How To Use MOSFET – Beginner’s Tutorial

In this article, I will go through some common questions people tend to raise when they are using MOSFET, and the basics of MOSFET. This tutorial is primarily written for non-academic people, so I will try to minimize the theory part and mainly focus on the practical side of things. However if you are into the theory how MOSFET work, I will share some useful academic articles and resources at the end of this post. MOSFET has some advantage and disadvantage over BJT, so choose carefully depends on your application.

What is MOSFET?

MOSFET stands for metal-oxide semiconductor field-effect transistor. It is a special type of field-effect transistor (FET).

Unlike BJT which is ‘current controlled’, the MOSFET is a voltage controlled device. The MOSFET has “gate“, “Drain” and “Source”  terminals instead of a “base”, “collector”, and “emitter” terminals in a bipolar transistor. By applying voltage at the gate, it generates an electrical field to control the current flow through the channel between drain and source, and there is no current flow from the gate into the MOSFET.

electronics_mosfet_schematic

A MOSFET may be thought of as a variable resistor, where the Gate-Source voltage difference can control the Drain-Source Resistance. When there is no applying voltage between the Gate-Source , the Drain-Source resistance is very high, which is almost like a open circuit, so no current may flow through the Drain-Source. When Gate-Source potential difference is applied, the Drain-Source resistance is reduced, and there will be current flowing through Drain-Source, which is now a closed circuit.

In a nutshell, a FET is controlled by the Gate-Source voltage applied (which regulates the electrical field across a channel), like pinching or opening a straw and stopping or allowing current flowing. Because of this property, FETs are great for large current flow, and the MOSFET is commonly used as a switch.

Okay, let me summarize the differences between BJT and MOSFET.

  • Unlike bipolar transistors, MOSFET is voltage controlled. While BJT is current controlled, the base resistor needs to be carefully calculated according to the amount of current being switched. Not so with a MOSFET. Just apply enough voltage to the gate and the switch operates.
  • Because they are voltage controlled, MOSFET have a very high input impedance, so just about anything can drive them.
  • MOSFET has high input impedence.

How to use MOSFET as a switch?

To use a MOSFET as a switch, you have to have its gate voltage (Vgs) higher than the source. If you connect the gate to the source (Vgs=0) it is turned off.

For example we have a IRFZ44N which is a “standard” MOSFET and only turns on when Vgs=10V – 20V. But usually we try not to push it too hard so 10V-15V is common for Vgs for this type of MOSFET.

However if you want to drive this from an Arduino which is running at 5V, you will need a “logic-level” MOSFET that can be turned on at 5V (Vgs = 5V). For example, the ST STP55NF06L. You should also have a resistor in series with the Arduino output to limit the current, since the gate is highly capacitive and can draw a big instantaneous current when you try to turn it on. Around 220 ohms is a good value.

This page shows some detail explanation how a MOSFET works as a switch. This page shows some advanced usage of MOSFET.

Types of MOSFET

MOSFETs come in four different types. There are three main categories we need to know.

  • N-Channel (NMOS) or P-Channel (PMOS)
  • Enhancement or Depletion mode
  • Logic-Level or Normal MOSFET

MOSFETS

N-Channel – For an N-Channel MOSFET, the source is connected to ground. To turn the MOSFET on, we need to raise the voltage on the gate. To turn it off we need to connect the gate to ground.

P-Channel – The source is connected to the power rail (Vcc). In order to allow current to flow the Gate needs to be pulled to ground. To turn it off the gate needs to be pulled to Vcc.

Depletion Mode – It requires the Gate-Source voltage ( Vgs ) applied to switch the device “OFF”.

Enhancement Mode – The transistor requires a Gate-Source voltage ( Vgs ) applied to switch the device “ON”.

Despite the variety, the most commonly used type is N-channel enhancement mode.

There are also Logic-Level and Normal MOSFET, but the only difference is the Gate-Source potential level required to drive the MOSFET.

