to remember which way current flows in a diode is to follow the direction of the arrow.

The arrow on a diode symbol indicates the direction that current will flow.

You’ll find most diodes these days made from two of the most popular semiconductor materials in electronics – silicon or germanium. But if you know anything about semiconductors, then you’ll know that in their natural state, neither of these elements conduct electricity. So how do we get electricity to flow through silicon or germanium? With a little magic trick called doping.

The Doping of Semiconductors

Semiconductor elements are strange. Let’s take silicon for example. It’s an insulator by day, but if you add impurities to it through a process called doping, then you give it the magical power to conduct electricity by night.

Because of their dual capabilities as both an insulator and conductor, semiconductors have found their perfect niche in components that need to control the flow of electric current in the form of diodes and transistors. Here’s how the doping process works in a typical piece of silicon.

• Grow it. First, silicon is grown in a tightly controlled laboratory environment. This is called a clean room, meaning that it’s free from dust and other contaminants.
• Dope it negatively. With silicon all grown up, it’s now time to dope it. This process can go one of two ways. The first is to dope silicon with antimony, which gives it a few extra electrons and allows silicon to conduct electricity. This is called n-type, or negative-type silicon because it has more negative electrons than usual.
• Dope is positively. You can also dope silicon in reverse. By adding boron to silicon, this removes electrons from the silicon atom, leaving a bunch of empty holes where the electrons should be. This is called p-type, or positive-type silicon.
• Combine it. Now that your pieces of silicon are both positively and negatively doped, you can put them together. By joining n-type and p-type silicon together, you create what’s called a junction.

It’s in this junction, which can be thought of like some kind of no man’s land, where all the magic in a diode happens. Let’s say you put n-type and p-type silicon together, and then connect a battery, creating a circuit. What will happen?

In this instance, the negative terminal is connected to the n-type silicon, and the positive terminal is connected to the p-type silicon. And the no man’s land space in between the two pieces of silicon? Well, it starts to shrink, and electric current starts to flow! This is the forward-biased state of a diode that we talked about in the beginning.

Connecting a battery properly to n-type and p-type silicon allows current to flow through the junction. (Image source)

Now, let’s say you connect your battery the other way around, with the negative terminal connected to the p-type silicon, and the positive terminal connected to the n-type silicon. What happens here is that the no man’s land between both pieces of silicon gets wider, and no current flows at all. This is the reverse-biased state that a diode can take on.

Hook a battery up in an unintended direction, and your diode will stop current from flowing between n-type and p-type. (Image source)

Forward Voltage & Breakdowns

When you work with diodes, you’ll come to learn that for one to allow current through, it requires a very specific amount of positive voltage. The voltage required to turn on a diode is called the forward voltage (VF). You might also see it referred to as cut-in voltage or on-voltage.

What determines this forward voltage? The semiconductor material and type. Here’s how it breaks down:

• Silicon Diodes. Using a silicon-based diode will require a forward voltage between 0.6 and 1V.
• Germanium Diodes. Using a germanium-based diode will require a lower forward voltage around 0.3V.
• Other Diodes. Specialized diodes like LEDs will require a higher forward voltage, whereas Schottky diodes (see below) will require a lower forward voltage. It’s best to check the datasheet for your specific diode to determine its forward voltage rating.

Now, I know we’ve been talking this whole time about diodes only allowing current to flow in one direction, but this rule can be broken. If you apply a huge negative voltage to a diode, then you can actually reverse the flow of its current! The specific amount of voltage that causes this reverse flow to occur is called the breakdown voltage. For normal diodes, the breakdown voltage is anywhere between -50V to -100V. Some specialized diodes are even designed to operate at this negative, breakdown voltage, which we’ll talk about later.

The Diode Family – Together at Last

There are a ton of diodes out there, each with their own specialized abilities. And while each shares a common foundation of restricting the flow of current, you can use this common basis to create a ton of different uses. Let’s check out each member of the diode family!

