Learn how the diode works to control the flow of electric current in a circuit with the use of n-type and p-type semiconductors.
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 cover the diode, the notorious control freak that only allows electricity to flow in one direction! If you’ve seen a LED in action, 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:
- Forward-Biased: When you insert a battery correctly into a circuit, current will be allowed to flow through a diode; this is called a forward-biased state.
- Reverse-Biased: When you slip a battery into a circuit backward, your diode will block any current from flowing, which is called a reverse-biased state.
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 open (off), and no current can flow through.
Diode Polarity & Symbols
Diodes are polarized components, meaning they have a very specific orientation that needs 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. Current can only move in a diode from the anode to the cathode, never the other way around.
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.
You’ll find most diodes these days made from two of the most popular semiconductor materials in electronics – silicon or germanium. If you know anything about semiconductors, then you’ll know that neither of these elements conducts electricity in their natural state. 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. However, if you add impurities to it through a process called doping, 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:
- First, silicon is grown in a tightly controlled laboratory environment. This is called a clean room, meaning it’s free from dust and other contaminants.
- 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.
- You can also dope silicon in reverse. Adding boron to silicon removes electrons from the silicon atom, leaving empty holes where the electrons should be. This is called p-type or positive-type silicon.
- 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 a junction.
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 connects to the n-type silicon, and the positive terminal connects to the p-type silicon. And the no man’s land space between the two pieces of silicon? Well, it starts to shrink, and the electric current starts to flow! This is the forward-biased state of a diode that we discussed initially.
Suppose 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.
Forward Voltage & Breakdowns
When you work with diodes, you’ll come to learn that for one to allow current through, it requires a particular amount of positive voltage. The voltage needed 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 require a higher forward voltage, whereas Schottky diodes (see below) require a lower forward voltage. It’s best to check the datasheet for your specific diode to determine its forward voltage rating.
I know we’ve been talking about diodes only allowing current to flow in one direction, but you can break this rule. 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. The breakdown voltage for normal diodes is between -50V to -100V. Some specialized diodes can even operate at this negative, breakdown voltage, which we’ll talk about later.
Unpacking the Diode family
There are a ton of diodes out there, each with its 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!
Your average diode. Standard diodes have a moderate voltage requirement and a low maximum current rating.
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.
This is the quirky cousin of the diode family. The Schottky diode comes in handy when you need to limit the voltage loss in your circuit. You can identify a Schottky diode on a schematic by looking for your typical diode symbol by adding two new bends (‘S’ shape) on the cathode pin.
The Zener diodes are the black sheep of the diode family. They send electric current flowing in the opposite direction! They do this by taking advantage of the breakdown voltage 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.
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 turn it into electrical current. You’ll find these for use in solar panels and optical communications.
Light-Emitting Diodes (LEDs)
LEDs are 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 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 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 affordable and last. That’s why you’ll find them used so much in traffic signals, displays, and infrared signals.
The three most common uses for diodes
Because diodes come in many shapes, sizes, and configurations, their use in our electronic circuits is equally as rich! Here are just a few of the ways you’ll see diodes used:
1. 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 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, your sole diode will clip out the negative part of the signal leaving only the positive input as a direct current wave.
- 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 converts all negative waves in an AC signal into positive waves for a DC-ready signal.
2. Controlling Voltage Spikes
Diodes can also be 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.
3. Protecting Your Current
Last, you’ll also find that diodes can protect sensitive circuits. If you’ve ever smashed a battery in the wrong way and nothing blew up, you can thank your friendly diode. Placing a diode in series with the positive side of a power supply ensures that current only flows in the right direction.
Get started with diodes
There you go, the control freak diode and all of its crazy family members! Diodes have many uses, from powering those colorful LED lights to converting AC to DC. But why hasn’t the diode received as much publicity as the transistor or integrated circuit? We think it comes down to too many cooks in the kitchen. The first diode was discovered nearly 150 years ago, and since then, hundreds of engineers and scientists have had their hands in the mix to improve this discovery. Despite its long history with many personalities, many people still consider the diode to be the fourth most important invention since the wheel.
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