How does it work in theory?

I will try to explain it in the simplest way I can, for more detail or if you are in doubt, check the references and links I provide at the bottom of the post.

MOSFET is a voltage controlled field effect transistor that differs from a JFET. The Gate electrode is electrically insulated from the main semiconductor by a thin layer of insulating material (glass, seriously!). This insulated metal gate is like a plate of a capacitor which has an extremely high input resistance (as high as almost infinite!). Because of the isolation of the Gate there is no current flow into the MOSFET from Gate.

When voltage is applied at the gate, it changes the width of the Drain-Source channel along which charge carriers flow (electron or hole). The wider the channel, the better the device conducts.

The MOSFET are used differently compared to the conventional junction FET.

  • The infinite high input impedance makes MOSFETs useful for power amplifiers. The devices are also well suited to high-speed switching applications. Some integrated circuits contain tiny MOSFETs and are used in computers.
  • Because the oxide layer is so thin, the MOSFET can be damaged by built up electrostatic charges. In weak-signal radio-frequency  work, MOSFET devices do not generally perform as well as other types of FET.

FAQ

Where to put the load to a MOSFET? Source or Drain?

Because load  has resistance, which is basically a resitor. For N-channel MOSFET the reason we usually put the load at the Drain side is because of the Source is usually connected to GND. 

If load is connected at the source side, the Vgs will needs to be higher in order to switch the MOSFET, or there will be insufficient current flow between source and drain than expected.

Heat Sink connected to the Drain?

Typically the heat sink on the back of a MOSFET is connected to the Drain! If you mount multiple MOSFETs on a heat sink, they must be electrically isolated from the heat sink! It’s good practice to isolate regardless in case the heat sink is bolted to a grounding frame.

What is the Body Diode For?

MOSFETs also have an internal diode which may allow current to flow unintentionally. The body diode will also limit switching speed. You don’t have to worry about it if you are operating under 1Mhz.

More Technical Resources and Academic information about MOSFET (Updating)

Understanding Modern Power MOSFETs

Theory behind MOSFET (Youtube Video Lecture)

25 thoughts on “How To Use MOSFET – Beginner’s Tutorial

  1. tony

    i want to repair heater speed control from a peugeot its only running at full speed ,its not the resister type looks digital.
    i think its got a mosfit on a large heat sink. am i right . tony

    Reply
  2. Nazrul Islam

    hello I need the information that silica sol or colloidal silica and mosfet with water what will be make would this intimate the volt and charge time delay pls. inform

    Reply
  3. bob

    Actually it’s the VGS(th) Gate Threshold Voltage of 2.0 to 4.0v on the IRFZ44N not the Vgs=10V – 20V for switching you should be looking at.

    Reply
  4. hernan

    hi OSCAR, thank you for this great discussion you’ve made.
    it really helps me a lot about understanding how MOSFET works…

    keep up the good work.
    Godbless..

    Reply
  5. Harish NR

    This is the components failure in variableDC power supply 0-32v &0-2A

    When I switched on source turn little bit preset ,our DC source. O/p shows around 50v
    And another problem DC voltage is ok, but current option not working please guid me which components failure & how to solve the problem
    Kindly reply ASAP

    Reply
  6. Reza

    I found Circuit on web, where use some 11N65C3 NChannel mosfet for RGB Led Color organ with music.

    But, I Don’t found this Mosfet. So nnow which Mosfet I should use for 5M RGB LED Strip.

    Reply
  7. Tomas

    I want a 12v control voltage to switch off a 5v supply. P-channel depletion seems to be the dogs bollox, but the devices I find require negative current to switch on in this configuration.

    Reply
  8. gerrit

    Gooday. Please can you assist me. I have parrel 10 IRFe50 mosfets on my power supply I’m busy building but when I put power dc on they don’t go on they not even gettig hot. My design is a 12v to 220v inverter. Can you maybe tell me how excact to connect them in parralel to switc on.

