Understanding Mosfets

Mosftets can be a bit of mystery and it took me a while to understand how exactly they worked and how best to configure them. In this article, we'll be exploring the basics of mosftets, the different types of mosfets and how you can use them in your electronics projects.

What exactly is a mosfet?

MOSFET stands for Metal Oxide Silicon Field Effect Transistor

That is quite a mouthful. Okay now try saying that 10 times :)

Anyways, I like to think of mosfets as an electronic part that you can use to turn on ON or OFF a part of your circuit - a bit like a switch.

If you are already familiar with transistors, mosfets are similar to transitors except that they get turned on and off with voltage as opposed to current.

Now there are four different types of mosfets.

  • Enhancement N-channel Mosfet
  • Depletion N-channel Mosfet
  • Enhancement P-Channel Mosfet
  • Depletion P-Channel Mosfet

We'll take a closer look at these differences in the next sections.

N-channel Mosfet (Enhancement)

All mosfets come with 3 pins, the Gate (G), the Drain (D) and Source (S). The way a mosfet is turned on diffs from between an n-channel and p-channel mosfet.

For n-channel mosfets, it will turn on when the Gate voltage is sufficiently GREATER than the Source voltage. Let's take a closer look at what that means.

Here is a basic TO-220 style n-channel enhancement mosfet - the IRLZ44N.

Now, if you are working with microcontrollers like Atmega or Arduinos, you want a logic level mosfet like this IRLZ44N.

Logic level mosfets simply are a specific breed of mosfets that are designed to operate with the 3V-5V range so is perfect for your 5V arduino.

To use a n-channel mosfet, you would typically create a circuit like this.

This is known as a low side configuration because the mosfet is connected to the ground and the load (for example, a light bulb that you want to turn on) is between VCC and the mosfet.

A simple rule for both n-channel and p-channel mosfets is that your load goes on the side of the Drain (D) if you are going to a standard configuration.

It is possible to connect the load to the Source but you would need a means of increasing the voltage at the gate (like a mosfet driver or charge pump) in order to drive the mosfet. In short, n-channel mosfets are much easier to use in low side configuration.

The resistor R1 (100K) is just a pull down resistor to pull the signal to 0V (GDN) when the controller is not turned on, sends a low signal or is in an undefined state.

The mosfet is turned ON by sending a voltage to the Gate. In this example, the controller might be your 5V Arduino.

At this point in time, the Vgs (the voltage difference between the Gate and source) is 5V with Gate being 5V and Source being ground (0V).

n-channel mosfets turn on when there is sufficient difference between the gate and source.

For this logic level mosfets like the IRLZ44N, it gets fully turned on when the Vgs reaches 3-5 volts.

So with Vgs at 5V, the mosfet turns on and current now flows from VCC > Load > Drain (D) > Source (S) and finally GDN.

When your controller goes LOW, Vgs would not be sufficient so current would then stop flowing between Drain and Source.

P-channel Mosfet (Enhancement)

So far we've seen that an n-channel mosfet gets turned ON when the Gate voltage is suffiently greater than the Source. Let's tak ea look at how an enhancement p-channel mosfet works.

A p-channel mosfet like the NDP6020P shown here, works when the Gate voltage is suffiently LOWER than the source voltage.

For logic level p-channel mosfets it turns on when the Vgs is -3V to -5V.

Here is a typical configuration for a enhancement p-channel mosfet.

Notice that your load now goes between the mosfet and ground. This is known as a high-side configuration because the mosfet sits between VCC and the load

You can do a low side configuration (where load is connected to source) but you would need a negative voltage to drive the mosfet.

The mosfet opens, when you controller (like an Arduino) sends a low signal, the Gate (G) connects to ground.

The difference between the Source (S) and Gate (G) becomes negative. The 5V (Source) - 0V (Gate) = -5V. Since the Vgs is -5V and exceeds the mosfet Vgs threshold, the mosfet is in an open state.

The mosfet closes when your controller sends a high signal.

This is because the Vgs is now at 0V with 5V (source) - 5V (Gate) = 0V.

Since p-channel mosfets turn ON when Vgs is negative, a possible disadvantage of this configuration is that your logic is inverted.

The mosfets turns on when you send a LOW signal and turned on when you send a HIGH signal.

P-channel with BJT

Well what if you absolutely wanted to turn on the mosfet with a HIGH signal and off with a LOW using a p-channel mosfet?

One way this can be accomplished is by using a n-channel BJT that sits between your controller and mosfet.

The above works because when when your controller sends a LOW signal, the BJT is closed with the base receiving no current. This results in the Gate (G) of the mosfet having the same potential as VCC and therefore, the mosfet remains in a closed state.

When your controller sends a HIGH signal, the BJT opens and the Gate (G) of the mosfet is pulled to ground. This causes the Vgs to be negative and opens the mosfet.

Depletion-Type Mosfet

So far we've only discussed the enhancement-type mosfet which is much more common. There is another type called the depletion-type mosfet.

With the enhancement-type mosfets we saw that when the Vgs is 0V the mosfet was turned off (refererred as normally OFF).

Depletion-type mosfets are simply the opposite and are in an ON state when Vgs is 0V. The logic is inverted and simply a depletion-type mosfet is normally ON.

What mosfet should I use?

The type of mosfet you should use really depends on your situation but if you are just starting off, an enhancement n-channel mosfet is easiest to work with and most commonly used. There are a few situations where you might need a p-channel mosfet so I tend to stock up on both.

It's also good to keep in mind that n-channel mosfets tend to be more efficient than p-channel mosfets.

About InverseQ

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I'm a senior front-end engineer by day and electronics inventor by night. I setup this site to share my explorations, discoveries and learnings with you.

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