Join Barron Stone for an in-depth discussion in this video What is a diode?, part of Electronics Foundations: Semiconductor Devices.
- [Instructor] The simplest semiconductor component is called a diode. Which consists of a piece of N-Type and a piece of P-Type material joined together. The junction between those two different types of materials behaves in a way that allows electrons to easily pass from the N-Type region to the P-Type region with just a small amount of voltage to push them but if the direction of that voltage is reversed, the diode makes it very difficult for electrons to flow in the other direction, from P to N.
The electrons need a lot of voltage to push them the other way. To use a diode, you don't need to understand why the PN junction behaves that way and the low-level physics that are going on here. So for this course, I'm just going to focus on how to use diodes for practical applications. I like to think of a diode like a turnstile that you might encounter at the entrance to a sports stadium or a subway. People pass through the turnstile in the same way that electrons pass through a diode.
When somebody enters through a turnstile in the correct direction, they simply push on the arm and it moves to allow them through. But if they try to go through the turnstile in the wrong direction, that arm locks up and blocks them. That one directional behavior makes diodes useful for controlling how current flows through parts of a circuit in the same way that turnstiles are used to control how people enter and exit a building. The schematic symbol for a basic diode look like a triangle with a bar attached to the tip.
The side at the base of the triangle corresponds to the P-Type section of the diode and is called the anode. And the side with the bar is called the cathode and corresponds to the N-Type material. The triangle acts like an arrow that points in the direction that current can flow through the diode from anode to cathode. When building a diode circuit, it's important to pay close attention to the polarity or direction of the diode to make sure it's inserted in the correct orientation.
Diodes often come in small axial packages with two metal leads like this one. There's usually a small marking on the package to indicate which side of the diode is the cathode. Now, I will admit, I tend to get the terms cathode and anode mixed up in my head all the time. And I can never seem to remember which one is the positive one and which one is negative. So instead of remembering that the marking on a diode corresponds to the cathode, I think of that marker line as representing the bar on the diode symbol.
Because I can easily remember which way current flows through the diode. A diode has two basic modes of operation called forward biased and reverse biased. Which basically mean that the diode is on or off. Whether a diode is forward or reverse biased will depend on the voltage across it and the current through it. It's common to plot the relationship between a diode's voltage and current to characterize its performance on what's known as an I-V curve.
On a I-V curve, the voltage across the diode is represented on the horizontal axis and the amount of current passing through it is represented on the vertical axis. A diode will become forward biased or turn on when a positive voltage is applied to push current through the diode in its forward direction. In an ideal world, a forward biased diode should act like a short circuit and allow current to flow through it, completely unrestricted. Since an ideal diode does not put up any resistance to that forward current, there won't be any voltage across it.
It's just acting as a pass through for the current. So, on the I-V curve the forward-biased region is represented by a vertical line when the voltage is equal to zero. No matter how much forward current flows through that diode, the voltage across it will never be greater than zero volts. If I flip the diode around to apply a negative voltage across it, which tries to push current through the diode in the other direction, then the diode will become reverse biased.
It basically turns off and acts like an open circuit to prevent any reverse current from passing through it. On the I-V curve for an ideal diode that reverse-biased region is represented by this horizontal line. No matter how much negative voltage I apply to the diode, it will not allow any current to pass through it. This plot represents the ideal behavior for a diode but in reality, the relationship between voltage and current through a diode looks more like this.
On the I-V curve for an actual diode the forward and reverse regions of operation look similar to the ideal diode regions, they're just slightly off. But there's also a new third distinct region of operation to the left called the breakdown region. Focusing in on the forward region which represents a diode's behavior when a positive voltage is applied across it. Unlike the ideal diode model, which allows forward current to flow completely unrestricted, an actual diode requires a small amount of voltage to be applied to it before it really opens up and allows current to flow through.
The forward voltage, labeled as Vf, is the voltage at which the diode will become forward biased and turn on. And for basic silicon diodes, Vf is usually around 0.7 volts. You can relate the forward voltage of a diode to how hard you need to push on a turnstile to get through. The arm on a turnstile is spring-loaded, so it puts up a small amount of resistance. So you have to give it a little push or voltage to move it aside so you can pass through in the forward direction.
When voltage is applied in the other direction, the diode becomes reverse biased where it turns off and prevents current from passing through. That's similar to when you try to go the wrong way through a turnstile. The arm locks up. Even if you push really hard on it, it won't budge. The turnstile is reverse biased. But if you push really, really, really hard on the turnstile, eventually you will break the arm and then you can pass through the turnstile in the wrong direction. And that corresponds to the breakdown region of the I-V curve.
If a large enough voltage is applied to the diode in the reverse direction, the diode will eventually turn on and allow reverse current to flow through it. This breakdown voltage, abbreviated as VBR, can be as low as negative a hundred volts or more for standard silicon diodes. Now, unlike the turnstile arm example, when a diode enters the breakdown region it's not necessarily broken. Operating in the breakdown region itself does not damage the diode.
However, under breakdown conditions the diode won't be limiting the amount of current through it. That means unless there's something else in the circuit to limit current through the diode, it can easily overheat and burn up. The same thing can happen to the diode when it's operating in the forward biased mode. For that reason, it's important to design your circuits in a way that will limit the current through the diodes operating in the forward biased or breakdown modes to prevent them from overheating.
- Semiconductor materials
- Diode applications
- Rectifying a signal
- Detecting the signal peak
- Protecting against large signals, reverse current, and flyback voltage
- Special purpose zener diodes, Schottky diodes, and photodiodes
- NPN and PNP bipolar junction transistors
- Using a BJT as a switch
- Field effect transistors
- Differences between BJTs and MOSFETs
- Operational amplifiers
- Op-amp applications
- Comparing signals
- Buffering signals
- Amplifying signals
- Filtering signals
- Combining signals