    Reply
  9. Sparklini

    This was very usefull. Really liked how you explained the difference between transitors and mosfet!

    I kept adding gate resistances because they were getting hot not realizing that was the problem to begin with.

    Thanks for writing this up.

    Reply
  10. Peter

    Hi Oscar
    I’ve never used a mosfet yet so no experience of using these devices and limited knowledge of anything electronic. I found your information very informative, but can you help me understand what the impedance does? I’m not sure I know what that means.
    You suggest using 220 resistor when using with a 5v rail. but the device is voltage driven with infinite impedance. so i think this means the current draw will be 0 A. what purpose is the resistor?

    Thanks
    Peter

    Reply
    1. Sean

      Peter,

      the resistor is to stop “in-rush current” you are correct that there is an enormous impedance when the device is “on” but during startup the impedance is capacitive (where the impedance can be worked out as 1/2*PI*Frequency*capacity), you can see with a large capacity resistance you end up with a very small impedance. Take that impedance and put it into ohms law “I=V/R” and you can see that current will be very large hence put a resistor there!

      Hope that helps.

      Reply
  11. H900

    Hi Oscar MY DEAR Friend!!!
    I was wondering why as example of transistor to pilot the power MOSFET you are proposing STP55NF06L, which is quite expansive and it’s a power MOSFET itself. Is there a specific reason to not using a very cheap low power MOS, instead? We only need to provide 10 or 12 volt on the MOSFET gate, no current is needed.
    Please correct me if I’m wrong.

    Regards

    Reply
  12. phu dinh

    Hi Oscar!!
    Very good education and easy to understand but hard to remember them.
    So for” the most commonly used type is N-channel MOSFET enhancement mode” you see the arrow go inside circle: from “Starter” of arrow ~(Source) to the “Header” of arrow~(gate); two of parallel line || just like “Capacitor” means no current please but only Voltage apply in Gate to control; also cause almost no current ampere ~(“The transistor requires a Gate-Source voltage ( Vgs ) applied to switch the device “ON”.”). so Gate of “N-channel” MOSFET ~negative ~ground to Gate and Source, then the last connector is the power rail (+)~ Drain and Loader. easy to remember that since how about other??? you guest similar same thing.

    Reply
  13. Jim

    Nicely done, without clouding the issue. Makes it a good jumping off point for a more definitive study of MOSFETs.
    Thanks.

    Reply
  14. Adamson

    I have a challenge with an electrolux washer W4180H.The motor is supplied via a motor control unit (MTU) which takes an upnut voltage of single phase and outputs three phase to drive the motor at 190volts AC.The variable speeds are controlled by the MCU.
    The machine has been giving me error E32(motor) too hot just after operating for about 8 minutes.When I physically check the motor the temperature is normal.
    I opened the MCU to check the status of the MOSFETS (K15N60) there are six of them.I removed them from the board and used an analog multimeter.One of them was giving me a deflection between Drain and source even when I interchanged the probes.Kindly help.I am not good at testing mosfets.

    Regards,
    Adamson

    Reply
  15. seymourfroggs

    Para 4, second last word. Should be CLOSED, shouldn’t it?

    But my question is, for a n-FET (its an IRF630), how do I find what voltage range allows the right current regulation?

    I understand about the Gate emf needing to reach (in my case) about 4.5 v before D – S flow occurs.
    But how do I control DS current?

    I know what the DS voltage is, and the Built-In resistance. To give a clear example, if Main Voltage is 15 V, and R is 2 ohm, I would get 7.5 A max.
    How do I control it for, say, 3 A and then for 6 A (brief pulses only)? How do I find the Vgs in each case?

    Reply
  16. Peter

    Does not distinguish between P and N channel operation until half way through page.
    ” By applying voltage at the gate, it generates an electrical field to control the current flow through the channel between drain and source…” does not say whether this is for N or P mosfet. Have to read several paragraphs down to realise that this is only true for one of them.

    Reply

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