Standard Diodes

Your average diode. Standard diodes have a moderate voltage requirement and a low maximum current rating.

A standard, everyday diode available from Digi-Key, notice the silver strip that marks the cathode end. (Image source)

Rectifier Diodes

These are the beefier siblings of the standard diodes and have a higher maximum current rating and forward voltage. They are mainly used in Power supplies.

The beefier siblings of the standard diode, the difference being the larger current rating and forward voltage.

Schottky Diodes

This is the quirky cousin of the diode family. The Schottky diode comes in handy when you need to limit the amount of voltage loss in your circuit. You can identify a Schottky diode on a schematic by looking for your typical diode symbol with the addition of two new bends (‘S’ shape) on the cathode pin.

Look for the bends in the cathode end of the diode to quickly identify is as a Schottky.

Zener Diodes

The Zener diodes are the black sheep of the diode family. These guys are used to send electric current flowing in the opposite direction! They do this by taking advantage of the breakdown voltage that we discussed above, also called the Zener Breakdown. By taking advantage of this breakdown ability, Zener diodes are great at creating a stable reference voltage at a specific spot in a circuit.

The Zener Diode looks strikingly different from the rest of the diode family and can send current from cathode to anode. (Image source)

Photodiodes

Photodiodes are the rebellious teenagers of the diode family. Rather than simply allowing current to flow through a circuit, photodiodes capture energy from a light source and turning it into electrical current. You’ll find these for use in solar panels as well as optical communications.

Photodiodes take it all in, capturing energy from light and turning it into an electrical current. (Image source)

Light-Emitting Diodes (LEDs)

The shining stars of the diode family. Like standard diodes, LEDs only allow current to flow in one direction, but with a twist! When the proper forward voltage is applied, these LEDs light up with some brilliant colors. Here’s the catch though, specific colors of a LED require different forward voltages. For example, a blue LED requires a forward voltage of 3.3V, whereas a red LED only needs 2.2V to start shining.

What makes these LEDs so darn popular?

• Efficiency. LEDs make light electronically without pumping out a ton of heat like traditional incandescent light bulbs. This allows them to save a ton of energy.
• Control. LEDs are also super easy to control in an electronic circuit. As long as they have a resistor placed before them, then they’re bound to work!
• Inexpensive. LEDs are also very inexpensive and are built to last. That’s why you’ll find them used so much in traffic signals, displays, and for infrared signals.

LEDs come in a ton of different shapes and colors, each of which requires a different forward voltage to light up! (Image source)

The Most Common Uses for Diodes

Because diodes come in so many shapes, sizes and configurations their use in our electronic circuits are equally as rich! Here are just a few of the ways you’ll see diodes used:

Converting AC to DC

The conversion process of translating Alternating Current (AC) into Direct Current (DC) can only be handled by diodes! This process of rectifying (converting) current is what allows you to plug in all of your everyday DC electronics into the AC wall outlet in your house. There are two types of conversion applications that a diode plays its part in:

• Half-Wave Rectification. This conversion only requires one diode. If you send an AC signal into a circuit, then your sole diode will clip out the negative part of the signal leaving only the positive input as a direct current wave.

A single diode in a half-wave rectifier circuit clipping out the negative end of an AC signal. (Image source)

• Full-Wave Bridge Rectification. This conversion process uses four diodes. And rather than just clipping out the negative part of an AC signal like the half-wave rectifier, this process actually converts all of the negative waves in an AC signal into positive waves for a DC-ready signal.

The full-wave bridge rectifier takes things a step further, converting an entire AC’s positive and negative signal into DC. (Image source)

Controlling Voltage Spikes

You’ll also find diodes being used in applications where unexpected voltage spikes can occur. Diodes in these applications can limit any damage that might occur to a device by absorbing any excess voltage that falls into the range of a diode’s breakdown voltage.