What is a diode?

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We use a lot of different components in our electronics devices but diodes are usually the first non-linear component people use in a circuit. Whether it’s because they’re turning an AC signal into a DC signal or because they’re using a light-emitting diode to show something, diodes appear quite frequently and their non-linear characteristics can make things a bit challenging at first. We’ll go into depth about those challenges and how to overcome them later. For right now, let’s just get an overview of a diode, identify some important characteristics, and get a good intuitive understanding of how they work.

At its most simple, a diode is a device that allows current to flow one direction and not the other. Ideally, it lets current flow one direction without any impedance or affect whatsoever while completely stopping all current flow in the other direction, with a clear line between the two states. If there is a reverse voltage, zero current can flow, no matter how great the negative voltage drop is. If there is a forward voltage, infinite current can flow without any voltage drop.

Figure 1. Ideal Diode IV Curve
Figure 1. Ideal Diode IV Curve

Obviously, this is not the case, but we do sometimes use this “Ideal Diode Model” because it makes our life simpler, and who doesn’t like it when things are easy? The reality is that diodes don’t start conducting until a certain forward voltage is reached, typically, but not always .7 volts. And then the current spikes up quickly but doesn’t immediately go to infinity.

Figure 2. Typical Diode IV Curve
Figure 2. Typical Diode IV Curve

This is a little trickier to run the calculations so sometimes a compromise between the Ideal Model and “perfectly” true-to-life model, something we call the Constant Voltage Drop model, or CVD model. In this case, the way I always imagine it, you’re taking the ideal model but shifting the point where it starts to conduct. So, in the CVD model, if you have anything less than 0.7V forward voltage, nothing conducts, but anything more than 0.7V, you have infinite current through the model.

Figure 3. Constant Voltage Drop IV Graph
Figure 3. Constant Voltage Drop IV Graph

Even though working with diodes using the Ideal Diode Model, or even the CVD model, is easier than the real thing, the reality is, it’s still non-linear. Resistors, capacitors, and inductors have a linear IV curve, meaning that the current through them has a straight line relationship with the voltage across them.

Figure 4. Linear or Ohmic IV Graph
Figure 4. Linear or Ohmic IV Graph

Obviously, this is not the case with the diode and makes it so that when you’re solving a circuit with a diode in it, you need to make some assumptions, do the calculations, and then check your assumptions. We have a tutorial that goes over this in depth - if you’re ready to actually start solving some circuits, go check that out now.

Figure 5. Diode Schematic Symbol
Figure 5. Diode Schematic Symbol

You may already be familiar with the schematic symbol for a generic diode but it’s simply an arrow with a flat bar on one side. The side with the bar is the cathode and, at the semiconductor level, is n-doped. The other side is the anode and, at the semiconductor level, is p-doped. We have several tutorials that go into how that works and why that matters, and while not necessary to know how to use a diode, eventually you’ll need to understand it if your end goal is to be an electrical or electronics engineer.

There are some important terms that you’ll need to know if you want to talk about diodes or understand a datasheet on them.

  • Forward bias - when the voltage on the anode is higher than the cathode, and if the voltage is greater than the forward voltage, current will flow. Other than Zener diodes, this is the typical operating mode of a diode.
  • Reverse bias - when the voltage on the cathode is higher than the anode. When a diode is reverse biased, ideally, no current will flow.
  • Forward voltage (VF) - the voltage at which a diode starts conducting. This is quite small for Schottky diodes but gets quite large for large power diodes.
  • Reverse voltage (VR) - the voltage at which, when reverse biased, the diode starts conducting. This is not usually desirable, so you typically don’t want to exceed this voltage.
  • Breakdown - what happens if you reverse bias a diode past its reverse voltage. Not a problem with zener diodes but may cause permanent damage with other diodes.
  • Forward current - amount of forward current your diode can safely conduct. There’s usually a relationship between forward voltage and forward current.
  • Reverse current or leakage current - how much current your diode conducts when it is reverse biased and supposedly not conducting. You want this number to be as low as possible, but there are always trade-offs.

And there are a few different types of diodes that you’ll hear about, summarized here:

  • Schottky Diodes - have a lower forward voltage and are more responsive but have a lower reverse breakdown voltage.
  • Zener Diodes - are designed to be reverse biased and have a very specific breakdown voltage.
  • Light Emitting Diodes - create light when there is a forward current.
  • Photodiodes - create electrical power when exposed to light.
  • Signal diodes - general diodes for lower current applications (less than 1 amp).
  • Power diodes - general diodes for higher current applications (1 amp or greater). These typically have a slightly higher forward voltage than signal diodes. The line between signal and power diodes is fuzzy and not clearly defined.

So we're done with the basics of diode and I hope this tutorial helps you understand how diodes work. If you’ve found this tutorial interesting or helpful, give it a like and if you have any questions, leave it in the comments below. See you in our next tutorial!

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