Intro: MOSFET Basics
In this Instructable, I’ll teach you the basics of MOSFETs, and by basics, I mean really basics. This video is ideal for a person who has never studied MOSFET professionally, but want to use them in projects. I’ll talk about n and p channel MOSFETs, how to use them, how they are different, why both are important, why MOSFET drivers and things like that. I will also talk about some little known facts about MOSFETs and much more.
Let’s get into it.
Step 1: Watch the Video.
The videos has everything covered in detail required for building this project. The video has some animations which will help in quickly grasping the facts. You can watch it if you prefer visuals but if you prefer text, go through the next steps.
Step 2: The FET.
Before starting MOSFETs, let me introduce you to its predecessor, the JFET or Junction Field Effect Transistor. It will make understanding the MOSFET a little easier.
The cross section of a JFET is shown in picture. The terminals are identical to MOSFETs terminals. The center part is called the substrate or body, and it’s just an n type or p type semiconductor depending on the type of the FET. The regions are then grown on the substrate having opposite type than that of the substrate are named gate, drain and source. Whatever voltage you apply, you apply to these regions.
Today, from practical point of view, it has very little to no importance. I won’t go for more explanation beyond this as it will get too technical and is not required anyways.
The symbol of JFET will help us to understand the symbol of MOSFET.
Step 3: The MOSFET.
After this comes the MOSFET, having a major difference in the gate terminal. Before making the contacts for the gate terminal a layer of Silicon Dioxide is grown above the substrate. This is the reason it is named Metallic Oxide Semiconductor Field effect Transistor. SiO2 is a very good dielectric, or you can say insulator. This increases the gate resistance in the scale of ten to the power ten ohms and we assume that in a MOSFET gate current Ig is always zero. This is the reason why it’s also called Insulated Gate Field Effect Transistor (IGFET).
A layer of a good conductor like aluminium is grown additionally above all the three regions, and then contacts are made. In the gate region, you can see that a parallel plate capacitor like structure is formed and it actually introduces a considerable capacitance to the gate terminal. This capacitance is called gate capacitance and can easily destroy your circuit if not taken in to account. These are also very important while studying on a professional level.
The symbol for MOSFETs can be seen in the picture attached. Placing another line on the gate makes sense while relating them to the JFETs, indicating the gate has been insulated. The arrow direction in this symbol depicts the conventional direction of electron flow inside a MOSFET, which is opposite to that of the current flow
Step 4: MOSFET Are a 4 Terminal Device?
One more thing I’d like to add is that most people think MOSFET is a three terminal device, while actually MOSFETs are a four terminal device. The fourth terminal is the body terminal. You might have seen the symbol attached for MOSFET, the center terminal is for the body.
But why almost all the MOSFETs have only three terminal coming out of it?
The body terminal is internally shorted to the source as it is of no use in the applications of these simple ICs, and after that the symbol becomes the one we are familiar with.
The body terminal is generally used when a complicated CMOS technology IC is fabricated. Keep in mind that this is the case for n channel MOSFET, the picture will be a bit different if the MOSFET is p channel.
Step 5: How It Works.
Ok, so now let’s see how it works.
A Bipolar Junction Transistor or a BJT is a current controlled device, that means the amount of current flow in its base terminal determines the current that will flow through the transistor, but we know that there is no role of current in MOSFETs gate terminal and collectively we can say that it is a voltage controlled device not because gate current is always zero but because of its structure which I’ll not explain in this Instructable because of its complicacy.
Let’s consider an n Channel MOSFET. When no voltage is applied in the gate terminal, two back to back diodes exists between the substrate and drain and source region causing the path between drain and source to have a resistance in the order of 10 to the power 12 ohms.
I grounded the source now and started increasing the gate voltage. When a certain minimum voltage is reached, the resistance drops and the MOSFET starts conducting and the current starts to flow from drain to source. This minimum voltage is called threshold voltage of a MOSFET and the current flow is due to the formation of a channel from drain to source in the substrate of the MOSFET. As the name suggests, in an n Channel MOSFET, the channel is made up of n type of current carriers i.e. electrons, which is opposite of the type of the substrate.
Step 6: But...
It has only started here. Applying the threshold voltage does not mean you are just ready to use the MOSFET. If you look at the data sheet of IRFZ44N, an n channel MOSFET, you will see that at its threshold voltage, only a certain minimum current can flow through it. That is good if you just want to use smaller loads like LEDs only, but, what is the point then. So for using bigger loads that draw more current you will have to apply more voltage to the gate. The increasing gate voltage enhances the channel causing more current to flow through it. To completely turn on the MOSFET, the voltage Vgs, which is the voltage between gate and source must be somewhere about 10 to 12 Volts, that means if the source is grounded, the gate must be at 12 Volts or so.
The MOSFET we just discussed are called enhancement type MOSFETs for the reason that the channel gets enhanced with increasing gate voltage. There is another type of MOSFET called depletion type MOSFET. The major difference is in the fact that channel is already present in the depletion type MOSFET. These type of MOSFETs is usually not available in markets. The symbol for depletion type MOSFET is different, the solid line indicates that channel is already present.
Step 7: Why MOSFET Drivers?
Now let’s say you are using a microcontroller to control the MOSFET, then you can only apply a maximum of 5 Volts or less to the gate, which will not be enough for high current loads.
What you can do is use a MOSFET driver like TC4420, you just have to provide a logic signal at its input pins and it will take care of the rest or you can build a driver yourself, but a MOSFET driver has a lot more advantages in the fact that it also takes care of several other things like the gate capacitance etc.
When the MOSFET is completely turned on, its resistance is denoted by Rdson and can be easily found in the datasheet.
Step 8: The P Channel MOSFET
A p channel MOSFET is just the opposite of the n channel MOSFET. The current flows from source to drain and the channel is made up of p type of charge carriers, i.e. holes.
The source in a p channel MOSFET must be at the highest potential and to completely turn it on Vgs must be negative 10 to 12 Volts.
For example, if source is tied to 12 Volts the gate at zero volts must be able to completely turn it on and that is why we generally say applying 0 Volts to the gate turn a p channel MOSFET ON and due to these requirements the MOSFET driver for n channel cannot be used directly with p channel MOSFET. The p channel MOSFET drivers are available in the market (like TC4429) or you can simply use an inverter with the n channel MOSFET driver. The p channel MOSFETs have relatively higher ON resistance than n channel MOSFETs but that doesn’t mean you can always use an n channel MOSFET for any possible applications.
Step 9: But Why?
Let’s say you have to use the MOSFET in the first configuration. That type of switching is called low side switching because you are using the MOSFET to connect the device to ground. An n channel MOSFET would be best suited for this job as Vgs is not varying and can be easily maintained at 12 Volts.
But if you want to use an n channel MOSFET for high side switching, the source can be anywhere between ground and Vcc, which will eventually affect the voltage Vgs as gate voltage is constant. This will have a huge impact on the proper functioning of the MOSFET. Also the MOSFET burns out if the Vgs exceeds than the mentioned maximum value which is around 20 Volts on an average.\
Hence, it is not a cake walk to use n channel MOSFETs here, what we do is we use a p channel MOSFET despite having a greater ON resistance as it has the advantage that Vgs will be constant throughout during a high side switching. There are also other methods like bootstrapping, but I'll not be covering them for now.
Step 10: Id-Vds Curve.
Lastly, let’s take a quick look at these Id-Vds curve. A MOSFET operated on three regions, when Vgs is less than the threshold voltage, the MOSFET is in cut off region, i.e. it is off. If Vgs is greater than the threshold voltage but less than the sum of voltage drop between drain and source and threshold voltage, it is said to be in triode region or linear region. In liner region, a MOSFET can be used as a voltage variable resistor. If Vgs is greater than the said voltage sum, then the drain current becomes constant it is said to be working in saturation region and to make the MOSFET act as a switch it should be operated in this region as the maximum current can pass through the MOSFET in this region.
Step 11: Parts Suggestions.
Step 12: That's It.
You must now be familiar with the basics of MOSFETs and able to decide the perfect MOSFET for your project.
But a question still remains, when should we use MOSFETs?
The simple answer is when you have to switch bigger loads that require more voltage and current. MOSFETs have the advantage of minimum power loss compared to BJTs even at higher currents.
If I missed anything, or am wrong, or you have any tips, please comment below.
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